PHYLOGENY OF URINARY AND SEXUAL ORGANS. 429 



VI. Sixth Period : Genitals and Kidneys of Cyclostoma. 



G. The sexual glauds (numerous in Acrania) coalesce into a pair. 



U. The primitive kidney ducts send out lateral branches which acquire 

 vascular coils {glomeruli) (the semi-pinnate primitive kidneys of Bdello- 

 stoma). 



XLIII. B. Second main division : the genital organs (G) and the urinary 

 organs (U) become united. (The sexual system and the urinary system 

 are united in the " urogenital system.") 



VII. Seventh Period : Urogenitals of Primitive Fishes (Selachii). 



The primary primitive kidney duct differentiates on each side, forming 

 two secondary canals ; the Wolffian duct, which develops into the seed-duct, 

 and the Mullerian duct, which develops into the oviduct. Both genital 

 ducts originally open behind the anus (Proselachii). 



VIII. Eighth Period : Urogenitals of Dipneusta. 



A cloaca is formed by the union of the urogenital opening and the cavity 

 of the anus. The single urinary bladder grows out from the anterior wall of 

 the rectum (Lepidosireri). 



IX. Ninth Period : Urogenitals of Amphibia. 



From the uppermost part of the primitive kidney which is in process of 

 atrophy, proceeds, in the male sex, the supplementary testis; in the female 

 sex, the supplementary ovary. The Wolffian duct yet acts, in both sexes, as 

 a urinary canal, and, in the male, also as the seed-duct. The Mullerian 

 duct acts in the female sex as oviduct ; in the male it is a rudimentary 

 organ (Ratbke's duct). 



X. Tenth Period : Urogenitals of Protamnia. 



The atrophied primitive kidney is replaced by the permanent secondary 

 kidney as the urinary organ. The urinary bladder grows out from the 

 ventral orifice of the embryo and forms the allantois. From the anterior 

 wall of the cloaca grows the sexual protuberance {phallus), which, in the 

 male, develops to the penis, in the female, to the clitoris. 



XL Eleventh Period : Urogenitals of Monotremes. 



The lower end of the oviduct enlarges on each side to a muscular 

 uterus. 



XII. Twelfth Period : Urogenitals of Marsupialia. 



The cloaca is separated by a partition into an anterior urogenital opening 

 apertura urogenitalis) and a posterior anal opening (amis'). From the 



