336 COMPARATIVE MORFHOLOGY 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 
ene 
Fic. 336.—Model of pelvic region of human embryo 25 mm. long, after Keibel. (Com- 
pare with fig. 335.) u, anal opening; /, lateral ligament of uterus; m, Miillerian duct; a, 
ovary; pu, primitive ureter (Wolffian duct); 7, rectum; s, symphysis pubis; sg, septum of 
genital protuberance; sug, urogenital sinus; «, 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, svg). 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 
