198 INDIVIDUALITY IN ORGANISMS 



regarded as very different from the agamic reproductive 

 processes. Actually, however, there are certain funda- 

 mental similarities between the two processes. I have 

 discussed this matter at some length elsewhere,^ and 

 need only review certain important points here. The 

 evidence indicates that the gametes, the two cells which 

 unite in sexual reproduction and which in their more 

 highly specialized forms we call egg and spermatozoon, 

 are physiologically subordinate parts of the body and 

 undergo differentiation with other parts ^ instead of being 

 composed of a mysterious, independent substance, the 

 germ plasm, as Weismann and many others have be- 

 lieved. Gametic maturity occurs at a relatively ad- 

 vanced physiological age in the organism, and the 

 gametes, like other parts of the body, are physiologi- 

 cally old cells with a low metabolic rate and are evi- 

 dently approaching death. Their isolation from other 

 parts of the body in those multicellular forms in which 

 complete isolation occurs has apparently no relation 

 to the range of dominance, but seems rather to be asso- 

 ciated with the completion of their period of growth 

 and differentiation. So far as the parent organism is 

 physiologically concerned, the isolation of the sex cells 

 may be compared with the casting off of other old 

 cells which have played their part and are approaching 

 death. In many cases, however, the egg remains in 

 the parent body until an earher or later stage of embry- ■ 

 onic development is reached, but even in such cases 

 the egg, after completing its developmental period, seems 

 to have little physiological relation to other parts of the 

 parent body. 



^ Child, Senescence and Rejuvenescence, 1915, Part IV. 



