DEVELOPMENT OF VERTEBRATES. 123 



seen and drawn. This similarity of relation of the epiblast to 

 the yolk in the two cases is a further confirmation of the 

 identity of the Selachian's yolk with the large yolk-spheres of 

 the previous eggs. 



The next stage, E III, is in many ways identical with the 

 corresponding stage in the last-described egg, and in the same 

 way as in that case the neural and alimentary canals are placed 

 in communication with each other. 



The mode in which this occurs will be easily gathered from 

 a comparison of E II and E III. It is the same for the Sela- 

 chians and Batrachians. The neural canal (n c] is by the stage 

 figured E III, completely formed in the way so well known in 

 the Bird, and between the roof of the canal and the external 

 epiblast a layer of mesoblast has already grown in. The floor 

 of the neural canal is the same layer marked ep in E II, and 

 therefore remains continuous with the hypoblast at x\ and when 

 by a simultaneous process the roof of the neural canal and the 

 ventral wall of the alimentary become formed by the folding 

 over of one continuous layer (the epiblast and hypoblast con- 

 tinuous at the point x], the two canals, viz. the neural and ali- 

 mentary, are necessarily placed in communication at their hind- 

 ends, as is seen in the diagram. 



There are several important points of difference between 

 E III and D III. In the first place, owing to the larger size of 

 the yolk mass in E III, the epiblast, accompanied by mesoblast, 

 has not proceeded nearly so far round it as in the previous case. 

 It is also worth notice that at the right as well as at the left end 

 of the germinal disc the epiblast is commencing to grow round 

 the yolk. The yolk has, however, become surrounded to a much 

 smaller extent on the right hand than on the left. Since, in the 

 earlier stage, the epiblast became continuous with the hypoblast 

 at x, it is not from sections obvious how this occurs. I have 

 therefore appended a diagram to explain it (E'). The blasto- 

 derm rests like a disc on the yolk and grows over it on all sides, 

 except at. the point where the epiblast and hypoblast are con- 

 tinuous (x). This point becomes as it were left in a bay. Next 

 the two sides of the bay coalesce, the bay becomes obliterated, 

 and the effect produced is exactly as if the blastoderm had 

 grown round the yolk at the point x (corresponding with the 



