By this stage the three layers of the embryo are definitely established. The epiblast, consisting from the first of two strata, arises from the small cells forming the roof of the segmentation-cavity. It becomes continuous at the lip of the blastopore with cells intermediate in size between the cells of which it is formed and the yolk-cells. These latter, increasing in number by additions from the yolk-cells, give rise to the mesoblast and to part of the hypoblast; while to the latter layer the yolk-cells, as mentioned above, must also be considered as appertaining. Their history will be dealt with in treating of the general fate of the hypoblast.
Urodela. The early stages of the development of the Newt have been adequately investigated by Scott and Osborn (No. [114]). The segmentation and formation of the layers is in the main the same as in the Frog. The ovum is without black pigment. There is a typical unsymmetrical invagination, but the dorsal lip of the blastopore is somewhat thickened. The most striking feature in which the Newt differs from the Frog is the fact that the epiblast is at first constituted of a single layer of cells ([fig. 75], ep). The roof of the segmentation cavity is constituted, during the later stages of segmentation, of several rows of cells (Bambeke, No. [95]), but subsequently it would appear to be formed of a single row of cells only (Scott and Osborn, No. [114]).
General history of the layers.
Epiblast: Anura. At the completion of the invagination the epiblast forms a continuous layer enclosing the whole ovum, and constituted throughout of two strata. The formation of the medullary canal commences by the nervous layer along the axial dorsal line becoming thickened, and giving rise to a somewhat pyriform medullary plate, the sides of which form the projecting medullary folds ([fig. 77] A). The medullary plate is thickened at the two sides, and is grooved in the median line by a delicate furrow ([fig. 72], r). The dilated extremity of the medullary plate, situated at the end of the embryo opposite the blastopore, is the cerebral part of the plate, and the remainder the spinal. The medullary folds bend upwards, and finally meet above, enclosing a central cerebrospinal canal ([fig. 74]). The point at which they first meet is nearly at the junction of the brain and spinal cord, and from this point their junction extends backwards and forwards; but the whole process is so rapid that the closure of the medullary canal for its whole length is effected nearly simultaneously. In front the medullary canal ends blindly, but behind it opens freely into the still persisting blastopore, with the lips of which the medullary folds become, as in other types, continuous. [Fig. 73] represents a longitudinal section through an embryo, shortly after the closure of the medullary canal (nc); the opening of which into the blastopore (x) is clearly seen.
Fig. 72. Transverse section through the posterior cephalic region of an early embryo of Bombinator. (After Götte.)
l. medullary groove; r. axial furrow in the medullary groove; h. nervous layer of epidermis; as. outer portion of vertebral plate; is. inner portion of vertebral plate; s. lateral plate of mesoblast; g. notochord; e. hypoblast.
On the closure of the medullary canal, its walls become separated from the external epiblast, which extends above it as a continuous layer. In the formation of the central nervous system both strata of the epiblast have a share, though the main mass is derived from the nervous layer. After the central nervous tube has become separated from the external skin, the two layers forming it fuse together; but there can be but little doubt that at a later period the epidermic layer separates itself again as the central epithelium of the nervous system.
Both the nervous and epidermic strata have a share in forming the general epiblast; and though eventually they partially fuse together yet the horny layer of the adult epidermis, where such can be distinguished, is probably derived from the epidermic layer of the embryo, and the mucous layer of the epidermis from the embryonic nervous layer.