POLYEMBRYONIC DEVELOPMENT IN TATUSIA 587 
passes from the horizontal groove to the attachment zone; for it is 
almost invariably the rule that the vesicle is located on that half 
of the zone lying adjacent to the groove along which it had passed 
from the fallopian tube. Thus, in a pregnancy in which the left 
ovary holds the corpus luteum, the vesicle will be found to be 
attached at some spot on the left side of the attachment zone. 
FORMATION OF THE ECTODERMIC VESICLE 
In the preceding paragraphs we have referred to the change in- 
volving the embryonic ectoderm which takes place shortly after 
the vesicle becomes attached to the mucosa. This change is per- 
haps anticipated in the more advanced of the free blastocysts in 
which a few divisions of the ectodermal cells are taking place (fig. 
47) ; but evidently does not express itself clearly until after the egg 
becomes quite well anchored to the uterine wall. As already in- 
timated, the change consists in the transformation of the flat, cir- 
cular plate of ectodermal cells into a spherical or ball-like mass 
(fig. 15). 
The entoderm in this stage is recognized as a distinct layer, 
and, although no longer connected with the ectodermic mass by 
protoplasmic strands, yet is still in close contact with it. The 
entodermal cells which lie directly beneath the ectoderm become 
distinctly thicker than before, and accompanying this change is 
the appearance of mitoses (fig. 15). 
The entodermal cells which lie beyond the limits of the embry- 
onic ectoderm, differ from those lying beneath the ectoderm in two 
important respects. First, the nuclei of the cells remain small, 
thus indicating that the cells are not in an active state of division; 
and second, these cells are flattened out against the trophoblast, 
with which they are in very close contact. These cells form a nar- 
row ring or annular zone around the margin of the ectoderm. 
This zone is of especial interest because it forms the axis about 
which the so-called inversion of germ layers revolves. _ 
The transformation of the plate of ectoderm into a spherical 
mass results eventually in its entire separation from the tropho- 
blast and its inclusion within an entodermal sac, thus leaving a 
JOURNAL OF MORPHOLOGY, VOL. 24, NO. 4 
