IV. VERTEBRATA: MAMMALIA. 



629 



Miillerian ducts into the urogenital sinus (ug}, the lower continua- 

 tion of the bladder. The gonad is connected with the Wolffian 

 duct. In the anterior wall of the urogenital sinus is a mass of 

 highly vascular tissue (cp), from which and a surrounding fold the 

 external genitalia are developed. Since the urogenital sinus opens 

 from in front into the intestine, there is always a claoca (cl) in 

 the embryonic stages, which persists throughout life in the mono- 

 tremes, and to a considerable extent in the female marsupials; in 

 all other vertebrates it is divided by a partition, the perinaeum, 

 into a urogenital opening in front and an anal opening behind. 



From this indifferent condition the male and female apparatus 

 are derived, the structures being closely similar in most males (fig. 

 652). The Miillerian duct vanishes, while the Wolman duct be- 

 comes 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 intromittent organ 

 (penis). In the female the Wolffian body and duct degenerate, 

 the Miillerian ducts become the reproductive canals. The modi- 

 fications of these become of great systematic importance. In the 

 monotremes both ducts open separately and become differentiated 

 into two parts (fig. 653, A), anterior oviducts with wide openings 



A. 



FIG. 653. Female genitalia of (^4) Echidna aculeata; (B) of Didelphys dorsigera; (O 

 Phascolom us wombat. (B and C, after Wiedersheim.) c/, cloaca; d, rectum; /*, 

 urinary bladder; n, kidney; o, ovary; od, oviduct; pu, month of ureters; s?t, uro- 

 genital sinus; f, ostium abdominale tubae; rt, uterus; it', opening into vagina; 

 wr, ureter; r, vagina; vb, vaginal blind sac. 



into the body cavity (od, t) and the uterus (). The ureters open 

 into the sinus (and not into the bladder) between the uterine 

 openings. In the marsupials (B and C) there are three divisions, 

 oviduct, uterus, and vagina; besides, the two Miillerian ducts may 

 approach, near the uterus (B}, or fuse in this region (C') in some 



