376 G. CARL HUBER 
cant that in the two rats in which the pathologic condition affects 
primarily the maternal tissue, the uterine mucosa, all of the con- _ 
tained ova are prone to degeneration. In the abnormal ova 
previously described, for which it was suggested that the causes 
for the abnormality were to be sought in the ova themselves, 
in the great majority of instances, only one abnormal ovum was 
found in each uterus along with a variable number of ova which 
are to be regarded as normal for the respective stage. 
IMPERFECT DEVELOPMENT OF THE ECTODERMAL VESICLE 
The series contains two ova, very favorably cut, ova in which 
the ectodermal vesicle with the antimesometrial portion of the 
proamniotic cavity does not seem to have developed normally. 
Stages showing the differentiation of the egg-cylinder, the for- 
mation of the ectodermal vesicle with the antimesometrial por- 
tion of the proamniotic cavity, the formation of the mesometrial 
portion of the proamniotic cavity in the extraembryonic ecto- 
derm, the union of the two primary proamniotic cavities to form 
a single space, are clearly shown in figures 26 and 27, Part I, in 
the series of closely approximated stages there portrayed. From a 
study of these figures, it will be observed that the antimesometrial 
portion of the proamniotic cavity develops within the ectodermal 
node before the mesometrial portion of this cavity develops in 
the extraembryonic ectodermal portion of the egg-cylinder. In 
the egg-cylinder shown in figure 8, rat No. 94, 8 days after the 
beginning of insemination, such is not the case. In the uterus 
of this rat there were found seven egg-cylinders, one of which, 
very favorably cut, is shown in C, figure 27, Part I. The other 
egg-cylinders obtained from this uterus, except the abnormally 
developed one to be discussed, though not favorably cut, present 
essentially the same form and structure as that figured under C 
of the figure above referred to. The egg-cylinder portrayed in 
figure 8 compares in size and form with those regarded as normal 
and taken from the same uterus. For the greater part it presents 
normal structure and normal relations of cells. The ectopla- 
cental cone, only in part included in the figure, and the parietal 
