STUDIES IN MAMMALIAN EMBRYOLOGY. 511 



trophoblast, as also of the participation of these different 

 regions in the processes of attachment and partly of nutrition 

 of the blastocyst. There now remains for our consideration 

 that yet more important portion of the trophoblast which takes 

 an active part in the formation of the placenta — the allantoi- 

 dean trophoblast. 



Its first appearance as an independent layer is coincident 

 with the formation and completion of the amnion. From the 

 very first (cf. fig, 7) the amnion fold (that can primarily be 

 observed behind the embryo) has an ever so much thicker outer 

 than inner fold. This diflfereuce in thickness must be set down 

 to the account of one of the component cell-layers — the epi- 

 blast, whereas the somatic mesoblast is and remains exceed- 

 ingly thin. I have reason to believe (though I will reserve the 

 consideration of this hypothesis to a later publication) that the 

 growth of the amnion fold is not really a slow turning over of 

 a spread-out layer, but that from the very first — even as long 

 as they are spread out flat — the inner and the outer curve can 

 be said to be distinct. 



In the earliest phase the thickened shield of epiblast is sharply 

 set off against the continuation of the epiblast external to it. 

 It seems to me very probable that from cells derived from 

 the first-named shield the epiblast entering into the inner fold 

 of the amnion is derived. The point of meeting between the 

 inner and the outer fold would then correspond to the rim of the 

 shield in the earliest phases. Inner and outer fold increase 

 simultaneously and at an equal rate. In Sorex they always 

 meet at a sharp angle ; they do not pass into each other along 

 a smooth curve. Be this as it may, the fact remains patent 

 that the outer fold of the amnion is composed of the somatic 

 mesoblast above referred to, and of a thickened layer of epi- 

 blast cells. This layer, as will be seen by reference to figs. 26, 

 27, and 75 — 79, is in many places two and sometimes more 

 cells thick : it is the allantoidean trophoblast. It cannot be 

 strictly defined topographically, as the allantoidean trophoblast 

 passes into the omphaloidean trophoblast quite gradually. 

 Still, the name is convenient for designating that portion of the 



