308 STUDY OF THE BLASTODERMIC VESICLE. 



scattered about in patches or isolated. As the cuboidal cells of the ectoderm 

 are confined to the region of the embryonic shield, the cells of the entoderm 

 outside of the shield lie close against the subzonal layer. Here they may be 

 more easily studied than in the shield itself. They are very much smaller than 

 the cells of the outer layer and contain each a nucleus with highly refringent 

 granules, which are now numerous and smaller than the somewhat similar 

 granules in the overlying nuclei of the ectoderm. The further away we proceed 

 from the edge of the embryonic shield, the fewer we find the entodermal cells. 

 The extent of their distribution varies greatly, and apparently more or less in 

 relation to the size of the blastodermic vesicle, since in the smallest vesicles of 

 this age we find the cells only a short distance beyond the edge of the shield, yet 

 in the largest vesicles they have expanded even past the equator. 



Vesicles at Six Days. At this age the vesicles are found more or less scat- 

 tered and isolated in position from one another through the upper half of the 

 uterus. They measure from i .o to 1.6 mm. , their walls are very transparent, and 

 the somewhat more opaque, round or oval embryonic shield can be readily distin 

 guished with a hand lens. Its size varies with the diameter of the vesicle, being 

 larger in the larger vesicles; but the proportions are not exact, for a vesicle of 

 given diameter may have an embryonic shield of either larger or smaller dimen- 

 sions than other vesicles of the same size. Hence, vesicles of different sizes may 

 have embryonic shields of similar dimensions. The actual diameter of the shield 

 is between 0.2 and 0.35 mm. The general structure of the vesicles is the same 

 as at five days, but certain differences may be noted. In preserved specimens 

 the external membrane is very apt to be wrinkled. The subzonal layer has very 

 much the same appearance as before, though the cells are somewhat smaller and 

 it has almost disappeared over the region of the embryonic shield. The manner 

 of its disappearance has not been definitely settled. There is no evidence that 

 the cells degenerate or are cast off, hence one inclines to the hypothesis that the 

 cells of the subzonal layer become incorporated in the inner layer of the cuboidal 

 ectodermal cells, for in sections shown at this stage the ectoderm is one-layered 

 in the region of the shield. Entodermal cells also have essentially the same 

 appearance as at five days, but they extend considerably further around the 

 vesicle, are more numerous, and form a more continuous layer. Sections show 

 that the subzonal layer outside of the shield is very thin, but its outer surface is 

 fitted to the inner surface of the zona pellucida. The center of each cell is some- 

 what thicker, projecting toward the interior of the vesicle. It is in this thicker 

 projecting portion that the nucleus is situated. Along the borders of the cells the 

 layer is of course thinner, and it is under these thinner parts that the thicker 

 nucleated portions of the entodermal cells are lodged. Hence, in surface views, 



