168 T. THOMSON FLYNN 



Further, considerable A'acuolation of the cells of the bilaminar 

 omphalopleure at certain stages mark it as being an absorptive 

 organ of some importance in embryotrophic processes. 



7. Concluding Remarks. 



Now that we have some clear idea of placentation in Pera- 

 m e 1 e s , the question arises what relation exists between this 

 and the placentation of other marsupials and of Eutheria 

 generally ? Further, can any light be thrown on the phylogeny 

 of the mammalian allanto-placenta by a consideration of this 

 question ? 



Before, however, entering on a discussion of these, it is, 

 I think, necessary to get a clear idea of the morpho-physio- 

 logical conception of the term ' Placenta '. 



(a) The Placental Conception. 



It is inevitable that the conception of placentation which 

 has arisen in the minds of investigators should, until fairly 

 recently, have been associated with the very complex and 

 highly advanced structure developed in the intimate fusion or 

 apposition of the foetal membranes of the commoner and best- 

 known Eutheria with the uterine mucosa. 



It might, however, be taken for granted that — in the case 

 of an organ so prominent in mammalian developmental pro- 

 cesses and rightly regarded ontogenetically and phylogeneti- 

 cally as of the highest importance — there would be no two 

 opinions as to its definitive structure or its physiological 

 significance. But such a supposition would be MTong, and 

 a very superficial examination of the \vTitings of the more 

 recent investigators soon shows that they hold radically different 

 views as to what is understood by the term ' placenta *. 



All the important recent works treating of the comparative 

 anatomy of the placenta to which 1 am able to refer (Strahl, 

 1905 ; Grosser, 1910 ; Jenkinson, 1913) are insistent that 

 the fundamental idea of placental formation lies in the apposing 

 of two blood-streams — one foetal, one maternal — to form 



