556 



CHORDATA 



ectodermal cloacal pit, which is less distinct in the female marsupials, 

 many insectivores, rodents and edentates. In all other mammals anus 

 and urogenital openings are separated by the formation of a partition, the 

 perineum. 



From this indifferent condition the male apparatus is derived, the 

 structures being closely similar in most species (fig. 603). The Miillerian 



FIG. 602. FIG. 603. 



FIG 602. Diagram of embryonic mammalian urogenital system (from Balfour, 

 after Thompson), d, cloaca; cp, genital process; go, genital cord; i, rectum; Is, ridge 

 * r ,y matlon of labia or scrot um; m, Mullerian duct; of, gonad; u%, urogenital sinus; 

 W, VVolffian body; u<, Wolffian duct; 3, ureter; 4, urinary bladder; 5, continuation 

 of latter to allantois furachus). 



FIG. 603. Urogenital system of male beaver (from Blanchard). a, castoreum 

 sacs; 6, openings of their ducts into preputial canal; c, tip of penis; d, preputial opening; 

 e anal glands;/, their ducts; g, anus; h, base of tail; i, corpora cavernosa; k, Cowper's 

 glands; I, seminal vesicles; m, vasa deferentia; n, testes; o, urinary bladder with ureters. 



duct vanishes, while the Wolffian duct becomes the vas deferens and its 

 accessories, serving as a canal for the genital products, while the external 

 genitals arise from the other parts mentioned, these forming an intro- 

 mittent organ (penis) . In the female the Wolffian body and duct degener- 

 ate, the Mullerian ducts become the reproductive canals. The modifi- 

 cations of these become of great systematic importance. In the ir.ono- 

 tremes both ducts open separately into the urogenital sinus and become 

 differentiated into two parts (fig. 604, A), anterior oviducts (od) with wide 



