288 H. J. VAN CLEAVE 



of development when it has tak n on the body form of the 

 adult, cell division either mitotic or amitotic, ceases to play any- 

 role whatever in the later history of the somatic cells. Among 

 the nmnerous specimens of five species of this genus which I 

 have examined there has not been, in any instance, the least 

 indication of further increase in the number of cells constituting 

 the body. This is evidenced by the entire lack of mitotic figures 

 and also by actual count of the cells present. This gives good 

 ground for doubting the generally accepted view that life must 

 be accompanied by cell division as expressed in the contention 

 that the cell has lost its lease on life as soon as it can no longer 

 divide. 



Nothing is known regarding the changes in Eorhjnichus ac- 

 companying the differentiation of the embryonic cells into the 

 tissues and organ.s of the adult, for no larval stages of any of 

 the American species have been discovered, and none of the inves- 

 tigations upon the European species contain references to the 

 changes in the structure of the cells at that time. In the later 

 stages of development some shght changes have been observed 

 in the structure of the nuclei but there are no strongly marked 

 general cytological modifications such as usually accompany the 

 advance of age. The nuclear change consists chiefly in the re- 

 arrangement of the chromatin. In the immature forms the nu- 

 clei, especially of the subcuticula and of the lemnisci, have their 

 chromatic substance irregularly scattered throughout the nucleus, 

 but with the advance to maturity this chromatic material assumes 

 the form of a more compact solid mass lying in the center of the 

 nucleus. 



The conditions of existence for all of the cells of the body are 

 so nearly uniform at all periods of life for the adult that it seems 

 probable that death must follow as a consequence of the termina- 

 tion of the reproductive period of the individual or as the result 

 of a combination of factors acting on the organism as a whole 

 rather than as a gradual senescence of the individual cells. 



