380 G. CARL HUBER 
beginning of insemination. The uterus of this rat contains 
eight egg-cylinders, all of which, except the one here figured, 
show normal structure, though presenting quite different stages 
of development. One of these, cut serially in cross-section, is 
figured in C, figure 32, Part I, as showing anlage of mesoderm 
with primitive streak and groove. Two of the other egg-cylinders 
show the anlage of the mesoderm, two others show late pre- 
mesoderm stages of the egg-cylinder, the remaining egg-cylinders 
are less fully developed, one showing a development which may 
be compared to B of figure 26, Part I, thus a much younger 
stage. By the end of the eighth day and with the early hours 
of the ninth day after the beginning of insemination in the 
albino rat, the two parts of the proamniotic cavity, which de- 
velop discretely, have joined to form a single space (C, fig. 27, 
Part I). The egg-cylinder shown in figure 9, presents normal 
development in all parts, except that there is as yet no union of 
the two parts of the proamniotic cavity. This egg-cylinder is 
most favorably cut, in longitudinal direction; the plane of see- 
tion being almost parallel to the mid-sagittal plane. This egg- 
cylinder, therefore, is easily followed through the several sections 
of the series into which it was cut. The irregularity of outline 
of the ectodermal vesicle, lower right of figure, it is believed, is 
not due to fixation shrinkage. Judging from size and structural 
differentiation of this egg-cylinder, union of the antimesometrial 
and mesometrial portions of the proamniotic cavity should have 
been completed before this stage of development was reached, 
with the primary embryonic ectoderm and the extraembryonic 
ectoderm forming a continuous layer, as shown in figure 29, Part I. 
The folding of the wall of the antimesometrial portion of the 
egg-cylinder, lower right of figure, evident in nearly all of the 
sections of the series, is regarded as indicating an abnormal 
growth of the primary embryonic ectodermal cells composing the 
wall of the ectodermal vesicle, as a result of retarded extension 
of the antimesometrial portion of the proamniotic cavity, perhaps 
an adjustment to meet the altered mechanical stress resulting 
from abnormal development. The condition here seen, it would 
seem, 1s foreshadowed in the egg-cylinder shown in figure 8. 
