Fig. 70. Section through Frog’s ovum at the close of segmentation. (After Götte.)
sg. segmentation cavity; ll. large yolk-containing cells; ep. small cells at formative pole (epiblast); x. point of inflection of epiblast; y. small cells close to junction of the epiblast and yolk.
The mesenteron is formed as in Petromyzon and Lepidosteus by an unsymmetrical form of invagination. The invagination first commences by an inflection of the epiblast-cells for a small arc on the equatorial line which marks the junction between the epiblastic cells and the yolk-cells ([fig. 70], x).
The inflected cells become continuous with the adjoining cells; and the region where the inflection is formed constitutes a kind of lip, below which a slit-like cavity is soon established. This lip is equivalent to the embryonic rim of the Elasmobranch blastoderm, and the cavity beneath it is the rudiment of the mesenteron.
The mesenteron now rapidly extends by the invagination of the cells on its dorsal side. These cells grow inwards towards the segmentation cavity as a layer of cells several rows deep. At its inner end, this layer is continuous with the yolk-cells; and is divided into two strata ([fig. 71] A), viz. (1) a stratum of several rows of cells adjoining the epiblast, which becomes the mesoblast (m), and (2) a stratum of a single row of more columnar cells lining the cavity of the mesenteron, which forms the hypoblast (hy). The growth inwards of the dorsal wall of the mesenteron is no doubt in part a true invagination, but it seems probable that it is also due in a large measure to an actual differentiation of yolk-cells along the line of growth. The mesenteron is at first a simple slit between the yolk and the hypoblast ([fig. 71] A), but as the involution of the hypoblast and mesoblast extends further inwards, this slit enlarges, especially at its inner end, into a considerable cavity; the blind end of which is separated by a narrow layer of yolk-cells from the segmentation cavity ([fig. 71] B).
In the course of the involution, the segmentation cavity becomes gradually pushed to one side and finally obliterated. Before obliteration, it appears in some forms (Pelobates fuscus) to become completely enclosed in the yolk-cells.
Fig. 71. Diagrammatic longitudinal sections through the embryo of a Frog at two stages, to shew the formation of the germinal layers. (Modified from Götte.)
ep. epiblast; m. dorsal mesoblast; m´. ventral mesoblast; hy. hypoblast; yk. yolk; x. point of junction of the epiblast and hypoblast at the dorsal side of the blastopore; al. mesenteron; sg. segmentation cavity.
While the invagination to form the mesenteron takes place as above described, the enclosure of the yolk has been rapidly proceeding. It is effected by the epiblast growing over the yolk at all points of its circumference. The nature of the growth is however very different at the embryonic rim and elsewhere. At the embryonic rim it takes place by the simple growth of the rim, so that the point x in [figs. 70] and [71] is carried further and further over the surface of the yolk. Elsewhere the epiblast at first extends over the yolk as in a typical epibolic gastrula, without being inflected to form a definite lip. While a considerable patch of yolk is still left uncovered, the whole of the edge of the epiblast becomes however inflected, as at the embryonic rim ([fig. 71] A); and a circular blastopore is established, round the whole edge of which the epiblast and intermediate cells are continuous.
From the ventral lip of the blastopore the mesoblast ([fig. 71], m´), derived from the small intermediate cells, grows inwards till it comes to the segmentation cavity; the growth being not so much due to an actual invagination of cells at the lip of the blastopore, as to a differentiation of yolk-cells in situ. Shortly after the stage represented in [fig. 71] B, the plug of yolk, which fills up the opening of the blastopore, disappears, and the mesenteron communicates freely with the exterior by a small circular blastopore ([fig. 73]). The position of the blastopore is the same as in other types, viz. at the hinder end of the embryo.