ESSENTIAL FEATURES OF SEXUAL PROCESS 1203 



all the chromatin is preserved, while in the other cell the thickened ends of 

 the chromosomes are cast off into the cytoplasm and degenerate, only the 

 thinner central portions being preserved. When these divide again, the 

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

 germ-cell, and the process is repeated during five or six divisions, after 

 which the chromatin elimination ceases and the two primordial germ- cells 



FIG. 554. 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 ohromatin (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. 



thenceforward 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 

 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, &c., are mortal. They pass through a certain number of 

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



