142 Heredity and Environment 



(Fig. 46, 1, pi). 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. 46, 2-4). On the an- 

 terior side of the egg a gray crescent is formed in a somewhat 

 similar 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 protoplasm. The yellow crescent goes into cleavage 

 cells which become muscles and mesoderm, the gray crescent into 

 cells which become nervous system and notochord, the slate blue 

 substance into endoderm cells and the colorless substance into 

 ectoderm cells. (Fig. 47; see also Figs. 10 and 11). 



Localization of Substances. — Thus within a few minutes after 

 the fertilization of the egg, and before or immediately after the 

 first cleavage, the anterior and posterior, dorsal and ventral, right 

 and left poles are clearly distinguishable, and the substances which 

 will give rise to ectoderm, endoderm, mesoderm, muscles, noto- 

 chord and nervous system are plainly visible in their character- 

 istic positions. 



At the first cleavage of the egg each of these substances is di- 

 vided into right and left halves (Fig. 46, 5). The second cleavage 

 cuts off two anterior cells containing the gray crescent from two 

 posterior ones containing the yellow crescent (Fig. 46, 6 and Fig. 

 47, 1). The third cleavage separates the colorless protoplasm in 

 the upper hemisphere from the slate blue in the lower (Fig. 47, 

 2). And at every successive cleavage the cytoplasmic substances 

 are segregated and isolated in particular cells, and in this way 

 the cytoplasm of the different cells comes to be unlike (Figs. 47 

 and 48). When once partition walls have been formed between 

 cells the substances in the different cells are permanently separ- 

 ated so that they can no longer commingle. 



What is true of Stycla in this regard is equally true of many 

 other ascidians, as well as of Amphioxus and of the frog (Figs. 

 9, 10, 11), though the segregation of substances and the differ- 



