Chapter Xii., Which, According to Kowalewsky and Others, Give Origin to the

free leucocytes.

We see, then, that the conception of a syncytial neuro-epithelial host holding in its meshes a number of free cells leads directly to the questions: What is the coelom? To which category does its lining membrane belong? and further, also, What is the origin of these free cells?

The Metazoa have been divided into two great groups--those which possess a coelom (the Coelomata; Lankester's Coelomocoela) and those which do not (Coelenterata; Lankester's Enterocoela). As an example of the latter we may take Hydra, because it is a very {473}primitive form, and because its development has been carefully worked out recently by Brauer.

In Hydra we find a dermal layer of cells and an inner layer of cells separated by a gelatinous mass known as mesogloea; in this mass between the dermal and inner layers scattered cells are found, the interstitial cells. Now, according to Brauer the position of the germ in Hydra is the interstitial cell-layer. One cell of the ovarium becomes the egg-cell, the others have their substance changed into yolk-grains, forming the so-called pseudo-cells, and as such afford pabulum to the growing egg-cell. Thus we see that in between the dermal and gastral layer of cells a third layer of cells is found, composed of free living germ-cells, some of which, by the formation of yolk-granules, become degraded into pabulum for their more favoured kinsfolk. These interstitial cells are said to arise from the dermal layer, or ectoderm, but clearly, as in other cases, germ-cells constitute a class by themselves and cannot be spoken of as originating from ectoderm-cells or from hypoderm-cells.

So also in Porifera, Minchin states: "In addition to the collared cells of the gastral layer, and the various cell-elements of the dermal layer, the body-wall contains numerous wandering cells or amoebocytes, which occur everywhere among the cells and tissues. Though lodged principally in the dermal layer, they are not to be regarded as belonging to it, but as constituting a distinct class of cells by themselves. They are concerned probably with the functions of nutrition and excretion, and from them arise the genital products." Further (p. 31): "At certain seasons some of these cells become germ-cells; hence the wandering cells and the reproductive cells may be included together under the general term archæocytes." Also (p. 51): "The mesogloea is the first portion to appear as a structureless layer between the dermal and gastral epithelia, and is probably a secretion of the former."

He also points out that in these, the very lowest of the Metazoa, the separate origin of these archæocytes can be traced back to a very early period of embryonic life. Thus in Clathrina blanca the ovum undergoes a regular and total cleavage, resulting in the formation of a hollow ciliated blastula of oval form. At one point, the future posterior pole of the larva, are a pair of very large granular cells with vesicular nuclei, which represent undifferentiated blastomeres and are destined to give rise to the archæocytes, and, therefore, also to the {474}sexual cells of the adult. Thus, as he says, from the very earliest period a distinction is made between the "tissue-forming" cells (my syncytial host) and the archæocytes.

We see, then, that the origin of all these free-living cells can be traced back to the very earliest of the Metazoa. Here between the dermal and gastral layers a gelatinous material, the mesogloea is secreted by these layers. This material is non-living, non-cellular. In it live free cells which may either be germ-cells, amoebocytes, or 'collencytes' (connective tissue cells). If this mesogloea were a fluid secretion, then we should have a tissue of the nature of blood or lymph; if it were solid, then we should have the foundation of connective tissue, cartilage, and bone.

From this primitive tissue it is easy to see how the special elements of the vascular, lymphatic, and skeletal tissues gradually arose, the matrix being provided by the cells of the syncytial host and the cellular elements by the archæocytes. In fact, we have no right to speak of these lowest members of the Metazoa as not being triploblastic, as possessing nothing corresponding to mesoblast, for in these free cells in the mesogloea we have the origin of the mesenchyme of the higher groups. Thus Lankester, talking of mesenchyme, says: "I think we are bound to bring into consideration here the existence in many Coelentera of a tissue resembling the mesenchyme of Coelomocoela. In Scyphomedusæ, in Ctenophora, and in Anthozoa, branched fixed and wandering cells are found in the mesogloea which seem to be the same thing as a good deal of what is distinguished as mesenchyme in Coelomocoela. These appear to be derived from both the primitive layers; some produce spicules, others fibrous substance, others again seem to be amoebocytes with various functions. It appears to be probable that, though it may be necessary to distinguish other elements in it, the mesenchyme of Coelomocoela is largely constituted by cells, which are the mother-cells of the skeletotrophic group of tissues, and are destined to form connective tissues, blood-vessels, and blood."

Thus we see that the earliest Metazoa were composed of a dermal and gastral epithelium, with a sub-epithelial nervous system connecting the parts together, which formed, as it were, a host, carrying around free living cells of varying function, all of which may be looked on as derived from archæocytes, i.e. germ-cells. From these the coelomatous animals arose, and here also we find, according to {475}present-day opinion, that the coelom arose in the first place in the very closest connection with the germ-cells or gonads. Thus Lankester, in his review of the history of the coelom, states:--

"The numerous embryological and anatomical researches of the past twenty years seem to me to definitely establish the conclusion that the coelom is primarily the cavity, from the walls of which the gonad cells (ova or spermata) develop, or which forms around those cells. We may suppose the first coelom to have originated by a closing or shutting off of that portion of the general archenteron of Enterocoela (Coelentera), in which the gonads developed as in Aurelia or as in Ctenophora. Or we may suppose that groups of gonad mother cells, having proliferated from the endoderm, took up a position between it and the ectoderm, and there acquired a vesicular arrangement, the cells surrounding the cavity in which liquid accumulated.

"The coelom is thus essentially and primarily (as first clearly formulated by Hatschek) the perigonadial cavity or gonocoel, and the lining cells of gonadial chambers are coelomic epithelium. In some few groups of Coelomocoela the coeloms have remained small and limited to the character of gonocoels. This seems to be the case in the Nemertina, the Planarians, and other Platyhelmia. In some Planarians they are limited in number, and of individually large size; in others they are numerous."

When Lankester says that "the lining cells of gonadial chambers are coelomic epithelium," that is equivalent to saying that the lining cells of the coelom form an epithelium which was originally gonadial, provided that, as seems to me most probable, his second suggestion, of the coelom being formed from gonadial mother-cells which have taken up an intermediate position between endoderm and ectoderm and there acquired a vesicular arrangement, is the true one. It does not seem to me possible to conceive of the gonads arising from cells of the epiblast or of the hypoblast, in the sense that such cells are differentiated cells belonging to a layer with a definite meaning. When we consider that the gonad gives origin to the whole of a new individual, that in the protozoan ancestors of the Metazoa their ultimate aim and object was the formation of gonads, it seems a wrong conception to speak of the gonads as formed from cells belonging either to the gut-wall or to the external epithelium. The gonads must stand in a category by themselves; they represent a whole, {476}while the other cells represent only a part; they cannot therefore be derived from the latter. They may, and indeed do, give rise to cells of a subordinate character, but they cannot rightly be spoken of as derived from such cells. The very fact mentioned by Lankester, that in the lowest coelomatous Metazoa, the Platyhelminthes, the coeloms are limited to the character of simple gonocoels, strongly points to the conclusion that all the coelomic cells were originally of the nature of gonadial cells, and therefore free-living and independent of the rest of the cells of the body. Whether the germ-cells appear, as in Hydra, to be derived from the ectoblast, or, as is usually stated, from the endoblast, in neither case ought they to be classed with the internal or external epithelium; they are germ-cells, and the epithelium which they form is neither epiblastic nor hypoblastic, but germinal, forming originally a simple gonocoele, afterwards, in the higher forms, the coelom with its cells of various function. Thus, to quote again from Lankester, "The coelomic fluid and the coelomic epithelium, as well as the floating corpuscles derived from that epithelium, acquire special properties and importance over and above the original functions subservient to the maturation of the gonadial cells ... the most important developments of the coelom are in connection with the establishment of an exit for the generative products through the body-wall to the outer world, and further in the specialization of parts of its lining epithelium for renal excretory functions."

Such exits led very early to the formation of coelomoducts, which are true outgrowths of the coelom itself (p. 14): "The coelomoducts and the gonocoels of which they are a part, frequently acquire a renal excretory function, and may retain both the function of genital conduits and of renal organs, or may, where several pairs are present (metamerized or segmented animals), subserve the one function in some segments of the body, and the other function in other segments."

The origin of the coelom and its derivatives from a germinal membrane, as suggested by Lankester, appears to me most probable, and, if true, it carries with it conclusions of far-reaching importance, for it necessitates that all the cells which line true coelomic cavities, and their derivatives, belong to the category of free-living cells, and are not connected with the nervous system. The cells in question are essentially those which line serous cavities and those which form excretory glands such as the kidneys. In the latter organ we ought especially to be able to obtain a clear answer to this question, for is {477}it not a gland which secretes into a duct and might therefore be expected to be innervated in the same way as other secretory glands? Although there is a strong primâ facie presumption in favour of the existence of renal secretory nerves, yet according to the universal opinion of physiologists no evidence in favour of such nerves has hitherto been found; all the phenomena of excretion of urine consequent on nerve stimulation are explicable by the action of nerves on the renal vessels, not on the renal cells. Not only is the physiological evidence negative up to the present time, but also, I think, the histological. On the one hand, Retzius has failed to find nerve-connections with kidney-cells; on the other, Berkley has obtained such evidence with the Golgi method, but failed entirely with methylene blue. I do not myself think that the evidence of the Golgi method alone is sufficient without corroboration by other methods, and, in any case, Berkley's evidence does not show the nerve-fibres terminating in the kidney-cells, in the same way as can be shown by modern methods to exist in the case of epithelial cells of the surface, etc. Quite recently another paper on this subject has appeared by Smirnow, who appears to have obtained better results than those given by Berkley.

Apart from these physiological and histological considerations, this question is also dependent upon the nature of the development of the excretory organs, for, according to Lankester, all excretory organs may be divided into the two classes of nephridial organs and coelomostomes, of which the former are largely derived from epiblast. We should, therefore, expect to find secretory nerves to nephridial organs, though possibly not to coelomostomes. The kidneys of the Mammalia are supposed to be true coelomostomes, although, according to Goodrich's researches, the excretory organs in Amphioxus are solenocytes, i.e. true nephridia.

As to the lining epithelium of the peritoneal, pleural, and pericardial cavities--i.e. the mesothelium--there is no definite evidence that these cells are provided with nerves. Such surfaces are remarkably insensitive in the healthy condition, and the pain in such cavities is essentially a pressure phenomenon and referable to special sense-organs, such as Pacinian bodies, etc., rather than to the mesothelium itself.

These sense-organs are identical in structure with those in the skin, and, as Anderson has shown, the nerves of these organs {478}medullate at the same time as those in the skin, and both obtain their medullary sheaths earlier than any other nerves, whether afferent or efferent. However difficult it may be to explain this fact, only one conclusion seems to me possible--these Pacinian bodies, like the skin Pacinians, originate from a nest of surface epithelial cells, a conclusion which is extremely probable on my theory of the origin of vertebrates, but not, as far as I can see, on any other.

At the present moment the weight of evidence is, to my mind, in favour of the lining endothelium of the coelomic cavities being composed of free cells, unconnected with the nervous system rather than the reverse, but I must confess that the question is undecided. If it be true that the coelomic lining is partly enterocoelic and partly gonocoelic, as Lankester teaches, then it would be natural that its cells should be in connection with the nervous system, to some extent at all events. This view is, however, based on very slender foundations. If the mesothelium is composed of cells capable of becoming free, it cannot give rise to the skeletal muscles, and it cannot therefore be right to speak of the skeletal muscles as derived from the lining cells of a part of the primary coelom. The phylogenetic history of the musculature of the different animals points strongly to its intimate connection with and derivation from surface epithelial cells rather than from coelomic mesothelial cells. Thus in the coelenterates, as seen in Hydra, the muscular layer arises directly from a modification of the surface epithelial cells; and right up to the annelids, even to the highest form in the Polychæta, we still see it stated that the musculature, both circular and longitudinal, arises from the ectoderm. In the Oligochæta and Hirudinea, according to Bergh, there are five rows of teloblasts on each side, of which four are ectodermic and give rise to the nerve-ganglia and the circular muscles, while one is mesoblastic and forms the nephridial organs and the longitudinal muscles. (The latter statement is, according to Bergh, well known, and is not particularly shown by him. These longitudinal muscle-bands always lie close against the nervous system at their first formation, and may well have been derived in connection with it.)

It is apparently only in the Vertebrata that the lining cells of the coelomic cavity are definitely stated to give origin to the body-musculature, and taking into account on the one hand the evidence of Graham Kerr as to the intimate connection between nerve-cell and {479}muscle-cell from the very beginning, and on the other the manner in which all the skeletal muscles of the adult are lined with a lymphatic endothelium, I am strongly inclined to believe that at the closing up of the myocoele, when the myomere separates from the mesomere, the lining cells remain scattered in among the forming muscle-cells and form the ultimate lymphatic tissue of the muscles. If this is really so, then the evidence in favour of the mesothelium being composed of free cells not connected with the nervous system would be much strengthened, for, on the one hand, an intimate relation exists between the connective tissue cells and the endothelium of the roots of the lymphatic vessels, a relation which, according to Virchow, has rendered it impossible to draw any sharp line of distinction between the two; and, on the other, the lymphatic endothelium merges into the lining cells of the great serous cavities of the body.

It is impossible to conceive of an animal possessing a nervous system which is not in connection with sensory and muscular tissues; an isolated nerve-cell is a meaningless possession; but it is equally natural to conceive of a germ-cell being isolated, capable of living an independent existence. Such a difference between the two kinds of tissues must have existed from the very commencement of the Metazoa, so that we must, it seems to me, imagine that in the formation of the Metazoa from the Protozoa the whole of the body of the latter did not break up into a mass of separate gonads, each capable of becoming a free-living protozoan similar to its parent, but that a portion proliferated into a multinucleated syncytium while the remainder formed the free-living gonads. This multinucleated syncytium, or host, as it might be called, would still continue to exist for the purpose of carrying further afield the immortal gonads, which need no longer be all shed at one time.

In such an animal as Volvox globator we have an indication of the very kind of animal postulated as connecting the single-celled Protozoa and the multi-cellular Metazoa, for it consists of a many-celled case which forms a hollow sphere, each of the cells being provided with flagella for the purpose of locomotion of the sphere, except a certain number which are not flagellated; the latter leave the case to swim freely in the fluid contained within the sphere, and forming spermaries and ovaries, conjugate, maturate, and then are set free by the rupture of the encircling locomotor host.

{480}This conception of the predecessors of the Metazoa being composed of a mortal host, holding within itself the immortal sexual products, leads naturally to the idea of the separate development of the host from that of the germ-cells ab initio, so that the study of the development of the Metazoa means the study of two separate constituents of the metazoan individual--on the one hand, the elaboration of the elements forming the syncytial host, on the other, of those derived from the free-living independent germ-cells. The elaboration of the host means the differentiation of the protoplasm into epithelial, muscular, and nervous elements, by means of which the gonads were carried further afield and their nourishment as well as that of the host ensured.

The rôle of the nervous system as the middleman between internal and external muscular and epithelial surfaces was, I imagine, initiated from the very earliest time. The further evolution of the host consisted in a greater and greater differentiation and elaboration of this neuro-epithelial syncytium, with the result of a steadily increasing concentration and departmental centralization of the main factor of the syncytium; in other words, it led to the origin and elaboration of a central nervous system. In the interstices of this syncytium the gonads were placed, and at first, doubtless, the life of the host ended when all the germ-cells had been set free. 'Reproduce and die' was, I imagine, the law of the Metazoa at its earliest origin, and throughout the ages, during all the changes of evolution, the reminiscence of such law still manifests itself even up to the highest forms as yet reached. With the differentiation of the syncytial host there came also differentiation of the free-living gonads, so that only some of them attained to the perfection of independent existence, capable of continuing the species; while others became subordinate to the first and provided them with pabulum, manufacturing within themselves yolk-spherules, and thus in the shape of yolk-cells ministered to the developing egg-cell. Thus arose a germinal epithelium of which only a few of the elements passed out of the host as perfect individuals, the remainder being utilized for the nutrition of these few. Such yolk-cells of the germinal epithelium would still, however, retain their character as free cells totally independent of the syncytial host, and, situated as they were between the internal and external epithelium, capable of amoeboid movement, would naturally have their phagocytic action {481}utilized either as yolk-cells for the providing of pabulum to the egg-cell, or as excretory cells for the removal and rendering harmless of deleterious products of all kinds. Thus the free cells of the body would become differentiated into the three classes of germ-cells, yolk-cells, and excretory cells.

Further, the mass of gonads, which originally occupied so large a space within the interior of the host, necessarily, as the tissues of the host differentiated more and more, took up less and less space in proportion to the whole bulk of the host and formed a germinal mass of cells between the outer and inner epithelial layers. This germinal mass formed an epithelium, some of the members of which acted as scavengers for the inner and outer layers of the host, with the result that fluid accumulated between the two parts of the germinal epithelium in connection respectively with the external and internal epithelial surfaces of the host, and thus led to the formation of a gonocoele, which, by obtaining an external opening, a coelomostome, gave origin to the coelom.

Again, with the longer life of the host, the setting free of the gonads no longer necessitating the destruction of the host, and also the gonads themselves requiring a longer and longer time to be fed up to maturity, the bulk and complexity of the whole organism increased and special supporting structures became a necessity. The host itself could and did provide these to a certain extent by secretions from its epithelial elements, but the intermediate supports were provided by the system of phagocytic cells utilizing the fluids of the body, at first in the shape of plasma-cells able to move from place to place, then settling down to form a connective tissue framework, and, later on, cartilage and bone.

So also were gradually evolved the whole of the endothelial structures; the lymph-cells, blood-cells, etc., all having their origin from the free cells of the body, which themselves originated in the extension of a germinal epithelium. Just as in a bee-hive the egg-cells may form the fully developed sexual animal, whether drone or queen bee, or the asexual host of workers, so in the body of the Metazoa the free cells may form either male or female germ-cells spermatozoa, or ova, or a host of workers, scavengers, repairers, food-providers, all useful to the community, all showing their common origin by their absolute independence of the nervous system.

Two points of great importance follow from this method of looking {482}at the problem. First, the evolution of the animal kingdom means essentially the evolution of the host, for that is what forms the individual; secondly, as the host is composed of a syncytium, the common factor of whose elements is the neural moiety, it follows that the tissue of central importance for the evolution of the host must be, as indeed it is, the nervous system. Further, seeing that the growth of the individual means the orderly spreading out of the epithelial moiety away from the neural moiety, it follows that the germ-band or germ-area from which growth starts must be in the position of the nervous system. If then, the nervous system in the animal is a concentrated one, then the growth will emanate from the position of such nervous system. If, on the other hand, the nervous system is diffused, then the growth will also be diffused.

In this book I have throughout argued that the ancestors of vertebrates belonged to a great group of animals which gave origin also to Limulus and scorpion-like animals; it is therefore instructive to see what is the nature of the development of such animals. For this purpose I will take the development of the scorpion, as given by Brauer, for he has worked out its development with great thoroughness and care. His papers show that the segmentation is discoidal, and results in an oval blastodermic area lying on a large mass of yolk. Very early there separates out in this area genital cells and yolk-cells, which latter move freely into the yolk and prepare it into a fluid pabulum for the nutrition of the cells of the embryonic shield or germ-band. These free yolk-cells do not take part in the formation of the germinal layers, nor does the endoderm when formed give origin to free yolk-cells.

The cells of the germ-band form a small compact area, in which by continual mitosis the cells become more than one-layered, and soon it is found that those cells which lie close against the fluid pabulum form a continuous layer and absorb the nutritious material for themselves and the rest of the embryo. While this area is thus increasing in thickness by continuous development, the group of genital cells remains always apart, increasing in number, but being always in a state of isolation from the cells of the rest of the growing area. Thus from the very first Brauer's observations on the development of the scorpion point to the formation of a syncytial host containing separate genital cells. The continuous layer of cells against the fluid pabulum, which is already functioning as a gut, and may {483}therefore be called hypoblast, spreads continuously over the yolk, as also does the surface epithelial layer, or epiblast. Such spreading is always a continuous one for both surfaces, so that the yolk is gradually enclosed by a continuous orderly growth from the germ-band, and not by the settling down of free cells in the yolk here and there to form the gut-lining. This steady orderly development proceeds owing to the nourishment afforded by the activity of the free cells or vitellophags and the absorbing power of the hypoblast, a steady growth round the yolk which results in the formation of the gut-tube, the outer covering and all the muscular and excretory organs. Where, then, is this starting-point, this germ-band from which the whole embryo grows? It forms the mid ventral area of the adult animal, it corresponds exactly to the position of the central nervous system. The whole phenomenon of embryonic growth in the scorpion is exactly what must take place on the argument deduced from the study of the adult that the animal arises as a neuro-epithelial syncytium, and we see that that layer of cells which is situated next to the food-material forms the alimentary tube. It is not a question whether such layer is ventral or dorsal to the neural cells, but whether it is contiguous to or removed from the food-material.

Take, again, a meroblastic vertebrate egg as of the bird. Again we find free cells passing into the yolk to act as vitellophags, the so-called periblast cells; again we see that the embryo starts from a germ-band or embryonic shield, and spreads from there continuously and steadily; again we see that the layer of cells which lies against the yolk absorbs the fluid pabulum for the growing cells; again we see that the area from which the whole process of growth starts is that of the central nervous system, and again we see that those cells which are contiguous to the food form the commencing gut, and are therefore called hypoblast, though in this case they are ventral not dorsal to the neural layer.

The comparison of these two processes shows that there is one common factor, one thing comparable in the two, one thing that is homologous and is the essential in the formation of that part of the animal which I have called the host, and that is the central nervous system. Whether the epithelial layer which lies ventrally to it or the one that is dorsal forms the gut depends upon the position of the food-mass. Where the food is, there will be the absorbing layer. {484}Where the food is not, there will be no gut formation, whatever may have been the previous history of that layer. If, then, we suppose, as I do, that the vertebrate arose from a scorpion-like animal without any reversal of dorsal and ventral surfaces, and that the central nervous system remained the same in the two animals, then the comparison of the development of the two embryos shows that the one would be derived from the other if the yolk-mass shifted from the dorsal to the ventral side of the nervous system. This would leave the dorsal epithelial layer of the original syncytium free from pabulum; it would no longer form the definite gut, but it would still tend to form itself in the same manner as before, would still grow from a ventrally situated germ-band dorsalwards to form a tube, would recapitulate its past history, and show how the alimentary canal of the arthropod became the neural canal of the vertebrate. Although this alimentary canal is formed in the same way as before, it is no longer recognized as homologous with the scorpion's alimentary canal, but because it no longer absorbs pabulum, and does not therefore form the definite gut, it is called an epiblastic tube, and, in the words of Ray Lankester, has no developmental importance.

All the arthropods are built up on the same type, and in all the development may in its broad outlines be referred to the type just mentioned. So also with the vertebrate group; in both cases the position of the central nervous system determines the starting area of embryonic growth. In both cases the absorbing layer shows the position of the definite gut. A concentrated nervous system of this type is common to all the segmented animals from the annelids to the vertebrates, and in all cases the germ-band which indicates the first formation of the embryo is in the position of this nervous system.

As far as the embryo is concerned, there is no great difficulty in the conception that the yolk-mass may have shifted from one side to the other in passing from the arthropod to the vertebrate, for in the arthropod the embryo at first is surrounded by yolk and then passes to the periphery of the egg. If it is permissible to speak of a dorsal and ventral surface to an egg, and we may imagine the egg held with such dorsal surface uppermost, then the yolk would be situated ventrally to the embryo, as in the vertebrate, if the protoplasmic cells of the embryo rose from their central position to the surface through the yolk, while if they sank through the yolk, the yolk would be situated dorsally to the embryo, as in the arthropod.

{485}In cases where there is no yolk, or very little, as in Lucifer and Amphioxus respectively, the embryo is compelled to feed itself at a very early age; such embryos form a free-swimming pelagic ciliated blastula, the invagination of which, for the purpose of collecting food material out of the open sea, is the simplest method of obtaining nutriment. Here, as in other cases, it is the physiological necessity which determines the method of formation of the gut, and such similarity of appearance as exists between the gastrula of Lucifer and that of Amphioxus, by no means implies that the gut of the adult Lucifer is homologous with the gut of Amphioxus.

I have compared two meroblastic eggs of the two classes respectively, because the scorpion's egg is meroblastic. I imagine that no real difficulty arises with respect to holoblastic eggs, for the experiments of O. Hertwig and Samassa show that by centrifugalizing, stimulating, and breaking down of large spheres the holoblastic amphibian egg may be converted into a meroblastic one, and then development will proceed regularly, i.e. in this case also the growth proceeds from the animal pole; the large cells of the vegetal pole, like the yolk-cells of the meroblastic egg, manufacture pabulum for the growing syncytial host.

SUMMARY.

Any attempt to discover how vertebrates arose from invertebrates must be based upon the study of Comparative Anatomy, of Palæontology, and of Embryology. The arguments and evidence put forward in the preceding chapters show most clearly how the theory of the origin of vertebrates from palæostracans is supported by the geological evidence, by the anatomical evidence, and by the embryological evidence. Of the three the latter is the strongest and most conclusive, if it be taken to include the evidence given by the larval stage of the lamprey.

The stronghold of embryology for questions of this sort is the Law of Recapitulation, which asserts that the history of the race is recapitulated to a greater or less extent in the development of the individual. In the previous chapters such recapitulation has been shown for all the organs of the vertebrate body. In this respect, then, embryology has proved of the greatest value in confirming the evidence of relationship between the palæostracan and the vertebrate, given by anatomical and geological study.

There is, however, another side to embryology, which claims that the tissues of all the Metazoa are built up on the same plan; that in all cases in the very early stage of the embryo three layers are formed, the epiblast, mesoblast, and hypoblast; that in all animals above the Protozoa these three layers are {486}homologous, the epiblast in all cases forming the external or skin-layer, the hypoblast the internal or gut-layer.

Such a theory, therefore, as is advocated in this book, which turns the gut of the arthropod into the neural canal of the vertebrate, and makes a new gut for the vertebrate from the external surface must be wrong, as it flatly contradicts the fundamental germ-layer theory.

Of recent years grave doubts have been thrown upon the validity of this theory, doubts which have increased in force year by year as more and more facts have been discovered which are not in agreement with the theory. So much is it now discredited that any criticism against my theory, which is based upon it, weighs nothing in the balance against the positive evidence of recapitulation already stated. If the germ-layer theory is no longer credited, upon what fundamental laws is embryology based?

In this chapter I have ventured to suggest a reply to this question, based on the uniformity of the laws of growth throughout the existence of the individual.

In the adult animal the body is composed of two kinds of tissues, those which are connected with or at all events are under the control of the nervous system, and those which are capable of leading a free life independent of the nervous system. These two kinds of tissues can be traced back from the adult to the embryo, and it is the task of embryology to find out how these two kinds of tissue originate.

The following out of this line of thought leads to the conception that, throughout the Metazoa, the body is composed of a host which consists of the master-tissues of the body, and takes the form of a neuro-epithelial syncytium, within the meshes of which free living independent organisms or cells live, so to speak, a symbiotic existence.

The evidence points to the origin of all these free cells from germ-cells, and thus leads to the conception that the blastula stage of every embryo represents two kinds of cells, the one which will form the mortal host being the locomotor neuro-epithelial cell, the other the independent immortal symbiotic germ-cell. Such conception leads directly to the conclusion that the blastula stage of every member of the Metazoa is the embryonic representation of a Protozoan ancestor of the Metazoa; an ancestor, whose nature may be illustrated by such a living form as Volvox globator, which, like a blastula, is composed of a layer of cells forming a hollow sphere. These cells partly bear cilia, and so form a locomotor host, partly are of a different character, and form male and female germ-cells. The latter leave the surface of the sphere, pass as free individuals into its fluid contents, form spermaries and ovaries, and then by the rupture of the mortal locomotor host pass out into the external medium, as free swimming young Volvox.

It is of interest to note that such members of the Protozoa are among the most highly developed of the members of this great group.

From such a beginning arose in orderly evolution, on the one hand, all the neuro-muscular and neuro-epithelial structures of the body--the so-called master-tissues; on the other, the germ-cells, the blood-corpuscles, lymph-corpuscles plasma and excretory cells, connective tissue cells, cartilage and bone-cells, etc., all of them independent of the central nervous system, all traceable to a modification of the original germ-cells.

{487}Such a view of the processes of embryology brings embryology into harmony with comparative anatomy and phylogeny, for it makes the central nervous system and not the alimentary canal the most important factor in the development of the host.

The growth of the individual, whether arthropod or vertebrate, spreads from the position of the central nervous system, regardless of whether that position is a ventral or dorsal one with respect to the yolk-mass. Where the pabulum is, there is the definite gut, the lining walls of which are called in the embryo, hypoblast; but when the pabulum is no longer there, although a tube is formed in the same manner as the alimentary canal of the arthropod, it is now called an epiblastic tube, and is known as the neural tube of the vertebrate.

This is the great fallacy of the germ-layer theory, a fallacy which consists of an argument in a vicious circle: thus the alimentary canal is homologous in all of the Metazoa, because it is formed of hypoblast, but there is no definition of hypoblast, except that it is always that layer which forms the definitive alimentary canal.

When, after the process of segmentation has been completed, a free swimming blastula results, unprovided with any store of pabulum in the shape of yolk, then the same physiological necessity causes such a form to obtain its nutriment from the surrounding medium. The simplest way to do this is by a process of invagination, in consequence of which food particles are swept into the invaginated part and then absorbed. For this reason in such cases true gastrulæ are formed, as in the case of Amphioxus among the vertebrates, and Lucifer among the crustaceans; such a formation does not in the least imply that the gut of the arthropod is homologous with that of the vertebrate. The resemblance between the two is not a morphological one, but due to the same physiological necessity. They are analogous formations, not homologous.

The muscular tissues are found to be formed in close connection with the nervous tissues, and in very many cases are described as formed from epiblast, so that there are strong reasons for placing them in a special category of the so-called mesoblastic tissues. If they be separated out, then it seems to me, the rest of the mesoblast would consist of the free-living cells of the body, which are not connected with the central nervous system. In watching, then, the formation of mesoblast, defined in this way, we are watching the separation out from the master-tissues of the body of the independent skeletal and excretory cells.

{488}CHAPTER XV

FINAL REMARKS

Problems requiring investigation--

Giant nerve-cells and giant-fibres; their comparison in fishes and in arthropods; blood- and lymph-corpuscles; nature of the skin; origin of system of unstriped muscles; origin of the sympathetic nervous system; biological test of relationship.

Criticism of Balanoglossus theory.--Theory of parallel development.--Importance of the theory advocated in this book for all problems of Evolution.

The discussion in the last chapter on the "Principles of Embryology" completes the evidence which I am able to offer up to the present time in favour of my theory of the "Origin of Vertebrates." There are various questions which I have left untouched, but still are well worth discussion, and may be mentioned here. The first of these is the significance of the giant nerve-cells and giant nerve-fibres so characteristic of the brain-region of the lower vertebrates. In most fishes two very large cells are most conspicuous objects in any transverse section of the medulla oblongata at the level of entrance of the auditory nerves. Each of these cells gives off a number of processes, some of which pass in the direction of the auditory nerves and one very large axis-cylinder process which forms a giant-fibre, known by the name of a Mauthnerian fibre. Each Mauthnerian fibre crosses the middle line soon after its origin from the giant-cell, and passes down the spinal cord on the opposite side right to the tail. Here, near the end of the spinal cord, it breaks up into smaller fibres, which are believed by Fritsch and others to pass out directly into the ventral roots to supply the muscles of the tail. Thus Bela Haller says: "The Mauthnerian fibres are known to give origin to certain fibres which supply the ventral roots of the last three spinal nerves, so that their terminal branches serve, in all probability, for the innervation of the muscles of the tail-fin." They do not occur in the eel, according to Haller, or in Silurus, according to Kölliker. {489}Their absence in those fishes, in which a well-developed tail-fin is also absent, increases the probability of the truth of Fritsch's original conclusion that these giant-fibres are associated axis-cylinders for certain definite co-ordinated movements of the fish, especially for the lateral movement of the tail.

In Ammocoetes, instead of two Mauthnerian fibres, a number of giant-fibres are found. They are called Müllerian fibres, and arise from giant-cells which are divisible into two groups. The first group consists of three pairs situated headwards of the level of exit of the trigeminal nerves. Two of these lie in front of the level of exit of the oculomotor nerves, and one pair is situated at the same level as the origin of the oculomotor nerves. The second group consists of a number of cells on each side at the level of the entrance of the fibres of the auditory nerves.

The Müllerian fibres largely decussate, as described by Ahlborn, and then become the most anterior portion of the white matter of the spinal cord, forming a group of about eight fibres on each side (Fig. 73). A few fibres are also found laterally, and slightly dorsally, to the grey matter. These giant-fibres pass down the spinal cord right to the anal region; their ultimate destination is unknown. Mayer considers that in the first part of their course they correspond to those tracts of fibres known as the "posterior longitudinal bundles" in other vertebrates. I imagine, therefore, that the spinal part of their course represents the two antero-lateral descending tracts. The second group of giant-cells, which appears to have some connection with the auditory nerves, may represent "Deiter's nucleus." The whole system is probably the central nervous part of a co-ordination mechanism, which arises entirely in the pro-otic or prosomatic region of the brain--the great co-ordinating and equilibrating region par excellence.

If we turn now to the arthropod it is a striking coincidence that in the crayfish and in the lobster the work of Retzius, of Celesia, of Allen, and of many others demonstrates the existence of an equilibration-mechanism for the swimming movements of the tail-muscles, which is carried out by means of giant-fibres. These giant-fibres are the axis-cylinder processes of giant-cells, situated exclusively in the brain-region, and they run through the whole ventral ganglionic chain in order to supply the muscles of the tail. In the ventral nerve-cord of the crayfish, according to Retzius, two specially large {490}giant-fibres exist, each of which breaks up, at the last abdominal ganglion, into smaller fibres, which pass directly out with the nerves to the tail-fin. Allen has shown that, in addition to these two specially large giant-fibres, there are a number of others, some of which, similarly to the Müllerian fibres of Ammocoetes, cross the middle line, while some do not. Each of these arises from a large nerve-cell and passes to one or other of the last pair of abdominal ganglia. The latter fibres, he says, send off collaterals, while the two specially large giant-fibres do not. The cells which give origin to all these large, long fibres are situated in or in front of the prosomatic region of the brain, similarly to the giant-cells, which give rise to the corresponding Müllerian fibres of Ammocoetes. I do not know how far this system is represented in Limulus or Scorpio.

It is, to my mind, improbable that the Mauthnerian fibres pass out directly as motor fibres to the muscles of the tail-fin; it is more likely that they are conducting paths between the equilibration-mechanism in connection with the VIIIth nerve and the spinal centres for the movements of the tail. Similarly, with respect to the arthropod, it is difficult to believe that the motor fibres for the tail-muscles arise in the brain-region. In either case, the striking coincidence remains that the movements of the tail-end of the body are regulated by means of giant-fibres which arise from giant-cells in the head-region of the body in both the Arthropoda and the lowest members of the Vertebrata.

The meaning of this system of giant-cells and giant-fibres in both classes of animals is well worthy of further investigation.

Another important piece of comparative work which ought to help in the elucidation of this problem is the comparison of the blood- and lymph-corpuscles of the vertebrate with those of the invertebrate groups. As yet, I have not myself made any observations in this direction, and feel that it is inadvisable to discuss the results of others until I know more about the facts from personal observation.

The large and important question of the manner of formation of the vertebrate skin has only been considered to a slight extent. A much more thorough investigation requires to be made into the nature of the skin of the oldest fishes in comparison with the skin of Ammocoetes on the one side, and of Limulus and the Palæostraca on the other.

The muscular system requires further investigation, not so much {491}the different systems of the striated voluntary musculature--for these have been for the most part compared in the two groups of animals in previous chapters--as the involuntary unstriped musculature, about which no word has been said. The origin of the different systems of unstriped muscles in the vertebrate is bound up with the origin of the sympathetic system and its relation to the cranial and sacral visceral systems. The reason why I have not included in this book the consideration of the sympathetic nervous system is on account of the difficulty in finding any such system in Ammocoetes. Also, so far as I know, the distribution of unstriped muscle in Ammocoetes has not been worked out.

One clue has arisen quite recently which is of great importance, and must be worked out in the future, viz. the extraordinary connection which exists between the action of the sympathetic nervous system and the action of adrenalin. This substance, which is obtained from the medullary part of the adrenal or suprarenal glands, when injected into an animal produces the same effects as stimulation of the nerves, which belong to the lumbo-thoracic outflow of visceral nerves, i.e. the system known as the sympathetic nervous system, which is distinct from both the cranial and sacral outflows of visceral nerves. The similarity of its action to stimulation of nerves is entirely confined to the nerves of this sympathetic system, and never resembles that of either the cranial or sacral visceral nerves.

Another most striking fact which confirms the great importance of this connection between the adrenals and the sympathetic nervous system from the point of view of the evolution of the latter system is that the extract of the adrenals always produces the same effect as that of stimulation of the nerves of the sympathetic system, whatever may be the animal from which the extract is obtained. Thus adrenalin obtained from the elasmobranch fishes will produce in the highest mammal all the effects known to occur upon stimulation of the nerves of its sympathetic system.

Further, the cells, which are always associated with the presence of this peculiar substance--adrenalin--stain in a characteristic manner in the presence of chromic salts. In Ammocoetes patches of cells which stain in this manner have been described in connection with blood-vessels in certain parts, so that, although I know of no definite evidence of the existence of cell-groups in Ammocoetes corresponding to the ganglia of the sympathetic system in other vertebrates, it is {492}possible that further investigation into the nature and connection of these "chromaffine" cells may afford a clue to the origin of the sympathetic nervous system. At present it is premature to discuss the question further.

Finally, another test as to the kinship of two animals of different species must be considered more fully than I have been able to do up to the present time. This test is of a totally different nature to any put forth in previous pages. It is known as the "biological test" of relationship, and is the outcome of pathological rather than of physiological or anatomical research. It is possible that this test may prove the most valuable of all. At present we do not know sufficiently its limitations and its sources of error, especially in the case of cold-blooded animals, to be able to look upon it as decisive in a problem of the kind considered in this book.

The nature of this test is as follows: It has been found that the serum of the blood of another animal, when injected in sufficient quantity into a rabbit, will cause such a change in the serum of that rabbit's blood that when it is added to the serum of the other animal a copious precipitate is formed, although the serum of normal rabbit's blood when mixed with that of another animal will cause no precipitate whatever. This extraordinary production of a precipitate in the one case and not in the other indicates the production of some new substance in the rabbit's serum in consequence of the introduction of the foreign serum into the rabbit, which brings about a precipitate when the rabbit's serum containing it is mixed with the serum originally injected. The barbarous name "antibody" has been used to express this supposed substance in accordance with the meaning of such a word as "antitoxin," which has been a long time in use in connection with preventive remedies against pathogenic bacteria. Now, it is found that the rabbit's serum containing a particular "antibody" will cause a precipitate only when added to the serum of the blood of the animal from which the "antibody" was produced or to the serum of the blood of a nearly related animal.

Further, if that animal is closely related a precipitate will be formed nearly as copious as with the original serum, if more distantly related a cloudiness will occur rather than a precipitate, and if the relationship is still more distant the mixture of the two sera will remain absolutely clear. Thus this test demonstrates the close relationship of man to the anthropoid apes and his more distant {493}relationship to monkeys in general. By this method very evident blood-relationships have been demonstrated, especially between members of the Mammalia.

I therefore started upon an investigation into the possibility of proving relationship in this way between Limulus and Ammocoetes, with the kind assistance of Mr. Graham Smith. I must confess I was not sanguine of success, as I thought the distance between Limulus and Ammocoetes was too great. Dr. Lee, of New York, kindly provided me with most excellent serum of Limulus, and the first experiments showed that the anti-serum of Limulus gave a most powerful precipitate with its own serum. Graham Smith then tried this anti-serum of Limulus with the serum of Ammocoetes, and to his surprise, and mine, he obtained a distinct cloudiness, indicative of a relationship between the two animals. This, however, is not considered sufficient, the reverse experiment must also succeed. I therefore, with Graham Smith, obtained a considerable amount of blood from the adult lampreys at Brandon, and produced an anti-serum of Petromyzon, which gave some precipitate with its own serum, but not a very powerful one. This anti-serum tried with Limulus gave no result whatever, but at the same time it gave no result with serum from Ammocoetes, so that the experiment not only showed that Petromyzon was not related to Limulus, but also was not related to its own larval form, which is absurd.

Considerable difficulties were encountered in preparing the Petromyzon anti-serum owing to the extreme toxic character of the lamprey's serum to the rabbit; in this respect it resembled that of the eel. It is possible that the failure of the lamprey's anti-serum was due to the necessity of heating the serum sufficiently to do away with its toxicity before injecting it into the rabbit. At this point the experiments have been at present left. It will require a long and careful investigation before it is possible to speak decisively one way or the other. At present the experiment is positive to a certain extent, and also negative; but the latter proves too much, for it proves that the larva is not related to the adult.

Some day I hope this "biological test" will be of use for determining the relationships of the Tunicata, the Enteropneusta, Amphioxus, etc., as well as of Limulus and Ammocoetes.

The origin of Vertebrates from a Palæostracan stock, as put forward in this book, gives no indication of the systematic position {494}of the Tunicata or Enteropneusta. Neither the Tunicata nor Amphioxus can by any possibility be on the direct line of ascent from the invertebrate to the vertebrate. They must both be looked upon as persistent failures, relics of the time when the great change to the vertebrate took place. The Enteropneusta are on a different footing; in their case any evidence of affinity with vertebrates is very much more doubtful.

The observer Spengel, who has made the most exhaustive study of these strange forms, rejects in toto any connection with vertebrates, and considers them rather as aberrant annelids. The so-called evidence of the tubular central nervous system is worth nothing. There is not the slightest sign of any tubular nervous system in the least resembling that of the vertebrate. It is simply that in one place of the collar-region the piece of skin containing the dorsal nerve of the animal, owing to the formation of the collar, is folded, and thus forms just at this region a short tube. My theory explains in a natural manner every portion of the elaborate and complicated tube of the vertebrate central nervous system. In the Balanoglossus theory the evolution of the vertebrate tube in all its details from this collar-fold is simple guesswork, without any reasonable standpoint. Similarly, the small closed diverticulum of the gut in Balanoglossus, which is dignified with the name of "notochord," has no right to the name. As I have already said, it may help to understand why the notochord has such a peculiar structure, but it gives no help to understanding the peculiar position of the notochord. The only really striking resemblance is between the gill-slits of Amphioxus and of the Enteropneusta. In this comparison there is a very great difficulty, very similar to that of the original attempts to derive vertebrates from annelids--the gill-slits open ventrally in the one animal and dorsally in the other. In both animals an atrial cavity exists which is formed by pleural folds, and in these pleural folds the gonads are situated so that the similarity of the two branchial chambers seems at first sight very complete. In the Enteropneusta, however, there are certain forms--Ptychodera--in which these pleural folds have not met in the mid-line in this branchial region, and in these it is plainly visible that these folds, with their gonads, spring from the ventral mid-line and arch over the dorsal region of the body. Equally clearly Amphioxus shows that its pleural folds, with the gonads, spring from the dorsal side of the animal, {495}and grow ventralwards until they fuse in the ventral mid-line (cf. Fig. 168).

As far, then, as this one single striking similarity between Amphioxus and the Enteropneusta is concerned it necessitates the reversal of dorsal and ventral surfaces to bring the two branchial chambers into harmony.

Al., alimentary canal; D.A., dorsal vessel; V.A., ventral vessel; g., gonads; NC., notochord; C.N.S., central nervous system.]

In a mud-dwelling animal, like Balanoglossus, which possesses no appendages, no special sense-organs, it seems likely enough that ventral and dorsal may be terms of no particular meaning, and consequently what is called ventral in Balanoglossus may correspond to what is dorsal in Amphioxus; in this way the branchial regions of the two animals may be closely compared. Such comparison, however, immediately upsets the whole argument of the vertebrate nature of Balanoglossus based on the relative position of the central nervous system and gut, for now that part of its nervous system which is looked upon as the central nervous system in Balanoglossus is ventral to the gut, just as in a worm-like animal, and not dorsal to it as in a vertebrate.

There is absolutely no possibility whatever of making such a detailed comparison between Balanoglossus and any vertebrate, as I have done between a particular kind of arthropod and Ammocoetes. In the latter case not only the topographical anatomy of the organs in the two animals is the same, but the comparison is valid even to microscopical structure. In the former case the origin of almost all {496}the vertebrate organs is absolutely hypothetical, no clue is given in Balanoglossus, not even to the segmented nature of the vertebrate. The same holds good with the evidence from Embryology and from Palæontology. I have pointed out how strongly the evidence in both cases confirms that of Comparative Anatomy. In neither case is the strength of the evidence for Balanoglossus in the slightest degree comparable. In Embryology an attempt has been made to compare the origin of the coelom in Amphioxus and in Balanoglossus. In Palæontology there is nothing, only an assumption that in the Cambrian and Lower Silurian times a whole series of animals were evolved between Balanoglossus and the earliest armoured fishes, which have left no trace, although they were able to hold their own against the dominant Palæostracan race. The strangeness of this conception is that, when they do appear, they are fully armoured, as in Pteraspis and Cephalaspis, and it is extremely hard luck for the believers in the Balanoglossus theory that no intermediate less armoured forms have been found, especially in consideration of the fact that the theory of the origin from the Palæostracan does not require such intermediate forms, but finds that those already discovered exactly fulfil its requirements.

One difficulty in the way of accepting the theory which I have advocated is perhaps the existence of the Tunicata. I cannot see that they show any affinities to the Arthropoda, and yet they are looked upon as allied to the Vertebrata. I can only conclude that both they and Amphioxus arose late, after the vertebrate stock had become well established, so that in their degenerated condition they give indications of their vertebrate ancestry and not of their more remote arthropod ancestry.

In conclusion, the way in which vertebrates arose on the earth as suggested in this book carries with it many important far-reaching conclusions with respect to the whole problem of Evolution.

When the study of Embryology began, great hopes were entertained that by its means it would be possible to discover the pedigree of every group of animals, and for this end all the stages of development in all groups of animals were sought for and, as far as possible, studied. It was soon found, however, that the interpretation of what was seen was so difficult, as to give rise to all manner of views, depending upon the idiosyncrasy of the observer. At his will he decided whether any appearance was coenogenetic or palingenetic, {497}with the result that, in the minds of many, embryology has failed to afford the desired clue.

At the same time, the geological record was looked upon as too imperfect to afford any real help; it was said, and is said, that the Cambrian and pre-Cambrian periods were so immense, and the animals discovered in the lower Silurian so highly organized, as to compel us to ascribe the origination of all the present-day groups to this immense early period, the animals of which have left no trace of their existence as fossils.

In consequence of, or at all events following upon, the supposed failure of embryology and of geology to solve the problem of the sequence of evolution of animal life, a new theory has arisen, which goes very near to the denial of evolution altogether. This is the theory of parallel development. It discards the old picture of a genealogical tree with main branches arising at different heights, these again branching and branching into smaller and smaller twigs, and substitutes instead the picture of the ribs of a fan, every rib running independently of every other, each group represented by a rib reaching its highest development on the circumference of the fan and coming nearer and nearer to a common point at the handle of the fan. This point of convergence, where all the groups ultimately meet, is so far back as to reach to the lowest living organisms.

This, in my opinion, unscientific and inconceivable suggestion has arisen largely in consequence of a conception which has become firmly fixed in the minds of very many writers on this subject--the conception that in the evolution of every group, the higher members of the group are the most specialized in the peculiarities of that group, and it is impossible to obtain a new group with different peculiarities from such specialized members. If, then, a higher group is to arise from a lower, it must arise from the generalized members of that lower group, in other words, from the lowest members or those nearly akin to the next lower group.

Similarly, the highest members of this latter group are too specialized, and again we must go to the more generalized members of the group. In this way each separate specialized group is put on one side, and so the conception of parallel development comes into being.

The evidence given in this book dealing with the origin of vertebrates strikes at the foundations of this belief, for it presents an {498}image of the sequence of evolution of animal forms in orderly upward progress, caused by the struggle for existence among the members of the race dominant at the time, which brought about the origin of the next higher group not from the lowest members of the dominant group, but from some one of the higher members of that group.

The great factor in evolution has been throughout the growth of the central nervous system; from that group of animals which possessed the highest nervous system evolved up to that time the next higher group must have arisen.

In this way we can trace without a break, always following out the same law, the evolution of man from the mammal, the mammal from the reptile, the reptile from the amphibian, the amphibian from the fish, the fish from the arthropod, the arthropod from the annelid, and we may be hopeful that the same law will enable us to arrange in orderly sequence all the groups in the animal kingdom.

This very same law of the paramount importance of the development of the central nervous system for all upward progress will, I firmly believe, lead to the establishment of a new and more fruitful embryology, the leading feature of which will be, as suggested in the last chapter, not the attempt to derive from the blastula three germ-layers common to all animals, but rather two sets of organs--those which are governed by the nervous system and those which are not--and thus by means of the development of the central nervous system obtain from embryology surer indications of relationship than are given at present.

The great law of recapitulation, which asserts that the past history of the race is indicated more or less in the development of each individual, a law which of late years has fallen somewhat into disrepute, owing especially to the difficulty of interpreting the embryological history of the vertebrate, is triumphantly vindicated by the theory put forward in this book. Each separate vertebrate organ, one after the other, as shown in the last chapter, indicates in its development the manner in which it arose from the corresponding organ of the arthropod. There is no failure in the evidence of embryology, the failure is in the interpretation thereof.

So, too, my theory vindicates the geological method. There is no failure here; on the contrary, the record of the rocks proclaims with startling clearness not only the sequence of evolution in the {499}vertebrate kingdom itself, but the origin of the vertebrate from the most highly-developed invertebrate race.

The study of the comparative anatomy of organs down to the finest details has always been a most important aid in finding out relationship between animals or groups of animals. My theory endorses this view to the uttermost, and especially indicates the study of the central nervous system and its outgoing nerves as that comparative study which is most likely to afford valuable results.

As for the individual, so for the nation; as for the nation, so for the race; the law of evolution teaches that in all cases brain-power wins. Throughout, from the dawn of animal life up to the present day, the evidence given in this book suggests that the same law has always held. In all cases, upward progress is associated with a development of the central nervous system.

The law for the whole animal kingdom is the same as for the individual. "Success in this world depends upon brains."

{501}BIBLIOGRAPHY AND INDEX OF AUTHORS

--------------+------------------------------------------+--------------- Author's name.| Title of Paper. | Pages of | | reference. --------------+------------------------------------------+--------------- AHLBORN |"Untersuchungen über das Gehirn der | 210, 489 | Petromyzonten" | | Zeitsch. f. wiss. Zool. Vol. 39. 1883 | | | |"Ueber die Segmentation des | 260 | Wirbelthierkörpers" | | Zeitsch. f. wiss. Zool. Vol. 40. 1884 | | | AICHEL |"Vergleichende Entwicklungsgeschichte | 424, 428 | und Stammesgeschichte der Nebennieren" | | Arch. f. Mikr. Anat. Vol. 56. 1900 | | | ALCOCK | | 135, 287, 288, | | 289, 304, 307, | | 347, 445 | | |"The Peripheral Distribution of the | 164, 171, 177, | Cranial Nerves of Ammocoetes" | 188, 202, 297, | Journ. of Anat. and Physiol. | 300, 310, 311, | Vol. 33. 1898 | 316 | | |"On Proteid Digestion in Ammocoetes" | 58, 213, 442, | Journ. of Anat. and Physiol. | | Vol. 33. 1898 | 452 | | ALLEN |"Studies on the Nervous System of | 489 | Crustacea" | | Q. J. Micr. Sci. Vol. 36. 1894 | | | ANDERSON, | | 448, 470 H. K. | | |"The Nature of the Lesions which hinder | 466, 467, 469 | the Development of Nerve-cells and their | | Processes | | Journ. of Physiol. Vol. 28. 1902 | | | |"On the Myelination of Nerve-fibres" | 467, 477 | Report of the Brit. Assn. 1898 | | | APATHY |"Das leitende Element des Nervensystems | 467 | und seine topographischen Beziehung zu | | den Zellen" | | Mitth. a. d. Zool. Stat. zu Neapel. | | Vol. 12. 1896 | | | ASSHETON |"On the Phenomenon of the Fusion of the | 42 | Epiblastic Layers in the Rabbit and in | | the Frog" | | Q. J. Micr. Sci. Vol. 37. 1894 | | | |"An Experimental Examination into the | 154 | Growth of the Blastoderm of the Chick" | | Proc. of Roy. Soc. Vol. 60. 1896 | | | ASSHETON |"On the Growth in Length of the Frog | 154 | Embryo" | | Q. J. Micr. Sci. Vol. 37. 1894 | | | |"A Re-investigation into the Early Stages | 154 | of the Development of the Rabbit" | | Q. J. Micr. Sci. Vol. 37. 1894 | | | |"The Primitive Streak of the Rabbit: the | 154 | Causes which may determine its Shape, | | and the part of the Embryo formed by its | | Activity" | | Q. J. Micr. Sci. Vol. 37. 1894 | | | BALFOUR |'Comparative Embryology.' Vol. 2 | 73, 74, 94, | London. 1881. Macmillan & Co. | 103, 104, 120, | | 181, 259, 424 | | |"On the Origin and History of the | 390, 392 | Urino-genital Organs of Vertebrates" | | Journ. of Anat. and Physiol. | | Vol. 10. 1876 | | | |"On the Nature of the Organ in Adult | 420 | Teleosteans and Ganoids, which is usually| | regarded as the Head-kidney or | | Pronephros" | | Q. J. Micr. Sci. Vol. 22. 1882 | | | BARKER |'The Nervous System' | 470 | London. 1901 | | | BATESON |"The Ancestry of the Chordata" | 11 | Q. J. Micr. Sci. Vol. 26. 1886 | | | |'Materials for the Study of Variation' | 387 | London. 1894 | | | BEARD |"The System of Branchial Sense Organs and | 262, 281, 283 | their Associated Ganglia in Ichthyopsida"| | Q. J. Micr. Sci. Vol. 26. 1885 | | | |"The Development of the Peripheral | 262, 281, 283 | Nervous System in Vertebrates" | | Q. J. Micr. Sci. Vol. 29. 1888 | | | |"The Old Mouth and the New" | 318 | Anat. Anzeiger. 1888 | | | |"The Source of Leucocytes and the True | 425, 426 | Function of the Thymus" | | Anat. Anzeiger. Vol. 18. 1900 | | | |"The Parietal Eye of the Cyclostome | 84 | Fishes" | | Q. J. Micr. Sci. Vol. 29. 1882 | | | BECK AND |"On the Muscular and Endo-skeletal | 171, 222, 224, LANKESTER | Tissues of Scorpio" | 247, 268-277 | Trans. Zool. Soc. Vol. 11. 1885 | | | BEECHER |"Natural Classification of the Trilobites"| 283, 351, 436, | Amer. Journ. of Sci. | 437 | Ser. 4. Vol. 3. 1897 | | | BELL, C. |'The Nervous System of the Human Body' | 155, 156, 183 | London. 1830 | | | BELLONCI |"Système Nerveux et Organes des sens du | 62, 90, 92, | Sphæroma serratum" | 101 | Archiv. Ital. de Biol. Vol. 1. 1882 | | | |"Sur la structure et les rapports des | 221, 225 | lobes olfactives dans les Arthropods | | superieurs et les Vertébrés" | | Archiv. Ital. de Biol. Vol. 3. 1883 | | | BENHAM AND |"On the Muscular and Endo-skeletal | 143, 171, 176, LANKESTER | Systems of Limulus" | 177, 247 | Trans. Zool. Soc. Vol. 11. 1885 | | | BERGER |"Untersuchungen über den Bau des Gehirns | 88-92, 97, | und der Retina der Arthropoden" | 100, 101 | Arbeit. a. d. Zool. Instit. Wien. | | Vol. 1. 1878 | | | BERGH |"Neue Beiträge zur Embryologie der | 478 | Anneliden" | | Zeitsch. f. wiss. Zool. Vol. 50. 1890 | | | BERKLEY |"The Intrinsic Nerves of the Kidney" | 477 | Bulletin of the Johns Hopkins Hospital.| | Vol. 4 | | | BERNARD |'The Apodidæ: a Morphological Study' | 284 | Nature Series. 1892 | | | BERTKAU |"Beiträge zur Kenntniss der Sinnesorgane | 369 | der Spinnen. 1. Die Augen der Spinnen" | | Archiv. f. mikr. Anat. Vol. 27. 1886 | | | BIEDERMANN |'Electro-physiology' | 20 | Translated by F. A. Welby. London. 1896 | | | BLANCHARD | Quoted by Huxley | 225 | | |'L'Organisation du Règne Animal. | 109, 177, 190, | Arachnides' | 206, 313, 315 | Paris. 1852 | | | BLES |"The Correlated Distribution of Abdominal | 431 | Pores and Nephrostomes in Fishes" | | Journ. of Anat. and Physiol. | | Vol. 32. 1898 | | | BOBRETSKY |'Development of Astacus and Palæmon' | 74 | Kiew. 1873 | | | BOURNE AND | See Lankester and Bourne. | LANKESTER | | | | BOVERI |"Die Nieren Canälchen des Amphioxus" | 392, 395, 402, | Zool. Jahrbuch. Vol. 5. 1892 | 407, 412, 426, | | 427 | | BRAEM |"Was ist ein Keimblatt" | 460, 461, 462 | Biol. Centralblatt. Vol. 15. 1895 | | | BRAUER |"Beiträge zur Kenntniss der | | Entwicklungsgeschichte des Skorpions" | 62, 167, 222, | Zeit. f. wiss. Zool. | 237, 281, 482 | Part I. Vol. 57. 1894 | | Part II. Vol. 59. 1895 | | | |"Beiträge zur Kenntniss der Entwicklung | 393, 394, 400, | und Anatomie der Gymnophionen." III. | 402 | "Die Entwicklung der Excretionsorgane" | | Zool. Jahrbuch. Vol. 16. 1902 | | | |"Ueber die Entwicklung von Hydra" | 473 | Zeit. f. wiss. Zool. Vol. 52. 1891 | | | BÜTSCHLI |"Notiz zur Morphologie des Auges der | 114 | Muscheln" | | Festschrift des Natur-hist-med. | | Vereins zu Heidelberg. 1886 | | | BUJOR |"Contribution a l'étude de la métamorphose| 135, 304 | de l'Ammocoetes branchialis en | | Petromyzon Planeri" | | Revue Biologique du Nord de la France. | | Vol. 3. 1891 | | | CARLSON | | 177, 315, 316 | | CELESIA |'Differenziamento della proprietà | 489 | inibitoria e dei funzioni coordinatrici | | nella catena gangliare dei crustacei | | decapodi' | | Genoa. 1897 | | | CLAUS |"Untersuchungen über den Organismus und | 90-92, 97, | Entwicklung von Branchipus und Artemia" | 100, 396 | Arbeit a.d. Zool. Institut. Wien. | | Vol. 6. 1886 | | | COPE |"On the Phylogeny of the Vertebrata" | 343 | Proc. Amer. Philos. Soc. Vol. 30. 1892 | | | CRONEBERG |"Ueber die Mundtheile der Arachniden" | 221-224, 241 | Archiv. f. Naturgeschichte. 1880 | | | CUÉNOT |"Études sur le sang et les glandes | 422 | lymphatiques dans la série animale; | | 2nd partie; invertébrés" | | Arch. d. Zool. exper. gen. | | 2nd Ser. Vol. 9. 1891 | | | CUNNINGHAM, |"The Significance of Kupffer's Vesicle, | 318 J. T. | with Remarks on other Questions of | | Vertebrate Morphology" | | Q. J. Micr. Sci. Vol. 25. 1885 | | | |"The Nephridia of Lanice conchilega" | 403 | Nature. Vol. 36. 1887 | | | DANA |"On Cephalization" | 53 | Mag. of Nat. Hist. 1863 | | | DEAN-BASHFORD |'Fishes, Living and Fossil' | 344 | New York. 1895 | | | |"On the Embryology of Bdellostoma | 405 | Stouti" | | Festschr. z. siebenzigsten Geburtstag. | | von C. v. Kupffer. Jena. 1899 | | | DENDY |"On the Parietal Sense-organs and | 80, 82 | Associated Structures in the New Zealand | | Lamprey (Geotria australis)" | | Q. J. Micr. Sci. Vol. 51. 1907 | | | DIETL |"Die Organisation des Arthropoden Gehirns"| 101 | Zeitsch. f. wiss. Zool. Vol. 27. 1876 | | | DOHRN |'Der Ursprung der Wirbelthiere und das | 14, 60, 185, | Princip des Functionswechsels' | 186, 317, 318 | Leipzig. 1875 | | | | Studien zur Urgeschichte des Wirbelthiere| 188, 195-198, | Körpers. VIII. "Die Thyroidea bei | 199, 212, 213 | Petromyzon, Amphioxus, und Tunicaten" | | Mitth. Zool. Stat. z. Neapel. | | Vol. 6. 1886 | | | |"Neue Grundlagen zur Beurtheilung der | 262, 263, 279 | Metamerie des Kopfes" | | Mitth. Zool. Stat. z. Neapel. | | Vol. 9. 1890 | | | | Studien zur Urgeschichte des Wirbelthiere| 167, 314, 337 | Gefässe Körpers. XIII. "Ueber Nerven | | und bei Ammocoetes und Petromyzon | | Planeri" | | Mitth. Zool. Stat. z. Neapel. | | Vol. 8. 1888 | | | DREVERMANN |"Ueber Pteraspis dunensis" | 29, 30 | Zeitschr. d. Deutsch. Geol. | | Gesellschaft. Vol. 56. 1904 | | | EDGEWORTH |"The Development of the Head-muscles in | 266 | Gallus domesticus, and the Morphology | | of the Head-muscles in the Sauropsida" | | Q. J. Micr. Sci. Vol. 51. 1907 | | | EDINGER |'Anatomy of Central Nervous System in Man | 17, 264 | and in Vertebrates' | | Translated by Hall. 1899 | | | v. EICHWALD |"Die Thier- und Pflanzenreste des alten | 327 | rothen Sandsteins und Bergkalks im | | Nowgorodschen Gouvernement" | | Bull. Sci. de l'Acad. Impér. | | d. St. Petersbourg. 1840 | | | EISIG |"Die Seiten-organe und becherförmigen | 357 | Organe der Capitelliden" | | Mitth. a. d. Zool. Stat. z. Neapel. | | Vol. 1. 1879 | | | |"Capitelliden" | 357 | Faun. u. Flor. d. Golfes v. Neapel. | | Vol. 16. 1887 | | | ELLIOTT |"On the Innervation of the Ileo-colic | 449 | Sphincter" | | Journ. of Physiol. Vol. 31. 1904 | | | EMERY | Quoted by Weldon | 420 | | FOSTER, M. | Text-book of Physiology | 108 | | FREUND |"Die Beziehungen der Schilddrüse zu den | 215 | weiblichen Geschlechtsorganen" | | Deutsch. Zeitsch. f. Chirugie. | | Vol. 18. 1883 | | | FRITSCH, G. |'Untersuchungen über den feineren Bau des | 488, 489 | Fischgehirns' | | Berlin. 1878 | | | FRORIEP |"Ueber Anlagen von Sinnesorganen am | 261, 262, 281, | Facialis, Glossopharyngeus und Vagus, | 283 | über die genetische Stellung des Vagus | | zum Hypoglossus, und über die Herkunft | | der Zungenmusculatur" | | Arch. f. Anat. u. Physiol; | | Anat. Abtheil. 1885 | | | FÜRBRINGER, |'Ueber die Spino-occipetalen Nerven der | M. | Selachier und Holocephalen' | 276-278, 409 | Fest-schrift für Carl Gegenbaur. 1897 | | | GAUBERT |'Recherches sur les organes des sens et | 364, 368-375 | sur les systèmes tegumentaire, | | glandulaire et musculaire des appendices | | des arachnides' | | Paris. 1892 | | | GEGENBAUR |"Anatomische Untersuchung eines Limulus" | 20, 358-360 | Abhandl. d. Naturforsch. Gesellsch. | | z. Halle. Vol. 4. 1858 | | | |"Ueber die Skeletgewebe der Cyclostomen" | 181 | Jen. Zeitschrift. Vol. 5. 1870 | | | | Untersuchungen zur vergleichende Anatomie| 151, 259, 261 | der Wirbelthiere III. Heft. 'Das | | Kopfskeletder Selachiern' | | Leipzig. 1872 | | | |'Grundriss der vergleichenden Anatomie' | 392 | Leipzig. 1878 | | | v. GEHUCHTEN |"De l'origine du pathétique et de la | 264 | racine supérieure du trijumeau" | | Acad. d. Sci. Belg. Bulletin. | | 3rd Ser. Vol. 29. 1895 | | | GOETHE | | 258 | | GÖTTE |'Entwicklungsgeschichte der Unke' | 101, 102, 114 | Leipzig. 1875 | | | GOLGI | | 72, 465, 477 | | GOODRICH |"On the Structure of the Excretory Organs | | of Amphioxus" | | Q. J. Micr. Sci. Vol. 45. 1902 | 395, 396, 477 | | |"On the Nephridia of the Polychæta." | 395 | Parts I., II., III. | | Q. J. Micr. Sci. Vols. 40, 41, 43 | | | |"On the Excretory Organs of Amphioxus" | 477 | Proc. Roy. Soc. Vol. 69. 1902 | | | GRABER |"Die Chordo-tonalen Sinnesorgane und das | 364, 369-371 | Gehör der Insecten" | | Archiv. f. Mikr. Anat. | | Vols. 20 and 21. 1882 | | | GRENACHER |'Untersuchungen über das Sehorgan der | 76, 100 | Arthropoden' | | Göttingen. 1879 | | | GUDDEN | Quoted in Obersteiner | 264 | | HAECKEL | | 461, 462 | | HALLER, BELA |"Untersuchungen über die Hypophyse und | 320, 321 | die Infundibulärorgane" | | Morph. Jahrbuch. Vol. 25. 1898 | | | |"Untersuchungen über das Rückenmark der | 488 | Teleostier" | | Morph. Jahrbuch. Vol. 23. 1895 | | | HARDY |"On the Histological Features and | 110, 159 | Physiological Properties of the | | Post-oesophageal Nerve-cord of the | | Crustacea" | | Phil. Trans. Roy. Soc. 1894. B. | | | HARDY AND |"On the Structure and Functions of the | 112, 206 MACDOUGALL | Alimentary Canal of Daphnia" | | Proc. Camb. Phil. Soc. Vol. 8. 1893 | | | HATSCHEK |"Die Metamerie des Amphioxus und des | 289, 300, 337 | Ammocoetes" | | Anat. Anzeig., 7 Jahrgang, 1892. | | Verhandl. d. Anat. Gesell. in Wien, | | p. 136 | | | |"Studien über Entwicklung des Amphioxus" | 407 | Arbeit. d. Zool. Inst. z. Wien. | | Vol. 4. 1881 | | | | Quoted by Lankester | 475 | | HAZEN | See Patten and Hazen. | | | HEIDENHAIN | | 258, 259 | | HEIDER | See Korschelt and Heider. | | | HENSEN |"Zur Entwicklung des Nervensystem" | 465, 466 | Virchows Archiv. Vol. 30. 1864 | | | HENSEN AND | Archiv. f. Opthalmol. Vol. 24. 1878 | 265, 266 VÖLCKERS | | | | HERTWIG, O., | Quoted in Zeigler's 'Lehrbuch der | 485 AND SAMASSA | vergleichenden Entwicklungsgeschichte | | der niederen Wirbelthiere.' 1902 | | | HIS |"Die Neuroblasten und deren Entstehung | 465, 466 | im embryonalen Mark" | | Archiv. f. Anat. u. Physiol. | | Anat. Abth. 1889 | | | HOFFMANN |"Ueber die Metamerie des Nachhirns und | 276 | Hinterhirns, und ihre Beziehung zu den | | segmentalen Kopfnerven bei Reptilien | | embryonen" | | Zool. Anzeiger. Vol. 12. 1889 | | | HOLM |"Ueber die Organisation des Eurypterus | 192, 240, 241, | Fischeri" | 306 | Mem. d. l'Acad. Imp. d. Sci. d. | | St. Petersbourg. Vol. 8. 1898 | | | HOYER |"Ueber den Nachweis des Mucins in Geweben | 131 | Mittelst der Färbe-Methode" | | Archiv. f. Mikr. Anat. Vol. 36. 1890 | | | HUXLEY |"Hunterian Lectures." 1869 | 124, 258, 259 | | |"On the Structure of the Mouth and Pharynx| 222, 225, 271 | of the Scorpion" | | Q. J. Micr. Sci. Vol. 8. 1860 | | | |"On the Anatomy and Affinities of the | 238 | Genus Pterygotus" | | Mem. of the Geol. Survey. | | Monograph I. 1859 | | | |"On Cephalaspis and Pteraspis" | 327 | Q. J. of Geol. Soc. Vol. 14. 1858 | | | JAEKEL |"Ueber Tremataspis und Patten's Ableitung | 329, 339, 340, | der Wirbelthiere von Arthropoden" | 351 | Protocoll der Deutschen Geolog. | | Gesellschaft, p. 84; in Zeitsch. d. | | Deutschen Geologischen Gesellsch. | | Vol. 55. 1903 | | | |"Ueber die Organisation und systematische | 345 | Stellung der Asterolepiden" | | Ibid., p. 41 | | | JOHNSON |"Contributions to the Comparative Anatomy | 70 | of the Mammalian Eye, chiefly based on | | Opthalmoscopic Examination" | | Phil. Trans. Roy. Soc. B. | | Vol. 194. 1901 | | | JOSEPH |"Ueber das Achsenskelett des Amphioxus" | 444 | Zeitsch. f. wiss. Zool. Vol. 59. 1895 | | | JULIN AND | Recherches sur l'Organisation des | 425 VAN BENEDEN | Ascidies simples. "Sur l'hypophyse," etc.| | Archives de Biologie. Vol. 2. 1881 | | | KAENSCHE |"Beiträge zur Kenntniss der Metamorphose | 135, 304 | des Ammocoetes branchialis in | | Petromyzon" | | Schneider's Beiträge. Vol. 2. 1890 | | | v. KENNEL |"Entwickelungsgeschichte von Peripatus | 398, 399, 411 | Edwardsii und Peripatus torquatus." | | II. Theil | | Arbeit. a. d. Zool. Zoot. Instit. | | Würzburg. Vol. 8. 1888 | | | KERR |"On some Points in the Early Development | 461, 466, 478 | of Motor Nerve-trunks and Myotomes in | | Lepidosiren paradoxa" | | Trans. Roy. Soc. Edin. Vol. 41. 1904 | | | KILLIAN |"Zur Metamerie des Selachierkopfes" | 262 | Verhandl. d. Anat. Gesell. | | Versamml. in München. 1891 | | | KISHINOUYE |"On the Development of Limulus | 167, 238, 252, | longispina" | 253, 273, 320, | Journ. of Coll. of Sci., Tokio. | 382 | Vol. 5. 1891 | | | KLEINENBERG | Quoted by Beard | 318 | | v. KÖLLIKER |"Die obere Trigeminus-Wurzel" | 280 | Arch. f. Mikr. Anat. Vol. 53. 1899 | | | v. KÖLLIKER | | AND | Handbuch der Gewebe-Lehre. 6th Auflage. | 264, 425, 488 TERTERJANZ | 1893 | | | KOHL |"Rudimentäre Wirbelthieraugen" | 94, 96, 99, | Bibliotheca Zoologica. Leukart und | 101 | Chun. Vol. 4 and Vol. 5 | | | KOHN |"Ueber den Bau und die Entwicklung der | 428 | sogenannten Carotis-drüse" | | Archiv. f. Mikr. Anat. Vol. 56. 1900 | | | KORSCHELT AND |'Text-book of the Embryology of the | 27, 73, 88, HEIDER | Invertebrates.' Translated by M. Bernard.| 114-116, 397, | 1900. Part III. and Part IV. | 429, 431 | | KOWALEWSKY |"Ein Beitrag zur Kenntniss der | 421 | Excretionsorgane der Pantopoden" | | Mem. d. l'Acad. d. Imp. d. Sci. d. St. | | Petersbourg. Ser. VII. Vol. 38. 1890 | | | |"Une nouvelle glande lymphatique chez le | 423 | scorpion d'Europe" | | Ibid. Ser. VIII. Vol. 5. 1897 | | | |"Étude Biologique sur les Clepsines" | 421 | Ibid. Ser. VIII. Vol. 5. 1897 | | | |"Ein Beitrag zur Kenntniss der | 420, 422, 472 | Excretionsorgane" | | Biologisches Centralblatt. 1889 | | | |"Weitere Studien über die | 409, 410 | Entwicklungsgeschichte des | | Amphioxus lanceolatus" | | Archiv. f. Mikr. Anat. Vol. 13. 1877 | | | KRIEGER |"Ueber das Centralnervensystem des | 101 | Flusskrebses | | Zeitsch. f. wiss. Zool. Vol. 33. 1880 | | | v. KUPFFER |'Studien zur vergleichenden Entwicklungs- | | geschichte des Kopfes der Kranioten.' | | Heft. 1. 'Die Entwicklung des Kopfes | 318, 319, 320, | von Acipenser' | 440 | München. 1893 | | | | Heft. 2. 'Die Entwicklung des Kopfes | 300, 440 | von Ammocoetes Planeri' | | München. 1894 | | | | Heft. 3. 'Die Entwicklung der | 228, 263, 282, | Kopfnerven von Ammocoetes Planeri.' | 283, 405, 458 | Dritter Abschnitt. 'Die Metamorphose | | des larvalen Nervensystems des Kopfes'| | München. 1895 | | | LANG |'Text-book of Comparative Anatomy.' | 357 | Translated by H. M. and M. Bernard | | | LANGERHANS |"Untersuchungen über Petromyzon | 94-101, 301, | Planeri" | | Bericht v. d. Verhandl. d. Naturforsch. | 405 | Gesellsch. z. Freiburg. 1873 | | | LANGLEY | Schäfer's 'Text-book of Physiology.' | 2, 3, 448 | Vol. 2. 1900 | | | LANKESTER | Article "Vertebrata" in the | 484 | 'Encyclopædia Britannica' | | | |"On the Skeleto-trophic Tissues and Coxal | 137, 139, 253, | Glands of Limulus, Scorpio, and Mygale | 320, 321 | Q. J. Micr. Sci. Vol. 24. 1884 | | | |"Limulus an Arachnid" | 62, 238, 241, | Q. J. Micr. Sci. Vol. 21. 1881 | 306, 361, 366 | | |'Extinct Animals' | 22, 150, 345 | London. Constable & Co. 1906 | | | | A treatise on Zoology. Edited by E. Ray | | Lankester. | | Part II. 'The Entero-coela and the | 472-478 | Coelomocoela' | | | LANKESTER AND |"A Monograph of the Fishes of the Old Red | 29, 275, 327, POWRIE | Sandstone of Britain." | 339, 345 | Part I. "The Cephalaspidæ" | | Palæontographical Soc. 1868 | | | LANKESTER, |"On the Muscular and Endo-skeletal Systems| 177, 222, 224, BENHAM, AND | of Limulus and Scorpio, with some Notes | 313 BECK | on the Anatomy and Generic Characters of | | Scorpions" | | Trans. Zool. Soc. Vol. 11. 1885 | | | LANKESTER AND |"The Minute Structure of the Lateral and | 74, 81-83 BOURNE | Central Eyes of Scorpio and Limulus" | | Q. J. Micr. Sci. Vol. 23 | | | LANKESTER AND |"The Development of the Atrial Chamber of | WILLEY | Amphioxus" | 409 | Q. J. Micr. Sci. Vol. 31. 1890 | | | LANKESTER AND |"Evidence in Favour of the View that the | 429 GULLAND | Coxal Gland of Limulus and of other | | Arachnids is a Modified Nephridium" | | Q. J. Micr. Sci. Vol. 25. 1885 | | | LATREILLE | | 221 | | LAURIE |"The Anatomy and Relations of the | 237 | Eurypteridæ" | | Trans. Roy. Soc. Edin. Vol. 37. 1893 | | | |"On a Silurian Scorpion and some | 238, 239 | Additional Eurypterid Remains from the | | Pentland Hills | | Ibid. Vol. 34. 1899 | | | LEYDIG | | 91 | | LOCY |"Contributions to the Structure and | 179, 262 | Development of the Vertebrate Head" | | Journ. Morph. Vol. 11. 1895 | | | LOEB, LEO, AND|"On Regeneration in the Pigmented Skin of | 470 R. M. STRONG | the Frog, and on the Character of the | | Chromatophores" | | Amer. Jour. of Anat. Vol. 3. 1904 | | | LOWNE |'The Anatomy, Physiology, Morphology, and | 369, 370, 375 | Development of the Blow-fly' | | London. 1895 | | | LUGARO | Quoted by Anderson | 467 | | LWOFF |"Ueber den Zusammenhang von Markrohr und | 444 | Chorda beim Amphioxus und ähnliche | | Verbältnisse bei Anneliden" | | Zeitsch. f. wiss. Zool. Vol. 56. 1893 | | | MAAS |"Ueber Entwicklungstadien der Vorniere und| 392, 402, 412, | Urniere bei Myxine" | 419 | Zool. Jahrbuch. Vol. 10. 1897 | | | MACBRIDE |"Further Remarks on the Development of | 410 | Amphioxus" | | Q. J. Micr. Sci. Vol. 43. 1900 | | | McDOUGALL | See Hardy and McDougall. | | | MACLEOD |"Recherches sur la structure et la | 169, 174 | signification de l'appareil respiratoire | | des Arachnides" | | Archiv. de Biol. Vol. 5. 1881 | | | MAGNUS |"Versuche am überlebenden Dünndarm von | 447 | Säugethieren" | | Archiv. f. d. Ges. Physiologie. | | Vols. 102, 103. 1904 | | | MARK | | 115 | | MARSHALL |"On the Head-cavities and Associated | 185, 186 | Nerves of Elasmobranchs" | | Q. J. Micr. Sci. Vol. 21. 1881 | | | |"The Segmental Value of the Cranial | 260 | Nerves" | | Journ. of Anat. and Physiol. | | Vol. 16. 1882 | | | MASTERMAN |"On the Diplochorda" | 16 | Q. J. Micr. Sci. Vol. 43. 1900 | | | MAURER |"Die Schilddrüse, Thymus und andere | 427, 428 | Schlundspaltenderivate bei den Eidechse" | | Morph. Jahrbuch. Vol. 27. 1899 | | | MAYER, F. |"Das Centralnervensystem von Ammocoetes" | 489 | Anat. Anzeig. Vol. 13. 1897 | | | MAYER, P. |"Ueber die Entwicklung des Herzens und der| 179 | grossen Gefässstämme bei den | | Selachiern" | | Mitth. a. d. Zool. Stat. z. Neapel. | | Vol. 7. 1887 | | | METSCHNIKOW | Quoted by Kowalewsky | 422 | | MEYER |"Studien über den Körperbau der | 403 | Anneliden" | | Mitth. a. d. Zool. Stat. z. Neapel. | | Vol. 7. 1887 | | | MILNE-EDWARDS |"Anatomie des Limules" | 157, 159, 176, | Annales des Sciences Naturelles. | 177, 313 | Ser. 5. Vol. 17. 1872 | | | MINCHIN | A treatise on Zoology. Edited by Ray | 473 | Lankester. Part II. "The Porifera and | | Coelenterata" | | | MITSUKURI |"On the Fate of the Blastopore, the | 179 | Relations of the Primitive Streak, and | | the Formation of the Posterior End of the| | Embryo in Chelonia," etc. | | Journ. Coll. Sci. Tokyo. | | Vol. 10. 1896 | | | MOTT |"Croonian Lectures of the Roy. Coll. of | 469 | Physicians," 1900 | | | MOTT AND |"On the Chemistry of Nerve-degeneration" | 469 HALLIBURTON | Phil. Trans. Roy. Soc. B. | | Vol. 194. 1901 | | | MÜLLER, J. | | 1 | | |"Vergleichende Anatomie der Myxinoiden" | 126 | Abhandl. d. Kgl. Akad. d. Wiss. | | Berlin. 1834 | | | MÜLLER, W. |"Ueber die Stammes Entwickelung des | 96-100, 105, | Sehorgans der Wirbelthiere" | 108 | Festgabe C. Ludwig. Leipzig. 1874 | | | NEAL |"The Segmentation of the Nervous System | 179, 266, 300 | in Squalus acanthias" | | Bull. of Mus. Comp. Zool. | | Harvard. Vol. 31. 1898 | | | NESTLER |"Beiträge zur Anatomie und | 168, 171, 175, | Entwicklungsgeschichte von Petromyzon | 445 | Planeri" | | Archiv. f. Naturgesch. Jahrgang, 56. | | Vol. I. 1890 | | | NIESKOWSKI |"Der Eurypterus Remipes aus den ober- | 26, 239, 240 | silurischen Schichten der Insel Oesel" | | Arch. f. d. Naturkunde Liv-Ehst-und | | Kurlands. 1st Ser. Vol. 3. 1858 | | | NUSBAUM, J. |"Einige neue Thatsachen zur | 320 | Entwicklungsgeschichte des Hypophysis | | Cerebri bei Säugethieren" | | Anat. Anzeiger. Vol. 12. 1896 | | | OBERSTEINER |'Central Nervous System.' Translated by | 264, 280 | Hill. 1896 | | | OKEN | | 258 | | OWEN |"Essays on the Conario-Hypophysial Tract, | 14 | and the Aspects of the Body in Vertebrate| | and Invertebrate Animals" | | | |"On the Anatomy of the American King-crab | 211 |(Limulus polyphemus)" | | Trans. Linn. Soc. Vol. 28. 1873 | | | PANDER |'Monographie der fossilen Fische des | 327 | Silurischen Systems des russisch- | | baltischen Gouvernements' | | St. Petersbourg. 1856 | | | PARKER, G. H. |"The Retina and Optic Ganglia in Decapods,| 91, 93, 97 | especially in Astacus" | | Mitth. a. d. Zool. Stat. z. Neapel. | | Vol. 12. 1895 | | | |"The Compound Eyes in Crustaceans" | 99, 100, 114 | Bull. of Harvard Mus. of Comp. Zool. | | Vol. 20. 1890 | | | |"The Function of the Lateral-line Organs | 357 | in Fishes" | | Bull. of the Fisheries Bureau. | | Washington. Vol. 24. 1904 | | | |"Studies on the Eyes of Arthropods" | 73, 79, 83-85, | Journ. of Morphology. | 114 | Vols. 1 and 2. 1887 and 1889 | | | PARKER, W. K. |"On the Skeleton of the Marsipobranch | 120, 125, 126, | Fishes" | 131 | Phil. Trans. Roy. Soc. 1883 | | | PATTEN |"On the Origin of Vertebrates from | 352, 353 | Arachnids" | | Q. J. Micr. Sci. Vol. 31. 1890 | | | |"On the Morphology and Physiology of the | 358-367, 371 | Brain and Sense-organs of Limulus" | | Q. J. Micr. Sci. Vol. 35. 1893 | | | |"New Facts concerning Bothriolepis" | 32, 351, 450 | Biological Bulletin. Vol. 7. 1904 | | | |"On the Structure and Classification of | 329 | the Tremataspidæ" | | Mem. d. l'Acad. Imp. d. Sci. de | | St. Petersbourg. Vol. 13. 1903 | | | |"On the Structure of the Pteraspidæ and | | Cephalaspidæ" | 415 | The American Naturalist. Vol. 37. 1903 | | | |"On the Appendages of Tremataspis" | 351 | The American Naturalist. Vol. 37. 1903 | | | |"On Structures Resembling Dermal Bones in | 346 | Limulus" | | Anat. Anzeig. Vol. 9. 1894 | | | PATTEN AND |"The Development of the Coxal Gland, etc.,| 408 HAZEN | of Limulus Polyphemus" | | Journ. of Morphol. Vol. 16. 1900 | | | PATTEN AND | Studies on Limulus. II. "The Nervous | 314, 315, 381, REDENBAUGH | System of Limulus Polyphemus" | 382 | Journ. of Morphol. Vol. 16. 1900 | | | PERLIA | Quoted by Edinger | 264 | | PICK | " " | 265 | | PLATT |"A Contribution to the Morphology of the | 253, 265-267, | Vertebrate Head, based on a Study of | 273, 274, 279, | Acanthias vulgaris" | 284 | Journ. Morphol. Vol. 5. 1891 | | | |"Fibres connecting the Central Nervous | 443 | System and Chorda in Amphioxus" | | Anat. Anzeig. 1892 | | | PRICE |"Development of the Excretory Organs of | 394 | Bdellostoma Stouti" | | Zool. Jahrbuch. Vol. 10. 1897 | | | RABL |"Ueber die Metamerie des Wirbel- | 258, 262 | thierkopfes" | | Verhandl. der Anat. Gesellsch. Versamml.| | in Wien. Anat. Anzeig. 1892 | | | |"Die Entwicklung und Structur der | 424 | Nebennieren bei den Vögeln" | | Arch. f. mikr. Anat. Vol. 38. 1891 | | | RAMÓN Y. CAJAL| | 72, 465 | | RATHKE |"Anatomie des Querders" | 161, 169, 304 | Naturforsch. Gesellsch. zu Dantzig. | | Vol. 2. 1827 | | | REDENBAUGH | See Patten and Redenbaugh. | | | REICHENBACH |"Entwicklungs-geschichte des Flusskrebses"| 98-100, 114 | Abhandl. d. Senckenbergischen | | Naturforsch. Gesellsch. Vol. 14. 1886. | | | RETZIUS |'Biologische Untersuchungen.' Vol. 1. | 20, 489 | 1890. "Zur Kenntniss des Nervensystem der| | Crustaceen" | | | ROHON | Die Obersilurischen Fische von Oesel. 1st| 32, 275, 276 | Theil. "Thyestidæ und Tremataspidæ" | | Mem. d. l'Acad. Imp. d. Sci. d. St. | | Petersbourg. 7th Ser. Vol. 38. 1892 | | | |"Weitere Mittheilungen über die Gattung | 327-330, | Thyestes" | 339-341, 382 | Bull. d. l'Acad. d. St. Petersbourg. | | 5th Ser. Vol. 4. 1896 | | | ROLPH |"Untersuchungen über den Bau des | 444 | Amphioxus lanceolatus" | | Morphol. Jahrbuch. Vol. 2. 1887 | | | RÜCKERT, J. |"Entwicklung der Excretionsorgane" | 392, 393, 400 | Merkel und Bonnet; Anat. Hefte. | | Vol. 1. 1891. | | | |"Ueber die Entstehung der Excretionsorgane| 403 | bei Selachiern" | | Archiv. f. Anatomie. 1888 | | | ST. HILAIRE |"Sur la Vertèbre" | 11 | La Revue Encyclopédique. 1822 | | | SAMASSA |"Bemerkungen über die Methode der | 462 | Vergleichenden Entwicklungsgeschichte" | | Biol. Centralblatt. Vol. 18. 1898 | | | SCHAFFER |"Ueber das Knorpelige Skelett von | 126-135 | Ammocoetes" | | Zeitsch. f. wiss. Zool. Vol. 61. 1896 | | | |"Ueber die Thymusanlage bei Petromyzon | 426-428 | Planeri" | | Sitzungsber. d. K. Akad d. Wiss. | | in Wien. Vol. 103. 1894 | | | SCHIMKÉWITSCH |"Sur la structure et sur la signification | 143-145, 342 | de l'Endosternite des Arachnides" | | Zool. Anzeig. 1893 | | | |"Anatomie de l'Epeire" | 369 | Ann. d. Sci. Nat. Vol. 17. 1884 | | | SCHMIDT |"Die Crustaceen-fauna der Eurypteren- | 190, 191, 236, | schichten von Rootziküll auf Oesel" | 240, 329, 341 | Mem. d'Acad. Imp. d. Sci. d. | | St. Petersbourg. Vol. 31. 1883 | | | SCHMIEDEBERG |"Ueber die chemische Zusammensetzung des | 147 | Knorpels" | | Arch. f. exper. Pathol. und Pharmak. | | Vol. 28. 1891 | | | SCHNEIDER, A. |'Beiträge zur Anatomie und | 128, 130, 172, |Entwicklungsgeschichte der Wirbelthiere' | 195, 197, 213, | Berlin. 1879 | 310, 445 | | SCHNEIDER, G. |"Ueber phagocytäre Organe und | 421 | Chloragogenzellen der Oligochæta" | | Zeitsch. f. wiss. Zool. Vol. 61. 1896 | | | SCOTT |"Notes on the Development of Petromyzon" | 42, 78, 111, | Journ. of Morphol. Vol. 1. 1887 | 112, 406 | | SEDGWICK |"A Monograph of the Development of | 397-400 | Peripatus capensis" | | Studies from the Morphological | | Laboratory, Cambridge. Vol. 4. 1888 | | | |"Development of the Kidney in its Relation| 390 | to the Wolffian Body in the Chick" | | Q. J. Micr. Sci. Vol. 20. 1880 | | | |"Early Development of the Wolffian Duct | 393, 394, 400 | and Anterior Wolffian Tubules in the | | Chick; with some Remarks on the | | Vertebrate Excretory System" | | Q. J. Micr. Sci. Vol. 21. 1881 | | | SEMON |"Das Excretionssystem der Myxinoiden" | 400, 419 | Festschrift f. Gegenbaur. Leipzig. 1897| | | SEMPER |"Die Stammesverwandschaft der Wirbelthiere| 390, 392 | und Wirbellosen" | | Arbeit. a. d. Zool. Zoot. Inst. | | Würzburg. Vol. 2. 1875 | | | |"Das Urinogenitalsystem der Plagiostomen | 390, 392 | und seine Bedeutung für die übrigen | | Wirbelthiere" | | Ibid. Vol. 2. 1875 | | | SHELDON |"On the Development of Peripatus | 400 | Nova-Zealandiæ" | | Studies from the Morphological | | Laboratory, Cambridge. Vol. 4. 1889 | | | SHERRINGTON |"On the Anatomical Constitution of the | 267 | Nerves of Muscles" | | Journ. of Physiol. Vol. 17. 1894. | | Proc. of Physiol. Soc. June 23 | | | SHIPLEY | | 334 | | |"On some points in the Development of | 167, 305, 378, | Petromyzon fluviatilis" | 401, 405, 406 | Q. J. Micr. Sci. Vol. 27. 1887 | | | v. SMIRNOW |"Ueber die Nervenendigungen in den Nieren | 477 | der Säugethiere" | | Anat. Anzeiger. Vol. 19. 1901 | | | SMITH, ELLIOT | | 17 | | SPANGENBERG |"Zur Kenntniss von Branchipus stagnalis"| 396 | Zeitsch. f. wiss. Zool. Vol. 25. 1875 | | | SPENGEL |'Die Enteropneusten' | 494 | Berlin. 1893 | | | STARR | Quoted by Edinger | 265, 266 | | STUDNIÇKA |"Sur les organes pariétaux de Petromyzon | 80, 81, 86 | Planeri" | | Sitzungsber. d. K. Gesell. d. | | Wiss. in Prag. 1893 | | | |"Ueber den feineren Bau der | 81, 86 | Parietalorgane von Petromyzon marinus" | | Sitzungsber. d. K. böhmischen Gesell. | | d. Wiss. Prag. 1899 | | | TAKAMINE |"The Isolation of the Active Principle | 423 | of the Supra-renal Gland" | | Journ. of Physiol. Vol. 27. | | Proc. of Physiol. Soc., Dec. 14, 1901 | | | TARNANI |"On the Anatomy of the Thelyphonides" | 190, 206-208 | Revue des Sciences Naturelles, | | St. Petersbourg. 1890 | | | |"Die genitalen Organe der Thelyphonus" | 190, 206-208 | Biol. Centralblatt. Vol. 9. 1889 | | | TRAQUAIR |"Report on Fossil Fishes collected by the | 343-345, 350 | Geological Survey of Scotland in the | | Silurian Rocks of the South of Scotland" | | Trans. Roy. Soc., Edin. Vol. 39. 1899 | | | VIALLANES |"Contribution à l'histologie du système | 100 | nerveux des Invertébrés; la lame | | ganglionnaire de la Langouste" | | Ann. Sci. Nat. Vol. 13 | | | VINCENT, |"The Carotid Gland of Mammalia and its | 424 SWALE | Relation to the Supra-renal Capsule, with| | some Remarks upon Internal Secretion and | | the Phylogeny of the latter Organ" | | Anat. Anzeiger. Vol. 18. 1900 | | | |"Contributions to the Comparative Anatomy | 424 | and Histology of the Supra-renal | | Capsules" | | Trans. Zool. Soc. Vol. 14. 1897 | | | VIRCHOW |"Transformation and Descent" | 479 | Journ. of Path. and Bacter. | | Vol. 1. 1893 | | | VOGT | | 258 | | VÖLCKERS | See Hensen and Völckers. | | | WAGNER | Quoted by Gaubert | | | WEISS |"Excretory Tubules in Amphioxus | 426 | Lanceolatus" | | Q. J. Micr. Sci. Vol. 31. 1890 | | | WELDON |"On the Supra-renal Bodies of Vertebrates"| 420, 424, 429 | Q. J. Micr. Sci. Vol. 25. 1885 | | | |"Note on the Origin of the Supra-renal | 424 | Bodies in Vertebrates" | | Proc. Roy. Soc. Vol. 37. 1884 | | | WHEELER |"Development of the Urino-genital | 402, 405 | Organs of the Lamprey" | | Zool. Jahrbuch. Vol. 13. 1899 | | | v. WIJHE |"Ueber die Mesodermsegmente des Rumpfes | 155-157, 172, | und die Entwicklung des Excretionsystems | 173, 188, 234, | bei Selachiern" | 258, 260, 262, | Archiv. f. Mikr. Anat. Vol. 33. 1889 | 263, 266, 273, | | 280, 308, 390- | | 393, 397, 400, | | 406-408, 412 | | |"Beiträge zur Anatomie der Kopfregion des | 410, 426-428 | Amphioxus lanceolatus" | | Petrus Camper. Deel. 1; Aflevering. 2| | | WILLEY | See Lankester and Willey. | | | WOLFF |"Die Cuticula der Wirbelthierepidermis" | 302 | Jen. Zeitsch. f. Naturwissenschaft. | | Vol. 23. 1889 | | | WOODWARD, H. |"A Monograph of the British Fossil | 235-240, 249, | Crustacea, belonging to the order | 251, 275 | Merostomata" | | Palæontographical Society. 1878 | | | WOODWARD, | | 339 SMITH | | |'Catalogue of Fossil Fishes in the British| 29, 326, 327, | Museum.' Part II. | 344, 349, 351 | London. 1891 | | | v. ZITTEL | Handbuch der Palæontologie | 190

GENERAL INDEX

Acilius larva, eye of, 78, 83 Acromegaly, 425 Actinotrocha, 438 Addison's disease, 423 Adelopthalmus, 249 Adrenalin, 423, 491 Adrenals, 423, 491 Agnathostomatous fishes, 29, 343 Alimentary canal, 433 " " Ammocoetes, 168, 405, 445 " " invertebrate, compared to tube of central nervous system of vertebrate, 43, 433 " " innervation of, 447 " " origin of, 444 " " position of vertebrate and invertebrate, 10 " " possibility of formation of new, 58 " " relationship between notochord and, 434 Ammocoetes, 32, 245 " an ancestral type, 35, 309 " alimentary canal, 168, 405, 445 " auditory organ, 378, 379 " brain, 39, 40, 41, 45, 46, 48, 54, 61 " branchial appendages, 161, 162, 163, 164 " " basket-work, 126, 128, 296, 331, 335 " " chamber, 161, 168, 162, 163 " " circulation in Limulus and, 174 " " diaphragms, 161, 167 " " lamellæ, 175 " " muscles, 171 " " nerves, 164 " " segments, 178, 312 " cartilage, hard, 133, 133, 293, 294, 377 " " muco, 130, 131, 291, 293, 294, 296, 330, 331, 333, 334, 335, 338 " " soft, 129, 130, 293, 294, 296, 335 " degeneracy, evidence of, 59, 94, 343 " development, 228, 458 " digestion, 58, 442 " epithelial cells of gills, 214 " epithelial cells of skin, 347 " " pits, 173, 200 " eye, 93 " " muscles, 267 " " median or pineal, 63, 75, 76, 77, 78, 80, 85, 86 " " " " left, 78, 79 " fat-column, 181, 182 " " in degenerated muco-cartilage, 333, 334 " ganglia in embryo, 229, 283 " gland-tissue round the brain, 209, 210, 379 " head-region, 128, 162, 163, 193, 293, 294, 296, 298, 335 " head-shield, 329, 331, 338 " liver, 442, 452 " lymphatic glandular tissue, 426 " Müllerian fibres, 489 " muscles, eye, 173, 267 " " lip, lower, 297 " " " upper, 305 " " respiratory, 171 " " somatic, 332, 336, 409 " " tubular, 173, 298, 309 " nerves, cranial, 141 " " facial, 186, 311 " " glossopharyngeal, 186 " " optic, 105 " " trigeminal, 282, 288, 288 " " vagus, 153, 173, 186 " nerve-fibres, medullation of, 20 " notochord, 182, 435 " olfactory tube, 219, 225, 227, 317 " oral chamber, 317, 243, 287, 458 " parasitism, 60, 286 " pituitary, 321 " prosomatic region, 243 " pronephric duct, 402, 405 " relationship to Ostracodermata, 326, 338, 344, 414, 416 " retina, 93, 111 " skin, 58, 346, 348, 442 " skeleton, 125, 126, 132, 291, 296, 335 " segments, comparison with segments of Eurypterus, 323 " " facial, 201 " " hyoid, 186, 201 " " prosomatic, 286 " septa between myomeres, 416 " tentacles of upper lip, 303 " test, biological, to show relationship with Limulus, 493 " thyroid, 192, 194, 196, 205, 213, 430 " transformation, 18, 59, 125, 168, 193, 199, 200, 220, 227, 228, 287, 291, 304, 307, 309, 331, 336, 347, 349, 389, 445 " velum, 228, 289, 298, 302 Amoebocytes, 473 Amphibia, 23, 345 Amphioxus, 33, 407 " atrial cavity, 409 " branchial nephric glands, 426 " endostyle, 198, 212 " excretory organs, 389, 395, 477 " neuropore, 220, 457 " notochord, 435, 436, 443 " pleural folds, 495 " septa between myomeres, 416 " somatic muscles, 409 " yolk, 485 Androctonus, 53, 54, 372, 423 Annelids, lateral sense-organs, 357, 367 " nephric organs, 390 " rigin of Arthropods from, 395 " parapodal ganglia, 283 " phagocytic glands, 421 Anthozoa, 474 Antiarcha, 29, 326, 343 Antibody, 492 Antitoxin, 492 Anus, 43, 457 Aponeuroses, 327, 342, 414 Apparatus, auditory, 355 " dioptric, 83 " respiratory, 148 " suctorial, of Petromyzon, 287 Appendages, branchial, of Ammocoetes, 161, 162, 163, 164 " " Limulus, 164 " " internal, 149 " derivation of suctorial apparatus of Petromyzon from, 290 " disappearance of, in transformation of Arthropod into Vertebrate, 386, 413 " evidence of, in prosomatic region of ancient fishes, 342 " muscles, in Limulus and Scorpion, 247 " prosomatic, of Gigantostraca, 234 " Trilobites, 351 Apus, 28, 137, 436, 437 Arachnids, eyes, 75, 87 " diverticula of stomach, 109 " lyriform organs, 364, 368 " segmental excretory organs, 423 Archæocytes, 473 Artemia, v. Branchipus Arthropleura, 249 Arthropoda, arrangement of organs, 10 " evolution, 11 " excretory organs, 396, 418 " eyes, 75, 89 " giant-fibres, 489 " musculature, 411 " olfactory organs, 220 " resemblance to ancient fishes, 29 Astacus, brain, 54 " digestive ferment in cells lining the carapace, 442 " optic chiasma, 101 " optic stalk, 91 " etina, 98 Asterolepis, 326, 342 Atrium, 410 Auchenaspis (Thyestes), 30, 31, 75, 275, 326, 327, 328, 338 Auditory apparatus, 355 Auerbach, plexus of, 447 Aurelia, 475 Autonomic nerves, 3

Balanoglossus, 12, 12, 433, 438, 494 Bdellostoma, 394, 405 Belinurus, 24, 249, 351 Bird, rhomboidal sinus, 46 Bladder, 449 " swim, 148 Blastula, 459, 471, 473 Blood, 463, 472, 474 " circulation, in Ammocoetes and Limulus, 174 " secretion of ductless glands into, 418 Bothriolepis, 29, 32, 239, 326, 351, 450 Bone, 344, 474, 481 Brain, Ammocoetes and Arthropod, 54, 61 " and brain-case of Ammocoetes, 40, 41, 46, 209 " caudal, of Thelyphonus, 450 " epithelial lining of, 38 " roof, 39 " Sphæroma serratum, 62, 90 " Thelyphonus, 56 " ventricles, 4 " vesicles, 48 Branchial basket-work of Ammocoetes, 126, 128, 296, 331, 335 Branchipus, 28 " brain, 51, 54 " eyes, lateral, 88 " " " retina of, 91, 97 " " median, 75 " excretory organs, 396 " (Artemia) diverticula of gut and retinal ganglion, 110, 111, 113 " nerves of appendages, 157 " segmentation, 159 " resemblance to Trilobite, 436 Bunodes, 24, 30, 249, 341, 351, 414 Bundle of Meynert, 48, 77 Bundles, posterior longitudinal, 489 Buthus, muscles, 270

Calcification in aponeuroses of Cephalaspis, 414 " cartilage, 140,330 " successive layers of the skin, 348 Camerostome, 221, 222, 223, 224, 241, 271 Canal, alimentary, formation of vertebrate, 58, 433, 446 " " innervation, 447 " " relationships between notochord and, 434 " " origin, 444 " Haversian, 329 " central, of spinal cord, 405, 439, 455 " spinal, 182 Capsule, auditory, 377, 379 Cartilage Ammocoetes, muco, 127, 130, 131, 200, 291, 303, 330, 333, 334, 344 " " hard, 133, 133, 377 " " soft, 126, 129, 130 " " spinal cartilages, 414 " Hypoctonus, 133, 142 " Limulus, hard, 142 " " muco, 139 " " soft, 20, 130, 137 " origin, 474, 481 " staining reactions, 131, 133, 139, 330, 336 Cavity, atrial, 409, 413 " coelomic, 167, 251, 266, 320, 389, 391, 408, 422, 430, 472 Cells, free-living, 463 Centre, vaso-motor, 468 Cephalaspis, diverticula of gut, 109 " eyes, lateral, 75, 275 " " median, 75 " head-shield, 327, 328, 330, 338 " muscles on head-shield, 269 " resemblance to Ammocoetes, 145, 291, 326, 329, 338, 348, 414 " " Arthropod, 29 " segmentation, 339 Ceratodus, 148 Cephalization, 51 Cephalodiscus, 438 Cephalopod, 23 Cerebellum, 47, 50 Chætopoda, 395 Chamber, oral, of Ammocoetes, 243, 287, 458 Cheliceræ, 235 Chiasma, optic, 101 Chilaria, 235, 238, 291, 301, 458 Chitin, 85, 119, 139, 205, 206, 302, 329, 346, 359, 440, 443 Cilia, 206 Circulation, branchial, 174 Cirri, 357 Clarke's column, 467 Clepsine, nephridial glands, 423 Cochlea, 378 Coelenterata, 465, 472 Coelolepidæ, 344 Coelom, 167, 251, 400, 472, 481 Coelomata, 472 Coelomocoela, 472, 475 Coelomostomes, 477, 481 Colleneytes, 474 Commissure, anterior, 49 " oesophageal, 14 " posterior, 48, 280 Comparison of brains of Ammocoetes and Arthropod, 61 " " invertebrate from Branchipus to Ammocoetes, 54 " " vertebrate, 40 " branchial circulation in Ammocoetes and Limulus, 174 " " lamellæ of Scorpion and Ammocoetes, 175 " " segments of Ammocoetes and Petromyzon, 169 " Cephalaspidian and Palæostracan fish, 31 " Coelom of Peripatus and Vertebrate, 400 " dermal covering of Pteraspis with chitin of Limulus or dentine of fish scales, 346 " entosternite or plastron of Limulus with trabeculæ of Ammocoetes, 145 " excretory organs of vertebrates and invertebrates, 389 " gut of Arthropod and tube of central nervous system of Vertebrate, 43, 244, 433, 440, 455, 457 " head-shield of Cephalaspis and Ammocoetes, 291, 329, 338 " hypophysial tube with olfactory tube of Arthropod ancestor, 229 " " " with position of palæostoma, 317 " mesosomatic region of Ammocoetes and Eurypterus, 192 " muscles, branchial, of Ammocoetes and appendage muscles of Scorpion, 171, 447 " " eye, of Vertebrate with dorso-ventral muscles of Scorpion, 267, 272, 459 " " of oral chamber of Ammocoetes and prosomatic musculature of Limulus, 247, 447 " " longitudinal body-muscles of Vertebrate and dorsal longitudinal muscles of Arthropod, 411, 447 " nerves, appendage of Limulus and Branchipus to lateral root system of Vertebrate, 157 " " cranial and spinal segmental, 152 " nervous systems of Vertebrate and Arthropod, 36 " pineal gland of vertebrates and median eyes of Arthropod, 63, 456 " pituitary body and coxal glands, 246, 319, 321 " prosoma and mesosoma of Limulus and Ammocoetes, 140, 141 " prosomatic region of Ammocoetes and Eurypterus, 244, 333 " retina in Ammocoetes and Musca, 97 " " compound in Arthropod and Vertebrate, 87 " skeleton of Limulus and Ammocoetes, 126, 136 " sense-organs of Arthropod appendages with auditory organs of Vertebrate, 375 " thyroid with endostyle, 198 " " " uterus of Scorpion, 205 Corneagen, 69 Corpora quadrigemina, 47 Corpuscles, Pacinian, Herbst, Grandry, etc., 470 Coxal glands, 242, 246, 319, 321, 389, 398, 403, 429 Cranium, 121, 145, 339 Crayfish, 442, 489 Crest, neural, 281 Cromatophores of frog, 470 Crura cerebri, 14 Crustacea, first appearance, 27 " eyes, 76, 87 " retina, 100 " segmental glands, 422 Ctenophora, 474 Cyathaspis, 29, 326, 340, 343 Cyclostomata, 165, 229, 343, 353, 424 Cysts, 50

Daphnia, 112 Degeneration, 17, 19, 59, 74, 78, 94, 107, 212, 309, 333, 336, 343 Deiters' nucleus, 489 Dendrites, 72 Development, parallel, 497 " of two types of eye, 73 " vertebrate retina, 101 Diaphragms, 161, 167 Didymaspis, 327, 338 Digestion, 441 Dinosaurs, 17 Dipnoans, 23, 45, 148 Diptera, 89, 369 Diverticula, optic, 102 Dogfish, skull, 121, 123 Drepanaspis, 344, 345, 450 Drepanopterus Bembycoides, 238

Ectognath, 238, 242, 271, 304, 342, 381 Eel, 488 Elasmobranchs, 23, 343, 423 Elastin, 435 Embryo, head of dogfish, 121, 123 " skull of pig, 121 Embryology, principles of, 455 Encepalomeres, 262 Endognath, 238, 271, 304, 381 Endostoma, 241, 306 Endostyle, 198, 212 Entapophysis of Limulus, 139 Enterocoela, 472 Enteropneusta, 438, 494 Entochondrites, 377 Entosclerite, 222, 271 Entosternite, 143 Epiblast, 444, 445, 459 Epithelium cells of Ammocoetes, 347 " of central nervous system of vertebrates, 38, 457 " coelomic spaces in annelids, 421 " optic diverticula, 103 " peritoneal, pleural, and pericardial cavities, 477 " velum of Ammocoetes, 301, 302 Equilibration, 358 Eukeraspis, 326 Eurypterus, 26, 150, 191, 237 " appendages, 150, 236, 237 " classification, 249 " comparison with Ammocoetes, 170, 323 " diagram of sagittal median section, 240, 245 " endostoma, 241, 306 " eyes, 275 " mesosomatic segments, 192 " muscles of carapace, 269 " operculum, 150, 190, 212 Evidence of alimentary canal, innervation, 446 " auditory apparatus and lateral line organs, 355 " coelomic cavities in Limulus, 251 " degeneracy in Ammocoetes, 59, 94, 343 " embryology, cartilage, 20, 129 " " eye-muscles, 263 " " excretory organs, 390 " " heart, 179, 451 " " nervous system, central, cerebral vesicles, 48, 458 " " " " " epithelial tube, 37, 42, 102, 244, 433, 455 " " " " " neurenteric canal, 37 " " " " " neuropore, 220, 457 " " " " " optic diverticula, 102 " " " " " spinal cord, 46 " " oral chamber, 228, 242, 243, 290 " " olfactory organ, 220, 227 " " palæostoma or old mouth, 317 " " pineal or median eyes, 15, 63, 74, 456 " " pituitary body and coxal glands, 246, 319 " " thyroid, 192, 194 " " segmentation, double, of head, 157, 234, 258 " " skeleton, cranial, 120, 153 " nervous system, central, 8 " notochord, origin from segmented region, 443 " olfactory apparatus, 218 " organs of vision, 68 " palæontology, 20, 497 " pineal or median eyes, 74 " prosomatic musculature, 247 " respiratory apparatus, 148 " segmentation in head-shield, 339 " skeleton, 119 Evolution, 8, 15, 20, 149, 482, 497 " of brain in brain-case, 210 " cranium of Vertebrate, 342 " excretory organs, 389 " eye of Vertebrate, 114 " nervous system, central, 34 " tissues, 19 " Vertebrate from Balanoglossus and Amphioxus, 33 Eyes, 68 " lateral, 87, 105, 108 " median or pineal, 74, 77, 78, 79

Fat-cells in muco-cartilage, 332 Fat-column of Ammocoetes, 181, 182 Fibres, Mauthnerian, 488 " Müllerian, of Ammocoetes central nervous system, 489 " " retina, 96, 107 Fishes, classification, 218 " ancient, classification, 326, 343 " " cloacal region, 450 " " dominance, 23 " " eyes, 75 " " head-shields. See Head-shields " " pleural folds, 414 Fissure, posterior, 43 Fittest, survival of, 16, 34 Flabellum, 359, 360, 362, 363, 366 Folds, pleural, 410, 414 Function of auditory organ, double, 358 " lateral line sense-organs, 357 " nerves, 448 " thyroid, 212, 215 Fusion of ganglia, 52

Galeodes, 230 " brain, and camerostome, 222, 223 " primordial cranium, 341 " racquet-organs, 369, 375 Ganglia, infraoesophageal, 4, 12, 14, 51, 221 " supraoesophageal, 4, 12, 14, 49, 52, 221, 225 " origin of, of cranial and spinal nerves, 281 Ganglion, epibranchial, 164, 282 " habenulæ, 48, 78 " optic of retina, 72, 89, 97 " of posterior root, 466 " cells of sympathetic system, 424, 428, 448 Ganoids, 23, 345 Gastrula theory, 165, 459 Genital corpuscles, 470 Geological record, 20 " strata, 22 Geotria australis, 80 Germ-band, 482 Germ-cells, 471 Giant-fibres, 489 Gigantostraca, 25, 234 Gills, 148, 161, 185, 214, 494 Glabellum, 339 Glands, carotid, 427 " coxal, 242, 246, 319, 321, 425, 429 " ductless, 418 " generative, of Limulus, 209 " internal secretion of, 214 " lymphatic, 418 " pineal, 15, 63, 75, 456 " pituitary, 244, 246, 319, 425 " segmental, of Crustacea, 422 " submaxillary, 466 " sweat, 448 " thymus, 425 " thyroid, of Ammocoetes, 193, 194, 196, 201, 205, 429 " tissue round brain of Ammocoetes, 209, 379 " uterine, of Scorpion, 202, 203, 204, 205 Gnathostomata, 60, 343 Goblet, 359, 360, 373 Goitre, 215 Gonad, 475, 479 Gonocoele, 475, 481 Grooves, ciliated, 188, 197, 212 " hyper-pharyngeal of Amphioxus, 410 " ventral, of apus and trilobites, 436 Gymnophiona, 393

Hæmocytes, 472 Head of embryo dogfish, 121, 123 Head-shield, dorsal, of Ammocoetes, 330, 331, 338 " " Auchenaspis, 29, 31, 338 " " Cephalaspis, 327, 328, 330, 338, 348 " " Cyathaspis, 340 " " Didymaspis, 338 " " evidence of segmentation, 339 " " Keraspis, 328 " " Ostreostraci, 327, 348 " " Palæostracan, 348 " " Pteraspis, 29 " " Thyestes, 29, 31, 327, 332, 338, 340, 341, 348 " ventral, Scaphaspis, 349 Heart, nerves, 2, 447 " origin of vertebrate, 179, 451, 459 " relative position in vertebrate and invertebrate, 175 " veins forming vertebrate, 180 Hemiaspis, 24, 25, 249, 250, 351, 414 Hemispheres, cerebral, 47 Hepatopancreas of Ammocoetes, 452 " Limulus, 211 Heterostraci, 29, 275, 326, 343 Hirudinea, 478 Histolysis in transformation of the lamprey, 59 Homology of branchial region of vertebrate and invertebrate, 149 " ductless glands and nephridial organs, 418 " external genital ducts of arthropods and nephridia of annelids, 429 " germinal layers in all Metazoa, 459 " pituitary body of Ammocoetes and coxal glands of Limulus, 319 " tubular muscles of Ammocoetes and veno-pericardial muscles of Limulus, 309 " ventral aorta of vertebrate and longitudinal venous sinuses of Limulus, 178 Hydra, 441, 465, 472, 476 Hydrophilus larva, eye, 84 Hyoid segment in Ammocoetes, 186, 267 Hypoblast, 434, 438, 444, 445, 459 Hypoctonus, cartilage cells in entosternite, 133 " operculum, 189, 207 Hypogastric plexus, 3 Hypogeophis, 393 Hypophysis, 229, 244, 317, 318, 340

Infundibulum, position, 122,132 " tube, the ancestral oesophagus, 4, 37, 244, 318 " " relation to neural canal, 14, 36, 318, 440, 457 " " " notochord, 318, 435,440 " " " olfactory tube, 220, 228, 318, 340 Insects, chordotonal organs, 364, 370 Invertebrate, heart, 175, 179 " excretory organs, 418 " nervous system, 13, 54 " segmental nerves, 152

Keraspis, 75, 328, 338 Kidney, 420, 459, 476 " nerves, 477 King-crab, v. Limulus

Labyrinthodont, 21, 28 Lamina terminalis, 49 Lamprey, v. Ammocoetes and Petromyzon Larva, v. Transformation of the Lamprey Lateral line system, 261, 355, 411, 470 Law of Progress, 19 " Recapitulation, 434, 456, 498 Layer, germinal, 459 " laminated, 347, 348 Leech, 421 Lens, formation, 83, 115 Lepidosiren, 148, 461, 466 Limulus or king-crab, 25, 140, 236, 240 " appendages, branchial, 138, 164, 175 " appendages, prosomatic, 381 " brain, 54 " circulation, 174, 176 " classification, 26, 249 " coelomic cavities, 252, 328 " coxal glands, 321, 389, 397, 403, 429 " eyes, median, 62, 74, 81 " entosternite or plastron, 142, 143 " flabellum, 360, 362, 363, 380, 381 " generative organs and ducts, 189, 202, 208, 209, 380 " heart, 180 " musculature, branchial, 170 " " prosomatic, 247 " " veno-pericardial, 177, 297, 309, 313 " nerves, appendage, 140, 157 " " cardiac, 314 " " segmental, tripartite division of, 157, 235, 267, 355 " segments, branchial, 152 " " first mesosomatic, 188 " " prosomatic, 233 " operculum, 189, 202, 235, 295 " sense-organs, poriferous, of appendages, 359 Lip, lower, of Ammocoetes, 246, 289, 297, 458 " upper, " 228, 243, 303, 336 Liver, Ammocoetes, 452 " Limulus, 209, 211 Lizard, pineal eye, 80 " suprarenals, 424 " tail, 50 Lobes, optic, 101 Lobster, 489 Lungs, 148 Lung-books of scorpions, 150 Lymph, 474 Lymph-corpuscles, 463, 490 Lymphocytes, 472

Malapterurus, 470 Mammal, dominance of, 21 Man, dominance of, 17 Marsipobranchs, 23, 35 Medullation of nerve-fibres, 20, 267, 467, 477 Membranes, basement, 436 Meroblastic egg, 485 Merostomata, 25, 249, 321 Mesencepalon, 48 Mesoblast, 444, 455, 459 Mesogloea, 474 Mesonephros, 389, 400, 424, 429 Mesosoma, 52 Mesothelium, 472, 477 Metanephros, 389 Metasoma, 52, 387, 411 Metastoma, 239, 246, 272, 289, 342, 458 Metazoa, 444, 459, 471, 472 Meynert's bundle, 48, 77 Mollusca, dominance of, 23 Mouth, old, or palæostoma, 14, 317, 322, 440, 458 " vertebrate, 317 Muco-cartilage, v. Cartilage Muscles, antagonistic, 447 " branchial, 170 " connection of, with central nervous system, 464 " eye, and their nerves, 263 " prosomatic, 243, 247 " phylogeny of origin of skeletal, 478 " rudimentary, in Ammocoetes, 289 " somatic trunk, origin of, 406 " striated, 20, 155 " tubular, of Ammocoetes, 309 " unstriped, 20, 447, 491 " visceral and parietal, 155, 172 " veno-pericardial of Limulus and Scorpion, 177, 297, 309 Muscle-spindles, 267 Mygalidæ, stomach, 109 " segmentation, 249, 306 Myomeres, 262, 337, 414, 479 Myotomes, 332, 337, 338, 391, 407, 408 Mysis, eyes, 100 " ductless glands, 422 Myxine, 220, 392, 402, 419

Nebalia, 144, 422 Nemertina, 475 Nephridia, 395, 421, 429 Nephrocoele, 430 Nephrotome, 393 Nerves, abducens, 155, 263, 266 " auditory, 356, 376 " autonomic, 3 " facial, 155, 156, 186, 188, 192, 311, 356, 378 " " ramus branchialis profundus, 311 " to flabellum, in Limulus, 361, 375 " glossopharyngeal, 155, 156, 186, 356 " hypoglossal, 156 " inhibitory, 447 " inedullation of, 20, 267, 467, 477 " occulomotor, 155, 234, 263, 274 " olfactory, 229 " optic, 101, 104 " " of pineal eye, 79 " origin of ganglia of cranial and spinal, 281 " to pecten of Scorpion, 375, 376 " preganglionic, 2 " of prosoma in Limulus, 235, 355 " regeneration of, 469 " roots, of Limulus, 157 " sacral, 448 " segmental, 152, 156 " segmental nature of cranial, 259, 411 " spinal, absence of lateral roots in, 388 " spinal accessory, 154 " trigeminal, 151, 155, 156, 234, 243, 257, 279 " " motor nucleus of, 280 " " of Ammocoetes, 288 " tripartite arrangement of cranial nerves, 154, 157, 235, 267, 355 " trochlear, 48, 155, 234, 263, 276 " vagus, 151, 154, 156, 173, 186, 356, 447, 449 Nervous system, central, comparison of Vertebrate and Arthropod, 36, 457 " " connection of, with muscular and epithelial tissues, 464 " " " with retina, 71 " " disease of, 50 " " evidence of, 8 " " evolution of, 34 " " importance of, 16, 463, 482, 498 " " invertebrate, 10, 13, 54 " " origin of, 480 " " relation of germ-band to, 483 " " segmentation of vertebrate, 51 " " tube of, 36-51, 102, 211, 433, 455, 457 " " vertebrate, 10, 13, 40, 41, 152 " enteric, 447 " sympathetic, 2, 424, 428, 448, 491 Neurenteric canal, 37 Neuroblast, 465 Neuromeres, 55, 247, 262, 312, 316 Neurones, 72, 92, 465 Neuropil, 71, 91 Neuropore, 220, 457 Nose, 219 " of Osteostraci, 329, 352, 458 Notochord, 120, 122, 180, 181, 220, 244, 295, 318, 405, 417, 433, 436, 494

Ocelli, 70 Oesophagus of Ammocoetes, 405 " Arthropod, compared to tube of infundibulum, 4, 244, 440 Olfactory apparatus, evidence of the, 218 " organs of the Scorpion group, 220 " tube of Ammocoetes, 219, 225, 244, 317 Oligochæta, 421, 478 Operculum of Eurypterus, 191, 212, 291 " Limulus, 189, 202, 235, 295 " Phrynus, 191 " Scorpion, 189, 206, 212, 372 " Thelyphonus, 189, 190, 206 Organs, arrangement of, 10 " auditory, of arachnids and Insects, 368 " branchial, innervation of vertebrate, 151 " " sense-organs of embryo vertebrate, 261, 281 " chordotonal, of insects, 364, 369, 370 " electric, 470 " generative, of Limulus, 208, 209 " " connection between Thyroid gland and, 215 " genital, of sea-scorpions, 206 " lateral line, 355, 411 " lyriform, of arachnids, 364, 369 " olfactory, of Scorpion group, 220 " phagocytic, 420 " racquet, of Galeodes, 369, 375 " segmental excretory, 389, 391, 408, 418, 459, 477 " sense, of appendages of Limulus, 358 " vestigial, 456 " of vision, evidence of, 68 " vital, 57 Origin of alimentary canal, 444 " arthropods from annelids, 395 " atrial cavity, 409 " auditory capsules and parachordals, 377 " coelom, 475, 481 " ductless glands, 428 " free cells, 472 " heart of vertebrate, 179 " lateral line organs, 356 " muscles, 478 " musculature, branchial, 170 " " somatic trunk, 406 " nervous system, central, 480 " notochord, 434 " segmental excretory organs, 389 " skeleton of vertebrates, 119 " vertebrates, 9, 36, 351, 433, 493 Ostracodermata, 326, 343 Osteostraci, 29, 75, 275, 326, 343 Otoliths, 378 Ovum, 473

Pacinian bodies, 470, 477 Palæmon, 20, 422 Palæontology, evidence of, 20, 497 Palæostoma, 317 Palæostraca, 27, 396 " median eyes, 74 " mesosomatic appendages, 188 " olfactory organs, 221 " segments, compared to Ammocoetes, 308 Pantopoda, glands, 423 Parachordals, 121, 132, 377 Parapodia, 357 Parapodopsis, foot glands, 422 Parathymus, 427 Parathyroids, 427 Parietal organ, 76 Pecten of scorpion, 114, 359, 366, 371, 372, 373, 374 Pedipalpi, 190 Periblast, 471 Peripatus, 396, 399, 400, 411, 421, 429 Petromyzon, alimentary canal, 405, 445 " auditory organ, 378 " branchial segments, 169 " life-history, 59 " olfactory tube, 219, 226 " pronephric duct, 402 " retina and optic nerve, 95 " skeleton, 125 " suctorial apparatus, 287, 304 " transformation, v. Transformation of the Lamprey Phagocytes, 420, 471 Pharynx of Amphioxus, 410 " Vertebrate, 440 Phoronis, 439 Phrynus, brain, 53 " caudal brain, 450 " carapace and carapace removed, 250 " coecal diverticula, 109 " evidence of segmentation of carapace, 249, 250, 341 " operculum, 191 " prosomatic appendages, 306 " crossing of dorso-ventral muscles, 271, 277 " stridulating apparatus, 368 Phyllodoce, 395 Phyllopoda, 321 Pigment, in Ammocoetes, in position of atrial cavity, 412 " epithelial lining of central nervous system, 43, 457 " choroid of vertebrate eye, 104, 107 " between glandular cells round brain of Ammocoetes, 211, 379 " tapetal layer of retina, 70 " white, of right pineal eye of Lamprey, 76, 80 Pineal body, 14, 15 " eyes, 74, 233, 244 " " of Ammocoetes, 80, 78, 85 " gland, 63, 75, 456 Pits, epithelial, of diaphragms in Ammocoetes, 164 " " skin in Ammocoetes, 173, 200 Pituitary body, 244, 246, 319, 321, 425, 430 Plasma-cells, 471 Plakodes, 283 Planarians, 475 Plastron, formation of cranial walls from the, 86, 322, 341 " of Limulus, 136, 142, 143 " Palæostracan, compared to trabeculæ of Ammocoetes, 145, 377 " muscles attached to the, 270 " of Thelyphonus, 143 Platyhelmia, 475 Pleuron, 410, 415 Plexus, of Auerbach, 447 " choroid, 38, 45, 49, 103 " hypogastric, 3 Polychæta, 357, 395 Pores, abdominal, 430 Porifera, 473 Pouch, formation of gill, 165, 166 Prestwichia, 24, 25, 249, 351 Principle of concentration and cephalization, 51 " embryology, 455 Pristiurus, 424 Progress, law of, 19 " result of, 56 Pronephros, 389, 397, 419, 424, 449 Prosencephalon, 48 Prosoma, 52 Protopterus, 148 Protostraca, 27, 396, 417 " dominance of, 28 Protozoa, 166, 479 Pseudoniscus, 25, 249 Pteraspis, 29, 30, 275, 326, 343, 344, 350 Pterichthys, 29, 31, 239, 326, 351 Pterygoid, pedicle of, 295 Pterygotus, 25, 27, 56, 170, 191, 221, 235, 238, 249, 276 Ptychodera, 494, 495

Ramus branchialis profundus of facial nerve, 311 " communicans, 2, 3 Raphe, 46 Recapitulation, law of, 434, 456, 498 Regeneration of nerves, 469 Reptiles, dominance of, 21 Retina, compound, 71 " development of, 101 " inversion of, in Vertebrates, 114 " inverted, 70 " layers of compound, 73 " " in Crustacean eye, 100 " of lateral eye of Ammocoetes, 93, 95, 111 " Musca, 89 " Pecten and Spondylus, 114 " upright compound, 72 " " simple, 69 Rhabdites, 69, 81

Saccus vasculosus, 244, 322 Scales, 345 Scaphaspis, 349 Schwann, sheath of, 469 Sclerotomes, 388 Scorpion, brain, 54 " branchial lamellæ, 175 " development, 482 " entochondrites, 377 " excretory organs, 397 " eyes, 75 " lung-books, 150, 170 " lymphatic glands, 423 " muscles, oblique, 278 " " recti, 271 " " respiration, 171 " " veno-pericardial, 177 " muscular system, 247, 268, 269 " nerves to Cheliceræ, 237 " olfactory organs, 220 " operculum of male, 189, 206, 212 " pecten, 359, 366, 371, 373, 374, 377 " under surface, 372 " uterus, 189, 202, 203, 204, 205, 212 Sea-scorpions, 25, 26, 27, 56, 150, 170, 191, 208, 221, 232, 235, 241, 349, 359 Segmentation, branchiomeric, 124 " body-muscles in vertebrate, 388 " eye-muscles, 248 " of head, double, 155, 157, 173, 234, 258, 411, 459 " of head-shield, 339 " history of cranial, 258 Segments, branchial of Ammocoetes, 161, 178, 186 " hyoid, in Ammocoetes, double, 186, 201, 267, 300 " innervation of branchial, 151 " first mesosomatic, in Limulus and its allies, 188 " mesosomatic, of Eurypterus, 192 " prosomatic of Limulus and its allies, 233, 249 " " Ammocoetes, 286 " of spinal region of Vertebrates, 388 " of trigeminal nerve-group, 257, 279 " tubular muscles of hyoid, 299 Sense-organs of Amphioxus, 34 " branchial, of Limulus, 359, 360 " lateral, of Annelids, 357, 367 " lateral-line system, 356, 411, 470 Serum, 492 Significance of the optic diverticula, 102 Silurus, 488 Sinus, longitudinal venous, of Limulus, 176, 312, 451 " rhomboidal of bird, 46 Skeleton, Ammocoetes, 126, 296, 335 " " branchial, 126, 126 " " basi-cranial, 132 " " muco-cartilaginous, 291, 296, 330, 331 " aponeurotic, 414 " Cephalaspis, 414, 415 " evidence of the, 119 " Limulus, cartilaginous, 126, 136 " " mesosomatic, 137 " " prosomatic, 142 " Petromyzon, 125 " Vertebrate, commencement of bony, 120, 121 Skin, digestive power of cells of, in Ammocoetes, 58, 442 " of Ammocoetes, 346 " nerves of, 448 Skull of dogfish, 123 " pig-embryo, 121 Slimonia, 27, 56, 170, 235, 238, 249, 276, 303 Solenocytes, 395, 477 Solpugidæ, 109 Sphæroma serratum, brain, 62, 90, 101, 225 Spiders, eyes, 75 " stomach, 109 Spina bifida, 50 Spinal cord, difference between brain and, 45 " " region of, 385 " " termination in bird-embryo, 51 Spondylus, retina of, 114 Squilla, eyes, 100 " glands, 422 Stomach, cephalic, 4, 43, 102, 244 Stylonurus Lagani, 27, 235, 239, 249 Substantia gelatinosa Rolandi, 44 Suprarenal body, 423 Surfaces, dorsal and ventral, 11 " reversal of, 15, 29, 36, 87, 175, 352, 433, 484 Synapse, 72 Syncytium, 464, 471, 479

Tail of lizards, 50 Tapetum, 69 Teleosteans, 23, 345, 420, 424 Tendon-organs, 470 Tentacles of Ammocoetes, 246, 289, 303 Tergo-coxal muscles, 247 Test, biological, of relationship of animals, 492 Thalainencephalon, 48 Thelodus, 344 Thelyphonus, 231 " brain, 53, 54, 56, 224 " " caudal, 450 " coecal diverticula, 109 " entosternite, 143 " genital organs, 206 " lyriform organs, 368 " olfactory passage, 226, 306 " operculum, 189, 190, 206, 207 Theory, gastræa, 444, 461 Theories of the origin of vertebrates, 9, 411, 433, 457 Thionin reaction, 131, 139, 213, 330, 336 Throat, formation of, 179 Thyestes, 30, 31, 275, 326, 328, 329, 339, 340, 341 Thymus, 425, 430 Thyroid gland of Ammocoetes, 61, 127, 192, 194, 196, 429, 459 " " evidence of the, 185 " " function of, in Ammocoetes, 213 Tissues, connective, 471, 474, 481 " evolution of, 19 " notochordal, 435 " two groups of, 463 Tongue of Ammocoetes, 246, 303 Tonsils, 427, 430 Torpedo, 262, 392, 470 Trabeculæ, 121, 132, 133, 145, 277, 295, 377 Transformation of the Lamprey, 18, 35, 59, 61, 125, 168, 193, 199, 200, 220, 227, 228, 287, 291, 304, 307, 309, 331, 336, 347, 349, 389, 445 Tremataspis, 32, 75, 275, 326, 351, 352 Trilobites, 24, 25, 26, 437 " appendages, 351, 437 " diagram of section through a trilobite-like animal, 413 " dominance of, 26 " excretory organs, 396 " eyes, 74, 88 " glabellum, 339 " relations of, 249, 283 " respiratory apparatus, 170 " ventral surface, 437 Tube of central nervous system, 37, 38, 42, 102, 211, 433, 455, 457 " from IVth ventricle to surface of brain in Ammocoetes, 209 " Fallopian, 431 " hypophysial, 229, 244, 317, 440 " meeting of four tubes in vertebrate, 318, 440 " notochord originally a, 436, 440 " olfactory, of Ammocoetes, 219, 225, 317, 440 " unsegmented, in segmented animal, 439 Tunicata, 16 " budding of, 441 " degeneration, 12, 17, 19, 60 " endostyle, 198, 212 " hypophysis, 425 " notochord, 438 " position of, 494

Unit, appendage, in non-branchial segments, 185 " branchial, 161, 165, 168, 185 Ureters, nerves of, 448 Uterus of Scorpion group, 189, 202, 203, 204, 205, 214 " vertebrate, nerves of, 448

Valve, ileo-colic, 449 " of Vieussens, 48 Variation in dominant races, 21, 88 " meristic, in spinal nerves, 154, 387 Veins, forming vertebrate heart, 180 Velum, 228, 289, 298, 302 Vertebrates, alimentary canal, innervation of, 446 " atrial cavity, 410 " auditory apparatus and lateral-line system, 356 " body-cavity, 401, 430 " brains, 40 " branchial organs, 151 " coelomic cavities in head region, 251, 266 " cranium, evolution of, 342 " egg of, 483 " evolution of, 11 " excretory organs, 389, 391, 408 " glands, ductless, 418 " " internal secretion of, 215 " heart, 175, 179, 180 " muscles, evidence of segmentation of eye, 248 " " oblique, 278 " " origin of somatic trunk, 406 " nervous system, central, 13 " nerves, segmental, 152 " notochord and gut, 434 " organs of, 10 " origin of, 9, 411, 433, 457 " segments, prosomatic, 257 " skeleton, commencement of bony, 120, 458 " spinal cord and medulla oblongata, 44 " spinal region, 385 " thyroid, connection between generative organs and, 215 " tubes, meeting of four, 318, 440 Vesicles, cerebral, formation of, 48, 458 Vitellophags, 471, 483 Volvox, 479

Wolffian body, 390

Xiphosura, 24, 26, 249

Yolk, 482

THE END

Notes.

N.B.--In addition to the nerves mentioned, C. Bell included, in his respiratory system of nerves, the fourth nerve or trochlearis, the phrenic and the external respiratory of Bell.

"The Origin of Vertebrates, deduced from the Study of Ammocoetes." Part X., "The Origin of the Auditory Organ: the Meaning of the VIIIth Cranial Nerve." Journ. Anat. and Physiol., vol. 36, 1902.

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Corrections made to printed text

Fig. 6: 'Dalmanites' corrected from 'Dalmatites' (which is an ammonite).

Fig. 15 caption: 'Pterichthys' corrected from 'Ptericthys'. So also on P. 239 and twice on P. 324.

P. 60: 'gnathostomatous condition' corrected from 'gnathostomotous ...'.

P. 409: 'well known' corrected from 'well know'.

P. 420: 'meso-nephros' corrected from 'neso-nephros'.

P. 432: 'had become a vertebrate' corrected from 'had became ...'.

Fig. 167 caption: 'nephrocoele' corrected from 'nephrocele'.

P. 474: 'Scyphomedusæ' corrected from 'Scyphomedusoe'.

P. 497: 'idiosyncrasy' corrected from 'idiosyncracy'.

Bibliography, Dietl: 'Gehirns' corrected from 'Gehirus'.

Bibliography, Goodrich: 'Polychæta' corrected from 'Polychoeta'.

Bibliography, Graber: 'Chordo-tonalen' corrected from 'Chordo-tonalem'.

Bibliography, Vincent: 'Phylogeny' corrected from 'Phyogeny'.

Index, Homology: 'Metazoa' corrected from 'Metozoa'.