groove in the shape of two parallel swellings. This groove is, of
course, the dorsal furrow, and the swellings are the dorsal or
medullary swellings; they form the first structure of the central
nervous system, the medullary tube. The medullary swellings now rise
higher; the groove between them becomes deeper and deeper. The edges
of the parallel swellings curve towards each other, and at last unite,
and the medullary tube is formed (Figures 1.83 m and 1.84 m). Hence
the formation of a medullary tube out of the outer skin takes place in
the naked dorsal surface of the free-swimming larva of the Amphioxus
in just the same way as we have found in the embryo of man and the
higher animals within the foetal membranes.

Simultaneously with the construction of the medullary tube we have in
the Amphioxus-embryo the formation of the chorda, the coelom-pouches,
and the mesoderm proceeding from their wall. These processes also take
place with characteristic simplicity and clearness, so that they are
very instructive to compare with the vermalia on the one hand and with
the higher vertebrates on the other. While the medullary groove is
sinking in the middle line of the flat dorsal side of the oval embryo,
and its parallel edges unite to form the ectodermic neural tube, the
single chorda is formed directly underneath them, and on each side of
this a parallel longitudinal fold, from the dorsal wall of the
primitive gut. These longitudinal folds of the entoderm proceed from
the primitive mouth, or from its lower and hinder edge. Here we see at
an early stage a couple of large entodermic cells, which are
distinguished from all the others by their great size, round form, and
fine-grained protoplasm; they are the two promesoblasts, or polar
cells of the mesoderm (Figure 1.83 p). They indicate the original
starting-point of the two coelom-pouches, which grow from this spot
between the inner and outer germinal layers, sever themselves from the