ATTACHMENT OF MAMMALIAN EMBRYO TO UTERUS. 175 



The cavity of the uterus is thus bounded on the raesometrial 

 side by an almost straight line, while the opposite wall is 

 circular. A body having the shape shown in tig. 4, in passing 

 down the tube by virtue of the continual slow contractions of 

 its walls, could hardly fail to become so fitted as to lie with its 

 flatter surface against the flatter wall of the uterus. 



Since the flatness of one surface of the blastodermic vesicle 

 is caused by the embryonic disc and changes connected there- 

 with being upon that surface, it follows that in the rabbit the 

 embryo always comes to lie up against the mesometrial side of 

 the uterus, which is, under the circumstances, by far the most 

 favorable position for its future development. For in this 

 position it will be less exposed to the tension which must 

 necessarily arise upon the opposite side in the subsequent 

 expansion of the uterus by the accumulation of fluid within 

 the blastodermic vesicle. 



The blastodermic vesicle does not by any means become 

 attached with the centre of its flat surface always exactly 

 adjoining the median cleft between the two placental lobes. 

 It may sometimes be a little to one side or the other, but it is 

 never very far out. 



As the blastodermic vesicle expands, it eventually fills up the 

 whole cavity of that section of the uterus in which it happens 

 to be. With further expansion it necessarily exerts a pressure 

 upon the walls of the uterus, and this pressure is made apparent 

 without by a swelling, or protuberance, occurring upon the 

 side of the uterus away from the mesometrium. 



The first visible sign of the commencement of the formation 

 of the placenta — or, at any rate, of those structures which 

 eventually cause the placenta to come into existence, appears 

 very shortly after the appearance of this swelling. I allude to 

 the papillae and protuberances of epiblast. 



I said just now the first visible sign because I believe that 

 the outgrowths are due really to a continuation of those 

 causes which have given to the blastodermic vesicle its present 

 form and shape, and described in a former paper. 



The immediate cause of the origin of the papillae seems to be 



