294 G. CARL HUBER 
of the two cross diameters. This may be seen from the series of 
drawings made of a blastodermiec vesicle cut cross-wise, taken 
from the uterus of rat No. 68, from which were also taken the 
two vesicles shown in D and E of figure 20. This series of figures 
is shown in figure 21, in which are reproduced in serial order 
the seven successive cross sections into which the vesicle was 
cut. It measures 65 » by 38 uw by approximately 70 u, and is 
found at the bottom of a mucosal fold, found at the mesometrial 
border, and is resting with one side on the epithelial lining of 
a shallow pit, the other wall of this mucosal fold, also showing 
a shallow pit, is shghtly retracted. From a study of this series 
Fig. 21 A complete series of cross-sections of an early stage of blastodermic 
vesicle of the albino rat. > 200. Rat No. 68, 4 days and 16 hours. A to C, 
sections through roof of vesicle, showing segmentation cavity; D to G, sections 
through floor of vesicle. 
of sections, I feel certain that the plane of section is cross and not 
oblique to the long axis of the vesicle. The roof of this vesicle 
passes through three sections, A, B and C. The segmentation 
cavity has thus a depth of less than 30 u. The overlapping of 
the cells surrounding the segmentation cavity is to be noted, 
especially as seen in B of this figure. This arrangement of the 
cells may explain how the cavity may be enlarged without a 
material increase in the number of the enclosing cells—in part, 
by a flattening out of the cells, in part by a rearrangement of the 
relations of the cells. In the figures of the sections passing 
through the floor of this vesicle, D to G, attention is drawn to 
the size, form and relations of the cells and to the fact that there 
is no distinct covering layer. In this series of sections, there are 
