The Cellular Basis 201 



plays a most important part in differentiation. While the chro- 

 matin is equally distributed to the daughter cells, except in the 

 case of the maturation divisions, the achromatin and the oxy- 

 chromatin of the nucleus are not always distributed equally and 

 this is probably an important factor in development. The divi- 

 sions of the cytoplasm of the egg are frequently differential and 

 such divisions are known to play a great part in embryonic differ- 

 entiation. 



(b) Isolation of Cytoplasmic Substances by Division Walls. — 

 In the differential divisions of the cytoplasm unlike substances 

 become localized in certain patts of the cell body, chiefly by means 

 of definite flowing movements of the cytoplasm, and when cell di- 

 vision occurs these substances become permanently separated by 

 partition walls. In this way irreversible differentiations are 

 formed. If the formation of partition walls is prevented the dif- 

 ferent substances within the cell body may freely commingle, es- 

 pecially during nuclear division when the cytoplasmic movements 

 are especially active; in such cases differentiation may be ar- 

 rested even though nuclear division continues. In the develop- 

 ing eggs of most animals partition walls between daughter cells 

 are necessary to prevent the commingling of different kinds of 

 substances, which are sorted by the movements within the cell and 

 are isolated by the partition walls. In some cases, as for example 

 in certain protozoa, the commingling of different kinds of pro- 

 toplasm within a cell may be prevented by the viscosity of por- 

 tions of the protoplasm, or by the formation of intracellular mem- 

 branes, or by a reduction to a minimum of the mitotic movements 

 within the cell by the persistence of the nuclear membrane dur- 

 ing division. In general the degree of differentiation may be 

 measured by the degree of unlikeness between different cells, and 

 by the completeness with which the protoplasm of different cells 

 is kept from intermingling. 



