SUMNER : KUPFFER'S VESICLE 67 



from the blastoderm margin, has been shown by him to arise in connection with the 

 latter, in fact, to arise from a portion of the latter. 1 In the egg of the reptile, the 

 connection of the plate of cells in which invagination occurs with the blastoderm has 

 not yet been established, but we cannot doubt that even here the invaginate part has, 

 in phylogeny at least, been detached from a primitive position on the margin. An 

 interesting parallel occurs between the fate of the prostomal cavity (KupfFer's Vesicle) 

 in the teleost and that of the invaginate cavity in the reptile. It has been shown 

 that the latter breaks through at its inner extremity, thus becoming connected with 

 a large sub-germinal cavity. Two regions of the archenteron, at first separate, are 

 thus brought into union. Now SOBOTTA ('98) at least I so interpret Sobotta's state- 

 ment and GREGORY ('99) have described a union, at a certain period, between the 

 cavity of KupfFer's Vesicle and the lumen of the gut in front of it. This parallel 

 was called to my attention by Professor MINOT. 



A, B and C. Showing three stages in the formation of the embryo of the toad-fish. (After Miss Clapp. ) 



This dichotomizing of the blastopore is, in the elasmobranchs and the sauropsida, 

 very evidently due to the great relative amount of the yolk, which renders the epi- 

 bolic growth of the blastoderm a slow process, thus making it necessary for the form 

 of the embryo to be established long before the close of gastrulation. In the case 

 of the teleost, the yolk as a rule is far smaller, so that the yolk blastopore is able to 

 close at a period when the embryo is much less advanced. In this group, accord- 

 ingly, we find the caudad growth of the embryo to take place at a rate about equal 

 to that of the blastoderm margin. Consequently, although the folding-off of the tail 

 end occurs here, as in the shark, resulting in the detachment of a portion of the blasto- 

 pore, the embryo retains throughout its connection with the border of the blastoderm. 

 That this is the true explanation of the difference in the position in the two groups 

 is shown by certain exceptional teleosts where this marginal position of the embryo 



1 It is true that the experiments of ASSHETON ('96) have not supported Duval's theory. [Since writing the fore- 

 going, I have fully confirmed such of Assheton's results as go to prove that the primitive streak of the bird does not 

 arise from the blastoderm border in ontogeny. This in no way disproves, however, that it so arose in phytogeny, and 

 I believe that there still remain strong reasons for such a view. September 21, 1900.] 



