556 



CHORDATA 



ectodermal cloacal pit, wliich 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 JMiiUerian 



.'^^ 



Fig. 602. Fio. 603. 



Fig. 602. — Diagram of enibn'onic mammalian urogenital SN'Stem (from Balfour, 

 after Tliompson). <-/, cloaca; cp, genital process; go, genital cord; /, rectum; /,;, ridge 

 for formation of labia or scrotum; m, Jiltillerian duct; ct, gonad; ug, urogenital sinus; 

 ir, Wolflian body; %c, WolQian duct; 3, ureter; 4, urinary bladder; 5, continuation 

 of latter to allantois (urachus). 



Fig. 603. — Urogenital system of male beaver (from Blanchard). a, caslorcum 

 sacs; i, openings of tlieir ducts into pre]iutial canal; c, tip of penis; (/, preputial opening; 

 f, anal glanris:/, their ducts; g, anus; h, liase of tail; /, corpora ca\'ernosa; A", Cov"]jer's 

 glands; /, seminal vesicles; m, vasa dcferentia; ;;, testes; 0, urinary bladder mill ureters. 



duct vanishes, while the Wolffian duct becomes the \as deferens and its 

 accessories, serving as a canal for the genital products, while the e-\lernal 

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

 mittent organ (penis). In the female the \\'olffian Iwdy and duct degener- 

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

 cations of these become of great systematic importance. In the mono- 

 tremes l)oth ducts open separately into the urogenital sinus and Ix-come 

 difl'erentiatcd into two parts (fig. 604, .1), anterior oviducts {od) with wide 



