248 ROLE OF CYTOPLASM [CH. 



example, CONKLIN writes as follows: "There are four or 

 five substances in the egg which differ in color, so that 

 their distribution to different regions of the egg and to 

 different cleavage cells may be easily followed, and even 

 photographed, while in the living condition. The peripheral 

 layer of protoplasm is yellow and it gathers at the lower 

 pole of the egg, where the sperm enters, forming a yellow 

 cap. 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. On the 

 anterior side of the egg a gray crescent is formed in a some- 

 what 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 muscle 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. 



"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, 

 notochord and nervous system are plainly visible in their 

 characteristic positions. 



"At the first cleavage of the egg each of these substances 

 is divided into right and left halves. The second cleavage 

 cuts off two anterior cells containing the gray crescent from 

 two posterior ones containing the yellow crescent. The third 

 cleavage separates the colorless protoplasm in the upper 

 hemisphere from the slate-blue in the lower. And at every 

 successive cleavage the cytoplasmic substances are segre- 



