segmentation-cavity of the blastula (Figure 1.38 C, h) is not quite
globular, but forms a flattened spheroid with unequal poles of its
vertical axis. While the blastula is being folded into a cup at the
vegetal pole of its axis, the difference in the size of the
blastodermic cells increases (Figure 1.38 D, E); it is most
conspicuous when the invagination is complete and the
segmentation-cavity has disappeared (Figure 1.38 F). The larger
vegetal cells of the entoderm are richer in granules, and so darker
than the smaller and lighter animal cells of the ectoderm.

But the unequal gastrulation of the amphioxus diverges from the
typical equal cleavage of the Sagitta, the Monoxenia (Figure 1.29),
and the Olynthus (Figure 1.36), in another important particular. The
pure archigastrula of the latter forms is uni-axial, and it is round
in its whole length in transverse section. The vegetal pole of the
vertical axis is just in the centre of the primitive mouth. This is
not the case in the gastrula of the amphioxus. During the folding of
the blastula the ideal axis is already bent on one side, the growth of
the blastoderm (or the increase of its cells) being brisker on one
side than on the other; the side that grows more quickly, and so is
more curved (Figure 1.39 v), will be the anterior or belly-side, the
opposite, flatter side will form the back (d). The primitive mouth,
which at first, in the typical archigastrula, lay at the vegetal pole
of the main axis, is forced away to the dorsal side; and whereas its
two lips lay at first in a plane at right angles to the chief axis,
they are now so far thrust aside that their plane cuts the axis at a
sharp angle. The dorsal lip is therefore the upper and more forward,
the ventral lip the lower and hinder. In the latter, at the ventral
passage of the entoderm into the ectoderm, there lie side by side a
pair of very large cells, one to the right and one to the left (Figure