EXCRETORY ORGANS. 



599 



Mammalia the dorsal part of the cloaca with the alimentary tract be- 

 comes tirst of all partially constricted off from the ventral, which then 

 forms a urinogenital sinus (fig. 

 407, ug). In the course of de- 

 velopment the urinogenital 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. 407, 3) open higher 

 up than the other ducts into the 

 stalk of the allantois which di- 

 lates to form the bladder (4). The 

 stalk connecting the bladder with 

 the ventral wall of the body con- 

 stitutes the urachus, and loses its 

 lumen before the close of em- 

 bryonic 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 Wolffian and 

 Miillerian ducts into the urino- 

 genital cloaca. 



In front of the urinogenital 

 cloaca there is formed a genital 

 prominence (fig. 407, cp), with a 

 groove continued from the uri- 

 nogenital opening ; and on each 

 side a genital fold {Is). In the 

 male the sides of the groove 

 on the prominence coalesce to- 

 gether, 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 urinoefenital sinus. 



Fig. 407. Diagram of the ukinogeni- 



TAL OKGANS OF A MaMMAL AT AN EARLY STAGE. 



(After Allen Thomson ; from Quain's Ana- 

 tomy.) 



The parts are seen chiefly in profile, 

 but the Miillerian and Wolffian ducts are 

 seen from the front. 



3. ureter; 4. urinary bladder; 5. ura- 

 chus; ot. genital ridge (ovary or testis); 

 W. left Wolffian body ; x. part at apex from 

 which coni vaseulosi are afterwards de- 

 veloped; w. Wolffian duct. m. Miillerian 

 duct ; gc. genital cord consisting of Wolffian 

 and Miillerian ducts bound up in a common 

 sheath; /.rectum; ug. urinogenital sinus; 

 cp. elevation which becomes the clitoris or 

 penis; Is. ridge from which the labia ma- 

 jora or scrotum are developed. 



