658 Comparative Morphology of Chordales 



intervening between them is known as the perineum (Fig. 494E). In 

 the adult, therefore, the dorsal part of the cloaca is represented in the 

 rectum, while the ventral part, from whose anterior region the urinary 

 bladder has grown out, is otherwise represented by the urinogenital 

 passages posterior to the bladder. 



In marsupials the partition between the dorsal rectal and ventral 

 urinogenital divisions of the cloaca is less strongly developed than in 

 placental mammals. It attains much less dorsoventral thickness (Fig. 

 496). Consequently the perineal region is less extensive and the anus 

 and urinogenital passage open fairly close together at the bottom of a 

 shallow superficial remnant of the cloaca, the surface of it covered 

 by proctodeal ectoderm (see p. 274). The pocket is surrounded by a 

 circular muscle (sphincter) which serves to contract its rim. A similar 

 shallow ectodermal cloaca occurs in a few placental mammals — 

 some insectivores and some rodents. 



The ureter, in both sexes of all Amniota, develops as an outgrowth 

 from the hind end of the Wolffian duct (Fig. 82). As the cloacal wall 

 expands, the posterior ends of the Wolffian duct and of the ureter 

 derived from it are carried apart so that they come to open separately 

 into the cloaca. They remain thus in adult monotremes, the ureters 

 opening not directly into the bladder but into the cloaca close to the 

 mouth of the bladder (Fig. 496). In the male monotreme, therefore, 

 two pairs of ducts open into the ventral region of the cloaca : the ureters 

 and the vasa deferentia (Wolffian ducts). The female cloaca likewise 

 receives two pairs of urinogenital ducts: the ureters and the oviducts 

 (their uterine regions), the Wolffian ducts having degenerated except 

 in so far as they may be represented in the hind ends of the ureters. 



In all mammals other than monotremes, the relations of the 

 ventral cloacal structures become modified by differential growth in 

 such a way that the hind ends of the ureters are shifted to the dorsal 

 surface of the bladder, while the proximal "neck" of the bladder 

 (Fig. 496) becomes more or less narrowed and elongated. In the male 

 (Fig. 492) the neck of the bladder joins the vasa deferentia and from 

 their point of junction a common urinogenital passage, the ure- 

 thra, extends to the tip of the penis. In the female (Fig. 495) the 

 neck of the bladder and the vagina (the joined oviducts) open into a 

 urinogenital sinus, the vestibule, which therefore corresponds to the 

 male urethra. All of the passages common to the urinary and genital 

 systems are of cloacal origin. 



Copulatory Organs 



The strategic position for copulatory organs is close to the posterior 

 ends of the genital ducts. Consistent with this fact, the copulatory 



