REPRODUCTION 



Where we find exceptions in these groups, they are of 

 secondary origin. It is equally certain, on the other 

 hand, that in other cases the separation of the sexes is 

 the primitive arrangement; as in siphonophorae, cteno- 

 phorae, bryozoa, cirripedia, and mollusks. In these 

 cases the hermaphrodism is clearly secondary in the 

 sense that the hermaphrodites descend originally from 

 gonochorists. 



It is only in a few sections of the lowest histona that 

 the two kinds of sex-cells arise without a definite loca- 

 tion in different parts of the simple tissue, as in a few 

 groups of the lower algae and in the sj^onges. As a rule 

 they are formed only at definite positions and in a special 

 layer of the tissue-body, and mostly in groups, in the 

 shape of sexual glands (gonadcs). These bear special 

 names in different groups of the histona. The female 

 glands are called archegonia in the crytogams, nnccllus 

 (formed from the macrosporangia of the pteridophyta) 

 in the phanerogams, and ovaries in the metazoa. The 

 male glands are called antheridia in the crytogams, 

 pollen-sacs (formed from the microsporangia of the 

 ferns) in the phanerogams, and testicles (as spcrmaria) 

 in the metazoa. In many cases, especially in aquatic 

 lower animals, the ovula (as products of the ovaries) 

 are discharged directly outward. But, in most of the 

 higher organisms, special sexual ducts (i^onodnctus) 

 have been formed to conduct both kinds of the gonocyta 

 out of the organism. 



While the two kinds of sexual glands are usually 

 located in different parts of the generating organism, 

 there are, nevertheless, a few cases in which the sex-cells 

 are formed directly and together from one and the same 

 gland. These glands are called hermaphroditic glands. 

 Such structures are very notable in several highly dif- 

 ferentiated groups of the metazoa, and have clearly 

 been developed from gonochoristic structures in lower 



249 



