ompactly arranged cells. The
ventral end of the entodermal wall is fused with the wall of a small
cavity, _li_, which may be traced several sections cephalad to this
plane. This cavity is a part of the system of hollow liver trabeculae
seen as a group of irregular masses of cells ventrad to the enteron at
the opening of the anterior intestinal portal. The large blood vessel,
_bv_, is the meatus venosus.

Figure 4I is just four sections caudad to the preceding. It passes
through the anterior intestinal portal, _aip_. The medial liver
trabecula into which the enteron was seen to open, in the preceding
figure, now opens ventrally to the yolk-sac as the anterior intestinal
portal. A few liver trabeculae are to be seen on either side of the
portal, but they show no lumena, and may be traced through only a few
sections. The extent of this uninclosed region, the midgut, is very
difficult to determine with accuracy, but, at this stage, it comprises
about one-half of all the sections of the series. The difficulty is due
partly to the unavoidable tearing of the tissues in removing the embryo
from the yolk-sac, and partly to the indefiniteness of the posterior
intestinal portal, where the walls of the enteron are very thin. As seen
in figure 4I the location of the anterior intestinal portal is very
distinct.

A short distance caudad to the anterior intestinal portal there is
constricted off from the roof of the midgut a narrow diverticulum,
figure 4J, _i_, the meaning of which is not apparent; it extends through
only ten to fifteen sections, tapering caudad till it disappears. The
region of the hindgut, at this stage, is about one-fifth of the entire
length of the embryo. Its anterior portion is wide and, as has been
said, rather indefinite in outline.

Figure 4K represents a typical section through the midgut region of an
embryo of about the age of the one from which the preceding figures were
drawn. This and the following figures of this stage were drawn from an
embryo in which the posterior region was in better condition than in the
embryo from which the other figures of the stage were taken. The
mesentery, _ms_, is here of considerable length and continues around the
yolk in a layer of diminishing thickness. The epithelium of this region
of the enteron consists of a single layer of fairly regular cells, which
are columnar in the dorsal region, just beneath the mesentery, and
cuboidal or even flattened in regions more distan