tion were employed in preserving the material. In
practically all cases the embryos were stained in toto with Borax
Carmine and on the slide with Lyon's Blue. Transverse, sagittal, and
horizontal sections were cut, their thickness varying from five to
thirty microns, depending upon the size of the embryos.

The first indication of the formation of the enteron is seen in the very
early embryo shown, from the dorsal aspect, in figure 1. The medullary
folds and notochord are evident at this stage, but no mesoblastic
somites are to be seen.

A sagittal section of approximately this stage, shown in figure 1A,
represents the foregut, _fg_, as a shallow enclosure of the anterior
region of the entoderm, while the wide blastopore, _blp_, connects the
region of the hindgut with the exterior. No sign of a tail fold being
present, there is, of course, no real hindgut. The entoderm, which has
the appearance of being thickened because of the fact that the notochord
has not yet completely separated from it, is continuous, through the
blastopore, with the ectoderm. Posterior to the blastopore the primitive
streak, _ps_, is seen as a collection of scattered cells between the
ectoderm and the entoderm, apparently formed by proliferation from the
ventral side of the ectoderm.

A slightly later stage is shown in figure 2, a dorsal view of an embryo
with five pairs of mesoblastic somites. A sagittal section of this stage
is shown in figure 2A. The foregut is here more inclosed, and the
notochord, _nt_, having separated from the entoderm, _en_, is seen as a
distinct layer of cells extending from the foregut to the blastopore.

A transverse section through the headfold of this stage is shown in
figure 2B. The foregut is seen as a wide cavity, _ent_, depressed
dorsally, apparently, by the formation of the medullary groove and the
notochord; it is wider laterally than in a dorso-ventral direction, and
its walls are made up of about three layers of closely arranged,
irregular cells; the wall is somewhat thinner on the dorsal side, just
below the notochord.

Figure 3 is a dorsal view of the next stage to be described; about
fifteen pairs of somites are present.

Figure 3A is a transverse section through this embryo near the anterior
end of the enteron, _ent_, which cavity, cephalad to this region, is
bluntly pointed. As seen in the figure the enteron is here wide from
side to side, and is depressed dorso-ventrally except for a wide groove