296 G. CARL HUBER 
passes through five sections of 10 » thickness, though one of the 
end sections, the fifth section of the series, seems to have fallen 
out during manipulations of staining and mounting, since the 
preceding, or fourth section does not quite complete the series. 
This vesicle lies free in the lumen of the uterus, and there is evi- 
dent only a shallow pit in the mucosa juxtaposed. In this 
vesicle the cells forming the roof of the segmentation cavity are 
relatively numerous, and are not markedly flattened, and in one 
an early mitotic phase is recognized. Here again cell prolifer- 
ation appears to have accompanied increase in size of segmenta- 
tion cavity. 
The vesicle shown in C of figure 22, measuring 130 u by 30 yu 
by approximately 40 u, lies free in a long, narrow fold of the 
uterine mucosa, in close proximity to a shallow mucosal pit, 
lined by cubic epithelium; the pit conforming in shape and 
extent to the form of the side of the vesicle presented to it. 
Therefore, it would seem that the form of the vesicle as seen in 
sections of the fixed material is essentially the same as that 
obtained in vivo. The two vesicles, typical sections of which 
are shown in B and C of this figure, are almost in identically the 
same phase of development, although their form as seen in sec- 
tions differs markedly. The plasticity of the living blastodermie 
vesicles is no doubt such that their form is in a great measure 
dependent on the shape of the mucosal fold in which they are 
lodged. In D of figure 22, there is reproduced a section of a 
blastodermie vesicle which points to a stage of development 
which is slightly more advanced than that shown in previous 
figures. The vesicle measures 100 » by 70 » by approximately 
50 uw. The roof enclosing the segmentation cavity is slightly 
folded; a portion of its wall is thus cut tangentially, as shown 
in the lower left of the figure. The segmentation cavity is 
distinctly larger than that shown in the preceding figures, and 
is bounded by a relatively large number of cells, fourteen in that 
portion of the roof sketched in this figure, one of which is in a 
mitotic phase. The mass of cells constituting the floor appears 
as slightly compressed, in consequence of a slight intravesicular 
pressure which aided in the enlarging of the segmentation cavity. 
