22: 



THE EVOLUTION OE SEX. 



reproductive cells. To quote Weismann's own words, "In each devel- 

 opment a portion of the specific germ-plasma which the parental ovum 

 contains is not used up in the formation of the offspring-, but is reserved 

 unchanged to form the germ-cells of the following generation." In 

 short, continuity is kept up by the plasma of nuclei, rather than by a 



FlG. 85. — The chromatin elements of the nuclei in coil (a), double star (i), and 

 almost di\ ided stages (c). — After Pfitzner. 



chain of cells. It will be observed, of course, that while early insula- 

 tion of definite germ-cells is a demonstrable fact, to be seen in a few 

 cases, though perhaps of wider occurrence than we know of, the 

 continuity of germ-plasma is strictly an hypothesis. 



This being so, reproductive maturity may be defined as the period 

 when the reproductive cells (bearing the inherited capital of germ- 

 plasma) have established themselves to that degree that they can start 

 fresh organisms, and have multiplied to an extent which in most cases 

 makes their liberation a physiological necessity. In the lower animals, 

 the maturity of the sexual functions is often as slightly marked as the 

 liberation of the elements is passive and random. In slightly differen- 

 tiated organisms, like sponges, there is little reason to suppose that 

 the distinction between cells preponderating in germ-plasma and the 

 ordinary cells of the body is much marked. Nor in such cases is the 

 anarchic opposition between body and reproductive cells at all 

 emphatic, especially as regards the female cells. It is only as the 

 differentiation increases, as the contrast between body-cells and sex- 

 cells becomes emphasized, as the asexual mode of getting rid of surplus 

 wanes, that the typical liberation of sex-elements which marks sexual 

 maturity becomes a striking fact in the life. That the male cells are 



