62 FORMATION OF THE LAYERS. 



all, formed of so-called yolk {Vide PL ill. fig. 4), which in 

 all forms the main mass of the egg. In the Amphibian the 

 yolk is segmented, and, though it is not segmented in the Elas- 

 mobranch, it contains in compensation the nuclei so often men- 

 tioned. In all the sides of the segmentation cavity are 

 formed by lower layer cells. In the Amphibian the sides 

 are enclosed by smaller cells (in the diagram) which corre- 

 spond 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. 1 and 2 B and 

 PI. IV. fig. 7, and for the Amphibian, fig. 8 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 

 Elasmobranch 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 



