380 



BERTRAM G. SMITH 



The impression gained through the study of sections is that 

 this differentiation is of the same general character throughout 

 the entire blastula stage, having its beginning in stages 5 and 6, 

 progressing rapidly in stages 7, 8, and 9, and continuing in some- 

 what different form through stage 10 (very late blastula). The 

 condition is one of bilateral symmetry, and its development ap- 



Fig. 19 Diagrams showing the direction of the axis of excentricity in the 

 superficial cleavage pattern of the upper hemisphere of the early blastula in 

 fifteen eggs of Cryptobranchus allegheniensis fertilized by seminal fluid applied 

 to the edge of the germinal disc on the side indicated by the arrows on the right- 

 hand margin of the figure. The arrow drawn through each circle indicates the 

 axis of excentricity; the head of the arrow is placed on the side occupied by the 

 larger and less numerous micromeres. 



pears to be fundamentally a single continuous process. That 

 the direction of differentiation of the micromeres, with reference 

 to the positions of the more stable macromeres, is unchanged 

 during this long period of development is of course not to be 

 assumed without proof. In the early stages, before the excentric 

 differentiation is well established, it seems likely to be subject to 

 changes in direction; in the later stages it assumes an aspect of 



