rrow groove in
the anterior floor of the mouth. As this groove is followed caudad its
ventral wall is seen to become much thickened, _tg_, to form the
_anlage_ of the thyroid gland. In the present section the walls of the
groove are just fusing, to cut off the cavity of the gland from the
dorsal part of the groove. The next section caudad to this shows the
thyroid as a round, compact mass of cells, with a very small lumen,
still closely fused with the bottom of the oral groove. The lumen may,
in this embryo, be traced for only a few sections, caudad to which the
thyroid is seen as a small, solid mass of cells unattached to the oral
groove. Close to the sides of the thyroid are seen two large blood
vessels, _ar_, the mandibular arches, which unite into the single
ventral aorta just caudad to the posterior end of the thyroid. High
power drawings of the thyroid just described are shown in figures 4E
and 4F.

Figure 4G is about fifty-five sections caudad to the preceding figure,
and passes through the middle region of the heart, _ht_. The enteron,
_ent_, is cut caudad to the last gill cleft, but it is nearly as large
as in the pharyngeal region described above; its walls are of a more
even thickness than in the more anterior sections, though there is an
area, just below the aorta, where the wall is still but one cell thick.
In the ventral wall of this part of the enteron, and, to some extent, in
the lateral walls, there seems to be a tendency for the nuclei to become
collected toward the side of the wall away from the digestive cavity;
this condition cannot be well seen in the figure owing to the amount of
reduction in reproduction.

Figure 4H is seventy-nine sections posterior to the last, and passes
through the foregut, _ent_, just cephalad to the anterior intestinal
portal and caudad to the heart. The outline of the enteron is here
almost a vertical slit, and the lining entoderm consists, in its dorsal
and lateral regions, of a single layer of columnar epithelium, while in
its ventral region, where it adjoins the liver trabeculae, it is made up
of several layers of cuboidal or irregular cells. The nuclei in the
dorsal and lateral regions of the entoderm are arranged in a very
definite layer at the basal ends of the cells, though an occasional
nucleus may be seen near the center of the layer. The mesoderm that
extends ventrad from the mesentery, on each side of the entoderm just
described, consists of a thick layer of c