COMPARISON OF THE GERMINAL LAYERS. 283 



parts the epiblast, accompanied by lower layer cells, grows round 

 the yolk or round the large cells which correspond to it. The 

 yolk-cells of the Amphibian embryo form a comparatively small 

 mass, and are therefore rapidly enveloped ; while in the case of 

 the Elasmobranch embryo, owing to the greater mass of the 

 yolk, the same process occupies a long period. The portion of 

 the blastoderm, where epiblast and hypoblast become continuous, 

 forms the dorsal lip of an opening the blastopore which leads 

 into the alimentary cavity. This cavity has the same relation in 

 all the three cases. It is lined dorsally by lower layer cells, and 

 ventrally by yolk-cells or what corresponds with yolk-cells ; a 

 large part of the ventral epithelium of the alimentary canal 

 being in both cases eventually derived from the yolk. In 

 Amphibia this epithelium is formed directly from the existing 

 cells, while in Elasmobranchii it is derived from cells formed 

 around the nuclei of the yolk. 



As in the earlier stage, so in the present one, the anatomical 

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

 branchii) are nearly identical with those of the yolk-cells to the 

 blastoderm in the other (Amphibia). 



The main features in which the two embryos differ, during 

 the stage under consideration, arise from the same cause as the 

 solitary point of difference during the preceding stage. 



In Amphibia 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 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 homo- 

 logy of this space with the primitive invagination cavity is never- 

 theless 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 : 

 (i) The continuity of epiblast and hypoblast at the dorsal lip 

 of the blastopore. (2) The continuous conversion of primitive 



