82 THE PROBLEMS OF EVOLUTION 



Autokatalysators sein muss, also eines Katalysa- 

 tors, . . . " ^ is justified, remains for the future to 

 prove, but we may be certain that the nucleus does 

 discharge substances into the cytoplasm, that it 

 does receive substances from the cytoplasm, and 

 that it does play the controlling part in the synthe- 

 sis of all living structures. Conklin ^ emphasizes 

 the discharge of materials into the cytoplasm dur- 

 ing cell division. This period certainly affords an 

 opportunity for the complete dissemination of 

 accumulated nuclear contents, but the normal 

 activity of established cells shows that an inter- 

 change must also take place without disruption of 

 the nuclear membrane, even though it may be 

 limited to a dialyzable portion of the available 

 substances. The ability of the nucleus to prepare 

 itself for repeated divisions is, in itself, proof that 

 it receives material from the cytoplasm, for the 

 process cannot be repeated indefinitely without 

 intervening periods of growth, but in addition 

 actual growth of nuclei without cell division is a 

 matter of record. 



As in the case of heritage and environment in 

 the existence of the organism, here is an association 

 of essentials. Cytoplasm is as necessary for the 

 expression of the genes as the nucleus is for the dif- 

 ferentiation of the cytoplasm ; neither can act with- 

 out the other, and we must bear in mind, as in the 



8 Op. cU., p. 91. Mm. Nat., Vol. LXIII, p. 11, 1929. 



