CHAPTER V. 

 Sexual Organs and Tissues. 



TT is the object of this portion of the book to continue the 

 analysis of sexual characters, but novv' in a deeper way, 

 reviewing successively the organs, tissues, and cells concerned 

 in sexual reproduction. The essential and auxiliary organs of 

 the two sexes, the frequent combination of these in hermaphro- 

 dite plants and animals, the sex-cells both male and female, 

 will be discussed in order. This survey will be for the most 

 part structural or morphological ; the special physiology of sexual 

 union and of fertilisation will be discussed at a later stage. 



§ I. Essential Sexual Organs of Animals. — It is now a well 

 established fact that among the ciliated infusorians, which 

 swarm especially in stagnant waters, a process occurs which 

 cannot but be described as in part sexual reproduction. Two 

 individuals, to all appearance alike be it noted, become tempor- 

 arily associated, and interchange some of the elements of their 

 accessory nuclear bodies. This process of fertiUsation is 

 essential to the continued vigour of the species, and will be 

 afterwards described at length. Such a very simple form of 

 sexual union differs from what occurs in higher animals, in two 

 conspicuous respects, — {a) the organisms are apparently quite 

 similar in form and structure; {b) they are unicellular, and 

 thus there is no distinction between "body" and reproductive 

 cells. What is fertilised by the mutual exchange in those 

 infusorians is, roughly speaking, the entire animal, for the 

 whole is but a unit mass of living matter. 



Among the protozoa, however, loose colonies of cells occur, 

 which bridge the gulf between unicellular and multicellular 

 animals. In these we find the first indications of the after- 

 wards conspicuous difference between "body" and reproductive 

 cells. From these loose colonies, certain of the units are set 

 adrift, and meeting with others more or less like themselves 

 fuse to form a double cell, virtually a fertilised ovum, from 



