THE CELLULAR BASIS 191 



posed 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 chromatin is equally distributed 

 to the daughter cells, except in the case of the 

 maturation divisions, the achromatin and the 

 oxychromatin 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 fre- 

 quently differential and such divisions are 

 known to play a great part in embryonic dif- 

 ferentiation. 



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 



