THE MARSUPIALS 



OR POUCH-BEARING MAMMALS 



(MARSUPIALIA). 



Non-placental mammals, with free digits bearing nails or claws. The young are born in a very imperfect condition, 

 and complete their development attached to teats situated in an external abdominal pouch (marsupium) 

 supported by two special bones (marsupial bones] attached to the pelvis. The dentition is usually complete 

 but permanent, except in the case of a single premolar, which is shed and renewed. The lower angle of the 

 lower jaw behind is turned inwards. 



The marsupials form along with the next 

 order, the monotremes, a separate group of 

 mammals, distinguished by characteristic fea- 

 tures in the mode of reproduction. All other 

 mammals produce in the ovary eggs of mi- 

 croscopic size, which, after leaving the ovary, 

 get lodged in the uterus or womb, with the 

 aid of a separate organ called the placenta, 

 formed partly from the egg itself and partly 

 from the walls of the uterus. By the rapid 

 circulation of the blood in this organ the 

 necessary materials for the growth of the 

 embryo are supplied to the egg, and it is 

 only by the constant exchange between the 

 blood of the mother and that of the embryo 

 that the development of the latter is rendered 

 possible. Now nothing of this kind takes 

 place in the two orders mentioned, which 

 have no placenta and are hence called non- 

 placental mammals. As in the Amphibia, 

 the eggs in these orders contain in them- 

 selves all the materials requisite for the de- 

 velopment of the embryo, and the latter does 

 not enter into relations with the organs of 

 the mother, but receives further supplies of 

 the materials required for the growth of the 

 embryo only from the fluids contained in the 



maternal organs. Among the marsupials 

 this embryo, which always lies completely 

 free in the uterus, is extruded in a very 

 undeveloped condition, but yet in a condition 

 so far advanced that it is able to obtain 

 further nutriment for itself by sucking the 

 teats of its mother, which secretes a very 

 abundant supply of milk. The small size of 

 these embryos at birth is something extra- 

 ordinary. The young of the great kangaroo, 

 which when full-grown is as large as a man, 

 is even less than an inch in size at the time 

 of its birth. In the case of the monotremes 

 a perfect egg with a thin shell is extruded. 



While, on the one hand, the relations 

 between the young and the mother are not 

 the same in the marsupials and monotremes, 

 there are also considerable differences in the 

 further development of the embryo. The 

 monotremes possess, indeed, separate marsu- 

 pial bones, but there is formed for the egg 

 and the embryo which proceeds from it only 

 a temporary brood-pouch, into which the 

 milk-glands open directly without teats; in 

 the marsupials, on the other hand, we find 

 for the most part numerous and often very 

 long teats, these teats being situated on the 



