704 



The orientation of the Teleost and Amphibian 

 on the egg. 



In any attempt to find a common explanation of the development 

 of the fish and frog, there is one fundamental fact to be kept in mind. 

 The embryos of the fish and frog lie over different portions of the 

 egg. The most anterior point of the head of the fish lies at the cen- 

 tral point of the early blastoderm i. e. at the point where the polar 

 qodies were extruded. From this point the embryo extends backwards. 

 In small pelagic fish-eggs the embryo closes in one hundred and eighty 

 degrees from the anterior end. In larger eggs although the embryo 

 is longer and larger, yet the embryo covers less than one hundred 

 and eighty degrees. 



On the other hand, in the frog the head is formed in the lower 

 hemisphere more than ninety degrees from the point where the polar 

 bodies were extruded, and the body extends through about one hun- 

 dred and twenty degrees over the lower hemisphere of the egg. 



Fig. 1. Pig. 2. 



Frog. Pish. 



The central arrow in the diagram of the fish indicates this median backward 

 growth of material. Nothing corresponding to this was found in the frog. 



In other words, the head of the frog lies on that portion of the 

 egg corresponding to the posterior portion of the body of the fish. 

 The posterior portion of the body of the frog lies over a portion of 

 the egg into which the body of the fish never extends, if in fact we 

 can speak of similar regions of eggs of different types. 



In an earlier preliminary paper (6) I have described experiments 

 that lead me to believe that the body of the fish was not formed by 

 a process of concrescence of the germ-ring. In a paper now ready 

 for the press I have described at length the results of these experi- 

 ments, and have modified to some extent my previous statement. It 



