In Elasmobranchs, owing probably to the larger bulk of the lower layer cells, the primitive hypoblast cells arrange themselves in their final position during segmentation, and no room is left for a true invagination; but instead of this there is formed a simple space between the blastoderm and the yolk. The homology of this space with the primitive invagination cavity is nevertheless proved by the survival of a number of features belonging to the ancestral condition in which a true invagination was present. Amongst the more important of these are the following:--(1) The continuity of epiblast and hypoblast at the dorsal lip of the blastopore. (2) The continuous conversion of primitive hypoblast cells into permanent hypoblast, which gradually extends inwards towards the segmentation cavity, and exactly represents the course of the invagination whereby in Amphibia the dorsal wall of the alimentary cavity is formed. (3) The obliteration of the segmentation cavity during the period when the pseudo-invagination is occurring. In the next stage there appear more important differences between the two types than in the preceding stages, though here again the points of resemblance predominate. Figs. 170 D and 174 C represent longitudinal sections through embryos after the closure of the medullary canal. The neurenteric canal is established; and in front and behind the epithelium of the ventral wall of the mesenteron has begun to be formed. The mesoblast is represented as having grown in between the medullary canal and the superjacent epiblast. There are at this stage two points in which the embryo Elasmobranch differs from the corresponding Amphibian embryo. (1) In the formation of the neurenteric canal, there is no free passage leading into the mesenteron from the exterior as in Amphibia (fig. 170 D). (2) The whole yolk is not enclosed by the epiblast, and therefore part of the blastopore is still open. The difference between Amphibia and Elasmobranchii in the first of these points is due to the fact that in Elasmobranchii, as in Amphioxus, the neural canal becomes first closed behind; and simultaneously with its closure the lateral parts of the lips of the blastopore, which are continuous with the medullary folds, meet together and shut in the hindmost part of the alimentary tract.