STUDIES IN MAMMALIAN EMBRYOLOGY. 505 



the stages of figs. 10 — 13 it has become much more important; 

 there, is a distinct blood-clot in which the separate blood-cor- 

 puscles can generally yet be traced in the sections, pressed 

 between the trophoblastic annulus and the uterine wall 

 (fig. 84). This blood-clot marks the lower end of what will 

 speedily become a tongue-shaped projection in the sections 

 (fig. 88), but which, in fact, is a thin cylindrical shred of de- 

 generating tissue, and which represents the remnants of the 

 proliferated omphaloidean region of the mucosa. Simultane- 

 ously with the resorption of these parts the placenta increases 

 in circumference. The trophoblastic annulus thus apparently 

 comes to be situated higher and higher up (figs. 11 and 12). 

 The area vasculosa is lifted ofi" from the surface with which it 

 has entertained relations of exchange, but which is now no 

 longer available for that purpose. 



Finally (fig. 13, uterus No. 130) the circumference of the 

 placentary region has so considerably increased that the tropho- 

 blastic annulus, as has already been indicated, is now connected 

 with the inner rim of the placenta by the same strip of tropho- 

 blast, and that could be noticed in its earliest stages on figs. 10 

 — 12, 48, 50, and 84, and that has now grown out to a mem- 

 brane. As such it has more and more asserted itself, its first 

 phases being figured as noticed above, its final stages being 

 indicated in figs. 13 — 15, and more considerably enlarged in 

 figs. 33 and 34. 



The last remnants of the omphaloidean trophoblast are visible 

 in the stage of fig. 13, though not indicated in that figure. 

 We find it in this stage as a mass of semi-resorbed cell-remains 

 mixed up with blood and nuclear detritus, and pressed in 

 between the outer surface of the trophoblastic annulus and 

 the outer rim of the placenta. In the stage of figs. 14 and 15 

 no further remains were present. The resorption has here 

 become final, and the allantoidean regions of mucosa and tro- 

 phoblast are now not only predominant, but entirely monopolise 

 the nutrition of the blastocyst. 



We are now enabled more clearly to picture to ourselves 

 what part the trophoblastic annulus has to play in all this. 



