segmentation-cells get more or less irregularly displaced, while the
segmentation-cavity enlarges in the centre of the morula; in the end
the former all lie on the surface of the latter, so that the foetus
attains the familiar blastula shape and forms a hollow ball, the wall
of which consists of a single stratum of cells (Figure 1.38 A to C).
This layer is the blastoderm, the simple epithelium from the cells of
which all the tissues of the body proceed.

These important early embryonic processes take place so quickly in the
Amphioxus that four or five hours after fecundation, or about
midnight, the spherical blastula is completed. A pit-like depression
is then formed at the vegetal pole of it, and in consequence of this
the hollow sphere doubles on itself (Figure 1.38 D). This pit becomes
deeper and deeper (Figure 1.38 E and F); at last the invagination (or
doubling) is complete, and the inner or folded part of the
blastula-wall lies on the inside of the outer wall. We thus get a
hollow hemisphere, the thin wall of which is made up of two layers of
cells (Figure 1.38 E). From hemispherical the body soon becomes almost
spherical once more, and then oval, the internal cavity enlarging
considerably and its mouth growing narrower (Figure 2.213). The form
which the Amphioxus-embryo has thus reached is a real "cup-larva" or
gastrula, of the original simple type that we have previously
described as the "bell-gastrula" or archigastrula (Figures 1.29 to
1.35).

As in all the other animals that form an archigastrula, the whole body
is nothing but a simple gastric sac or stomach; its internal cavity is
the primitive gut (progaster or archenteron, Figure 1.38 g, 1.35 d),
and its aperture the primitive mouth (prostoma or blastoporus, o). The
wall is at once gut-wall and body-wall. It is composed of two simple