302 G. CARL HUBER 
In figure 23, there are reproduced representative sections of 
five blastodermic vesicles falling to the end of the sixth day after 
insemination. None of these five vesicles can be regarded as 
well fixed. All show a certain amount of distortion, much more 
evident were the entire series of each of the respective vesicles 
shown. The form of the blastodermic vesicle of the albino 
rat at this stage of development, as indicated by the molding 
of the uterine mucosa, is ellipsoid. Their size as in vivo, when 
distended and of regular outline, again as indicated by the 
molding of the uterine mucosa, is slightly larger than would be 
Fig. 23 Sections of blastodermic vesicles or blastocysts of the albino rat. 
X 200. A and C, rat No. 99, 6 days; B, D, E, rat No. 100, 6 days. y.ent., yolk 
entoderm; p.ent., parietal layer of entoderm; p.ect., parietal or transitory 
ectoderm. 
supposed from the drawings presented. By reason of this dis- 
tortion, exact measurements of size cannot be given. 
In A of figure 23, there is reproduced that portion of one of the 
sections of a blastocyst (rat No. 99, 6 days) which passes through 
its floor; the thin roof of this vesicle was so folded that its inelu- 
sion in the drawing was deemed undesirable. However, its 
floor or the germinal disc, seems to have retained its normal 
form and structure, presenting when traced through the series 
a regular concavo-convex, discoidal form. It consists in the 
main of three layers of cells of polyhedral type; toward the 
border of the disc, of two layers of somewhat flattened cells, the 
peripheral layer being continuous with the single layer of cells 
