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, but 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.