Physical Basis of Heredity and Evolution 



25 



As in the case of all organs, the cytoplasm and the nucleus 

 are associated with special functions. This does not mean 

 that each does a certain thing and nothing else, but that each 

 is conspicuous in connection with a certain kind of work. 

 Especially unsafe is it to 

 ascribe certain definite 

 functions to these organs of 

 the protoplast, because pro- 

 toplasm itself is so little 

 understood. In any event, 

 the cytoplasm seems to be 

 conspicuousl}- associated 

 with the metabolic acti\'i- 

 ties of the cell; and it is 

 certain that the nucleus is 

 conspicuously associated 

 with cell division. When 

 division occurs, and one 

 cell gives rise to two cells, 

 this process almost invari- 

 ably begins with the nu- 

 cleus, which may thus be 

 said to initiate cell di^^sion. 

 It must be understood 

 clearly that we are speak- 

 ing of visible changes in 

 structure, behind which and 

 accompanying which there are certainly numerous invisible 

 physical and chemical changes. 



It is e^'ident that the problem of heredity is involved 

 in the process of cell di\-ision, for through this process the 

 old cell transmits whatex'er determines the characters of 



Fig. I. — Cells from a moss leaf; each 

 of the complete cells shows the well- 

 dehned nucleus; since the leaf is green, 

 there are also numerotis jjreen protoplas- 

 mic organs (chloroplasts) ; the remaining 

 granular-looking groimd substance is the 

 cytojilasm. 



