No. 1.] ORIGIN OF HUMAN MONSTERS. 79 
to be a small amnion, and this condition may therefore be 
designated multiple amnion. The second specimen, No. 78, 
is much like the one just described. Again the stem of the 
vesicle is encircled by a layer of epithelial cells, and within it 
there are a couple of vesicles lined with the same kind of 
cells. There are also some blood-vessels within the stem. 
In the third specimen (No. 247) of this group the vesicle 
is detached from the chorion; it is pear-shaped and is lined 
‘with a single layer of epithelial cells. The outer layer is rela- 
tively thick, is composed of mesoderm in which are located 
numerous large spaces filled with blood; there are no blood- 
vessels in the chorion. Within this mesodermal layer there 
are a number of sharply-defined vesicles lined by a single 
layer of epithelial cells which is unlike that of the main vesicle. 
It is natural to conclude that the large vesicle belongs to the 
yolk sac and is lined with endoderm, and that the smaller 
vesicles form multiple amnions and are lined with ectoderm. 
Table II shows that the age of the specimens (from Nos. 11 
to 78) increases with the size of the chorion. The same is 
true regarding the last group, Nos. 21 to 147, which is also 
arranged according to the size of ova. 
Complete destruction of the amnion.—In the specimens just 
discussed it has been shown quite conclusively, I think, that 
radical changes may take place in the amnion and embryo of 
very young ova when the chorion is affected. The remaining 
seven specimens of this group show still greater changes in 
the embryonic mass, i. ¢., both the amnion and embryo are 
destroyed entirely, leaving only the umbilical vesicle. That it 
should be so, and not the opposite, is quite natural when we 
take the order of development into consideration. The embryo 
and amnion receive their nutrition in early stages from the | 
umbilical vesicle, which in turn draws upon the fluid within 
the exoceelom. This in turn is acted upon by the exchange of 
fluid with the villi. 
In these specimens (Nos. 21 to 147) it is seen that the 
changes in the villi are more pronounced than in those in 
