390 EYCLESHYMER. [Vol. X. 



constant increase in yolk, as compared with the mass of em- 

 bryonic material. 



In Fig. I I have attempted to represent the method of embryo 

 formation in Amphibia, based on the study of Amblystoma. 

 At a time when the blastopore extends around the entire blasto- 

 dermic margin (i b.) the first outlines of the embryo are barely 

 visible ; this portion of the embryo, extending to a point 

 somewhere in the vicinity of b, is the part which is to be re- 

 garded as having formed through differentiation i?i situ. 



Successive stages in the closure of the blastopore are repre- 

 sented at I, 2, and 3. The closure in the earlier stages is 

 effected through a convergence of the entire margin, which 

 gradually changes to a lateral convergence, the margins finally 

 coalescing to form the primitive streak {p.s). From the posi- 

 tion of the primitive streak one naturally infers that the ventral 

 portion of the blastoderm must pass over the yolk most rapidly; 

 the explanation probably lies in the fact that the effect of over- 

 growth is here least obscured by infolding. In order to com- 

 prehend the backward extension of the embryo from b to c, it 

 is necessary to recall that at the dorsal lip of the blastopore 

 there is an area in which the cell activity is greatest ; otherwise 

 the decided infolding at this point would appear incompatible 

 with such an extension. 



If we next consider a condition Fig. 2 (Coregonus and other 

 Teleosts), we find the embryo has reached the same stage of 

 differentiation as in Amphibia at a time when the blastoderm 

 has extended over about one-third of the yolk. In other words, 

 the embryo differentiates at a relatively earlier period than in 

 Amphibia. 



With the change in quantity and quality of yolk a corre- 

 sponding modification has arisen in the process of gastrulation ; 

 there is no longer as extensive an infolding as in the preceding 

 forms, while overgrowth must be considered as playing a more 

 important part. We must also bear in mind the fact that the 

 most active or formative area at b is farther removed from that 

 portion of the germ-ring destined to form the tail of the em- 

 bryo ; so that during the time the germ-ring is passing over 

 the yolk (i, 2, 3, 4) the embryo becomes well differentiated. 



