76 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



to suppose that the epiblast, where it becomes continuous with 

 the hypoblast, in reality becomes involuted, and that from 

 this involuted epiblast are formed the whole mesoblast and 

 hypoblast. 



In this case we would be compelled to suppose that the mass 

 of lower layer cells which forms the embryonic swelling is used 

 as food for the growth of the involuted epiblast, or else em- 

 ployed solely in the growth over the yolk of the non-embryonic 

 portion of the blastoderm ; but the latter possibility does not 

 seem compatible with my sections. 



I do not believe that it is possible, from the examination of 

 sections alone, to decide which of these two views (viz. whether 

 the epiblast is involuted, or whether it becomes merely conti- 

 nuous with the lower layer cells) is the true one. The question 

 must be decided from other considerations. 



The following ones have induced me to take the view that 

 there is no involution, but that the mesoblast and hypoblast are 

 formed from the lower layer cells. 



(1) That it would be rather surprising to find the mass of 

 lower layer cells which forms the " embryo swelling " playing no 

 part in the formation of embryo. 



(2) That the view that it is the lower layer cells from which 

 the hypoblast and mesoblast are derived agrees with the mode 

 of formation of these two layers in the Bird, and also in the 

 Frog ; since although, in the latter animal, there is an involu- 

 tion, this is not of the epiblast, but of the larger cells of the 

 lower pole of the yolk, which in part correspond with what 

 I have called the lower layer cells in the Dog-fish. 



If the view be accepted that it is from the lower layer cells 

 that the hypoblast and mesoblast are formed, it becomes ne- 

 cessary to explain what the continuity of the hypoblast with 

 the epiblast means. 



The explanation of this is, I believe, the keystone to the 

 whole position. The vertebrates may be divided as to their 

 early development into two classes, viz. those with holoblastic 

 ova, in which the digestive canal is formed by an involution with 

 the presence of an "amis of Rusconi" 



This class includes "Amphioxus," the "Lamprey." the "Stur- 

 geon," and " Batrachians." 



