THE DEVELOPMENT OP THE EMBflYO ITSELF. 117 



In both sexes the most posterior portion of the Wolffian duct 

 gives rise to the metanephros, or what becomes the permanent 

 kidney and m-eter ; in the male also to the vas deferens, testicle, 

 vas aberrans, and seminal vesicle. 



The ovary has a similar origin to the testicle ; the germinal 

 epithelium furnishing the cells, which are transformed into 

 Graafian follicles, ova, etc., and the mesoblast the stroma in 

 which these structures are imbedded. 



In the female the parovarium remains as the representative 

 of the atrophied Wolffian body and duct. 



The bladder and urachus are both remnants of the formerly 

 extensive allautois. The final forms of the genito-urinary or- 

 gans arise by differentiation, fusion, and atrophy : thus, the 

 cloaca or common cavity of the genito-urinary ducts is divided 



AL. 



^- 



CLi 

 Via. 126. Fig. 187. 



Pig. 128. Fig. 129. 



Figs. 186 to 189.— Diagrams illustrating the evolution of the posterior passages (after 



Landois and Stirling). 

 Pig. 126.— AUantois continuous with rectom. 

 Fig. 127.— Cloaca formed. 

 Fig. 188. — Early condition in male, before the closure of the folds of the groove on 



the posterior side of the penis. 

 Pig. 129. — Early female condition. 

 .A, commencement of proctodseum; .4iiii, allantois; i?, bladder; C, penis; CX, cloaca; 



M, MflUerian duct; B, rectum; U, urethra; S, vestibule; SU, urogenital sinus; Y, 



vas deferens in Fig. 188, vagina in Fig. 139. 



by a septum (the perineum externally) into a genito-urinary 

 and an intestinal (anal) part ; the penis in the male and the 

 corresponding clitoris in the female appear in the region of the 

 cloaca, as outgrowths which are followed by extension of folds 

 of integument that become the scrotum in the one sex and the 

 labia in the other. 



The urethra arises as a groove in the under surface of the 



