PLACENTA IN PERAMELES 171 



E u t li e r i a , and it is due to the fact that the most detailed 

 investigation has been expended on this group and to the 

 prominence of the allantoic placenta in it that other features 

 of embryonal intra-uterine life have been for so long over- 

 looked. 



Viewed in the light of what we already know of the morpho- 

 logy and physiology of the foetal memln-anes in the two groups 

 of viviparous mammals, it is evident that placentation, as 

 generally understood, is but part of a much larger conception 

 which has to do with the whole physiological intimacy, during 

 intra-uterine life, between foetus and mother. 



Thus I am fully in accord with Assheton's suggestion (1909) 

 that the term ' placenta ' should be applied to all organs 

 consisting of an intimate apposition or fusion of the foetal 

 membranes with the uterine wall for the purpose of carrying 

 out physiological processes destined for the well-being of the 

 embryo. 



Such a conceptioii would include tlie following types of 

 placenta : 



(a) That in which the trophoblast is vascularized from the 

 allantois — a 1 1 a. n t o p 1 a c e n t a . 



(h) That in which the trophoblast is vascularized from the 

 yolk-sac — o m p h a 1 o p 1 a c e n t a . 



(c) That in which no foetal blood-vessels are concerned. 

 This is the case of the bilaminar omphalopleure of marsupials 

 whether there is a fusion of part of this with the uterine mucosa 

 (Dasj'urus, Phascolarctos), or merely, as is more usual, intimate 

 apposition. For this type of placenta I propose the term 

 ' metrioplacenta '. These may be illustrated by referring 

 to Perameles, the genus which is the subject of investiga- 

 tion in the present paper. 



(h) Placental Phases in Perameles. 



Preliminary Phase. — During this period the blasto- 

 cyst is formed and the physiological processes are carried on 

 by means of the trophoblastic cells. There is no union in 



