130 PRIMITIVE ANIMALS 



at any rate succeed in getting into communication 

 with the maternal tissues, whereas this only happens 

 in a single case. 



The evidence is conclusive that the Marsupials are 

 descended from a form of Mammal which had a func- 

 tional allantoic placenta, but whether this placenta 

 was ever very much more highly developed than it is 

 in Perameles remains open to question. 



At birth we have stated that the small and little- 

 developed foetus is transferred by the mother to her 

 marsupium and affixed to one of the nipples there. 

 The question arises as to whether the possession of a 

 marsupial pouch is a primitive characteristic of the 

 early Mammalia or whether it is a special acquisition 

 of the Marsupialia. We may answer with confidence 

 that this is a primitive character, firstly because a 

 marsupium is possessed by the Monotreme Echidna, 

 and secondly in the young of a great many of the 

 Eutherian Mammals two ridges are developed on 

 either side of the mammary glands representing the 

 rudiment of a marsupial pouch which subsequently 

 disappears. 



The young of the Marsupials spends a very long 

 time in the pouch, and it is characteristic of this class 

 of Mammals that the suckling period is much more 

 prolonged than in the higher Eutheria. In connection 

 with this lengthened suckling period a peculiarity in 

 the development of the milk-teeth is to be noticed. 



