124 HEREDITY AND ENVIRONMENT 



following the sperm nucleus, up to the equator 

 of the egg on the posterior side and there 

 forms a yellow crescent extending around the 

 posterior side of the egg just below the equator 

 (Fig. 27, 3). On the anterior side of the egg 

 a gray crescent is formed in a somewhat simi- 

 lar manner and at the lower pole between these 

 two crescents is a slate blue substance, while at 

 the upper pole is an area of colorless proto- 

 plasm. The yellow crescent goes into cleavage 

 cells which become muscle and mesoderm, the 

 gray crescent into cells which become nervous 

 system and notochord, the slate blue substance 

 into endoderm cells and the colorless sub- 

 stance into ectoderm cells. (See also Figs. 

 9 and 10). 



Thus within a few minutes after the ferti- 

 lization of the egg, and before or immediately 

 after the first cleavage, the anterior and pos- 

 terior, dorsal and ventral, right and left poles 

 are clearly distinguishable, and the substances 

 which will give rise to ectoderm, endoderm, 

 mesoderm, muscles, notochord and nervous 

 system are plainly visible in their character- 

 istic positions. 



