326 



SENESCENCE AND REJUVENESCENCE 



The diminished cell S T of the two- cell stage produces a definite part 

 of the ectoderm, and the cells 5 2 , S 3 , and S 4 of following generations 

 each have a definite fate. In other words, various portions of the 

 soma or body are segregated before the so-called germ plasm. 



Mesoderm I 



and 

 Stomodeum 



Primitive germ cells 



Mesoderm II 



FIG. 142. Diagram of the cell lineage in the early cleavage of Ascaris megalo- 

 cephala: the black circles represent cells before chromatin diminution and the primitive 

 germ cells which do not undergo diminution; the unshaded circles with four black dots 

 about them represent the cells which undergo diminution, and the unshaded circles 

 alone, the cells after diminution. The further history of the various groups of cells 

 is indicated by the words, "ectoderm," etc. After Boveri, '10. 



The undiminished cells show in all cases a slower rate of division 

 than those in which diminution has occurred, and there is no evi- 

 dence to show that the differences in the behavior of the chromatin 

 are anything more than visible indications or expressions of differ- 

 ences in rate of metabolic activity. It is quite possible that the 

 undiminished cells become germ cells because they have a low 





