permanent gut;

2. above, at the dorsal side, the axial rod or chorda; and

3. the two coelom-sacs, which immediately sub-divide into two
structures:--

3A. above, on the dorsal side, the episomites, the double row of
primitive or muscular segments; and

3B. below, on each side of the gut, the hyposomites, the two lateral
plates that give rise to the sex-glands, and the cavities of which
partly unite to form the body-cavity. At the same time, the neural or
medullary tube is formed above the chorda, on the dorsal surface, by
the closing of the parallel medullary swellings.

All these processes, which outline the typical structure of the
vertebrate, take place with astonishing rapidity in the embryo of the
Amphioxus; in the afternoon of the first day, or twenty-four hours
after fertilisation, the young vertebrate, the typical embryo, is
formed; it then has, as a rule, six to eight somites.

The chief occurrence on the second day of development is the
construction of the two permanent openings of the gut--the mouth and
anus. In the earlier stages the alimentary tube is found to be
entirely closed, after the closing of the primitive mouth; it only
communicates behind by the neurenteric canal with the medullary tube.
The permanent mouth is a secondary formation, at the opposite end.
Here, at the end of the second day, we find a pit-like depression in
the outer skin, which penetrates inwards into the closed gut. The anus