THE CELLULAR BASIS OF HEREDITY 245 



by flowing movements within cells which are associated especially with 

 differential cell division. 



In all these processes of heredity and development cell division plays 

 a particularly important part. If cell divisions were always exactly 

 alike there could be no initial difference between the daughter cells, and 

 unless acted upon by different stimuli all cells would remain exactly 

 alike. But there is much evidence that daughter cells are often unlike 

 from the time of their formation, and that different stimuli act upon 

 them to still further increase this initial difference. 



(a) Differential and Non-differential Cell Division 



When each half of any dividing unit is like the other half the divi- 

 sion is non-differential. So far as we now 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 chromosomes 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 supposed to 

 grow and divide into equivalent halves, there may be an unequal distri- 

 bution 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 a most 

 important factor in development. The divisions of the cytoplasm of 

 the egg are frequently differential and such divisions are known to play 

 a great part in the embryonic differentiation. 



In the differential divisions of the cytoplasm unlike substances be- 

 come 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, especially during nuclear division 

 when the cytoplasmic movements are especially active; in such cases 

 differentiation is arrested even though nuclear division continues. In 

 the developing eggs of most animals partition walls between daughter 

 cells are necessary to prevent the commingling of different kinds of sub- 

 stances, 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 prevented by the viscosity of portions of the protoplasm, 

 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 



