scribed in detail in
another paper. Compared to the size of the gill clefts the cavity of the
pharynx is, at this stage, comparatively small.

Followed caudad the pharynx becomes depressed until, in the region shown
in figure 5B, it is a mere narrow slit, _g_, extending transversely
across the embryo and opening through the gill clefts to the exterior on
each side.

Figure 5C passes through the posterior region of the pharynx, _ph_, the
tip of the forebrain, _fb_, the anterior edge of the heart, _ht_, and
the curve of the tail, _t_. The chief point of interest in this section
is the thyroid gland, _tg_. It now lies deep in the tissue of the floor
of the pharynx, entirely separated from the pharyngeal epithelium. It
consists of a compact mass of cells, now showing a bilobed structure in
its anterior end, and extending through about twenty-five ten-micron
sections. It is solid throughout most of its extent, but, in the section
figured, which is near the anterior end, the lobe on the right side
shows a small but distinct cavity scarcely visible in the figure.

Caudad to the region just described the pharynx contracts suddenly to
form the oesophagus, a narrow, V-shaped slit, which soon divides into an
upper and a lower cylindrical tube, figure 5D, _ent_.

Followed caudad the lower of these tubes divides into the two bronchial
rudiments, figure 5E, _lu_, which, in the embryo here figured, extend
through nearly one hundred sections. In the region shown in figure 5E
the three tubes, _oe_ and _lu_, lie at the angles of an imaginary
equilateral triangle, while in the region of the liver, where the
bronchial rudiments end, the tubes lie in the same horizontal plane.

A short distance caudad to the ends of the bronchial rudiments the
oesophagus turns suddenly ventrad and becomes much enlarged to form the
stomach, figure 5F, _i'_, which may be traced through twenty-five or
thirty sections in this series. The epithelium of the stomach is fairly
thick, and consists of five or six layers of compact, indistinctly
outlined cells with spherical nuclei. Ventrad to the stomach is seen, in
figure 5F, a section of the duodenum, _i_, which extends, with gradually
diminishing caliber, for twenty-five or thirty sections caudad to the
posterior limit of the stomach, where it opens to the yolk-sac and is
lost.

The section that cut this embryo in the posterior region of the stomach
also passed through the hindgut in the region of the poster