POLYEMBRYONIC DEVELOPMENT IN TATUSIA 655 
SE 
2.638>= ~ 
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i 
is 
y 
“ey 
Fig. 19 Median section in a right-left plane of specimen No. 234. The devel- 
opment of the mesoderm has made considerable progress, and the smaller of the 
two vesicles shows at ‘x’ the point where the entoderm joins the chorionic ecto- 
derm (trophoblast). > 142. 
Fig. 20 Asectionsimilar to the preceding, from specimen No. 256. The vesicle 
has collapsed and thus appears very much flattened in the figure. The chorionic 
ectoderm has sloughed off and disappeared. The mesodermal vesicles have ex- 
panded until they now fill up the entire extraembryonic cavity. X 113. 
