288 



ORDERS OF MAMMALIA. 



so-called " marsupial hones," attached to the hrim of the pelvis. 

 The corpus callosum is very small, and has been asserted to he 

 absent. The young Marsup)ials are born in a very imperfect 

 condition, of very small size, and at a stage v:hcn their develop- 

 ment has proceeded to a very limited degree only. There is no 

 placenta or vascular communication between the mother and 

 foetus, parturition taking place before any necessity arises for 

 such an arrangement. As the young are born in such an im- 

 perfect state of development, special arrangements are required 

 to secure their existence. When born, they are therefore, in the 

 great majority of cases, transferred by the mother to a peculiar 

 pouch formed by a folding of the integument of the abdomen. 

 This pouch is hiotvn as the " marsupiiim," and gives the name 

 to the order. Within the marswpium are contained the nip>plcs, 

 ivhich are of great length. Being for some time after their 

 birth extremely feeble, and unable to perform the act of 

 suction, the young within the pouch are nourished invol- 

 untarily, the mammary glands being 

 provided with special muscles which 

 force the milk into the mouths of 

 the young. At a later stage the 

 young can suckle by their own ex- 

 ertions, and they leave the pouch 

 and return to it at will. In a few 

 forms there is no complete marsu- 

 pium as above described ; but the 

 structure of the nipples is the same, 

 and the young are carried about by 

 the mother, adhering to the lengthy 

 teats. 



The so-called " marsupial bones " 

 (fig. 598) doubtless serve to sup- 

 port the marsupial pouch and its 

 contained young, but this cannot be 

 their sole function, since they occur 

 in the Monotremes, in which there 

 is no pouch. They consist of two small bones, which 

 spring from the brim of the pelvis, and which are merely 

 ossifications of the internal tendons of the " external oblique " 

 muscles of the abdomen. 



Fig. 598.— One side of the pel- 

 vis of a Kangaroo, showing the 

 " marsupial bones " (m). After 

 Owen. 



