ATTACHMENT OF MAMMALIAN EMBRYO TO UTEEUS. 179 



these thickened spots is shown to be on the point of effecting 

 an attachment. It may be noticed to be wedge-shaped in 

 section ; it is a little blunt cone. Practically each pa})illa 

 assumes this shape, and is being pressed against the epithelium 

 lining the uterus. In this figure (and almost any number 

 might be drawn showing the same characters) the papilla 

 certainly looks as though it were piercing the epithelium by 

 reason of the pressure from within the vesicle. 



Of course the actual hydrostatic pressure will be the same at 

 a. as it is at e., but nevertheless a greater pressure will be 

 exerted on the uterine wall at the apices of any knobs on the 

 wall of the vesicle than at the area between them if the uterine 

 wall is in a state of tension, which undoubtedly it is at this 

 time. 



If we consider that in all probability the uterine epi- 

 thelium is a softer material than the muscular and con- 

 nective tissue outside it, it is all the more probable that the 

 softer (if it is softer) uterine epithelium will give way between 

 the two. 



I think it quite possible that this may be the only neces- 

 sary cause, and by this means the " papilla " reaches the 

 capillary system of blood-vessels in the uterine connective 

 tissue, a point of the utmost importance to the welfare of the 

 embryo. 



On the other hand, although I have no doubt that the 

 additional pressure which exists at the points of these knobs is 

 of much importance in that it causes very close contact 

 between the uterine epithelium and parts of the wall of the 

 blastodermic vesicle, yet it is more than possible that the 

 breaking down of the uterine epithelium at those points is 

 not due to mechanical pressure alone, but to a chemical or a 

 physiological process, such as absorption of the uterine cells 

 by the vital activity of the cells of the knobs. But the possi- 

 bility should not, I think, be lost sight of that the first 

 breaking through of the uterine epithelium at these points 

 may be entirely due to mechanical pressure alone. 



A point to which special attention must here be drawn is 



