Hardly less striking in appearance than the pouch are the pair of flat rod-like bones called the epipubic or marsupial bones, which lie in the deepest part of the muscular abdominal wall and are attached to the front or pubic border of the pelvis. These mar supial bones are present in both sexes and therefore are not ex clusively related to the support of the female's pouch but seem rather to be remnants of the skeletal floor of the abdomen, which in primitive reptiles extends from the pubis to the sternum. Similar epipubic bones are found in the monotremes or egg-laying mammals. In some marsupials (including the thylacine or mar supial wolf) the epipubic bones are reduced to vestiges or com pletely lost.
At least in the larger marsupials the new-born animals are very small and in general poorly developed but it is remarkable that at this stage their fore-limbs are very large and tipped with strong claws, while the hind legs are hardly more than embryonic buds. In the case of the Virginia opossum, immediately after birth the young animal uses these large fore-limbs and claws in climbing along the under-side of the mother's body from the cloaca to the mammary field, where it attaches itself to one of the teats. The teat then swells and becomes fastened firmly in the mouth of the young; the windpipe of the young becomes prolonged upward and forward to fit into the back part of the nasal tunnel; thus the milk is prevented from getting into the windpipe and the air is delivered directly to the lungs. The teats vary in number from two in certain diprotodonts to 27 in Peramys.
The large size of the maternal teat and the prolonged attach ment of the young to it may have crowded the tooth-forming part of the jaws to such an extent that the milk teeth are delayed in formation, only the hinder upper and lower premolars, at the back of the jaw, having room to develop in most marsupials, while even these milk teeth become reduced in size in the marsupial wolf (Thylacinus) and completely eliminated in the dasyures, wom bats, koalas and marsupial anteaters. In this connection it is inter esting to note that tiny vestiges of deciduous incisors and pre molars, which never break through the gum, have been found in some marsupials.
At birth the young of the large forms are about an inch in length, entirely naked, blind, the ears hardly visible, the hind limbs small and the fore limbs more robust and with well developed claws. The period of gestation in large kangaroos is about three weeks. At parturition the female sits with her tail brought forward between her legs and spends some of her time scratching at her pouch with the fore-paws and licking it. When the young animal emerges from the cloaca of the female it climbs by its clawed fore-limbs into the pouch, reaching the teats, one of which it eventually seizes with its mouth, the more easily as the teats at this time are pointed and rather turgid.
Shortly after the teat is received in the mouth of the young animal, lactation begins and the teat becomes somewhat flaccid. The body of the little kangaroo increases in size but the mouth that holds the teat does not at this time enlarge. The tip of the teat expands within the mouth so that the young cannot be released without rupturing the sides of its mouth. Numerous striated muscle fibres pass between the lobes of the mammary gland, indi cating that the milk is forcibly expressed from it.
By the end of the Australian winter, September or October, the young kangaroo has grown considerably and is ready to leave the pouch. For some time before finally leaving the pouch it has been free from the teat and has eaten vegetation that it reached by leaning from the pouch while its mother was feeding.
The female reproductive organs of marsupials differ markedly from those of higher or placental mammals in the following respects : the vaginal tubes of the right and left sides always have at least a kink or sharp bend, one on each side, just above the place where the ureters pass downward to open into the bladder. These kinks usually join in the mid-line and from the point of junction is developed a cul-de-sac of varying lengths, leading back ward toward the cloaca or short passage to the exterior. In females that have never produced young the median cul-de-sac ends blindly at the lower end. After the egg has passed down from the ovary into the oviduct it is fertilized by some of the sperm which has been kept in the vaginal caecum or kink above de scribed. During parturition (in members of several families) the embryo passes down into the median cul-de-sac, which becomes prolonged backward, forming a median canal. A hole is formed in the median canal and the young escapes into the cloaca without going through the lateral canals. Thus the typical marsupials have a contorted vaginal canal with one median and two lateral passages, the median one being formed from the coalescence of part of the two laterals. In placental mammals, on the other hand, no traces of the three-way vaginal canal or of the kinks in the vaginal canals are ever formed but the lower part of the right and left oviducts tend to unite in a single median uterus. It is therefore inferred that the marsupial arrangement is a peculiar specialization not developed in the common ancestors of marsupials and placentals. Another remarkable feature of the reproductive system of mar supials is the peculiar relation of the two sacks or outgrowths from the ventral wall of the embryo, either one of which forms the functional placenta or organ of contact of the embryo with the uterus. In certain marsupials a double contact with the uterine wall is formed both by the true or allantoic placenta and by the omphalopleure or yolk-sack wall. In the bandicoots the true or allantoic placenta forms an intimate relationship with the uterine wall, essentially like that in the true placental mammals. On the other hand, in Dasyurus the allantois is degenerate and the yolk sack alone makes contact with the uterine wall. Essentially similar conditions are found in the American opossum (Didelp/iys). In the higher placental mammals, on the other hand, only the true placenta is of functional importance in supplying the growing embryo with maternal nutriment. Thus the marsupials specialized in the early and brief internal development of the embryo; which depends for food chiefly upon its own yolk-sack and which com pletes its development after birth while attached to the teat. The higher or placental mammals gave the young a longer and better uterine development and a more flexible system of nursing, with greater maternal responsibility.