theory existed already in Tyson's time, and was discussed by him. But to-day we have a more correct knowledge of their mode of reproduction, which so long remained clouded with mystery.
An animal born so premature as the little opossum must necessarily perish from exposure, were it not for the curious provision for its protection and the constant supply of milk afforded in the pouch of the female. The internal cavity of the adult female marsupium seems to be formed by an infolding of the external skin. From its opening on the median line of the abdomen the pouch extends backward and laterally, forming a kind of bilateral pocket. From the posterior wall of this about thirteen teats project. To these the young are attached after birth. The two so-called marsupial bones are found in both the male and female Virginian opossums, as well as in some of the South American opossums, which have only a rudimentary pouch, and the monotremes, which have no pouch at all. The investigations of Prof. Owen have shown that these bones are no essential part of the marsupium, although formerly regarded as such; they attach to the anterior border of the pelvis and lie against the mammary glands, where the cremaster muscle winds around them, and makes them act to compress the glands and force out the milk into the throats of the young, which at first seem too feeble to suck.
The young opossums are born as almost helpless little bodies, with mouth and fore-limbs well developed. The transfer of the embryo from the uterus to the pouch has not been observed, but this must be done as with the kangaroo, where it is believed that the mother takes each new-born embryo between her lips and places it upon one of the nipples, which it grasps firm with its mouth and the claws of its fore-feet. Immediately after birth, the young opossums are found hanging upon the mammary glands fixed in the above manner, each with the hind part of its body free and pendant. At first, the mouth is a transverse, gaping fissure; but, when attached to the nipple, its corners soon grow up, leaving only a small, round pore surrounding the neck of the teat, which enlarges, so that the suckling cannot let go nor fall off, but hangs on without any exertion. Each of the largest fœtal specimens (Fig. 3) I have examined was covered with scattered hairs. The nose was large and blunt, unlike that of the adult. These measured, from the tip of the nose to the ear, 17 millimetres; from the ear to the base of the tail, 39 millimetres; length of the tail, 20 millimetres. Those of the second size (Fig. 4, A) were much smaller, and, in general appearance, looked more like opossums than the next larger size. Perhaps they were of a different species. These were, from the tip of the nose to the ear, 8 millimetres; from the ear to the base of the tail, 27 millimetres; length of the tail, 10 millimetres. The other specimens formed a very good series down to those of the smallest size, which were taken from the uterus. These smallest specimens (Figs. 5, 6) measured, from the tip of the nose to the ear,
