172 T. THOMSON FLYNN 



Perameles between the troplioblast and the uterine 

 wall, and there is no absorption by means of foetal blood- 

 vessels. 



The work of this phase is carried on in later intra-uterine 

 life by the bilaminar omphalopleure. 



Intermediate Phase. — This is the stage of the vascular 

 yolk-sac placenta. It conies into being with the functional 

 formation of the vascular area. Inhere is a close apposition 

 between the foetal and maternal blood-vessels. This phase 

 reaches its most active condition before the attachment of the 

 allantois, and although, maybe, less efficient, endures, with the 

 existence of the vascular area, until the end of pregnancy. 



Final Phase. — The allantoic attachment takes place and 

 the allantoic placenta is completed. 



It is evident that in marsupials, with the exception of 

 Perameles, the preliminary and intermediate stages are the 

 more important, in fact the only ones present, while in general, 

 in Eutheria, the preliminary and the final phases are of 

 the greater value. 



Under these circumstances we can denote the placental 

 periods in Perameles as metricoplacental, omphaloplacental, 

 or allantoplacental, according to the type of placenta which is 

 the dominant one for the period concerned. 



(c) Placental Phenomena in Marsupials 

 g e n e r a 1 1 y . 



In reviewing these I will commence with the most specialized 

 groups. 



M a c r o p o d i d a e . — The works of Owen (1884-7, M a c r o - 

 pus major), Semon (1894, Aeprymnus rufescens), 

 and Hill (1895, M . p a r m a , M . r u f i c o 1 1 i s , M . r o b u s t u s , 

 and M. major) emphatically show that in these forms the 

 allantois throughout life remains small, buried in the splanchno- 

 coele. From my own observations I am able to state that 

 this is also the case for P o t o r o u s t r i d a c t y 1 u s and 

 Bettongia cuni cuius. It is possible that in some 

 Macropods the allantois reaches the chorion, although Cald- 



