COMPARATIVE ANATOMY OF THE UROGENITAL SYSTEM 279 



as a vagina to receive the penis. In mammals above marsupials the two vaginae fuse to a 

 single vagina (hence the name Monodelphia). There is also generally more or less fusion of 

 the two uteri (Fig. 63). When only the posterior portions of uteri are fused, the fused portion 

 is called the body of the uterus and the separate portions the horns of the uterus. In man and 

 other primates the uteri are fused along their entire length producing the single uterus or 

 womb. The young of the placental mammals develop only in the uterine part of the oviducts • 

 in those forms with partially fused uteri, only in the horns. 



5. The evolution of the cloaca. — The cloaca is found in all vertebrates except cyclostomes, 

 teleostomes, and the placental mammals. It receives the termination of the intestine and the 

 urinary and genital ducts. From the preceding account it will be evident that in the males 

 of elasmobranchs and Amphibia the cloaca receives only the Wolffian ducts, while in the 

 females both oviducts and Wolffian ducts enter it (Fig. 62A and E). It commonly happens 

 however, that in the males of these groups vestiges of the oviducts are present. In the males 



Fig. 64. — Diagrams to illustrate the changes in the cloaca in mammals during development. 

 A, early embryonic stage, showing the cloaca receiving the urinary bladder, the rectum, and the Wolffian 

 duct, as in the lower vertebrates. B, later stage, showing the beginning of the fold which divides the 

 cloaca into a ventral urogenital sinus which receives the urinary bladder, Wolffian ducts, and ureters, 

 and into a dorsal part which receives the rectum. C, further progress of the fold, dividing the cloaca 

 into urogenital sinus and rectum; the ureter has separated from the Wolffian duct and is shifting 

 anteriorly. D, completion of the fold, showing complete separation of the cloaca into ventral uro- 

 genital sinus and dorsal rectum. Note in D that the ureter has shifted farther so that it opens into the 

 urinary bladder. 



of reptiles and birds the cloaca receives the Wolffian ducts (vasa deferentia) and the ureters; 

 in the females the oviducts and the ureters (Fig. 62C and D). In addition, in many fishes, 

 Amphibia, reptiles, and the embryos of birds and mammals the urinary bladder opens into 

 the ventral wall of the cloaca. Adult birds have no urinary bladder; mammals have one, 

 but it is no longer attached to the digestive tract. 



In placental mammals marked changes occur in the relations of the terminal portions of 

 the urogenital 'ducts. In the embryo the cloaca becomes divided by a fold which extends 

 posteriorly to the body wall and separates the cloaca into two parts, each with its own opening 

 to the exterior (Fig. 64). The dorsal part includes the intestine only; this terminal portion 

 of the intestine is called the rectum and opens to the exterior by the anus. The ventral part 

 separated from the cloaca is called the urogenital sinus. It receives the stalk of the bladder 

 and the excretory and genital ducts. The excretory ducts (either Wolffian ducts or ureters) at 

 first open into the urogenital sinus, but subsequently the ureters shift so as to open into the blad - 

 der, in all of the placental mammals (Fig. 64) . Thus, the ureters pass into the bladder while the 

 Wolffian ducts (vasa deferentia) in males or the vagina in females unite with the duct of the 

 bladder, named the urethra, forming a common tube or chamber, the urogenital sinus, which 

 opens externally in front of the anus by a urogenital aperture. In the females only of the 



