ut they do not fuse at any point.
These two head-cavities are the only ones to be seen, in this animal,
unless the small evaginations from their walls represent other cavities
fused with these. Their walls are thin but distinct, and consist of a
single layer of cells. These cells are completely filled with their
large, round nuclei, so that the wall has the appearance, under higher
magnification than is used in this figure, of a band of closely strung,
round beads.

Figure 4B represents the eighteenth section caudad to the one just
described. It passes through that region of the enteron, _ph_, which may
be called the pre-oral gut, since it lies cephalad to the now open
mouth. Owing to the plane of the section the upper angle of the first
gill cleft, _g^1_, is seen on the left, although this would not
naturally have been expected in a section through the pre-oral gut. The
evagination to form the hypophysis, _p_, is seen against the floor of
the forebrain, _fb_. The wall of this region of the enteron is
comparatively thin, and consists of not more than two layers of
compactly arranged cells with round nuclei.

Figure 4C is about forty sections caudad to the one just described. It
passes through the mouth, seen as a vertical opening between the two
mandibular arches, _md_. The hyomandibular cleft, _g^1_, the only one
which opens to the exterior in this embryo, is very wide, and may be
traced through a number of sections; in this section the opening is seen
only on the left. The pharynx, _ph_, is very wide; as it is followed
caudad its ventral opening is gradually closed by the approach of the
two mandibular folds. The dorsal wall of this region of the pharynx is
very thin, consisting of a single layer of flat cells with round nuclei;
while the ventral wall, leading through the mouth and lining the
mandibular folds, is composed of two or three layers of compactly
arranged cells.

Figure 4D is through a plane sixteen sections caudad to the last. In
this region, which is just caudad to the otic vesicles, the pharynx has
still its rectangular outline, and its walls are of the same character
as in the preceding figure. The posterior edges of the hyomandibular
clefts are seen projecting in a ventro-lateral direction, _g^1_; while
dorsal to these are the wider, second pair of clefts, _g^2_. Where the
mandibular folds come together posterior to the mouth, they fuse first
at their outer or ventral border, which leaves a deep, na