PLACENTA IN PERAMELES 177 



in which there is no union between the trophoblast and mucosa 

 in the allantoic placental region, must have reached this condi- 

 tion, in the course of their phylogeny, by a process of secondary 

 simplification. 



It is evident enough then that the attempt made by Strahl 

 (1906) to group Perameles with the Ungulates and others 

 in the Semiplacenta breaks down. At the same time it is 

 not easy to suggest any arrangement by which Perameles 

 will take its proper place in placental classification. 



The only possible course at this stage appears to be to 

 examine briefly what is the relation of the allantoic placentation 

 in Perameles to some one or other of the groupings at 

 present in use. 



Assheton's suggestion to divide the P la cent alia into 

 P la cent alia cumulata and plicata seems to be the 

 most promising, since in addition to having a structural basis 

 these divisions are to some extent physiological. Assheton has 

 given a table of the characteristics of the two groups as he 

 conceived them (1909), and even a cursory glance at these will 

 indicate that the allantoplacenta of Perameles structurally 

 and physiologically occupies a place somewhere between the 

 two but more primitive than either. 



Thus in the ' heaping up ' of the trophoblast — a very funda- 

 mental point — it agrees with the cumulate type, while in many 

 other features, absence of lacunae and the mildness of attack 

 on maternal tissues, it approaches the plicate type. The 

 secretion of the placental glands appears to be at first of minor 

 importance in Perameles, but, later, absorption by the 

 allantoic vessels is direct, increasing the value to the embryo 

 of glandular secretion tremendously. 



It is apparent that if we regard the above grouping as a 

 rational one, the allantoic placenta can be regarded as being 

 of a central primitive type from which development in either 

 direction might easily have proceeded. 



And the very real relation of the allantoplacentation in 

 Perameles to that of the Carnivora — the relation of the 

 simple to the slightl}^ more complex — shows that the Carni- 



NO. 265 N 



