376 EYCLESHYMER. [Vol. X. 



The results of my observations are : First, an area at the 

 apical pole forms the basis of the head of the embryo. 



Second, the area at the dorsal lip of the blastopore lies in 

 the posterior half of the embryo. 



Consequently, the anterior half of the embryo, at least, is 

 formed from the material lying between the apical pole and the 

 dorsal lip of the blastopore. 



The formation of the posterior portion of the embryo is so 

 intricate that I hesitate to attempt a description, realizing that 

 more careful study is necessary before anything like a complete 

 explanation can be offered. We know that two processes, that 

 of overgrowth and infolding, are going on during gastrula- 

 tion; that these vary in rate in the different portions of the 

 blastoporic rim, the most active area being at the dorsal lip. 

 This variable rate of infolding, in connection with the pres- 

 sure necessarily resulting from the thickened areas of the me- 

 dullary folds, make the closure of the blastopore an extremely 

 complicated process. 



That this closure may occur in a number of different ways is 

 undoubtedly true, as the observations of Schanz ('87), Morgan 

 ('89), Erlanger ('9i), Robinson and Assheton ("91), Ziegler ('92), 

 Jordan ('93), and others, amply testify. I cannot consider this 

 or that variation as indicative of the method of embryo-forma- 

 tion, but rather as secondary modifications of the more primi- 

 tive process of uniform convergence. 



The above facts indicate that the greater portion of the body 

 of the embryo is formed in the dark hemisphere by differentia- 

 tion in situ, while the tail arises from the coalescence of the 

 blastoporic lips. 



5. The Primitive Streak and Groove. 



Throughout the literature scarcely two statements can be 

 found which agree as to the nature and extent of the primitive 

 streak. In the Amphibia this is especially true, as the follow- 

 ing brief references indicate : 



1. The primitive streak is in front of the blastopore, Alice 

 Johnson ('84), Schultze ('88), Erlanger ('90). 



2. The primitive streak lies behind the blastopore, Minot (92). 



