190 HEREDITY AND ENVIRONMENT 



which are associated especially with cell 

 division. 



In all these processes of heredity and de- 

 velopment cell division plays a particularly 

 important part. If cell divisions were always 

 exactly alike there could be no initial differ- 

 ence between the daughter cells, and unless 

 acted upon by different stimuli all cells would 

 remain exactly alike. But there is much evi- 

 dence that daughter cells are often unlike from 

 the time of their formation, and that different 

 stimuli act upon them to increase still further 

 this initial difference. 



(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 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- 



