OPOSSUMS AND THEIR YOUNG. 155 



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 I'epro- 

 duction, which so long- remained clouded with mystery. 



An animal born so premature as the little opossum must neces- 

 sarily 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 tlie median line of the abdomen the pouch extends back- 

 ward 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 oj^ossums, Avhich have only a rudi- 

 mentary pouch, and the monotremes, Avhich 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 embi-yo 

 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 firmW Avith 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, Avhen 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 

 fcEtal 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. Tiiese smallest 

 specimens (Figs. 5, 6) measured, from the tip of the nose to the ear. 



