in the ventral wall. This groove is lined with rather more closely
arranged cells, and marks the region where the mouth will break through
at a somewhat later stage. A short distance caudad to this region the
groove disappears and the pharynx is reduced to a shallow slit extending
almost to the superficial ectoderm on either side; then the slit-like
pharynx becomes suddenly reduced in a lateral and increased in a
dorso-ventral direction, to assume the outline shown in figures 3B and
3C. At a point about one-third of the length of the embryo from the tip
of the head, the enteron opens to the yolk-sac, so that what now may be
called the foregut has this considerable extent. There is, however, not
the slightest indication of a tail-fold, so that there is no inclosed
hindgut at all. As is shown in figure 3D, the neurenteric canal, _nc_,
still opens ventrally, though the medullary canal, _mc_, has now no
dorsal opening to the exterior. The medullary canal continues for a
short distance (about fifteen sections of five microns thickness)
posterior to the opening of the neurenteric canal.

Figure 4 is a surface view of the next stage to be described. There are
here about twenty pairs of somites, though the exact number cannot be
determined. Although not visible externally in the surface view shown,
the gill clefts are beginning to form, and the first one opens to the
exterior as will be seen in sections of another embryo of this stage.
The mouth has now broken through, putting the wide pharynx into
communication with the exterior; probably the mouth opening is formed at
about the time of the opening of the first gill cleft.

Figure 4A represents a transverse section through the head of an embryo
of the approximate age of the one just described; it passes through both
forebrain, _fb_, and hindbrain, _hb_; through the extreme edge of the
optic vesicles, _ov_, and through the anterior end of the notochord,
_nt_. It is just cephalad to the anterior end of the pharynx and to the
hypophysis. The chief purpose in showing this section is to represent
the two large head-cavities, _hc_. The origin of these cavities may be
discussed at a later time. They are irregularly oval in cross section,
and extend in an antero-posterior direction for a distance about equal
to their long axis as seen in cross section. The two cavities project
towards each other in the middle line, and are almost in contact with
the notochord, in the region figured, b