144 Heredity and Environment 



2). And at every successive cleavage the cytoplasmic substances 

 are segregated and isolated in particular cells, and in this way 

 the cytoplasm of the different cells comes to be unlike (Figs. 47 

 and 48). When once partition walls have been formed between 

 cells the substances in the different cells are permanently sepa- 

 rated so that they can no longer commingle. 



What is true of Styela in this regard is equally true of many 

 other ascidians, as well as of Amphioxus and of the frog (Figs. 

 9, 10, n), though the segregation of substances and the differ- 

 entiation of cells are not so evident in the last named animals be- 

 cause these substances are not so strikingly colored. Indeed the 

 segregation and isolation of different protoplasmic substances 

 in different cleavage cells occurs during the cleavage of the egg 

 in all animals, though such differentiations are much more marked 

 in some cases than in others. 



This same type of cell division, with equal division of the 

 chromosomes and more or less unequal division of the cell body, 

 continues long after the cleavage stages, indeed throughout the 

 entire period of embryonic development. Sometimes the division 

 of the cell body is equal, the daughter cells being alike; sometimes 

 it is unequal or differential, but always the division of the chro- 

 mosomes is equal and non-differential. When once the various 

 tissues have been differentiated the further divisions in these 

 tissue cells are usually non-differential even in the case of the 

 cell bodies. 



Significance of Cleavage. There can be no doubt that this re- 

 markably complicated process of cell division has some deep 

 significance; why should a nucleus divide in this peculiarly in- 

 direct manner instead of merely pinching in two, as was once 

 supposed to be the rule ? What is the relation of cell division to 

 embryonic differentiation ? In this process of mitosis, or indirect 

 cell division, two important things take place: (i) Each chro- 

 mosome, chromomere and centrosome is divided exactly into two 

 equal parts so that each daughter structure is at the time of its 



