770 ANATOMY OF VERTEBRATES. 



adheres. In the new-born Opossum the oral pore is singularly- 

 minute, and the mother's nipple has an obtuse but not expanded 

 termination. 



In the Phascogale, in which the nipples are relatively larger 

 than usual, and of a subcompressed clavate form, the young, 

 when grown too large to be carried in the pouch, are dragged 

 along by the mother, if she be pursued, hanging by the nipples. 

 604 The number of nipples bears re- 



lation to that of the young brought 

 forth at a birth ; although, from the 

 circumstance of the produce of two 

 gestations being for a short time 

 suckled simultaneously, the nipples 

 are never so few. Thus the unipar- 

 ous Kangaroo has four nipples, of 

 which the two anterior are generally 

 those in use : the Petaurists, which 



Nipple, and head of Mammary Foetus, bring forth tWO yOUng at a birth, have 



Kangaroo. a ] g0 f our n jppi es . w hilst the multipar- 



ous Virginian Opossum has thirteen nipples, six on each side and 

 the thirteenth in the middle. In the Didelphys Opossum there are 

 nine nipples, four on each side and one in the middle. The Di- 

 delphys dorsigera has the same number of nipples, although six is 

 the usual number of young at a birth, fig. 605. In the Phasco- 

 gale penicillata there are eight nipples arranged in a circle. The 

 Perameles nasuta has the same number of nipples arranged in two 

 slightly curved longitudinal rows; this Marsupial has three or four 

 young at a birth. In all Marsupials the milk exudes from six to 

 ten minute orifices arranged round the apex. The nipple increases 

 in size with the growth of the mammary foetus appended to it. 



The development of the marsupial pouch is in an inverse ratio 

 to that of the uteri and directly as that of the complicated vaginas : 

 thus it is rudimental in the Dorsigerous Opossum, which has the 

 longest uteri and the simplest vaginae : we may conclude therefore 

 that the young undergo a greater amount of development in the 

 womb in this and allied species ; and here, if in any Marsupials, 

 beginnings of a placental structure may be found. In the Kanga- 

 roos and Potoroos, which have the shortest uteri and longest vaginal 

 tubes and cul-de-sac, the marsupial pouch is wide and deep. It 

 is composed of a duplicature of the integument, of which the ex- 

 ternal fold is supported by longitudinal fasciculi of the panniculus 

 carnosus converging below to be implanted in the symphysis 

 pubis. The mouth of the sac is closed by a strong cutaneous 



