394 JAS. P. HILL. 



Hubrecht's account, we have to do with a proliferation of cells 

 from the under surface of the uterine epithelium. These pro- 

 liferated cells eventually form crypts, between which vessels 

 penetrate. The crypts, however, play only a temporary 

 role in the formation of the placenta, and take no ultimate 

 part in its development. 



In Perameles, on the other hand, we have to do not with a 

 proliferation of cells, but of nuclei in a continuous syncytial 

 layer ; and what is more important is the fact that here this 

 transformed epithelium persists to form the maternal portion 

 of the functional placenta. 



Stages C and D. 



General Account of the Fcetal Membranes. 



Before proceeding to the detailed consideration of Stages 

 C and D, it is advisable here to give a general account of the 

 foetal membranes, so far as they can be made out from these 

 two stages. 



In Peraraeles the fcetal membranes have the same general 

 arrangement as in Phascoiarctus, my two stages exhibiting 

 characters corresponding to the stages described and figured 

 by Caldwell (12) and Semon (8). 



Owing to the mode of growth and the development of an 

 exceedingly voluminous proamnion the embryo is found, at 

 the stage when the amnion is complete, sunk down into the 

 cavity of the yolk-sac, and partially surrounded by the upper 

 portion of the yolk-sac wall (text fig., y. $pl.), which is thus 

 invaginated into the yolk-sac cavity. Semon distinguishes 

 this invaginated portion of the yolk-sac wall (or briefly 

 " yolk-sac splanchnopleure ") simply as '' inneres Blatt." 



The space in which the embryo, enclosed in its amnion, lies 

 is, of course, the extra-embryonic splanchnocoele (text fig., 

 COS.), and is closed externally by a discoidal area of true chorion.^ 

 It is with this discoidal area that the allantois fuses, and over 

 it the allantoic placental connection is eventually established. 



1 We use the term chorion here in the sense specified by Minot (' Human 

 Embryology,' p. 286), viz. the true chorion is that part of the extra-embryonic 

 somatopleure which remains after separation of the amnion. 



