ESSENTIAL FEATURES OF THE SEXUAL PROCESS 1255 



same process is repeated in only one of the daughter cells derived from a 

 germ cell, and this recurs during five or six divisions, after which the 

 chromatin elimination ceases and the two primordial germ cells thence- 

 forward give rise only to other germ cells in which the entire chromatin is 

 preserved. Thus " the original nuclear constitution of the fertilised egg is 

 transmitted, as if by a law of primogeniture, only to one daughter cell, and 



FIG. 567. Origin of the primordial germ cells and casting out of chromatin in 

 the somatic cells of Ascaris. ( WILSON and BOVERI.) 



A, two-cell stage dividing ; s, stem cell, from which arise the germ cells. B, the 

 same from the side, later in the second cleavage, showing the two types of mitosis 

 and the casting out of chromatin (c) in the somatic cell, c, resulting four-cell 

 stage; the eliminated chromatin at c. D, the third cleavage, repeating the foregoing 

 process in the two upper cells. 



by this again to one, and so on, while in other daughter cells the chromatin 

 in part degenerates, in part is transformed, so that all of the descendants 

 of these side-branches receive small reduced nuclei " (Boveri, quoted by 

 Wilson). 



The immortality, which was the property of all the unicellular ancestors 

 of the metazoa, has in the latter descended only to the germ cells. All the 

 other cells of the body, which form the nervous and muscular tissues, 

 glands, skin, etc., are mortal. They pass through a certain number of 

 divisions ; but although this number is large, it is limited, and on the number 

 of divisions which are possible depends the normal duration of life of the 

 organism to which the cells belong. We may thus regard the egg cell as 

 dividing into two parts. From one part will be formed by differentiation 



