2o6 Heredity and Environment 



in different cells. A somewhat similar view was advanced by 

 deVries in his theory of "intra-cellular pangenesis." However, as 

 we have seen already, there is good evidence that the chromosomes 

 do not undergo progressive differentiation in the course of devel- 

 opment; they always divide with exact equality, and even in 

 highly differentiated tissue cells their number and form usually 

 remain as in embryonic cells. 



On the other hand the cytoplasm undergoes progressive dif- 

 ferentiation, and when by pressure or centrifugal force it is 

 brought into relations with other nuclei the differentiations of 

 the cytoplasm are not altered thereby, thus showing that the dif- 

 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. 



