. Outlines of zoology. But the bent proximal parts of the vaginse some-times fuse and form a ctecum, which, according to thedegree of fusion, may be a single tube or divided by a CLASSIFICATION OF MARSUPIALS. 745 partition. Moreover, in Bennetts kangaroo, the caecumopens independently into the sinus between the aperturesof the distal portions of the vaginae, and forms the so-calledthird vagina. In Perameles, although such a medianpassage does not exist in the young female, it is formed bya process of rupture at the period of parturition. The truevaginse are apparently too narrow for the passag
. Outlines of zoology. But the bent proximal parts of the vaginse some-times fuse and form a ctecum, which, according to thedegree of fusion, may be a single tube or divided by a CLASSIFICATION OF MARSUPIALS. 745 partition. Moreover, in Bennetts kangaroo, the caecumopens independently into the sinus between the aperturesof the distal portions of the vaginae, and forms the so-calledthird vagina. In Perameles, although such a medianpassage does not exist in the young female, it is formed bya process of rupture at the period of parturition. The truevaginse are apparently too narrow for the passage of theembryos. The allantois in Perameles, as already seen, forms a true allantoicplacenta; in Phascolarctos it fuses with the subzonal membrane,becomes highly vascular, and functions as an embryonic respiratoryorgan, but does not unite with the uterine wall; in all other Mar-supials, so far as is known, it is small, only projects slightly into theextra-embryonic body cavity, and is apparently functionless. Accord-. FlG. 404.—Lower jaw of kangaroo.»,, Inflected angle ; /., single incisor. ing to Hill, the condition seen in Perameles is primitive, and the otherMarsupials show degeneration. The wall of the umbilical vesicle oryolk-sac is highly vascular, and may unite with the uterine wall tc forma yolk-sac placenta. The gestation is short, only lasting a fortnight in theopossum, about five weeks in the kangaroo; whereas thatof the mare, for instance, is about eleven months. Exceptin some opossums, there is a marsupial pouch, usually with aforward-directed aperture. Within this pouch are the teats,and here the delicate young are nurtured after birth. Asthey are unable to suck, the milk is forced down theirthroat, the mammary gland being compressed by thecremaster muscle which covers it. Vague vestiges of amarsupium are said to be visible in some Placentals. Classification of Marsupials.—The Marsupials are dividedinto two sub-orders, each of which contains four families. Th
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
