174 T, THOMSON FLYNN 



physiology, and ontogeny of the foetal membranes of most 

 marsupials. Particularly in such primitive genera as Thyla- 

 c i n u s and S a r c o p h i 1 u s , it may be expected that investiga- 

 tion will help to shed a clear light on the phylogeny of the 

 placenta in this group. 



Another point of importance (of greater value, I think, than 

 Assheton would have had us believe) lies in the behaviour of the 

 uterine mucosa. Of this our knowledge in the marsupials is 

 particularly meagre. Yet it is extremely important, since 

 there is naturally a mutual reaction of embryo and uterus. 

 An investigation of the modifications of the uterine mucosa 

 during pregnancy would, there is not the slightest doubt, be 

 of great value in shedding a light on ancestral placental arrange- 

 ments in marsupials. 



Pseudochirus cooki is instructive in this regard. 

 Preliminary investigations which I have already made in the 

 case of this diprotodent marsupial have shown that the uterine 

 epithelium in a very early stage of pregnancy consists of a single 

 layer of very high columnar cells with correspondingly elongated 

 deeply-staining nuclei. Below the epithelium the connective 

 tissue is condensed to form a layer in which run the capillaries. 

 This stage can be recognized as being very similar to one occur- 

 ring in many E u t h e r i a . 



At a later stage of gestation, cell outlines have disappeared 

 and a vascular syncytium is formed similar to that of Pera- 

 meles, except that it is composed apparently not only of the 

 epithelial cells but of those of the sub-epithelial capillary layer. 

 These capillaries now ramify at the surface as is the case in 

 P e r a m e 1 e s . 



Here without doul^t can be recognized the remains of an 

 ancestral trophospongial proliferation. 



From the consideration of the above facts, particularly as 

 regards the condition of the foetal membranes in Perameles, 

 Dasyurus, and Phascolarctus, bearing in mind the 

 complementary modifications of the uterine wall where they 

 are known, it must be evident that these conditions in marsu- 

 pials represent a degeneration from a more complex system 



