340 G. CARL HUBER 
The egg-cylinder presented in figure 29 constitutes the final 
mesoderm-free stage, the final stage in which no distinct bilateral- 
ity may be determined. I assume that the egg-cylinder pre- 
sented in the figure is cut in the sagittal plane. This assumption 
is based on the fact that the primary embryonic ectoderm ex- 
tends slightly farther toward the mesometrial pole on the one 
side than on the other. In good frontal sections one side of the 
egg-cylinder in this stage of development should present a mir- 
ror picture of the other side. The side on which the primary 
embryonic ectoderm extends farther toward the mesometrial 
pole, the left in the figure, is regarded as containing the caudal 
end of the future embryo. In the primary embryonic ectoderm 
of this region, it is believed, will develop the primitive streak and 
groove, and thus the anlage of the mesoderm. Not in all the 
egg-cylinders of this stage of development found in my series can 
the caudal end of the future embryonic area be postulated prior 
to the anlage of the mesoderm, and in cross-sections no such 
differentiation can be made. The proamniotic cavity of the 
ege-cylinder shown in figure 29 presents a regular and nearly 
smooth contour, not divisible into regions such as described 
for a similar stage for the mouse by Selenka, Melissinos, and 
Sobotta. A very slight constriction is to be observed only in 
the region where the primary embryonic and extraembryonic 
ectoderm are joined in a continuous layer. I am thus wholly 
in accord with Widakowich, who in describing a similar stage in 
one of his preparations, states: ‘‘Das war die einzige konstante, 
bald stirker, bald schwiicher ausgeprigte.Einschniirung der 
Proamnionhohle,” as previously quoted. 
A series of figures of critical regions taken from a series of 
cross-sections of an egg-cylinder of a stage nearly identical with 
that shown in figure 29, though of a shghtly smaller egg-cylinder, 
is given in figure 30, rat No. 42, 8 days, 16 hours, after insemina- 
tion. The sections chosen for the several drawings, A to D, are 
from the following regions, as may be ascertained by compari- 
son with figure 29; A, through about the middle of the ectoplacen- 
tal cone; B, through the proamniotic cavity just below its meso- 
metrial end; C, through the proamniotic cavity just above the 
