nd may be traced through a large
number of sections, being in some regions solid and of a smaller
diameter than in the present section.

Figure 6D is in the region of the line 500 in figure 6A. The thymus
rudiments, _ty_, have about the same appearance as in the preceding
figure, except that they are somewhat larger. The pharynx, _ph_, is much
smaller than in the last section, and though somewhat crescentic in
outline, its convex side is dorsal instead of ventral in position. The
pharyngeal walls are here thicker, and consist of two or three layers of
cells, instead of the single layer of more anterior sections.

In the median plane the floor of the pharynx is pushed down, as a solid
tongue of cells, _gs_, the anterior edge of the glottis. Ventrad and
laterad to the glottis a crescentic condensation of mesoblast represents
the beginning of the laryngeal cartilages, _la_.

Two or three sections caudad to the one just described, the two layers
of which the tongue of cells from the floor of the pharynx is composed
separate slightly at the bottom to form a small cavity, the trachea,
_ta_; this condition is shown in figure 6E, which represents part of a
section through the plane 532 of figure 6A.

The oesophagus, _oe_, is here a solid, crescentic mass of cells, the
lumen being completely obliterated. The dorsal part of the tongue of
cells, mentioned above, connects the ventral side of the oesophagus with
the trachea, like a sort of mesentery. Above the oesophagus, on either
side, is the thymus rudiment, _ty_, in this section practically a solid
mass of cells instead of a tube. The epithelium of the trachea here
consists of three or four layers of compactly arranged cells; this
epithelium is surrounded by a dense mass of mesoblast which is
responsible for the greater thickness of the trachea as seen in figure
6A. As has been said, the oesophagus here has no lumen, and when
examined under high magnification its walls are found to be completely
fused, not merely in close contact. The same is true of the tongue of
cells between the oesophagus and trachea. Two or three sections caudad
to the one under discussion this tongue of cells loses its connection
with the trachea, and the latter structure is entirely independent of
the oesophagus.

The solid condition of the oesophagus continues through about fifty
sections of this series, the horns of the crescent gradually shortening
until only the central part remains as the hollow