146 Organisms from Eggs 



streams to the point of entrance of the sperm, and these 

 movements may lead to the segregation of different kinds 

 of plasma in different parts of the egg and to the unequal 

 distribution of these substances in different regions of the 



egg- 



One of the most striking cases of this is found in the 

 Ascidian Styela in which there are four or five different 

 kinds of substances in the egg which differ in colour, 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 when it gathers at the 

 lower pole of the egg where the sperm enters it forms a 

 yellow cap. This yellow substance then moves following 

 the sperm nucleus, up to the equator of the egg on the poste- 

 rior 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 grey 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 colourless protoplasm. The yellow 

 crescent goes into cleavage cells which become muscle and 

 mesoderm, the grey crescent into cells which become ner- 

 vous system and notochord, the slate-blue substance into 

 endoderm cells, and the colourless 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. 1 



1 Conklin, E. G., loc. cit., p. 117. 



