POLYEMBRYONIC DEVELOPMENT IN TATUSIA 589 
not be doubted, and this effect seems to be limited entirely to the 
region in direct contact. 
The sections are cut somewhat obliquely, and hence the section 
passing through the middle of the embryonic ectoderm does not 
cut the region of the primitive placenta, or point of attachment 
(fig. 16). Consequently the exact relation of the entoderm to the 
other parts of the blastocyst can only be made out by referring to 
several of the sections of the series. In plate 2 is shown a series 
of four photographs which will demonstrate this relation. 
In figure 43, which representssthe section from which the draw- 
ing was made (fig. 16), the entoderm is a well organized layer pass- 
ing around the sphere of ectoderm. On approaching the low- 
er side of the ectoderm the entoderm at each side passes laterally 
and upwards to join on to the inner surface of the trophoblastic 
wall (fig. 16, x). The layer of entoderm lying directly beneath 
the ectoderm belongs to that portion which connects the entoder- 
mal sae with the left wall of the trophoblast. A few sections to the 
right in the series the lower layer of entoderm disappears, except 
a small group of cells (fig. 44). In this section the entoderm, on 
leaving the ectoderm, passes outwards and upwards as before,to 
join the inner surface of the trophoblast. Aside from the group of 
entodermal cells lying directly beneath it, the embryonic ectoderm 
is open below to a cavity which is bounded beneath by the placental 
portion of the trophoblast, and which represents what is later to 
become an extraembryonic cavity. In sections still farther to the 
right, at a point where the section cuts just the tip of the ecto- 
derm (fig. 45), the extraembryonic cavity is almost free from cells 
of any kind. This section passes through the center of the Trager 
(fig. 31). Beyond the limits of the ectoderm (fig. 46) we see noth- 
ing but a line of entodermal cells, which represents the right lat- 
eral portion of the entodermal layer as it passes outwards to unite 
with the trophoblast on this side of the blastocyst. 
To sum up: The conditions revealed in the series of photo- 
graphs shows that the ectoderm, upon assuming a spherical form 
pushes up into the cavity of the blastocyst, carrying before it the 
well established layer of entoderm, and creating behind a cavity 
which gives rise to the extraembryonic cavity, and which will 
