Diagrammatic longitudinal sections of Bombinator igneus. Reproduced with modifications from Götte.
Epiblast without shading. Mesoblast black with clear outlines to the cells. Lower layer cells and hypoblast with simple shading.
ep. epiblast. l.l. lower layer cells. y. smaller lower layer cells at the sides of the segmentation cavity. m. mesoblast. hy. hypoblast. al. alimentary cavity. sg. segmentation cavity. nc. neural cavity. yk. yolk-cells.
A is the youngest stage in which the alimentary involution has not yet appeared. x is the point from which the involution will start to form the dorsal wall of the alimentary tract. The line on each side of the segmentation cavity, which separates the smaller lower layer cells from the epiblast cells, is not present in Götte's original figure. The two shadings employed in the diagram render it necessary to have some line, but at this stage it is in reality not possible to assert which cells belong to the epiblast and which to the lower layer.
B. In this stage the alimentary cavity has become formed, but the segmentation cavity is not yet obliterated.
x. point where epiblast and hypoblast become continuous.
C. The neural canal is already formed, and communicates posteriorly with the alimentary.
x. point where epiblast and hypoblast become continuous.
The relation of the yolk to the blastoderm in the Elasmobranch embryo at this stage of development very well suits the view of its homology with the large cells of the Amphibian ovum. The only essential difference between the two ova arises from the roof of the segmentation cavity being in the Elasmobranch embryo formed of lower layer cells, which are absent in the Amphibian embryo. This difference no doubt depends upon the greater quantity of yolk particles present in the Elasmobranch ovum. These increase the bulk of the lower layer cells, which are thus compelled to creep up the sides of the segmentation cavity till they close it in above.