t from the median
plane.

Figure 4L, through the region of the hindgut, shows at _i_ the
completely inclosed intestine; it is a comparatively narrow tube, lined
with columnar epithelium outside of which is a dense layer of mesoblast
continuous with the mesentery. In the center of the figure the
allantois, _al_, is seen as an irregular cavity, lined with a single
layer of columnar or cuboidal cells, and surrounded by a thick mass of
loosely arranged, stellate mesoblast cells. The allantois is probably
somewhat larger here than in the other embryos used for this stage, in
which it was torn away. The tail, _t_, of the embryo is shown at the
lower side of the figure, surrounded by the amnion; it is cut in the
region of a curve so that the caudal intestine, _i_, is cut
longitudinally and has the outline of an elongated ellipse. In this
embryo the caudal intestine could be followed to the end of the tail,
through several dozen sections; for some distance posterior to the
allantois it is extremely narrow, so that its lumen is almost
obliterated, and its walls are made up, in any one place, of not more
than a dozen cuboidal cells. Towards the posterior end of this region
the intestine is considerably enlarged as seen in figure 4L.

Figure 4M passes through the region where both the allantois and the
Wolffian ducts open into the hindgut. The union of the allantois and the
gut accounts for the elongated outline of the enteron in this section.
The openings of the Wolffian ducts, _wdo_, are seen at the lower end of
the section of the enteron. The cells lining the Wolffian ducts are
smaller than those lining the enteron. In the lower side of the figure
are seen the structures of the tail, including the outline of the tiny
caudal intestine, _i_, mentioned above. No sign of a cloacal
invagination could be made out with certainty.

The next stage to be studied is shown in surface view in figure 5.

Figure 5A represents a section through the head region of this embryo.
Owing to the obliquity of the plane of the section the figure is quite
asymmetrical. The pharynx, _ph_, is lined with a comparatively thin
epithelium and opens, on the left, at two places, one the mouth and the
other the second gill cleft, _g^2_. In the dorsal wall of this cleft, as
well as in the corresponding wall of the opposite cleft, is seen a
thickening of the epithelium; these thickenings, _ty_, are the rudiments
of the thymus gland, whose development may be de