184 HEREDITY AND ENVIRONMENT 



differential. So far as we know the di- 

 visions of all the smallest elements of the cell 

 are of this sort ; there is no good evidence that 

 the plastosomes, the chromomeres, or the chro- 

 mosomes ever divide into unlike halves, though 

 in the maturation divisions the separation of 

 whole chromosomes leads to the appearance of 

 a differential division of the chromosomes. 

 But while all of the cell elements may be sup- 

 posed to grow and divide into equivalent 

 halves there may be an unequal distribution 

 of these halves 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 chromosomes are 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 the embryonic 

 differentiation. 



