ESSENTIAL FEATURES OF SEXUAL PROCESS 1349 



any cell is constant for the species. Thus every human somatic cell 

 has sixteen chromosomes in its nucleus. The same number is found 

 in types so far apart as the ox, the guinea-pig, and the onion. In the 



FIG. 553. Diagram showing the changes which occur in the centrosomes 

 and nucleus of a cell in the process of mitotic division. (SCHAFER.) 



The nucleus is supposed to have four chromosomes. 



mouse, the salamander, and the lily the number is twenty-four. 

 Other types, such as the crustacean Artemia, are said to have as many 

 as 168 chromosomes, while in Ascaris the cells only contain two or 

 four chromosomes. All these changes, which are included under the 



