cell-layers, the familiar primary germinal layers. The inner layer or
the invaginated part of the blastoderm, which immediately encloses the
gut-cavity is the entoderm, the inner or vegetal germ-layer, from
which develop the wall of the alimentary canal and all its appendages,
the coelom-pouches, etc. (Figures 1.35 and 1.36 i). The outer stratum
of cells, or the non-invaginated part of the blastoderm, is the
ectoderm, the outer or animal germ-layer, which provides the outer
skin (epidermis) and the nervous system (e). The cells of the entoderm
are much larger, darker, and more fatty than those of the ectoderm,
which are clearer and less rich in fatty particles. Hence before and
during invagination there is an increasing differentiation of the
inner from the outer layer. The animal cells of the outer layer soon
develop vibratory hairs; the vegetal cells of the inner layer do so
much later. A thread-like process grows out of each cell, and effects
continuous vibratory movements. By the vibrations of these slender
hairs the gastrula of the Amphioxus swims about in the sea, when it
has pierced the thin ovolemma, like the gastrula of many other animals
(Figure 1.36). As in many other lower animals, the cells have only one
whip-like hair each, and so are called flagellate (whip) cells (in
contrast with the ciliated cells, which have a number of short lashes
or cilia).

In the further course of its rapid development the roundish
bell-gastrula becomes elongated, and begins to flatten on one side,
parallel to the long axis. The flattened side is the subsequent dorsal
side; the opposite or ventral side remains curved. The latter grows
more quickly than the former, with the result that the primitive mouth
is forced to the dorsal side (Figure 1.39). In the middle of the
dorsal surface a shallow longitudinal groove or furrow is formed
(Figure 1.79), and the edges of the body rise up on each side of this