386 H. H. Newman and J. T. Patterson. 



vesicle develops the Trager assumes a saucer-shaped form, as 

 seen in vesicles 10 and 18 (figs. 12 and 14). 



It will have been noted that, owing to the inversion of germ 

 layers, the whole yolk-sac region of the vesicle is covered exter- 

 nally with entoderm, and that the trophoblast layer of this region, 

 which in species with a diffuse placenta ultimately forms the outer 

 lining of the villi, has practically disappeared. In the Trager 

 region, however, the original trophoblastic epithelium persists 

 in a somewhat modified form. This region of the vesicle consists 

 of an inner layer of mesoderm, at this time rather thin and free of 

 blood vessels, and an outer trophoblastic layer of true epithelial 

 character, from the surface of which protrude branching and anas- 

 tomosing cords of trophoblast tissue, which give to the Trager a 

 characteristic rough or ridged appearance (fig. 12). These cords 

 of cells appear to function at first as adhesive pads in that they 

 no doubt serve to give the vesicle a firmer grip upon the uterine 

 wall. 



In the primitive streak stage these Trager cords, when exam- 

 ined histologically, show themselves to be composed of solid 

 masses of cells with large nuclei and deeply staining cytoplasm, 

 surrounded by a rather flattened layer of epithelium continuous 

 with that covering the general surface of the Trager. Mitotic 

 figures are of frequent occurrence among the cord cells, showing 

 rapid cell proliferation. In spme respects the appearance of the 

 tissue suggest a glandular function, and it may well be that from 

 it a secretion is given off which subsequently facilitates the pene- 

 tration of the villi into the uterine mucosa. That these cords 

 of cells are of trophoblastic origin seems certain, for the meso- 

 derm, the only other layer in this region of the vesicle, is a thin 

 membrane entirely separate from the trophoblast, which at this 

 period it has not begun to invade. The Trager cords then must 

 be formed by a process of rapid local cell proliferation which causes 

 masses to protrude from the surface and frequently to overgrow 

 it to such an extent that they appear to be almost completely 

 constricted off (fig. 9). 



Taking the primitive streak stage as the last phase of the prim- 

 itive placentation, we may note thac the Trager occupies roughly 



