SEXUAL ORGANS AND TISSUES. 

 53 



CHAPTER V. 

 SEXUAL ORGANS AND TISSUES. 



Tt is the object of this portion of the book to continue the analysis 

 of sexual characters, but now in a deeper way, reviewing suc- 

 cessively the organs, tissues, and cells concerned in sexual repro- 

 duction. The essential and auxiliary organs of the two sexes, the 

 frequent combination of these in hermaphrodite 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 fertilization 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 can not but be 

 described as in part sexual reproduction. Two individuals, to all 

 appearance alike be it noted, become temporarily associated, and 

 interchange some of the elements of their accessory nuclear bodies. 

 This process of fertilization is essential to the continued vigor 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 fertilized 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 afterwards 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 fertilized ovum, 

 from which by continuous division a fresh colony is then developed. 

 In these transition-forms there are thus reproductive cells of slight 

 distinctness, but as yet obviously no sexual organs. 



When we pass to the sponges, we find colonies consisting or 

 myriads of cells, among which there is a considerable division 01 

 labor. An outer layer (or ectoderm) usually consisting of much 

 subordinated cells, an inner layer (or endoderm) of predominantly 



