The Cellular Basis 207 



(a) -Differential and Non-differential Cell Division When 

 each half of any dividing unit is like the other half the division 

 is non-differential. So far as we know the divisions of all the 

 smallest elements of the cell are of this sort; there is no good 

 evidence that the plastosomes, the chromomeres, or the chromo- 

 somes ever divide into unlike halves, though in the maturation 

 divisions the separation of whole chromosomes leads to the ap- 

 pearance of a differential division of the chromosomes. But 

 while all of the cell elements may be supposed to grow and divide 

 into equivalent halves there may be an unequal distribution of 

 these elements in cell division, so that the two daughter cells are 

 unlike. This is what is known as differential cell division and it 

 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 parts 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 



