ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 667 



else employed 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 continuous 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 involution, this is not of the epiblast, but of 

 the larger cells of lower pole of the yolk, which in part cor- 

 respond 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 h^^poblast and mesoblast are formed, it becomes 

 necessary 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 holo- 

 blastic ova, in which the digestive canal is formed by an 

 involution with the presence of an " anus of Rusconi." 



This class includes " Amphioxus," the " Lamprey," the 

 " Sturgeon," and " Batrachians." 



The second class are those with meroblastic ova and no 

 anus of Rusconi, and with an alimentary canal formed by the 

 infolding of the sheet of hypoblast, the digestive canal re- 

 maining in communication with the food-yolk for the greater 

 part of embryonic life by an umbilical canal. 



This class includes the " Elasmobranchii,^^ " Osseous 

 fish," " Reptiles,^' and " Aves.^' 



The mode of formation of the alimentary canal in the 

 first class is clearly the more primitive ; and it is equally 

 clear that its mode of formation in the second class is an 

 adaptation due to the presence of the large quantity of food- 

 yolk. 



In the Dog-fish I believe that we can see, to a certain 



