38o OSBORN: [Vol. I. 



uterine wall, while the remaining surface of the chorion lies 

 nearer to the centre of the uterus, but is, nevertheless, in close 

 contact with the folds of the uterine walls (see Fig. i, ^^ 7itJ^). 

 There is one fact which seems, however, to demonstrate that the 

 entire nutritive and respiratory function at this advanced period 

 devolves upon the yolk-sac placenta; this is, that in some 

 cases, where two or three healthy embryos are found united 

 by the fusion of the chorions, the yolk-sac placentas are free 

 and intact, while the entire remaining membrane surfaces of the 

 three embryos are tightly twisted into a single cord, in which 

 only the large vessels of the vascular discs can be distinguished. 

 In other cases, where such coalescence has not taken place, the 

 allantoic region of the chorion is often widely extended. 



The arrangement of the attaching cells over the vascular area 

 and of the flattened subzonal cells over the non-vascular area 

 in Didclphys is directly the reverse of what obtains in PJiasco- 

 larctos and Halmatiirits, as observed by Caldwell, and renders 

 it probable that other marsupials will show still further variations 

 in the growth of the yolk-sac placentae, while they may retain 

 in common the area of attaching cells over some portion of the 

 yolk-sac. The disposition of the blood-vessels in the fold of 

 the uterus, as represented in Fig. 4, indicates that the attaching 

 cells have functional relations directly with the maternal circula- 

 tion as well as indirectly through the medium of the utricular 

 glands. However, having once been near the fire of inferential 

 biology, I will leave the question of the physiology of the yolk- 

 sac placenta in the able hands of Selenka. 



