324 G. CARL HUBER 
ectoplacental cone. The cells are of irregular polyhedral form, 
compactly grouped, showing as yet no definite arrangement. 
Cell proliferation as evidenced by mitoses is active, amply ac- 
counting for the increase in length of this structure. The vis- 
ceral entoderm encloses the long egg-cylinder as a single layer 
of cells and is continuous at its base with the parietal entoderm, 
well shown at the left of the figure. The ectoplacental cone of 
this vesicle is very favorably cut in a plane parallel to the long 
axis of the uterus. This vesicle was unusually well fixed and 
may be regarded as showing normal relations of the thin mem- 
branous wall, derived from the parietal ectoderm, and of the 
egg-cylinder, which reaches quite to the antimesometrial end 
of the vesicle. 
Vesicle C of figure 26, obtained from the same uterus as was 
vesicle B (rat No. 81, 7 days, 22 hours), differs from that shown 
under B, in that it presents the anlage of a mesometrial portion 
of the proamniotic cavity. In the extraembryonic ectoderm, 
near its junction with the base of the ectoplacental cone, two 
irregular spaces may be observed. These are distinctly evident, 
passing through the entire section, only in the section figured. 
The antimesometrial portion of the egg-cylinder is not cut 
quite through its center, so that the primary embryonic ectoderm 
of the ectodermal vesicle appears as a stratified epithelium, and 
the antimesometrial portion of the proamniotic cavity appears 
as relatively small, this owing to a slight curvature shown by 
this egg-cylinder. The other features presented by this vesicle 
are sufficiently well portrayed in the figure to obviate the neces- 
sity of further description. 
In figure 27, there are shown three further stages of egg-cylinder 
differentiation, showing progressively older stages than shown 
in the preceding figure. Under A of this figure, there is re- 
produced a representative sectionof a vesicle taken from the same 
uterus as were vesicles B and C of figure 26 (rat No. 81, 7 days, 
22 hours). The figure is not of a single section, but is com- 
bined from two sections, superimposed so as to give correct 
dimensions and relations. The egg-cylinder of A of this figure 
differs from that shown in C of figure 26, in that the mesometrial - 
