78 THE PROBLEMS OF EVOLUTION 



The relationship may differ somewhat in the 

 germ cells and in the specialized cells of the body. 

 Years ago, before cytological studies disclosed the 

 true facts, it was supposed that the hereditary 

 units of the germ cells were distributed differen- 

 tially to the developing cells of the body, so that a 

 specialized cell not only had characteristic differ- 

 entiations of cytoplasm but also a corresponding 

 nuclear condition. Boveri ^ showed in his studies 

 of Ascaris that the chromatin is diminished in 

 quantity in the cells of the body and that the re- 

 mainder is broken up into small particles, but even 

 in this case the differentiation is not comparable 

 to that of the cytoplasm. In by far the greater 

 number of known cases the full chromosome com- 

 plex of the species seems to persist in all cells of 

 the body which retain nuclei capable of mitotic 

 division. Even the marked contrast between the 

 germinal cytoplasm and that of the most highly 

 specialized cells is not accompanied by comparable 

 nuclear differentiation. The differentiation of the 

 cytoplasm thus becomes one of the great problems 

 of development, and the relations of nucleus and 

 cytoplasm in reproductive cells are part of the 

 general problem of interaction of cellular constit- 

 uents. 



The nucleus is commonly accepted as a control- 



* Die Enticicklung von Ascaris megalocephala, etc., Festschr. f. C. von 

 Kupffer. 1899. 



