368 EYCLESHYMER. [Vol. X. 



The stage shown (PL XX, Fig. i) is soon followed by a 

 sinking in of the cells on either side of the line, forming a 

 depression which, in 8 hrs., becomes a crescentic groove 

 (Fig. 2). 



A corresponding stage of Rana is shown in Fig. 9. The 

 blastopore {bp?^ appears in the same region as in Amblystoma; 

 very early the line of invagination {pp.) assumes the form of a 

 more or less acute angle (Fig. 9), but to this angle but little 

 significance can be attached beyond its slight bearing on the 

 theory of concrescence. 



As is well known, different opinions are held concerning the 

 formation of the gastrula in Amphibia. A brief summary will 

 define the views of a number of investigators. 



The archenteron is formed by invagination, as held by Scott 

 and Osborn ('79), Hertwig (-82), Schultze (-88), Perenyi ('89), 

 Jordan ("93), Morgan and Tsuda ("94). 



These authors are, however, not agreed as to the precise 

 method ; the majority believe the process to be one of 

 infolding (embolic invagination). Morgan and Tsuda consider 

 it largely a process of overgrowth (epibolic invagination), while 

 Jordan believes both processes are involved. 



The archenteron is not formed by invagination, but by 

 delamination, according to Moquin-Tandon ('76), Houssay ('90), 

 Robinson and Assheton ('91), and Marshall ('93). 



With the aid of the mechanical devices described, I have 

 been able to follow certain cells, which from individual peculiar- 

 ities could be easily distinguished, in their course from the 

 point which they occupied at the rim of the blastopore, step by 

 step, until they disappeared from view. It is evident, therefore, 

 that there is either an infolding of the surfaces on either side of 

 the line of invagination, or that the surface at the dorsal lip, is 

 overgrowing the other, and at the same time infolding, giving 

 such an appearance. Sections show that at this stage the ecto- 

 blast has overgrown the entoblast to a slight extent only, so that 

 while there is some indication of epiboly, the appearance of 

 sections and the actual infolding of cells observed on the sur- 

 face can only be interpreted by considering the process as one 

 of modified emboly. 



