646 J. T. PATTERSON 
first form a fenestrated structure which later is transformed into 
a continuous layer (figs. 8-14, 36-42, pp. 572-585). 
9, After the entoderm becomes a continuous layer, it splits 
from the ectoderm, and its free margin passes beyond the limits 
of the ectodermal mass until the area covered by the entoderm 
equals an arc of about 80 degrees on the circumference of the 
blastocyst. The margin of the entoderm now unites with the 
trophoblastic wall. The remaining 280 degrees of trophoblas- 
tic wall never becomes lined with entoderm (figs. 15, 47, 48; 
p. 585). 
10. The blastocyst becomes attached at the embryonic pole 
to the uterine wall. (figs. 16, 17, 48-46, 49, 50; pp. 585-587). 
11. After the entoderm is split from the embryonic ectoderm, 
the latter rounds up into a spherical mass, which upon parting 
company with the trophoblast, pushes into the cavity of the ves- 
icle and becomes included in the entodermal layer (figs. 15-17, 
43-46, 48-50; pp. 587-590). 
12. The ectodermal vesicle is formed by a vacuolization proc- 
ess, which results in disintegrating the core of the ectodermal 
sphere. When complete, the ectodermal vesicle consists of a 
single layered pole turned toward the Trager,and a uniformly 
thick pole, which faces the now inverted entoderm or yolk-sac 
(figs. 16, 17, 18, 43-46, 49-51; pp. 590-593). ¢ 
13. During the process of inclusion of the ectodermal mass, 
there is created an extraembryonic cavity, which lies between the 
Trager and the endothelial-like pole of the ectodermal vesicle 
(figs. 17, 18, 49-51; pp. 592-593). 
14. The extraembryonic mesoderm arises through cell prolifer- 
ations from the ectodermal vesicle. These proliferations occur 
around the entire vesicle, at the angle formed by the vesicle and 
the entoderm, where the latter turns out to join the trophoblast. 
The mesoderm cells are given off in groups, which are quickly 
transformed into vesicular-like structures that fuse together to 
form a continuous lining or mesothelium for the extraembryonic 
cavity (figs. 19, 20, 55-60; pp. 593-595). 
15. On the right and left sides of the ectodermal vesicle, the 
primary diverticula or buds appear from thickened areas that 
