primitive gut, and provide the cellular material for the middle layer.

Immediately after their formation the two coelom-pouches of the
Amphioxus are divided into several parts by longitudinal and
transverse folds. Each of the primary pouches is divided into an upper
dorsal and a lower ventral section by a couple of lateral longitudinal
folds (Figure 1.82). But these are again divided by several parallel
transverse folds into a number of successive sacs, the primitive
segments or somites (formerly called by the unsuitable name of
"primitive vertebrae"). They have a different future above and below.
The upper or dorsal segments, the episomites, lose their cavity later
on, and form with their cells the muscular plates of the trunk. The
lower or ventral segments, the hyposomites, corresponding to the
lateral plates of the craniote-embryo, fuse together in the upper part
owing to the disappearance of their lateral walls, and thus form the
later body-cavity (metacoel); in the lower part they remain separate,
and afterwards form the segmental gonads.

In the middle, between the two lateral coelom-folds of the primitive
gut, a single central organ detaches from this at an early stage in
the middle line of its dorsal wall. This is the dorsal chorda (Figures
1.83 and 1.84 ch). This axial rod, which is the first foundation of
the later vertebral column in all the vertebrates, and is the only
representative of it in the Amphioxus, originates from the entoderm.

In consequence of these important folding-processes in the primitive
gut, the simple entodermic tube divides into four different
sections:--

1. underneath, at the ventral side, the permanent alimentary canal or