THE CELLULAR BASIS 183 



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 

 which are associated especially with cell 

 division. 



In all these processes of heredity and de- 

 velopment cell division plays a particularly 

 important part. If cell divisions were always 

 exactly alike there could be no initial differ- 

 ence between the daughter cells, and unless 

 acted upon by different stimuli all cells would 

 remain exactly alike. But there is much evi- 

 dence that daughter cells are often unlike from 

 the time of their formation, and that different 

 stimuli act upon them to increase still fur'ther 

 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- 



