236 THE GASTKULA OF ELASMOBRANCIIII. 



In Elasmobrancbs, 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 nvimber of features belonging to the ancestral condition 

 in which a true invagination was present. Amongst the more im- 

 portant 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 be- 

 tween 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. 



i There are at this stage two points in which the embryo Elasmo- 

 branch 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 ej)iblast, 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 simul- 

 taneously with its closure the lateral parts of the lips of the blasto- 

 pore, which are continuous with the medullary folds, meet together 

 and shut in the hindmost part of the alimentary tract. 



The second point is of some importance for understanding the 

 relations of the formation of the layers in the amniotic and the non- 

 amniotic Vertebrates. Owing to its large size the whole of the yolk 

 in Elasmobranchii is not enclosed by the epiblast at the time when 

 the neurenteric canal is established ; in other words a small pos- 

 terior and dorsal portion of the blastopore is shut off in the forma- 

 tion of the neurenteric canal. The remaining ventral portion be- 

 comes closed at a later period. Its closure takes place in a linear 

 fashion, commencing at the hind end of the embryo, and proceeding 

 apparently backwards ; though, as this part eventually becomes folded 

 in to form the ventral wall of the embryo, the closure of it really 



