122 HEREDITY AND ENVIRONMENT 



stances in different cleavage cells occurs dur- 

 ing 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 chromosomes is 

 equal and non-differential. When once the 

 various tissues have been differentiated the 

 further divisions in these tissue cells are usu- 

 ally non- differential even in the case of the 

 cell bodies. 



There can be no doubt that this remarkably 

 complicated process of cell division has some 

 deep significance ; why should a nucleus divide 

 in this peculiarly indirect manner instead of 

 merely pinching in two, as ^\^as once supposed 

 to be the rule? What is the relation of cell 

 division to embryonic differentiation? In this 



