136 T, THOMSON FLYNN 



Hubrecht's term ' tropliohlast ' for the outer ectodennal layer 

 of the inammahaii lilastocyst without, howe\er. associating 

 it with the more recent theoretical and hypothetical meaning 

 with which Hubrecht has invested it. 



I shall also use Minot's expression ' trophoderm ' for that 

 portion of the trophoblast which proliferates and enters into 

 relationship with the maternal epithelium. As will be seen 

 later, this, in Peranieles, undergoes changes comparable 

 to those occurring in the E u t h e r i a . Similarly we find 

 a complementary structure, the maternal ' trophospongia ', used 

 to indicate (Hul)recht, 1909) ' maternal cell proliferation, 

 specially intended for tlie tixation of the blastocyst '. 



5. Desckiptive Account of Mateiual. 

 Stage 1. Perameles obesula, 6-1 mm. 



This is perhaps the most important stage which has yet 

 been examined in the placentation of Perameles. Its 

 investigation definitely shows the fundamental connexion 

 between the placentation of this animal and that of the 

 E u t h e r i a . 



The specimen of Perameles obesula, on the examina- 

 tion of which the following account is based, was trapped by 

 myself some miles from a small town in the Tasmanian midlands. 

 It had apparently been dead in the trap for about an hour, but 

 no sign of post-mortem change was to be detected. It w^as 

 dissected on the spot and both uteri w^ere found to be swollen. 

 Conditions at the time made any dissection of the uteri inadvis- 

 able. They were opened slightly and placed in fixing solution 

 (Hill's fluid). Later detailed examination showed that the 

 uterus of the right side was pregnant, but. unfortunately, 

 the delicate foetal membranes had been somewhat damaged. 

 The general condition was that the chorion was already 

 attached to the uterine syncytium over a small area, but that 

 the allantois had not yet come into relation with the conjoint 

 layer so formed. 



Pregnant U t e r u s .— This was found to contain an 



