144 Heredity and Environment 



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 

 formation quantitatively and qualitatively precisely like its mother 

 structure. (2) Accompanying the formation of radiations, which 

 go out from the centrosomes into the cell body, diffusion currents 

 are set up in the cytoplasm which lead to the localization of dif- 



trosome are at the lower pole near the point of entrance ; in 2 and 3 they 

 have moved up to the eqUator on the posterior side of the egg; in 4 the 

 egg and sperm nuclei have come together and the sperm centrosome has 

 divided and formed the cleavage spindle ; in 5 the egg is dividing into right 

 and left halves; in 6 it is dividing into anterior and posterior halves. 



