Part 2

Digby is frankly deterministic in his description of embryonic development:

Take a beane, or any other seede, and putt it into the earth, and lett water fall upon it; can it then choose but that the beane must swell? The beane swelling, can it choose but breake the skinne? The skinne broken can it choose (by reason of the heate that is in it) but push out more matter, and do that action which we may call germinating.... Now if all this orderly succession of mutations be necessarily made in a beane, by force of sundry circumstances and externall accidents; why may it not be conceived that the like is also done in sensible creatures; but in a more perfect manner.... Surely the progresse we have sett downe is much more reasonable, then to conceive that in the meale of the beane, are contained in litle, severall similar substances.... Or, that in the seede of the male, there is already in act, the substance of flesh, of bone, of sinewes, of veines, and the rest of those severall similar partes which are found in the body of an animall; and that they are but extended to their due magnitude, by the humidity drawne from the mother, without receiving any substantiall mutation from what they were originally in the seede. Lett us then confidently conclude, that all generation is made of a fitting, but remote, homogeneall compounded substance: upon which, outward Agents working in the due course of nature, do change it into an other substance, quite different from the first, and do make it lesse homogeneall then the first was. And other circumstances and agents, do change this second into a thirde; that thirde, into a fourth; and so onwardes, by successive mutations (that still make every new thing become lesse homogeneall, then the former was, according to the nature of heate, mingling more and more different bodies together) untill that substance be produced, which we consider in the periode of all these mutations....

Digby thus makes a good statement of epigenetic development. He attempts, without success, a physiochemical explanation of the mechanisms of development, finally admitting:

I persuade my selfe it appeareth evident enough, that to effect this worke of generation, there needeth not be supposed a forming vertue ... of an unknowne power and operation.... Yet, in discourse, for conveniency and shortnesse of expression we shall not quite banish that terme from all commerce with us; so that what we meane by it, be rightly understood; which is, the complexe, assemblement, or chayne of all the causes, that concurre to produce this effect; as they are sett on foote, to this end by the great Architect and Moderatour of them, God Almighty, whose instrument Nature is.

Digby's general theory thus represents a strange mixture of epigenesis and pangenesis, and is not entirely devoid of "virtues." It is, however, a bold attempt to explain embryonic development in terms commensurate with his time, and it embodies the same optimistic belief that the mechanism of embryogenesis lay accessible to man's reason and logical faculties that similarly led Descartes and Gassendi to comprehensive interpretations of embryonic development comprising a maximum of logic and minimum of observations.

The traditionalist reaction to the attack upon treasured and intellectually comfortable interpretations of development was not slow to set in. A year after the appearance of Digby's Nature of Bodies, Alexander Ross published a treatise with a title indicating its goals and content: The Philosophicall Touch-Stone; or Observations upon Sir Kenelm Digbie's Discourses of the nature of Bodies, and of the reasonable Soule: In which his erroneous Paradoxes are refuted, the Truth, and Aristotelian Philosophy vindicated, the immortality of mans Soule briefly, but sufficiently proved. Ross supports the Galenist tradition that the liver, not, as Digby claimed, the heart, forms first in development. It can be no other way, he says, since the blood is the source of nourishment and the liver is necessary for formation of the blood. Furthermore, he contends, "the seed is no part of the ... aliment of the body ... the seed is the quintessence of the blood." Ross is an epigeneticist, to be sure, but so was Aristotle, and Ross prefers to maintain the supremacy of logic and the concepts of the Aristotelian tradition as a guide to the interpretation of development.

In 1651, Nathaniel Highmore, a physician at Sherborne in Dorset, published The History of Generation, which, he informs us, is an answer to the opinions expressed by Digby in The Nature of Bodies. Highmore's book is an important one in the history of embryology, since it is the first treatment of embryogeny from the atomistic viewpoint and because it contains the first published observations based upon microscopic examination of the chick blastoderm. Admittedly, the drawings illustrating Highmore's observations upon generation are, to use a word often applied to modern art, "interesting," but they do derive from actual observations of developing plant and animal embryos. His observations on the developing chick embryo are quite full, complete, and exact, and he also records some interesting facts regarding development of plant seeds.

Highmore's theory of development appears to have emerged directly out of his observations of development. In this sense, his theory rests upon a more solid base than does the developmental theory of Digby. His theory is a mixture of vitalism and atomism, designed to eliminate the "fortune and chance" resident in Digby's concept. "Generation," he says,

...is performed by parts selected from the generators, retaining in them the substance, forms, properties, and operations of the parts of the generators, from whence they were extracted: and this Quintessence or Magistery is called the seed. By which the Individuals of every Species are multiplied...

From this, All Creatures take their beginning; some laying up the like matter, for further procreation of the same Species.

In others, some diffus'd Atomes of this extract, shrinking themselves into some retired parts of the Matter; become as it were lost, in a wilderness of other confused seeds; and there sleep, till by a discerning corruption they are set at liberty, to execute their own functions. Hence it is, that so many swarms of living Creatures are from the corruption of others brought forth: From our own flesh, from other Animals, from Wood, nay, from everything putrified, these imprisoned seminal principles are muster'd forth, and oftentimes having obtained their freedom, by a kinde of revenge feed on their prison; and devour that which preserv'd them from being scatter'd. Accounting thus for sexual and spontaneous generation, Highmore defines two types of seminal atoms in the seed--"Material Atomes, animated and directed by a spiritual form, proper to that species whose the seed is; and given to such matter at the creation to distinguish it from other matters, and to make it such a Creature as it is." The seminal atoms come from all parts of the body, the spiritual atoms from the male, and the material atoms from the female. The atoms of Democritus are thus transmuted into the "substantial forms" and endowed either with the efficient cause of Aristotle or, permitted to remain material, with Aristotle's material cause. According to Highmore, the atoms are circulated in the blood, which is a "tincture extracted from those things we eat," and these various atoms retain their formal identity despite corruption. The testicles abstract some spiritual atoms belonging to each part and, "As the parts belonging to every particle of the Eye, the Ear, the Heart, the Liver, etc. which should in nutrition, have been added ... to every one of these parts, are compendiously, and exactly extracted from the blood, passing through the body of the Testicles." Being here "cohobated and reposited in a tenacious matter," the particles finally pass out of the testes. A similar extraction of the female seed occurs in the ovaries. The female seed

...containing the same particles, but cruder and lesse digested, from a cruder matter, by lesse perfect Organs, is left more terrene, furnished with more material parts; which being united in the womb, with the spiritual particles of the masculine seed; everyone being rightly, according to his proper place, disposed and ordered with the other; fixes and conjoynes those spiritual Atomes, that they still afterwards remain in that posture they are placed in.

The theories of development promulgated by Digby and Highmore reveal the chief formulations of mechanistic rationalism, more or less free of empiricism, that were emerging as the vitalism of the sixteenth and seventeenth centuries waned. There was little new in these theories: both Digby's and Highmore's theories included different combinations of elements of ancient lineage. Digby's concept was essentially free of vitalistic coloring; akin to the embryological efforts of Descartes in its virtual independence from observations of the developing embryo, it was similarly vulnerable to Voltaire's criticism of Descartes, that he sought to interpret, rather than study, Nature. This criticism is not so applicable to Highmore, whose theory of development is more vitalistic than Digby's, and is more akin to the concepts developed by Gassendi than those of Descartes. Highmore had experience with the embryo itself, and his actual contribution as an observer of development, although hardly epochal, is worthy of note. But despite this empirical base, Highmore has final recourse to a hypothesis blending many ancient ideas and substituting the Aristotelian material and efficient causes for the "fortune and chance" he objected to in Digby's hypothesis. It was not easy in the seventeenth century to avoid falling back upon some variety of cause or force.

In 1651, about two months before publication of Highmore's History of Generation, a work appeared which marks another period in seventeenth-century English embryology. William Harvey, De Motu Cordis almost a quarter of a century behind him, now published De Generatione Animalium, the work he said was calculated "to throw still greater light upon natural philosophy." This book is, perhaps, not as well known as Harvey's treatise demonstrating circulation of the blood, but it is an important work in the history of embryology and it occupies a prominent position in the body of English embryological literature.

In De Generatione, Harvey provides a thorough and quite accurate account of the development of the chick embryo, which, in particular, clarified that the chalazae, those twisted skeins of albumen at either end of the yolk, were not, as generally believed, the developing embryo, and he demonstrated that the cicatricula (blastoderm) was the point of origin of the embryo. The famous frontispiece of the treatise shows Zeus holding an egg, from which issue animals of various kinds. On the egg is written Ex ovo omnia, a legend since transmuted to the epigram Omne vivum ex ovo. The legend illustrates Harvey's principal theme, repeated constantly throughout the text, "that all animals were in some sort produced from eggs."

If Harvey made no contribution beyond emphasizing the origin of animals from eggs, he would deserve a prominent place in the history of embryology. But the work is also significant in its espousal of epigenesis, and, supported as his argument was by observation and logic, it became the prime formulation of that concept of development during the seventeenth and eighteenth centuries. His statement of epigenetic development is clear:

In the egg ... there is no distinct part or prepared matter present, from which the fetus is formed ... an animal which is created by epigenesis attracts, prepares, elaborates, and makes use of the material, all at the same time; the processes of formation and growth are simultaneous ... all its parts are not fashioned simultaneously, but emerge in their due succession and order ... Those parts, I say, are not made similar by any successive union of dissimilar and heterogeneous elements, but spring out of a similar material through the process of generation, have their different elements assigned to them by the same process, and are made dissimilar ... all its parts are formed, nourished, and augmented out of the same material.

Actually, Harvey's exposition of epigenesis, albeit clear, is not totally impressive, since it is largely a reflection of Aristotle's influence. The main importance of Harvey's vigorous and cogent defense of epigenesis is that it provided some kind of counterbalance to the increasingly dominant preformationist interpretations of embryonic development.

Harvey did not break with Aristotelianism; on the contrary, he lent considerable authority to it. Unable to escape the past, he was not completely objective in his study of generation. Everywhere the pages of his book reveal his indebtedness to past authorities. Robert Willis, who provided the 1847 translation of De Generatione, expresses this well:

Even Harvey's concept of the egg reveals a strong Aristotelian bias. Actually, Harvey attained to his conclusion that all animals derive from eggs by assuming that

on the same grounds, and in the same manner and order in which a chick is engendered and developed from an egg, is the embryo of viviparous animals engendered from a pre-existing conception. Generation in both is one and identical in kind: the origin of either is from an egg, or at least something that by analogy is held to be so. An egg is, as already said, a conception exposed beyond the body of the parent, whence the embryo is produced; a conception is an egg remaining within the body of the parent until the foetus has acquired the requisite perfection; in everything else they agree; they are both alike primordially vegetables, potentially they are animals.

The ovum, for Harvey, is in essence "the primordium vegetable or vegetative incipience, understanding by this a certain corporeal something having life in potentia; or a certain something existing per se, which is capable of changing into a vegetative form under the agency of an internal principle." The ovum is for Harvey more a concept than an observed fact, and, as stated by one student of generation, "The dictum ex ovo omnia, whilst substantially true in the modern sense, is neither true nor false as employed by Harvey, since to him it has no definite or even intelligible meaning."

Harvey's treatise on generation is clearly a product of his time. It advances embryology by its demonstration of certain facts of development, by its aggressive espousal of epigenesis and the origin of all animals from eggs, and by its dynamic approach stressing the temporal factors in development and the initial independent function of embryonic organs. However, the strong Aristotelian cast of Harvey's treatise encouraged continued discussion of long outdated questions in an outdated manner and, combined with his expressed disdain for "chymistry" and atomism, discouraged close cooperation between embryologists of different persuasions. It is perhaps easy to underestimate the impact and general importance of Harvey's work in view of these qualifications, and so it should be remarked that both positive and negative features of De Generatione influenced profoundly subsequent embryological thought.

It will be recalled that the title of The Philosophicall Touch-Stone identified Digby as the object of Alexander Ross's ire. In comparable manner, the latter's Arcana Microcosmi, published in 1652, declares its purpose to be "a refutation of Dr. Brown's Vulgar Errors, the Lord Bacon's Natural History, and Dr. Harvy's book De Generatione." Let us pause a brief moment in memory of a man so intrepid as to undertake the refutation of three of England's great intellects in one small volume, and then proceed to examine the embryological concepts of one of the trio, Sir Thomas Browne.

Browne's Religio Medici, composed as a private confession of faith around 1635, is known to all students of English literature, as is his later, splendid work on death and immortality, Hydrotaphia, Urne-Buriall. One of the greatest stylists of English prose, Browne was also a physician and a student of generation who deserves our attention as an early chemical embryologist pointing the way to a form of embryological investigation prominent in the last half of the seventeenth century.

Browne's embryological opinions are found particularly in Pseudodoxia Epidemica, The Garden of Cyrus, and in his unpublished Miscellaneous Writings. Browne, a well-read man, was educated at Oxford, Montpellier, Padua, and Leyden, and he was thoroughly imbued with the teaching of the prophets of the "new learning." This is evident throughout his writings, as witness his admonition to the reader of the Christian Morals:

Let thy Studies be free as thy Thoughts and Contemplations, but fly not only upon the wings of Imagination; Joyn Sense unto Reason, and Experiment unto Speculation, and so give life unto Embryon Truths, and Verities yet in their Chaos.

Browne greatly admired Harvey's work on generation, considering it "that excellent discourse ... So strongly erected upon the two great pillars of truth, experience and solid reason." Browne carried out a variety of studies upon animals of all kinds, in them joining Sense unto Reason, and "Experiment unto Speculation." Thus in his studies of generation, he made observations and also performed certain simple chemical experiments. Noting that "Naturall bodyes doe variously discover themselves by congelation," Browne studied experimentally the chemical properties of those substances providing the raw material of development. He observed the effects of such agents as heat and cold, oil, vinegar, and saltpeter upon eggs of various animals, recording such facts as the following: