Part 89

The black shank color results when melanin appears in the epidermis. Two forms of black pigment occur in the epidermis: granules in both layers and pigment cells in the rete Malpighii.

The green shank is produced where there is pigment in the epidermis and numerous melanin pigment cells in the upper corium. It is an optical effect due to melanin lying under the semi-transparent yellow epidermis. There is no melanin in the epidermis.

In the beak the corium of the skin is represented by a thin layer located between the =periosteum= and the stratum Malpighii. Numerous blood-vessels pass into it, and in the soft horn-like skin, occur sensory nerve fibers.

The nails originate from the epidermis, of which they are a modification. The nails of the toes are bent downward. They present a convex dorsal surface and are concave ventrally. The dorsal surface consists of a horny plate, which is set in a nail matrix. The ventral portion merges with the sides of the upper half, and, as the lower portion is the softer and wears faster, the nail has a sharp point and edge. The matrix is formed by a growth of the Malpighian layer of the cutis. A fold of skin lies over its posterior part. The epidermic cells of the dorsal, or nail, part, and the base of the ventral part grow fast. The outer cell layer gradually becomes horn-like.

The spurs are conical with a flat base. The basal part rests upon an enlargement of the shank bone. Soft structure is found between the horny spur and the bone. The upper cells, like those of the nails and of the skin, are constantly being worn or cast off, and new cells push up from the lower layers of cells. These newly formed flattened cells soon become cornified. The oldest formation is found at the tip and the youngest at the base.

EMBRYOLOGY OF THE CHICK

That a new individual may be brought into existence, there must be accomplished the union of the male element, or spermatozoon, with the female element, the ovum. This union is called fertilization. In the fowl this fertilization is accomplished at the anterior portion of the oviduct, after the calyx has ruptured and discharged its yolk, and before the albumen has been formed around it. The blastoderm is found on the surface of the yolk. One spermatozoon is all that is required; in fact, only one can be used in this union.

=Spermatogenesis.=—The spermatozoa are formed by the seminiferous tubules of the testis. From these cells, called the spermatogonia, are formed other cells called spermatocytes, which in turn form the spermatids, the immediate forerunners of the spermatozoa. During the period of multiplication the spermatogonia divide repeatedly by mitosis. Numbers of small cells are thus produced, each containing in its nucleus the number of chromosomes typical of the somatic cell of that fowl. In the second period the cells become larger and spermatocytes of the first order are formed. Then comes the period of maturation, during which two succeeding divisions rapidly occur. The first division results in the formation of two cells exactly alike. These are spermatocytes of the second order. They differ from the somatic cells in that they contain only one-half the typical number of chromosomes. The second division produces two similar spermatids from one spermatocyte of the second order. Therefore four spermatids exactly alike may be formed from one spermatocyte of the first order. From these spermatids the spermatozoa are formed (Fig. 55). The heads of the spermatozoa contain the nuclei derived from the spermatids; the necks contain the centrosomes; and the tail, consisting of three parts is probably formed from the protoplasm. Three parts of the tail are as follows: first, the pars conjunctionis, which unites the tail to the neck; second, the pars principalis, which constitutes the main length of the tail; and, third, the pars terminalis, which consists of an axial filament which transverses the entire tail and is surrounded by a protoplasmic sheath.

=Oögenesis.=—The ovum during its formation passes through three stages.

FIG. 81.—Section of ovum in a hen. 1. Nucleolus. 2. Nucleus. 3. Liquor folliculi. 4. Stratum granulosum. 5. Follicular cells. 6. Theca folliculi. 7. Peripheral stroma. ]

The first stage, that of division, takes place before the chick is hatched, and, according to Bradley, comes to an end about the time of hatching. This stage consists of the rapid formation of ova in the female chick. In the second stage, which begins about the time of hatching, there is an increase in the size of the units of the ovary, accompanied by yolk formation. At this time each ovum is in its own follicle (Fig. 81), and is surrounded by a layer of cuboidal cells and a theca. The theca is formed from the adjacent fibrous stroma. The third stage, that of maturation, commences during the development of the yolk and is complete after it has escaped to the oviduct. Maturation consists of each cell’s dividing into two unequal parts. In each division the cell is split into a small cell known as the polar body, which is cast off and disappears, and a larger cell, which is the ovum proper. In this process half of the original chromosomes are cast off.

=Fertilization.=—The sperm travels rapidly; experiments have shown eggs to be fertile laid twenty-four hours after service by a male. When the ovum is discharged into the ovarian pocket it is surrounded by spermatozoa.

After the male pronucleus has united with the female pronucleus in the single-celled ovum, there is a cleavage of the cell in the long axis of the egg, making two cells; and then a cleavage at right angles, which progressively continues, makes the mulberry-like mass. The remaining content of the egg consists of food for the development of the embryo. From this mass of cells before the egg is laid the blastoderm is formed. The cells of the blastoderm are differentiated into two layers. The superficial layer is the ectoderm and the lower layer is the entoderm. In the newly laid egg the blastoderm may be observed. It is about 4 millimeters in diameter. It has a transparent central area, the zona pellucida, which is located over the subgerminal region. There is a peripheral, less transparent area called the zona opaca.

In the fertile egg, as soon as it is subjected to the proper temperature, cell multiplication in the blastoderm begins. The first signs of such change are noted in the pellucid area of the blastoderm where embryonal traces appear in the form of the parallel lines called the plicæ primitivæ, which diverge to form the cephalic dilatation. At about this time takes place the formation of the myelencephalous columns, in which the blood lakes expand in the surrounding halones and in the tracts along which pass colorless blood particles. These tracts extend from below the cephalic expansion to the peripheral sinuses, as the proto-vertebræ, which begin to appear at the sides of the myelon. The red color is acquired by the blood, and the heart by its movements, is made more manifest as the punctum saliens. A distinct membrane, the serous layer, is formed upon the germ and the blastoderm. The cephalic end of the embryo rises from the surface of the blastoderm, and then, curving down, sinks into it, forming for itself a kind of hood of the serous layer. This hood gradually extends from the margin of the fossa over the body, and, meeting a similar fold formed by the projecting and incurved tail, closes over the germ on the upper side, making a circumscribed cavity, which is the amnion. The progress of differentiation of layers of the blastoderm has, meantime, gone on beneath. The serous layer is in part reflected from the vascular and from the mucous layer. The mucous layer is concerned in the formation of the intestinal canal; and beyond this part, which is at first an open groove, the mucous layer expands over the yolk, which it ultimately incloses, the margins of the vitellicle so formed contracting and uniting at the side opposite the embryo at a sort of cicatrix, to which the last part of the abdominal yolk adheres. The vitellicle is richly vascular, and the surface next to the yolk is augmented by rugæ.

The fowl’s egg, at about the fortieth hour, shows the buds from which the limbs are developed. A vesicle is seen to protrude near the anal end of the intestine, which, rapidly expanding, spreads over the embryo, acquiring a close adhesion to the amnion, but remaining distinct from the vitellicle, over which it spreads. It finally encloses the albumen and interposes itself between the latter and the lining membrane of the shell. Umbilical vessels are associated with this membrane. Hunter called this membrane the allantois from its containing urine, and Owen states that the sac which surrounds the albumen acts as the chorion or placenta; for it is most probable that from this surface the albumen is absorbed and the chick supported on its developmental food. The external part of the sac apparently acts as lungs as it comes into contact with the shell of the egg through pores of which there is an exchange of air. Oxygen is consumed and carbon dioxide is given off. The blood in the vessels of this membrane is in color more like arterial blood and that in the interior more like venous blood.

The embryonic mass of the incubating egg always floats to the top side. As the embryo grows it turns upon its left side, exhibiting a profile view; it then indents the yolk, and finally almost divides it into two parts.

The peripheral layers of cells rise from the margin of the germ mass, and extend and contract toward the opposite pole. This tract of germ substance is the primitive streak. Along the median line it next forms a furrow, which stops short of the ends of the streak. This streak terminates opposite the point from which the germ begins, and swells into the head. The median furrow expands upon it. The cephalic borders are next united by a thin layer of epithelial cells above the furrow, converting it into a cavity, or ventricle. The myelonal furrow is similarly covered by a layer, uniting the lateral columns. The embryonal trace becomes longer, narrower, and bends round the vitellus. A layer of epithelial cells forms a network over the whole dorsal surface of the embryo. Oblique striæ appear in the broadening germ mass radiating from the primitive streak. These indicate divisional segments. These beginnings of aponeurotic septa probably accompany and support nervous productions from the myelon columns.

Two transverse constrictions begin to divide the cephalic enlargements into three lobes, the second and the third of which expand into vesicles. An accumulation of cells at the side of the middle expansion appears to add greatly to its breadth. This forms the basis of the eyes.

The differentiation and the confluence of the cell constituents of the primitive streak have led to the formation of a pair of albuminous cords along the sides of the median furrow, forming the myelon proper. The cells exterior to and above them are converted into muscle and fibrous septa; and beneath the column is a jelly-filled cylinder, with a transversely striated sheath, pointed at both ends, forming the notocord. Its anterior point passes a little in advance of the acoustic vesicle. Beneath the notocord and surrounding the blastema is stretched the vegetative, or mucous, layer of cells, in contact with the yolk. Both the head and the tail of the now cylindrical embryo are liberated from the surface of the yolk. A fold of the blastema, reflected from the under part of the head, sinks like a pouch into the yolk, and soon includes the rudiment of the heart, like a bent cord, which begins to oscillate about the seventh day. From the midline of the inferior surface of the embryo, or its mucous layer, two longitudinal plates descend, diverging into the yolk-substance, and form the primitive intestinal groove.

The ophthalmic vesicle elongates and curves outward until the two ends almost come into contact. Between these two ends and beneath the delicate tegumentary layer connecting them the crystalline lens is formed. About the same time, the otoliths appear in the acoustic vesicles, which have now acquired a cartilaginous case. The cerebral lobes begin to be formed by a small fold, rising laterally and overlapping the forepart of the second enlargement, which has expanded to greater breadth. The olfactory cavities appear as small cutaneous follicles.

The two myelonal columns, expanding between the ear sacs and receding so as to show the notocord beneath, bend upward and inward, and unite, to be continued into the posterior of the optic lobes, thus commencing the cerebellar bridge across the epencephalic ventricle. The encephalic vacuities have begun to be filled by the granular basis of the cerebellar substance.

The intestinal groove begins to be converted into a canal at its two ends. Beneath the anterior end, and behind the heart, there gradually accumulates the cellular basis of the liver.

The commencement of the development of the organ of hearing is by a superficial depression of the cephalic blastema to meet the process from the encephalon, which forms the acoustic nerve. The lining of the depression becomes, on closure of the slit, the proper tunic of the labyrinth.

The vesicle of the labyrinth swells into four dilatations, of which three are ampullar and the fourth cochlear. The ampullar dilatations extend into very slender canals, at first almost in the same plane, by which they are brought into mutual communication. As the canals expand and elongate, they assume their characteristic relative positions as external, superior, and posterior, the posterior end of the external canal being extended beneath the posterior canal. The cochlear dilatation curves as it elongates. An inner layer becomes distinct from the common membrane and forms the acoustic lamina.

As in the development of the ear, so in the development of the eye, the production of the nerve process from the cerebral center is the first step; the infolding of the superficial blastema to meet the nerve is the next. The so-called cutaneous follicle becomes a circumscribed sac or vesicle, in which the changes and the development next proceed, converting the vesicle into an acoustic labyrinth or into an eyeball. In each case neural elements of two vertebræ become modified to lodge and to protect the sense organs, forming respectively the recess called otocrane and that called the orbit. The one is located between the occipital and the parietal vertebræ, and the other between the frontal and the nasal vertebræ. The part of the outer blastemal layer of the head which sinks to meet the process from the mesencephalic dilatation, rapidly changes its follicular into a vesicular state. The vesicle thus formed elongates, bending around the cell mass in which the crystalline lens is formed, and the meeting of the two ends results in forming the choroid fissure at the lower part of the eyeball.

The mesencephalic process, or optic nerve, expands at the posterior of the circular sac, and, in the course of mutation into eyeball, lines its posterior part with a layer which becomes the retina. The transparent layer covering the forepart of that sac and the inclosed lens is formed into the cornea. Other layers of the sac are formed into the choroid, the ciliary processes, the iris, and the pecten.

Of the appendages of the eye the membrana nictitans is the first to develop. Then develop the lower lid and, last, the upper lid.

After the development of the essential organs of sense, the skin is developed. Modifications of the skin form the outer ear and the eyelids. Then are formed the maxillary arch, the hyoidean arch, and the scapular arch.