134 T. THOMSON FLYNN 



no mention is made of any gland alteration, nor would any such 

 be likely to occur under Hill's conception of a degenerating 

 chorion with a passive syncytium — yet distinct traces of such 

 alteration is evident in this tigure. It will be seen that up to the 

 point where the gland enters the presumable syncytium its 

 epithelium consists of the somewhat low cu])ical cells so charac- 

 teristic of many of the glands of this stage. From this point to 

 the opening of the gland, however, there is abundant evidence 

 of degeneration. Apparently this consists of a syncytialization 

 similar to — though not as marked as — that occurring in the 

 Carnivora. It was the evidence of this phenomenon in my 

 own material which first drew my attention to the possibility 

 of the occiTrrence of a more complex process in Perameles 

 than was described by Hill, and in itself lends sufficient colour 

 to the view I have expressed above that there is something 

 more to be reckoned with in the placentation of this animal 

 than a simple degeneration of the foetal ectoderm. 



Accepting the fact that the trophoblast of the placental area 

 in Perameles proliferates and that the uterine epithelium 

 after a preliminary preplacental extension remains afterwards 

 passive, then we can come to the conclusion that placental 

 phenomena in Perameles can now be brought more or less 

 into line with similar phenomena in the Eutherian mammals. 

 Here, further, I may be allowed to state that I will be able to 

 show by the aid of my own material that the chorionic ectoderm 

 after attachment proceeds to form by proliferation two 

 structures : 



(rt) a plasmodiblast . plasmo ditropho blast , or 

 Plasmodium, 



(6) a c y t o b 1 a s t or c y t r o p h o b 1 a s t , 

 and that Hill failed to recognize the presence of the plasmodi- 

 blast nuclei, the structure which he calls the chorionic ectoderm 

 being really only the cytoblastic portion of that layer. 



8. Material. 

 At my disposal for the examination of the foetal membranes 

 of Perameles, I have two intra-uterine stages both of which 



