No. 1] ORIGIN OF HUMAN MONSTERS. 197 
stem of the vesicle is quite extensive, in which are embryo 
blood-vessels filled with blood. Many of them extend into the 
chorion and some of them into villi. The walls of the vesicle 
are composed of three distinct layers. The inner is com- 
posed throughout of a single layer of sharply defined cubical 
epithelium, the entoderm. Immediately next to this is an ex- 
tensive mesoderm, which continues into the mesodermal layer 
of the stem to the chorion. Near the attachment of the vesicle 
to the chorion there is a sharp invagination of the vesicle 
which is lined with a thick layer of epithelial cells, the ecto- 
#e ig . * = 
cy ee 
Fic. 134b—Photograph of the invagination shown in dark in Fig. a. 
derm. This layer lines only the invagination and does not 
extend over the rest of the vesicle. Beyond and on the distal 
side of the invagination the mesoderm is arranged in five 
groups of cells which suggest in every way myotomes. In this 
region there are embryonic blood-vessels filled with blood. 
The syncytium is very extensive. 
The blood clot from the mother within the ccelom is recent, 
as is shown by the fact that there are present may red blood 
corpuscles. In the periphery of the clot next to the chorion 
