436 JAS. P. HILL. 



plification, to say the least, not wholly improbable. Thus, iu 

 our view, it is unnecessary to trace the placental ancestry of 

 Eutheria back into the marsupial group. The occurrence 

 there of a true allantoic placenta,, and its absence in the 

 majority of members of the order, do no doubt, at first sight^ 

 suggest that in this group we must find the first beginnings of 

 the organ. But we believe that the explanation is to be found 

 in the fact that marsupials are, after all, a markedly specialised 

 group, and that in it conditions have obtained producing 

 placental disappearance, just as conditions (probably identical 

 in character) have determined the degeneration of other early 

 nutritional arrangements, i. e. the milk-teeth. We therefore 

 fall back upon the view that the Metatheria and Eutheria are 

 the divergent branches of a common ancestral stock, which 

 was not only diphyodont but also placental. 



We may next inquire whether the facts and conclusions 

 detailed in the present paper have any bearing upon the 

 question of the condition of the foetal membranes in these 

 primitive Placentalia. 



We believe that the facts of placentation in Perameles 

 most strikingly confirm and support the opinion of Balfour 

 (19), that in the primitive types of Placentalia both the allan- 

 toic and yolk-sac vessels may have been concerned in main- 

 taining a placental circulation. We have insisted on the 

 fundamental similarity of the placentation of Perameles with 

 that of the more generalised Eutheria ; and if we select Eri- 

 naceus as representing a fairly generalised Eutherian type, we 

 find that here, according to Hubrecht's account (9), just as in 

 Perameles, an extensive yolk-sac (omphalopleural) placental 

 connection is developed at an early stage, only to be replaced 

 later by the formation of the definitive allantoic placenta, 

 through the union of a large vesicular and vascular allantois 

 with the non-vascular chorion. Now if we leave aside the 

 trophoblastic differentiation in the one and the formation of a 

 maternal syncytium in the other, the type of placentation 

 occurring in these two generalised Metatherian and Eutherian 

 forms is an essentially similar one. This fact can in our 



