cylinder seen in
figure 6F, _oe_, which is a section through plane 650 of figure 6A. From
about this point to its opening into the stomach the oesophagus has
essentially the same structure. Its epithelium is of the simple columnar
type, the cells being long, with generally basally located nuclei.

In the section under discussion the trachea, _ta_, is of about the same
size as the oesophagus, but its epithelium is thicker and consists of
two or three layers of cells. The trachea extends, as a separate and
distinct structure, through about one hundred and fifteen sections, and
then, at a point four or five sections caudad to the present section, it
divides suddenly into the two bronchial tubes. Each bronchus, like the
trachea, is lined with an epithelium of three or four layers of cells;
but the epithelium is surrounded by a thin layer of much condensed
mesoblast. The bronchi continue caudad, with slightly increasing
caliber, through about fifty sections, when they suddenly enlarge to
form the lungs. As seen in figure 6A the lungs are irregularly conical
in outline and lie on either side of the posterior end of the
oesophagus.

Figure 6G is a section through the plane 750 of figure 6A. The
oesophagus, _oe_, is seen as a small, circular opening between two much
larger openings, the lungs, _lu_. The epithelium of the oesophagus is
the same here as in the more anterior regions described above; that of
the lung rudiments is very variable in thickness, even in different
parts of the same section, being in some places composed of a single
layer of cuboidal or even flattened cells, in other places consisting of
four or five layers of cells (not well shown in the figure). Surrounding
the epithelium of the lung rudiments is a thin layer of quite dense
mesoblastic tissue. A fairly well defined mesentery, _ms_, is now
present in this region.

Filling the greater part of the body cavity, below the oesophagus and
lung rudiments, is the liver, _li_; and ventrad to the liver the section
passes through a loop of the duodenum, _d_.

The epithelium of the duodenum consists of four or five layers of
compactly arranged cells, near the center of an oval mass of fairly
dense mesoblast. In a lateral projection of this mass of mesoblast lies
a small, circular opening, the bile duct, _bd_. Its epithelium consists
of a single layer of columnar cells. In more anterior sections the bile
duct is larger in cross section, being about one-half the