THE CELLULAR BASIS 189 



On the other hand the cytoplasm undergoes 

 progressive differentiation, and when by pres- 

 sure or centrifugal force it is brought into rela- 

 tions with other nuclei the differentiations of 

 the cytoplasm are not altered thereby, thus 

 showing that the different nuclei are essentially 

 alike and that differentiations are mainly lim- 

 ited 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. Nevertheless differentiations do not take 

 place in the absence of nuclear material, and it 

 seems probable that the interaction of nucleus 

 and cytoplasm is necessary to the formation of 

 the new cytoplasmic substances which appear 

 in the course of development. 



2. Segregation and Isolation of Different 

 Substances in Cells. — But differentiation con- 

 sists not only in the formation of different 

 kinds of substances in cells but also in the sep- 

 aration of these substances from one another. 

 This separation is brought about to a great 

 extent by flowing movements within cells 



