THE PLACENTATION OF PERAMELES. 423 



adlierent to tlie irregular surface of the syucytiuni, while iu 

 otliers it has become separated from the latter (figs. 28 

 aud 29). 



The allantoic cavity (fig. 28^ all. c.) is distinct and con- 

 tinuous, but its entodermal lining is no longer distinguishable. 

 It contains here and there an irregular cellular detritus. The 

 walls of the allantois have altered considerably in character. 

 The inner (coelomic) wall (fig. 28, cce. iv.) is somewhat thicker 

 than the outer (all. mes'.), and is now composed of a dense 

 mesodermal layer, carrying embedded in it in pairs the branches 

 and factors of the allantoic arteries and vein. 



The allanto-chorionic mesenchyme of the outer (placental) 

 wall [all. mes'.) has also become quite compact in appearance. 

 On its outer surface the allantoic capillaries project (figs. 28 

 and 29, all. cap.). They now contain enucleated foetal blood- 

 cells, and can still in places be seen to fit in and adhere to 

 corresponding depressions in the syncytial surface (fig. 29). 



It will be apparent from figs. 28 and 29 that no essential 

 change has taken place in the constitution of the placenta iu 

 the period intervening between Stage D and the time of birth. 



The question whether or not the allantois is resorbed in 

 situ is at once settled positively by the occurrence in the 

 outer allantoic wall, and to a lesser degree in the inner as 

 well, of numbers of polynuclear leucocytes similar to those 

 already described as existing so abundantly in the placental 

 syncytium (figs. 32 — 34, j9. /ewe). 



There is not the least doubt but that these leucocytes 

 migrate from the syncytium into the allantoic walls. In 

 sections the leucocytes are found not only at the surface of the 

 syncytium, but actually in the spaces which exist here and 

 there between the syncytium and the outer allantoic wall, and 

 they are even to be seen just in process of entering the latter. 



Absorption of the allantois has, however, not yet actively 

 begun, still it is breaking up in portions of its extent (cf. 

 fig. 28), and the cellular detritus iu the allantoic cavity can 

 only have arisen through disintegration of its walls. 



And if, as we have seen, the foetal portion of the placenta is 



