292 G. CARL HUBER 
cavity, eccentrically placed and passing clearly through two 
of a series of five sections of 10 » thickness, is evident. The small 
cleftlike cavity is bounded by the surrounding cells, the outline 
of which is distinct. So far as may be judged from the appear- 
ance noted as presented in the two sections in which this cavity 
is found, this arose as a single space and as a result of the separa- 
tion of the enclosing cells. 
In C, of figure 20, there is presented a slightly older stage 
showing the blastodermic vesicle formation and measuring 80 u, 
by 50 uw, by approximately 50 yu, comprising as is estimated, 34 
to 36 cells. Unfortunately, the lower part of this vesicle is 
slightly folded as is shown in the lower left of the figure. The 
appearances presented in the sections are reproduced as faith- 
fully as could be. Owing to the folding, a portion of the thin 
wall is cut tangentially. The more darkly colored curved line 
represents in reality the outer boundary of this portion of the 
vesicle. The segmentation cavity in this vesicle is distinctly 
larger than that shown in B of this figure. In the section re- 
produced the segmentation cavity is bounded for the greater 
part by four somewhat flattened cells, the increase in the size 
of the cavity being accompanied, it would seem, by a flattening 
of the enclosing cells. 
In these three closely approximated stages, which, since 
they are taken from the same uterus are probably separated in 
time of development by only short intervals, the cells though 
varying in size and shape, show no essential or fundamental 
difference in structure, neither in cytoplasm nor nuclei; nor do 
they show any regularity in arrangement. Only few mitotic 
figures are to be observed; none in the morula mass shown in A, 
and but two in each of the other two stages, shown in B and C. 
Judging from these preparations, one would be led to con- 
clude that segmentation cavity formation in the albino rat is 
not associated nor accompanied by active cell proliferation. This 
point will be referred to again after the presentation of further 
material at hand. In slightly older stages of the blastodermic 
vesicle than here considered, the thicker portion of the vesicle 
is designated by Sobotta and others as its floor, which is directed 
