SOMATOGENESIS 259 



in the cytoplasm of the fertilized egg of many forms 

 qualitative differences may already be detected that 

 prophesy clearly the course which differentiation is 

 to take. Conklin cites the illuminating case of the 

 ascidian Styela, in whose egg the cytoplasm in differ- 

 ent regions varies distinctively in color so that these 

 parts may be unquestionably followed in subsequent 

 cleavage and their fate definitely discovered. 



For example, the peripheral area of the cytoplasm 

 of this egg containing yellow coloring matter finds its 

 way into the cleavage cells which become muscles and 

 mesoderm; a gray area is differentially assorted into 

 cells that become nervous system and notochord; a 

 slate-blue part proves to be the source of epidermal 

 cells and a region of colorless substance gives rise to 

 ectoderm cells. 



Most egg-cells are more reticent than Styela in 

 revealing the part that their cytoplasm is to play in 

 ontogenesis, but it has been possible in many instances 

 to trace cell-lineage through the cleavage stages until 

 the results of differentiation are unmistakable in the 

 tissues. 



The fact that so many eggs clearly show polarity 

 and indicate the future symmetry of the organism be- 

 fore development has begun at all is further evidence of 

 the important part that the cytoplasm plays in soma- 

 togenesis. For example, when the fertilized frog's egg 

 divides for the first time into the two-cell stage these 

 two cells are the ancestors of the right and left sides 

 of the animal and the cleavage plane between them 

 marks the future long axis of the body. 



