Fig. 54—Discoid gastrula (discogastrula) of a bony fish. e ectoderm, i entoderm, w border-swelling or primitive mouth, n albuminous globule of the nutritive yelk, f fat-globule of same, c external membrane (ovolemma), d partition between entoderm and ectoderm (earlier the segmentation-cavity.)
Essentially different from the wide-mouthed discoid gastrula of most of the selachii is the narrow-mouthed discoid gastrula (or epigastrula) of the amniotes, the reptiles, birds, and monotremes; between the two—as an intermediate stage—we have the amphigastrula of the amphibia. The latter has developed from the amphigastrula of the ganoids and dipneusts, whereas the discoid amniote gastrula has been evolved from the amphibian gastrula by the addition of food-yelk. This change of gastrulation is still found in the remarkable ophidia (Gymnophiona, Cœcilia, or Peromela), serpent-like amphibia that live in moist soil in the tropics, and in many respects represent the transition from the gill-breathing amphibia to the lung-breathing reptiles. Their embryonic development has been explained by the fine studies of the brothers Sarasin of Ichthyophis glutinosa at Ceylon (1887), and those of August Brauer of the Hypogeophis rostrata in the Seychelles (1897). It is only by the historical and comparative study of these that we can understand the difficult and obscure gastrulation of the amniotes.
Fig. 55—Longitudinal section through the blastula of a shark (Pristiuris). (From Ruckert.) (Looked at from the left; to the right is the hinder end, H, to the left the fore end, V.) B segmentation-cavity, kz cells of the germinal membrane, dk yelk-nuclei.
The bird’s egg is particularly important for our purpose, because most of the chief studies of the development of the vertebrates are based on observations of the hen’s egg during hatching. The mammal ovum is much more difficult to obtain and study, and for this practical and obvious reason very rarely thoroughly investigated. But we can get hens’ eggs in any quantity at any time, and, by means of artificial incubation, follow the development of the embryo step by step. The bird’s egg differs considerably from the tiny mammal ovum in size, a large quantity of food-yelk accumulating within the original yelk or the protoplasm of the ovum. This is the yellow ball which we commonly call the yolk of the egg. In order to understand the bird’s egg aright—for it is very often quite wrongly explained—we must examine it in its original condition, and follow it from the very beginning of its development in the bird’s ovary. We then see that the original ovum is a quite small, naked, and simple cell with a nucleus, not differing in either size or shape from the original ovum of the mammals and other animals (cf. Fig. 13 E). As in the case of all the craniota (animals with a skull), the original or primitive ovum (protovum) is covered with a continuous layer of small cells. This membrane is the follicle, from which the ovum afterwards issues. Immediately underneath it the structureless yelk-membrane is secreted from the yelk.
The small primitive ovum of the bird begins very early to take up into itself a quantity of food-stuff through the yelk-membrane, and work it up into the “yellow yelk.” In this way the ovum enters on its second stage (the metovum), which is many times larger than the first, but still only a single enlarged cell. Through the accumulation of the store of yellow yelk within the ball of protoplasm the nucleus it contains (the germinal vesicle) is forced to the surface of the ball. Here it is surrounded by a small quantity of protoplasm, and with this forms the lens-shaped formative yelk (Fig. 15 b). This is seen on the yellow yelk-ball, at a certain point of the surface, as a small round white spot—the “tread” (cicatricula). From this point a thread-like column of white nutritive yelk (d), which contains no yellow yelk-granules, and is softer than the yellow food-yelk, proceeds to the middle of the yellow yelk-ball, and forms there a small central globule of white yelk (Fig. 15 d). The whole of this white yelk is not sharply separated from the yellow yelk, which shows a slight trace of concentric layers in the hard-boiled egg (Fig. 15 c). We also find in the hen’s egg, when we break the shell and take out the yelk, a round small white disk at its surface which corresponds to the tread. But this small white “germinal disk” is now further developed, and is really the gastrula of the chick. The body of the chick is formed from it alone. The whole white and yellow yelk-mass is without any significance for the formation of the embryo, it being merely used as food by the developing chick. The clear, glarous mass of albumin that surrounds the yellow yelk of the bird’s egg, and also the hard chalky shell, are only formed within the oviduct round the impregnated ovum.
Fig. 56—Longitudinal section of the blastula of a shark (Pristiurus) at the beginning of gastrulation. (From Ruckert.) (Seen from the left.) V fore end, H hind end, B segmentation-cavity, ud first trace of the primitive gut, dk yelk-nuclei, fd fine-grained yelk, gd coarse-grained yelk.