650 Comparative Morphology of Chordates 



of these animals. A few Australian mammals, definitely "marsupial" 

 in other respects, lack the marsupium (e.g., the "marsupial" anteater, 

 Myrmecobius), and in some species of the American opossum the 

 pouch is poorly developed or lacking. 



In the oviparous mammals, Ornithorhynchus and Echidna. 

 the right oviduct is shorter than the left and the anterior mouths of 

 the Fallopian tubes are not fimbriated. The large eggs, discharged 

 from the ovary, are fertilized before they arrive in a slightly differ- 

 entiated uterine region of the duct (always the left?) where they linger 

 while the shell is deposited by the uterine wall. In Echidna the shell has 

 a horny texture, but in the duckbill it is hardened by deposition of 

 calcium. In these two animals no vaginal region is differentiated. The 

 uterine chambers of the right and left oviducts open directly and 

 independently into a cavity into which open also the urinary bladder 

 ventrally and the pair of ureters dorsolaterally. It is, accordingly, 

 a urinogenital sinus, and is the more anterior region of the 

 embryonic cloaca. The sinus continues backward into the definitive 

 cloaca, which is also entered by the more dorsal rectum. These animals, 

 therefore, have only a single median posterior external opening, the 

 cloacal aperture. 



In Echidna a temporary marsupial fold of skin exists during the 

 period of incubating the egg and early development after hatching. 

 In Ornithorhynchus no pouch is formed at any time. The epipubic 

 bones, however, are present in both of these animals. 



Testis and Wolffian Ducts 



The testis, like the ovary, develops in close relation to the meso- 

 nephros but, unlike the ovary, it acquires functional relation with the 

 mesonephros in that the sperm-producing tubules of the testis become 

 connected with tubules of the more anterior part of the mesonephros 

 (Fig. 78), thus providing the testis with an outlet into the meso- 

 nephric (Wolffian) duct, which thus becomes the spermatic duct 

 (vas deferens or ductus deferens). At a later stage of the embryo, 

 the metanephros replaces the mesonephros as functional kidney, but 

 those tubules of the mesonephros which have become functionally 

 necessary to the testis persist as the vasa efferentia connecting the 

 tubules of the testis with the spermatic duct. The vasa efferentia, 

 together with the much-elongated and closely coiled anterior portion 

 of the mesonephric duct, constitutes the epididymis which, in the 

 adult, forms a flattened mass closely joined to the outer surface of the 

 testis. Emerging from the epididymis, the vas deferens continues 

 backward to the terminal urinogenital passage, the urethra (Fig. 492). 

 Just before entering the urethra, each vas deferens widens and its 



