372 



condition of the Träger epithelium, and later forms the basis for tiie 

 origin of the definitive placental discs of advanced embryonic stages. 



During the formation of the Träger a number of significant changes 

 appear in the embryonic region of the vesicle, chief among which is 

 the so-called "inversion of germ layers'' — a process tliat is now known 

 to represent a characteristic phase in the development of certain 

 rodents and in at least two species of the armadillo. In our species 

 of the armadillo germ layer inversion seems to be of a very primitive 

 type, for here the epiblast simply caves into the cavity of the vesicle, 

 carrying before it the layer of hypoblast, and leaving behind the 

 overlying Träger. During this change the margin of the hypoblast 

 extends down the inner side of the trophoblast until one-half of the 

 animal hemisphere of the vesicle is lined by it: but upon the approach 

 of the inversion process the edge of tiie hypoblast becomes united 

 to the inner surface of the trophoblast, and this union forms a fixed 

 point about which the inversion of the layers turns. 



The inversion results in the formation of two secondary incomplete 

 vesicles or sacs, one lying within the other. The innermost of these 

 is the thicker of the two and constitutes the ectodermic sac; the 

 outermost is composed of a single layer of cells (the hypoblast) and 

 is the entodermic sac (Fig. 3). 



The ectodermic sac, which is derived from the epiblast, is at 

 first open on the proximal side, and its cavity communicates directly 

 with the space lying between the Träger and the sac. This opening 

 soon becomes closed and the ectoderm then forms a true vesicle, in 

 which the distal wall is three or four cells thick and the proximal 

 wall but a single cell thick. 



The entodermic sac differs from its fellow in that it never be- 

 comes closed on the proximal side. As already stated, the entoderm 

 becomes united to the trophoblast at a point that separates the Träger 

 portion of the trophoblast from the free portion. 



During the course of further development the free portion of 

 the trophoblast becomes greatly attenuated and folded and eventually 

 sloughs off, leaving the entoderm directly exposed to the uterine 

 cavity. In Fig. 4 is shown a vesicle in which the trophoblast is on 

 the point of sloughing off. In many respects this vesicle is much 

 more advanced than in tlie preceding stage. Undoubtedly the most 

 significant difference is seen in the appearance of the mesoderm, 

 wliicii arises as two bilaterally arranged pouches (M) at the angles 



