No. 1.] ORIGIN OF HUMAN MONSTERS. 147 
Diameter of stem 
$a ae eee a NSA 0.4. mm. 
Wengthet Stent. a) ase wee eee OAs; 
Lengtheofovesicle. va. soit aan ame is ear 
Widthmotevesicle ;2 wt aaa sees 10 Speke 
Lencile of invarination= 450.06 aa Oar: 
Widihroteinvacination <¢ 2ifiess ae ee OnGig o+ 
Diameter of opening of invagination.... 0.03 “ 
The sections and reconstruction show that the embryonic 
vesicle is attached to the chorion by means of a stem. The 
greater part of the vesicle itself is composed of two layers. 
ectoderm and mesoderm. In the neighborhood of the em- 
Fic. 11d.—Section through the stem uniting the umbilical vesicle with 
the chorion. X 50 times. The cavity within the stem lined with 
epithelium is the allantois. 
bryonic stem there is a third outer layer which shows all of 
the characteristics of the ectoderm. Just beside the attach- 
ment of the vesicle to the stem there is a sharp, deep and 
narrow invagination of all three embryonic membranes. 
Within the stem there is a sharply defined allantois which 
communicates with the cavity of the vesicle just below the 
cavity of the ectoderm. The ectodermal plate of the invagi- 
nation is very broad but not of equal thickness throughout 
its whole extent. It extends to the outside of the vesicle and 
ends quite abruptly in the neighborhood of the stem. The 
blood-vessels of the mesodermal layer extend to the stem but 
do not enter it, nor are there any blood-vessels in the chorion. 
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