CHAPTER XVIII. 



Special Physiology of Sex and Reproduction. 



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

 of sexual and reproductive functions. The fundamental physi- 

 ology of the essential functions has been the subject of preced- 

 ing chapters ; the details will be found in the standard works 

 on Physiology, Botany, and Zoology. For the sake of com- 

 pleteness, 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 general biology of the subject. 



§ IVeismann's Theory of'''' Contitiiiity of the Gei'm-Flas?na." — 

 Thanks, esi)ecially 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 im- 

 probable. In a minority of cases, already quoted, the repro- 

 ductive 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 fertilised parent ovum intact, they con- 

 tinue 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 separate 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 fertilised 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 ordinary body-cells modified into reproductive 



