PLACENTA IN PERAMELES 145 



a layer bounding a central space. This is indicated 

 in j&gs. 1, 2, and 3, whilst in other figures, 4, 7, and 9, I have 

 drawn attention to the fact that the trophoblastic proliferations 

 bear a definite relation to the groups. Particularly in the next 

 stage, it will be seen that the foetal nuclei invade and fill the 

 nest with consequent more or less complete disappearance of 

 the maternal nuclei. Under these circumstances it does not 

 require any extraordinary stretch of imagination to recognize 

 in this highly characteristic and important phenomenon the 

 remains of a much more elaborate system of placental forma- 

 tion. The conclusion is certainly obvious to me that here in 

 Perameles, in the formation of the peculiar syncytial 

 groups, there is to be recognized an abortive attempt at the 

 formation of crypts such as occur in the placental area of 

 Sorex, and further, while in the latter crypt-building is con- 

 fined to the placental area, in Perameles the comparable 

 phenomenon occurs at all points of the uterine epithelium, 

 although in a lesser degree opposite to the omphalopleure 

 than in the placental area. 



(h) General Ee marks on the Fixation of 

 the Embryo. 



Here it will be convenient to interpolate a few remarks on 

 the method of fixation of the blastocyst and on the general terms 

 used to express the nature of the structures taking part in it. 



Fixation is brought about in Perameles as in others by 

 the junction of a circumscribed portion of the trophoblast, 

 the chorion, with a corresponding area of preplacentally 

 proliferated maternal tissue, the trophospongia. 



There is a fundamental difference in the character of the 

 two uniting layers — the foetal being an active, the maternal 

 a quite passive layer. To a mobile, virile formation of the 

 former type — of foetal origin — the general term Plasmo- 

 dium is applied, while the corresponding multinucleate 

 structure, of maternal origin, usually acting as a pabulum 

 for the foetal plasmodium, is known as a syncytium (see 

 Schoenfeld, 1903). 



NO. 265 L 



