PHYISOLOGY OF SEX AND REPRODUCTION. 221 



CHAPTER XVIII. 

 SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 



TT is no part of our purpose to discuss in detail the physiology of 

 sexual and reproductive functions. The fundamental physiology of 

 the essential functions has been the subject of preceding chapters ; the 

 details will be found in the standard works on Physiology, Botany, 

 and Zoology. For the sake of completeness, however, it is necessary 

 to take a brief survey of some of the facts, which are in themselves of 

 supreme importance, and which further elucidate the genera! biology 

 of the subject. 



I. Weismann's Theory of " Continuity of the Germ- 

 Plasma." — Thanks, especially to Weismann, the view that ordinary 

 cells of the "body" become at a certain epoch changed into special 

 reproductive cells, may now be put aside as exceedingly improbable. 

 In a minority of cases, already quoted, the reproductive cells, or the 

 rudiments of sexual organs, are demonstrably set apart at an early 

 stage, before the differentiation of the embryo has proceeded far. 

 They thus include some of the original capital of the fertilized parent 

 ovum intact, they continue the protoplasmic tradition unaltered, and, 

 when liberated in turn, they naturally enough develop as the parent- 

 ovum did. Following out this important fact, various naturalists have 

 reached the conception of a continuous necklace-like chain of sex-cells 

 from generation to generation, — a continuous chain upon which the 

 mortal individual organisms arise and drop away, like so many sepa- 

 rate and successive pendents. 



But in the majority of cases, such a conception, as Weismann has 

 justly insisted, gives a false simplicity to the facts. A chain of insulated 

 sex-cells, connecting the parental fertilized ovum with the germ-cells 

 which develop into offspring, is, so far as we yet know, only rarely 

 demonstrable. In other words, the rudiments of the reproductive 

 organs often appear at a relatively late stage in the development. 

 Where do they come from ? Are somatic, or ordinarv body-cells 

 modified into reproductive elements? Weismann's answer is a decided 

 negative. Although no continuous chain of germ-like cc//s is demon- 

 strable, there is a strict continuity of germ-fi/asma. Part of the double 

 nucleus of the fertilized ovum keeps its characteristics unaltered, in 

 spite of manifold divisions persists intact, and is finally established in 

 the rudiment of reproductive organs. Or in other words, those cells 

 in which the original germ-plasma most predominates become the 



