FACTORS IN ONTOGENY 257 



"It is readily seen that the culmination of the process lies in the 

 splitting of the chromosomes and the separation of their component 

 halves to form the two new daughter nuclei." 



The obvious distinction in capacity of development shown 

 by the various cells which compose an animal's body leads us 

 to ask whether we can distinguish differences associated with 

 these different potentialities in the fine structure of the cells 

 themselves, and especially in their behavior during the process 

 of multiplication. For the fate or future character of any 

 cell must largely depend on the nature of its origin, the character 

 of its inheritance. Now in some cases this difference in poten- 

 tiality of the undifferentiated dividing cells is plainly shown 

 by differences in the details of the process of division. A 

 conspicuous and important instance of this, and one bearing 

 directly on our subject of the relation of the structure and 

 character of the germ plasm to the fully developed organism, 

 is the distinction, usually easy to make, between the body or 

 so-called somatic cells and the reproductive or germ cells of 

 any organism. 



" Every multicellular organism arises by a process of division from 

 a single cell, the fertilized germ or egg cell, which in turn has been cut 

 off from the cells of a preexisting individual. Out of the group of 

 cells which result from the continued division of the germ cell and its 

 descendants are differentiated the various tissues and organs of the 

 body through which the vital functions are carried on. Those tissues 

 and organs which perform functions pertaining directly to the existence 

 of the individual have been termed 'somatic/ and their constituent 

 cells the 'somatic' or body cells, in contradistinction to the repro- 

 ductive tissues or cells whose function concerns the continuance of 

 the species. In some forms these groups of cells, the somatic and the 

 reproductive, become isolated from each other quite early in develop- 

 ment; in one case, indeed, the differentiation of reproductive cells 

 from the somatic ones has been traced by Boveri back to the first 

 division of the egg. This case of Ascaris megalocephala is so striking 

 and of such fundamental theoretical importance that it must not be 

 passed without notice, for in it we find marked differences between 

 the somatic and reproductive cells in their nuclear structure, their 

 relative amount of chromatin, and mode of division. The egg of 

 Ascaris has been the classical object for cytological studies on account 



