STUDIES IN MAMMALIAN EMBRYOLOGY. 513 



Before describing those processes in detail it will be well 

 once more to summarise them. Superficially it would seem as if 

 the phenomena could be thus characterised: — the trophoblastic 

 knobs become inserted in the crypts and vascularised by the 

 allantois, and by further growth and division of all these parts 

 the full-grown placenta comes into existence. Nothing is, how- 

 ever, further from the truth. It can be easily understood that 

 the number of crypts is limited when once the trophoblast has 

 come to be applied against the latter. Thus new crypts and 

 knobs cannot possibly arise in the same way as the original 

 ones after the adhesion of the trophoblast against the maternal 

 surface. 



Neither do I find traces, when once the trophoblastic pro- 

 tuberances have become firmly fastened in the crypts and have 

 received allantoidean villi in their subsequent cavities, of any 

 penetration of trophoblastic tissue into the proliferated 

 maternal tissue between the already existent maternal 

 crypts. 



The positive interpretation of the facts which I desire to sub- 

 stitute for the insufficient hypothetical suggestions just brought 

 forward is the following : — After the maternal crypts have 

 received the plugs of trophoblast in their cavities a destruction 

 of the maternal epithelium as far as it is in contact with the 

 trophoblastic plugs follows. 



Trophoblastic proliferation and histolysis in the surrounding 

 maternal tissue going hand in hand, the firm adhesion of the 

 blastocyst in this placental region is very soon brought about. 



And it is to obtain this degree of very firm connec- 

 tion that the crypts and protuberances have to a great 

 extent served. The trophoblast has, so to say, become 

 anchored by its wart-like protuberances in the maternal pro- 

 liferation, and it is now going to prepare the placenta by its 

 own activity. For this the firm adhesion between trophoblast 

 and mucosa which has now been established is of the highest 

 importance. It is henceforth possible for the trophoblastic 

 tissue to expand into a syncytium of considerable size, and to 

 tap the maternal circulation without any danger of uuservice- 



VOL. 35, PART 4. NEW SER. N N 



