layers of cells.
The allantois is cut near its opening into the hindgut; its walls are
thin, the epithelium consisting of but a single layer of more or less
flattened cells.

Figure 7 represents a reconstruction of the enteron of an embryo of 42
mm. crown-rump length. Because of the body flexure and large size of the
embryo the head was amputated, in the plane _a-b_, and cut sagitally,
while the body was cut transversely in the direction shown by the
section planes. In the present figure the outline of the embryo,
including the eye, appendages, and umbilical stalk, is shown by fine
dotted lines; the outlines of the lungs and liver are shown by heavier,
broken lines; while the outlines of the enteron proper and the trachea
are shown in solid lines, filled in which fine stippling. For the sake
of simplicity only one lung and one bronchus are shown.

Since the head has now quite a reptilian form, the oral cavity, _m_, has
more or less of the adult outline. A transverse groove near the anterior
end of the lower jaw marks off the tongue, _tn_; and the rudiments of
teeth are seen but not shown in the figure because of the low
magnification used.

The pharynx, _ph_, is a very extensive cavity that is sharply separated
from the mouth by a prominent transverse fold of skin, the velum
palitum, _vp_, just in front of the posterior nares, _pn_, and by a less
marked fold from the base of the tongue; it is these two valves that
enable the adult alligator to open its mouth under the surface without
getting water into the lungs. The mouth and pharynx are lined at this
stage with a thin, stratified epithelium, which consists of a basal
layer of rather tall columnar cells and one or two superficial layers of
flattened cells. The pharyngeal epithelium is rather thicker than that
of the oral cavity.

In the embryo from which this reconstruction was made the pharynx was in
direct communication with neither the oesophagus nor the trachea, though
the separation in each case was by a mere membrane. The trachea, _ta_,
opens, except for this membrane, into the pharynx a short distance back
of the transverse, dorsal and ventral folds mentioned above, and almost
directly ventrad to the posterior nares. The anterior end of the
oesophagus, _oe_, is in contact with the extreme postero-ventral wall of
the pharynx.

The trachea, which is already surrounded by distinct cartilaginous
rings, is long, and of about the same diameter throughout. In the