200 Heredity and Environment ■ 



ferent nuclei are essentially alike and that differentiations are 

 mainly limited to the cytoplasm. Thus the differentiations of cells 

 are not due to the differentiations of their nuclei, but rather the 

 reverse is true, such differentiations of nuclei as occur are due 

 to differentiations of the cytoplasm in which they lie. Never- 

 theless differentiations do not take place in the absence of nu- 

 clear material, and it seems probable that the interaction of nucleus 

 and cytoplasm is necessary to the formation of the new cytoplas- 

 mic substances which appear in the course of development. 



2. Segregation and Isolation of Different Substances m Cells. 

 — But differentiation consists not only in the formation of differ- 

 ent kinds of substances in cells but also in the separation of these 

 substances from one another. This separation is brought about 

 to a great extent by flowing movements within cells which are 

 associated especially with cell division. 



In all these processes of heredity and development cell divi- 

 sion plays a particularly important part. If cell divisions were 

 always exactly alike there could be no initial difference between 

 the daughter cells, and unless acted upon by different stimuli all 

 cells would remain exactly alike. But there is much evidence 

 that daughter cells are often unlike from the time of their for- 

 mation, and that different stimuli act upon them to increase still 

 further this initial difference. 



(a) Differential and Non-differential Cell Division — When 

 each half of any dividing unit is like the other half the division 

 is non-differential. So far as we know the divisions of all the 

 smallest elements of the cell are of this sort; there is no good 

 evidence that the plastosomes, the chromomeres, or the chromo- 

 somes ever divide into unlike halves, though in the maturation 

 divisions the separation of whole chromosomes leads to the ap- 

 pearance of a differential division of the chromosomes. But 

 while all of the cell elements may be supposed to grow and divide 

 into equivalent halves there may be an unequal distribution of 

 these elements in cell division, so that the two daughter cells are 

 unlike. This is what is known as differential cell division and it 



