DEVELOPMENT OF ELASMOBRANCH FISHES. 63 



relations of the yolk to the blastoderm in the one case (Elasmo- 

 branch) are nearly identical with those of the yolk cells to the 

 blastoderm in the other (Amphibian). The main features in 

 which the two embryos differ, during the stage under considera- 

 tion, arise from the same cause as the solitary point of differ- 

 ence during the preceding stage. 



In Amphibians, the alimentary cavity is formed coincidently 

 with a true ingrowth of cells from the point where epiblast and 

 hypoblast become continuous, and from this ingrowth the dorsal 

 wall of the alimentary cavity is formed. The same ingrowth 

 causes the obliteration of the segmentation cavity. 



In the Elasmobranchs, owing to the larger bulk of the lower 

 layer cells caused by the food-yolk, these have been compelled 

 to arrange themselves in their final position during segmenta- 

 tion, and no room is left for a true invagination ; but instead 

 of this there is formed a simple split between the blastoderm 

 and the yolk. The homology of this mth the primitive 

 invagination 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 anus of Rus- 

 coni. (2) The continuous conversion of indifferent lower layer 

 cells into hypoblast, which gradually extends backwards towards 

 the segmentation cavity, and exactly represents the course of 

 the invagination whereby in Amphibians the dorsal wall of 

 the alimentary cavity is formed. (3) The obliteration of the 

 segmentation cavity during the period when the pseudo-invagi- 

 nation is occurring. 



The asymmetry of the gastrula or pseudo-gastrula in Cyclo- 

 stomes. Amphibians, Elasmobranchs and, I beUeve, Osseous 

 Fishes, is to be explained by the form of the vertebrate body. 

 In Amphioxus, where the small amount of food-yolk present is 

 distributed uniformly, there is no reason why the invagination 

 and resulting gastrula should not be symmetrical. In other 

 vertebrates, where more food-yolk is present, the shape and 

 structure of the body render it necessary for the food-yolk to 

 be stored away on the ventral side of the alimentary canal. 

 This, combined with the unsymmetrical position of the anus. 



