ior
appendages, figure 5G. There the intestine, _i_, is a distinct,
cylindrical tube which extends, with not much variation in caliber, and
with little variation in position, from this point to the cloaca.
Followed cephalad, towards the posterior intestinal portal, it gradually
diminishes in caliber, as did the foregut on approaching the anterior
intestinal portal. The epithelium consists here of three or four layers
of compactly arranged cells, and has about the same appearance as in the
oesophagus and duodenum.

Figure 5H represents a section through the cloacal region, _cl_, showing
the openings into the cloaca of the Wolffian ducts, _wdo_. Just anterior
to these openings the cloaca opens ventrally into a small,
anteriorly-projecting pouch, the rudiment of the allantois.

Caudad to the openings of the Wolffian ducts the cloaca extends ventrad
as a narrow, solid tongue of epithelium towards the exterior, figure 5I,
and fuses with the superficial ectoderm at the caudal end of a prominent
ridge that lies in the mid-ventral line between the posterior
appendages. In this embryo the cloaca has no actual opening to the
exterior; the walls of the part that projects towards the exterior are
in close contact, except in the region of the openings of the Wolffian
ducts, as is shown in figure 5H.

Owing to the coiling of the end of the long tail the plane of the
section, as is seen in figure 5I, passes through the posterior end of
the embryo no less than four times. In the most posterior of these four
sections of the tail, beginning slightly caudad to the section here
shown, is seen a small cavity which may be called the post-anal gut,
_pag_. It has thick walls, and extends for about thirty-five sections in
the series under discussion. Its lumen is very large in its caudal
region, figure 5I, _pag_, and tapers gradually cephalad until it
disappears. Posteriorly the post-anal gut ends quite abruptly not very
far from the extreme tip of the tail.

Figure 5J is a composite drawing from reconstructions of the enterons of
two embryos of approximately this stage. One of these reconstructions
was plotted on paper from a series of transverse sections; the other was
made in wax from a series of sagittal sections. For the sake of
simplicity the gill clefts are not represented, and the pharynx, mouth,
and liver are represented in outline only. For the same reason the lung
rudiment of one side only is shown.

The relative size of the ph