130 T. THOMSON FLYNN 



Thus it is definite enough, according to Hill's views, that 

 the chorionic ectoderm takes no share in the formation of the 

 placenta proper. 



The completion of the placenta is brought about l)y the allan- 

 toic capillaries coming into intimate relationship with the 

 maternal capillaries ramifying on the surface of the maternal 

 syncytium. 



The above is, I am inclined to think, an accurate precis 

 of Hill's results. His interpretation of the facts and his 

 strongly-expressed opinion that there is nothing of the nature 

 of a fusion between the foetal ectoderm and maternal tissue 

 has given rise to the idea — almost generally accepted — that 

 the placentation of Perameles is of 'a peculiar type not 

 met with anywhere else * (Jenkinson, 1913, p. 216). 



The correctness of Hill's conclusions has been questioned by 

 A. A. W. Hubrecht in his famous essay on ' The Early Onto- 

 genetic Phenomena in Mammals ' (Hubrecht, 1909). 



Hubrecht's criticism :- — 



The opinion of this gifted investigator was arrived at evidently 

 on somewhat theoretical grounds, but he w^as confirmed in his 

 ideas by an examination of material placed at his disposal by 

 Professor Hill. It is important to understand Hubrecht's 

 standpoint thoroughly, because, in my opinion, it is to some 

 extent justified. He believes that the foetal ectoderm, so 

 far from disappearing, penetrates into the maternal syncytium 

 to form a ' mixed syncytium ', corresponding to what Schoen- 

 feld has described (1903) for the dog. ' The Perameles 

 placenta may be said to be a somewhat simpler — because 

 thinner — form of placenta than that of the Carnivora, 

 but at the same time to approach more closely to that type ; 

 whereas amongst the Insectivora, Sorex provides us 

 with an example of a yet more extensive proliferation of the 

 material uterine epithelium before the allantoic attachment 

 of the blastocyst comes about than even Perameles. 

 At all events, the placentation of Perameles, characterized 

 by so intimate a fusion between foetal and maternal elements, 

 should never be classified amongst those forms of placenta 



