UROGENITAL SYSTEM 14 ! 



which may be regarded as a uterus. Preliminary to the separa- 

 tion of the sexes an unequal development of the double sexual 

 apparatus in one and the same individual takes place, being on 

 the one hand unduly increased, on the other arrested. The 

 Nematoda, Chaetopoda and Radiata, in all of which the sexes 

 are separate, possess tubular testes and ovaries ; the Nema- 

 toda having seminal vesicles, oviducts and uterus. The sper- 

 matozoa of the Annelida are clearly motile and have frequently 

 a condensed head part. In the Arthropoda, in spite of their 

 variability, progress from a lower to a form more closely re- 

 sembling the vertebrates is to be traced, one of the most 

 important signs thereof being the increased tendency to separa- 

 tion of the sexes, hermaphroditism occurring only among the 

 lower types. Where the sexes are distinct, the reproductive 

 organs are either single (of the Crustaceans, the Copepoda and 

 certain of the Arachnida), or paired and bilateral-symmetrical 

 (Branchiopoda, Arthostraca and, of the Arachnida, the Galeodes 

 and Araneida, besides all the Insecta). The Insecta are more 

 highly organised, in that they possess a vesicula seminalis in 

 the male sex, and a receptaculum seminis in the female sex. 

 The Brachiopoda, parts of the Lamellibranchiata, and a large 

 proportion of the Cephalophora are hermaphrodite ; in the 

 remaining Cephalophora, and all the Cephalopoda, the sexes 

 are separate. To the latter belongs the peculiarity of having 

 the spermatozoa enclosed in a spermatophore. 



We have seen that in the Invertebrates the germinal glands 

 are at first undifferentiated, and that, even after this character 

 has disappeared, hermaphroditism is still very widely spread. 

 Now it is one of the most eminently distinctive characters of the 

 Vertebrates that the germinal glands are strictly differentiated 

 into organs for the formation, respectively, of ova and semen, 

 and the sexes are represented by separate individuals, although, 

 as far as has been ascertained, both the germinal glands originate 

 in the same embryonic rudiment, the Wolffian duct. Another 

 character common to all the Vertebrates is the development of 

 ovarian follicles from groups of cells in the primitive ovarian 

 tube. Common to man and the mammals is the peculiar con- 

 struction of the ovarian follicle the ovum being contained 

 within a Graafian follicle filled with fluid and the minuteness 



