336 



COMPARATIVE MORIHOLOGY OF VERTEBRATES. 



the embryonic excretory organs (mesonephroi) and their (Wolffian) 

 ducts; the permanent kidneys (metanephroi) and the ureter; the gonads; 

 the Miillerian ducts; the cloaca and the anlagen of the external genitalia, 

 which arise in the anterior or ventral wall of the urogenital sinus. 



In the embryonic stages the Wolffian and Miillerian ducts and the 

 ureters open into the cloaca (fig. 335). Then a part of the latter, with 

 the openings of these ducts, is cut off to form the allantois, a portion 

 of which becomes the urinary bladder, this part receiving the ureters 



FIG. 336. Model of pelvic region of human embryo 25 mm. long, after Keibel. (Com- 

 pare with fig. 335-) 0"> anal opening; /, lateral ligament of uterus; m, Miillerian duct; o, 

 ovary; pu, primitive ureter (Wolffian duct); r, rectum; s, symphysis pubis; sg, septum of 

 genital protuberance; sug, urogenital sinus; u, ureter; ub, urinary bladder; ur, recto-uterine 

 excavation. 



(except in monotremes) while the Wolffian and Miillerian ducts open 

 into the basal part of the allantoic outgrowth which is separated from 

 the bladder by a narrower stalk which becomes the urethra. This 

 part, into which the two pairs of ducts and the urethra empty, forms 

 the urogenital sinus (fig. 336, sng). With the formation of the per- 

 manent kidneys the mesonephros largely disappears (see p. 341) and 

 the same fate extends to one or the other pair of ducts, the Miillerian 

 largely disappearing in the male, the Wolffian in the female. The 

 parts which persist are more specialized than in any other group of 

 vertebrates, this being in part due to the fact that usually a large part 



