. Chordate morphology. Morphology (Animals); Chordata. 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
. Chordate morphology. Morphology (Animals); Chordata. 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; thi
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