_SS_, skeletogenous layer; _Ob_, _Ub_, dorsal and ventral processes
of _SS_; _C_, notochord; _Cs_, sheath of notochord; _Ee_, elastic
external layer of sheath; _F_, fatty tissue; _M_, spinal marrow;
_P_, sheath of _M_.]

The nervous system on the dorsal surface of the notochord consists
of the brain in the head and the spinal marrow running down the
back. The brain of all except the very lowest vertebrates consists
of four portions: 1. The cerebrum, or cerebral lobes, or simply
"forebrain," the seat of consciousness, thought, and will, and from
which no nerves proceed. Whether the primitive vertebrate had any
cerebrum is still uncertain. 2. The mid-brain, which sends nerves to
the eyes, and in this respect reminds us of the brain of insects.
Its anterior portion appears from embryology to be very primitive.
3. The small brain, or cerebellum, which in all higher forms is the
centre for co-ordination of the motions of the body. 4. The medulla,
which controls especially the internal organs. The spinal marrow, or
that portion of the nervous system which lies outside of the head,
is at the same time a great nerve-trunk and a centre for reflex
action of the muscles of the body. But the development of these
distinct portions and the division of labor between them must have
been a long and gradual process.

We have every reason to believe that here, as in insects, the head
has been formed by annexation of segments from the rump and the
fusion of their nervous matter with that of the brain. But here,
instead of only three segments, from nine to fourteen have been
fused in the head to furnish the material for the brain. Notochord
and backbone may be the most striking and apparent characteristic of
vertebrates, but their predominant characteristic is brain. On this