GENETICS 89 



other. The comparison is valuable but it must 

 obviously be used with care lest it lead to implica- 

 tions which cannot be supported. 



Since it is impossible, in view of the uninter- 

 rupted continuity of the cytoplasm, to ascribe to 

 the chromosomes unlimited ability to synthesize 

 their own materials of construction, the suggested 

 origin of these fundamental parts of the cell as 

 differentiations of an originally more homogeneous 

 unit is not difficult to accept, and their roles as 

 complementary parts are significant. We have 

 already noted the tendency in evolution toward 

 greater independence of environment; here is an 

 initial step whereby the vital controlling factors 

 are removed from direct association with external 

 forces. Granting the presence of a favorable en- 

 vironment, the resulting cytoplasm is able to take 

 in everything necessary to the life of the entire 

 cell and, under the control exerted by the nucleus, 

 to play its part in the normal processes of life. 

 Modifying conditions impinge upon the cytoplasm. 

 Unless they are extreme, its intake of materials 

 may still be sufficient for the cell to maintain a 

 normal composition through selective synthesis, 

 so that the normal functions of the nucleus are not 

 prevented. Temperature changes may, of course, 

 reach all parts of the cell, but within normal lati- 

 tude they merely retard or hasten its processes. 

 Various rays may penetrate the entire substance of 



