THE PLAOENTATION OF PEEAMBLES. 433 



Peraraeles has iu no whit served to weaken our previously ex- 

 pressed opinion. 



In view of the present very incomplete state of our know- 

 ledge regarding the condition of the foetal membranes in other 

 Australian polyprotodont Marsupials, especially in Dasyurus 

 and Myrraecobius, and even of the precise uterine changes in 

 Phascolarctus and other Diprotodonts whose foetal membranes 

 have been examined, it is impossible to decide finally between 

 these two views, which alone seem to us worthy of considera- 

 tion. In the concluding section of the paper just referred to 

 we presented in brief form a case in favour of the second 

 alternative. And in this summing up we dealt with the bear- 

 ings upon the case of facts relating to the dentition, the 

 placentation, and the mammary function. Here it is pro- 

 posed rather to treat shortly of those facts and considerations 

 which, in the opinion of the writer, tend to negative the first 

 alternative. 



From the preceding account of the placentation phenomena 

 in Perameles I think we may justly conclude that the pro- 

 cesses of utero-gestation in that form are fundamentally the 

 same as those occurring in the more generalised Eutherians. 

 Such differences in detail as exist are, in my opinion, to be 

 regarded either as evidences of primitiveness of type on the 

 part of the Perameles placenta, or as physiological adaptations 

 such as Hubrecht has pointed out we may expect to find in 

 different types of placentation, in view of '' the great youth of 

 the placenta as compared with the other chief components of 

 the organisation of a mammal" (9, p. 388). 



I wish here more especially to lay emphasis on my convic- 

 tion that it is just as impossible to draw a hard and fast line 

 between the placentation phenomena as they occur in Pera- 

 meles and in the lower Eutherians, as it is to arbitrarily 

 mark off from each other the various types of placental forma- 

 tion occurring among the Eutheria themselves. 



Now it seems on a priori grounds exceedingly improbable 

 that an allantoic placenta should have been twice indepen- 

 dently acquired, and in such a fundamentally similar manner 



