STUDY OF RABBIT BLASTODERMIC VESICLES IN ALCOHOL. 



169 



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 are nearly spherical and 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 distinguished with a hand lens (Fig. 124).' Its size varies 

 with the diameter of the vesicle, being larger in the larger vesicles; but the pro- 

 portions are not exact, for a vesicle of given diameter may have an embryonic 

 shield of either larger or smaller dimensions 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 disap- 

 pearance has not been definitely settled. There is no FIG. 124. BLASTODERMIC VESI- 

 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. The entodermal cells 

 also have essentially the same appearance as at five days, but they extend considerably 

 farther 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 somewhat 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, the nuclei of the two layers are seen to alternate more or less with one an- 

 other. This characteristic disposition is not kept^ everywhere, but is subject to 

 considerable variations. In the very most advanced ova of six days a small spot 

 sometimes can be observed in the embryonic shield which is noticeable from its 

 greater opacity. This spot corresponds to Hensen's knot, but it does not usually 

 show itself distinctly until considerably later. 



Vesicles at Seven Days. Vesicles at this age vary greatly in size, and the stage 



CLE OF A RABBIT OF Six DAYS 

 AND ONE AND ONE-HALF HOURS. 

 FROM AN ALCOHOLIC SPECIMEN. 

 X 20 diams. 



