retained. The oviduct in this group secretes albumin and 

 applies a shell to the eggs as in the reptilian antecedents of 

 the mammals. 



The genital tract of the marsupial is similar except that 

 the cloaca is much reduced (Figure 10-36). The urogenital 

 and anal openings, the former below the latter, are con- 

 tained in a shallow cavity closed by a sphincter. The genital 

 system is bipartite. The terminal part of the Miillerian 

 duct, opening into the urogenital sinus, is differentiated into 

 a thin-walled vagina. Each vagina forms an S-shaped loop, 

 coursing first laterally, then posteriorly, then medially and 

 anteriorly. Anteriorly the vaginae usually meet at the mid- 

 line above the bladder and fuse. The vaginae remain sep- 

 arate in Perameles and the wombat and also in some species 

 or individuals of opossums. In the Virginia Opossum or in 

 Dasycercus, fusion occurs only between the posterior parts of 

 the medial limbs, while in the kangaroo and many other 

 species fusion produces a large medial chamber or pouch 

 into which the uteri open. This medial pouch extends for- 

 ward and may contact the urogenital sinus and even open 

 into it as a medial vagina. In some species, such a medial 

 opening is formed only at the time of birth of the young 

 when the young rupture through the walls of the pouch 

 and sinus. A similar event occurs in the wombat where 

 young can rupture from either vaginal loop into the uro- 

 genital sinus. 



The thick-walled uterus continues the duct forward and 

 laterally from the vagina or from the medial vaginal pouch. 

 The duct has a tube section and a fimbriated ostium. 



Among placental mammals there is some variation of 

 structure. Most have a single medial vagina opening to the 

 exterior separately, or nearly separately, from the urethra, 

 the exceptions being the sloth, anteater, armadillo, the ro- 

 dent Pedetes, and the rabbit-like pika. The tubular uteri may 

 open separately into the median vagina as in the rat; this is 

 the duplex condition. The uteri may be fused posteriorly 

 and open together into the vagina, while anteriorly they ex- 

 tend as separate tubes to either side; this is the bipartite type 

 (pig). When the uteri are more extensively fused and have 

 but short lateral extensions, this is the bicornuate type 

 (horse, cat). Complete fusion without distinct horns — with 

 only the thin-walled tubes extending away to either side — 

 identifies the simplex type (man). 



The gonad of the male or female arises from a genital ridge 

 associated with the kidney. This interrelationship in the two 

 sexes and the different kinds of mammals is summarized in 

 Figure 10-37. 



Reptiles and birds 



The male The male reptile has the testis suspended in the 

 body cavity like the primitive mammal (Figure 10-8). In 

 the interior of the testis, the looped, but not anastomosing, 

 seminiferous tubules open into a central net or canal which 

 connects through the efferent tubules with the marginal 



mesonephros 



Mullerian duct 



metanephros 



mesonephros 

 gonad 

 Mullerian duct 



A PRIMITIVE TETRAPOD PRIMITIVE MAMMAL B 



metanephros 

 epididymis^ 



kidney 



mesonephric vestige 



glomeru 



Muller 



nephric duct 



mesonepriros 

 ^^j/^nef 

 ^ Mullerian duct 



HUMAN F 



neural arch 



centrum 



Mullerian duct 



broad ligament 

 vagina and uterus 



HUMAN 



H 



Figure 10-37. Cross sections of o hypothetical primitive tetropod 

 and several kinds of mammals to show the interrelationships between 

 the kidney stages and the reproductive tracts of males and females. 

 (After van den Broek, and Patten, 1946) 



THE REPRODUCTIVE SYSTEM 



323 



