XII.] EXTERNAL GENERATIVE ORGANS. 417 



sinus becomes, in all Mammalia but the Ornithodelphia, 

 completely separated from the intestinal cloaca, and the 

 two parts obtain separate external openings. The 

 ureters (Fig. 140, 3) open higher up than the other 

 ducts into the stalk of the allantois which here dilates 

 to form the bladder. That part of the stalk which con- 

 nects the bladder with the ventral wall of the body 

 constitutes the urachus, and loses its lumen before the 

 close of embryonic life. The part of the stalk of the 

 allantois below the openings of the ureters narrows to 

 form the urethra, which opens together with the Wolffi an 

 and Mullerian ducts into the urogenital cloaca. 



In front of the urogenital cloaca there is formed 

 a genital prominence (Fig. 140 cp) with a groove con- 

 tinued from the urinogenital opening, and on each side a 

 genital fold (Is). In the male the sides of the groove on 

 the prominence coalesce together, embracing between 

 them the opening of the urinogenital cloaca, and the 

 prominence itself gives rise to the penis, along which the 

 common urinogenital passage is continued. The two 

 genital folds unite from behind forwards to form the 

 scrotum. 



In the female the groove on the genital prominence 

 gradually disappears, and the prominence remains as the 

 clitoris, which is therefore the homologue of the penis : 

 the two genital folds form the labia majora. The urethra 

 and vagina open independently into the common uro- 

 genital sinus. 



THE ALIMENTARY CANAL AND ITS APPENDAGES. 



It is convenient to introduce into our account of the 

 organs derived from the hypoblast, a short account of 

 F. & B. 27 



