192 HEREDITY AND ENVIRONMENT 



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 

 the mitotic movements within the cell by the 

 persistence of the nuclear membrane during 

 division. In general the degree of differentia- 

 tion may be measured by the degree of un- 

 likeness between different cells, and by the 

 completeness with which the protoplasm of 

 different cells is kept from intermingling. 



