BILATEEALITY IN CRYPTOBRANCHUS 387 



become slightly thinner on one side, which we shall see becomes 

 the dorsal side of the embryo, but on this side the floor of the 

 blastocoele dips abruptly downward to form a small crevice, 

 while on the opposite side it curves gradually upward. On the 

 thinner side of the roof of the blastocoele, the cells are more 

 columnar in form: on this side of the egg, below the level of the 

 blastocoele, the micromeres usually, but not always, extend a 



Fig. 31 Meridional section taken in the median plane of bilate^-al symmetry of 

 an egg of Cryptobranchus allegheniensis in a very late blastula stage (late phase 

 of stage 10). S, segmentation cavity or blastocoele; Y, yolk. The drawing was 

 made with the aid of a camera lucida. X 15. 



little further toward the vegetal pole. When the plane of the 

 section is transverse, the structure revealed is symmetrical. 



So far as one can judge from the study of sections, this differ- 

 entiation of the very late blastula is genetically continuous with 

 the excentric development of the micromeres in earlier stages, 

 certainly as early as stages 8 and 9 (figs. 22 to 25), In these 

 earlier stages the roof or lateral wall of the blastocoele is decidedly 

 thicker on the side where the more rapid multiplication of cells 



JOURNAL OF MORPHOLOGT, VOL. 36, NO. 3 



