Vertebrate Embryology

The zygote is moored in the centre of the albumen by the axial strand of albumen of a denser, tougher consistency—the "chalaza." If the egg-shell is rolled over, the chalaza, while keep ing the zygote at its proper distance from the poles of the shell, allows it to rotate about the long axis of the shell, itself twisting in the proCess. Consequently the apical pole of the zygote, with its germinal disc less heavily weighted by yolk, always keeps upper most next the warm body of the incubating hen even when the shell is turned over.

Segmentation.—The first visible phase in development is the segmentation or cleavage, by which the unicellular zygote resolves itself into the mass of cells constituting the embryo. As in other cases (see EMBRYOLOGY), the character of the segmentation is greatly influenced by the relative amount of the yolk and still more by its distribution within the zygote. Thus in the ordinary mammal, where there is practically no yolk, the zygote simply into two equal blastomeres, each of these again into two equal daughter-cells and so on.

Gastrulation.—In the Vertebrata, as in so many other cases, the process of segmentation, resulting in a blastula or hollow sphere of cells, is succeeded by gastrulation, resulting in the forma tion of a more or less cup-shaped gastrula, composed of two layers of cells—ectoderm and endoderm—surrounding a cavity, the archenteron, with a wide opening to the exterior the primitive mouth or protostoma.

Gastrulation is seen amongst vertebrates in its most primitive form in Amphioxus where the abapical hemisphere of the blastula, marked by its larger cells, becomes first flattened and then in voluted (invaginated) into the interior of the apical hemisphere. The widely open protostoma becomes gradually narrowed through one lip of the gastrula, shown by later development to be the anterior lip, growing actively backwards so as gradually to cover in the cavity or archenteron, except at its hind-end where the persisting part of the protostoma remains as a small pore—the blastopore. The study of subsequent stages shows that the por tion of the embryo formed by this process of backgrowth, i.e., the roof of the archenteron, becomes the dorsal side of the em bryo. It should be noted that there are two distinct processes at work : (I) the process of involution or invagination in which one wall of the blastula becomes inverted into the other, and (2) the process of overgrowth by which the archenteron becomes roofed in.

The modifications in gastrulation accompanying increase

in the amount of yolk are well seen in amphibians or dipnoans, where the relative amount of yolk is intermediate between that of Amphi oxus and that in meroblastic eggs. Here again segmentation re sults in the formation of a blastula but, owing to the far greater amount of yolk stored in the abapical cells, the abapical wall of the blastula is so thick that by no possibility could it be invo luted into the interior, as it was in Amphioxus. The result is that, to arrive at the stage corresponding to the end of gastrulation in Amphioxus, a somewhat different route is followed. Involution begins but makes little headway: overgrowth however takes place actively, the anterior lip of the gastrula growing backwards and roofing in the archenteron just as in Amphioxus. A new process however now makes its appearance, for the layer of small-celled ectoderm spreads gradually over the surface of the egg by a process of delamination or splitting off from the large underlying cells. In this way the whole of the large-celled yolky cells come to be completely covered in and the stage corresponding to that of Amphioxus with the small blastopore is reached.

In the Amniota below mammals the egg is of similar large di mensions. In the reptiles, it is still possible to recognize distinctly the processes of involution and overgrowth, but they are clearly diminishing in importance and the archenteron to which they give rise is of little moment in the later development. In various rep tiles the blastopore has been seen to take on eventually the form of a longitudinal slit, the side lips of which eventually undergo fusion over the greater part of its extent, and the line of fusion remaining marked by a kind of seam or scar along which the outer layer of cells or ectoderm is continuous with the underlying cells. This line along which such continuity exists is termed the prim itive streak. In the birds, all obvious involution and overgrowth have disappeared, but there still appears as a conspicuous struc ture during early stages the primitive streak which reptilian embryology shows to be a last vestige of a blastopore.