THE CELLULAR BASIS 185 



In the differential divisions of the cytoplasm 

 unlike substances become localized in certain 

 parts of the cell body, chiefly by means of 

 definite flowing movements of the cytoplasm, 

 and when cell division 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 different substances 

 within the cell body may freely commingle, es- 

 pecially during nuclear division when the cyto- 

 plasmic movements are especially active; in 

 such cases differentiation may be arrested even 

 though nuclear division continues. In the de- 

 veloping eggs of most animals partition walls 

 between daughter cells are necessary to pre- 

 vent 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 cer- 

 tain protozoa, the commingling of different 

 kinds of protoplasm within a cell may be pre- 

 vented by the viscosity of portions of the pro- 

 toplasm, or by the formation of intracellular 

 membranes, or by a reduction to a minimum of 



