FORMATION OF THE LAYERS. 275 



enclosed by smaller cells (in the diagram) which correspond 

 exactly in function and position with the lower layer cells of the 

 Elasmobranch blastoderm. 



The relation of the yolk to the blastoderm in the Elasmo- 

 branch 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. 



In the next stage for the Elasmobranch, fig. I and 2 B and 

 PI. 7, fig. 7, and for the Amphibian, fig. 3 B, the agreement 

 between the two types is again very close. In both for a small 

 portion (x) of the edge of the blastoderm the epiblast and hypo- 

 blast become continuous, while at all other 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 ovum form a comparatively small mass, and are 

 therefore rapidly enveloped ; while in the case of the Elasmo- 

 branch ovum, owing to the greater mass of -the yolk, the same 

 process occupies a long period. In both ova the portion of 

 the blastoderm, where epiblast and hypoblast become continuous, 

 forms the dorsal lip of an opening the anus of Rusconi which 

 leads into the alimentary cavity. This cavity has the same 

 relation in both ova. It is lined dorsally by lower layer cells, 

 and ventrally by yolk or what corresponds with yolk ; the 

 ventral epithelium of the alimentary canal being in both cases 

 eventually supplied by the yolk cells. 



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

 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. 



