118 HEREDITY AND ENVIRONMENT 



This yellow substance then moves, 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 similar man- 

 ner and at the lower pole between these two 

 crescents is a slate blue substance, while at the 

 upper pole is an area of colorless protoplasm. 

 The yellow crescent goes into cleavage cells 

 which become muscle and mesoderm, the gray 

 crescent into cells which become nervous sys- 

 tem and notochord, the slate blue substance 

 into endoderm cells and the colorless substance 

 into ectoderm cells. 



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. 



