STUDIES IN MAMMALIAN EMBRYOLOGY. 499 



of these changes, and at the same time to trace the detail of 

 the very important interaction which henceforth becomes 

 established between maternal and embryonic growths — 

 (a) In the region of the yolk-sac. 



{b) In the region where the allantois completes its 

 growth. 



As far as the embryo is concerned, it is obvious that these 

 two regions are always very distinct. In the maternal tissue, 

 on the contrary, there is direct continuity between that portion 

 of it which is going to be in contact with the yolk-sac and that 

 which is preparing for the adhesion of the allantois, although 

 certain important differences have been noticed in the preceding 

 paragraph (formation of crypts, &c.). 



The continuity ceases in the later stages of pregnancy, when 

 the yolk-sac is again removed from any contact with the 

 maternal tissue, because then the corresponding portion of the 

 maternal tissue is gradually resorbed and disappears. But 

 even before this final disappearance certain landmarks can be 

 noticed, by which it is possible to distinguish an allantoidean 

 from an omphaloidean region in the uterine wall. As early as 

 the stage of fig. 10, connective tissue is seen to penetrate in 

 wedge shape between the proliferated epithelium cells that 

 shall contribute towards the placenta and those that are in 

 contact with the yolk-sac. This is no full separation of the 

 two regions, but all the same a valuable indication of their 

 extension. 



The actual contact between the outer layer of the blastocyst 

 and the maternal tissue is the first step of a new series of trans- 

 formations. It is a zonary region of the blastocyst that first 

 becomes attached to the maternal proliferation. This zonary re- 

 gion is equatorially situated with respect to the embryonic area; 

 the latter is, moreover, always facing the concavity of the uterus 

 lumen opposite the mesometrium. There is great probability, 

 as far as I can see, that the zonary region here alluded to is 

 already present before the attachment has yet come about. 

 The thickened trophoblast cells that were figured in vol. xxxi, 

 PI. XXXVII, fig. 27, of this Journal, in my paper on the " Ger- 



