me. This was a later production. The primitive skeleton was
the notochord, still appearing in the embryos of all vertebrates and
persisting throughout life in fish. This is an elastic rod of
cartilage, lying just beneath the spinal marrow or nerve-cord, which
runs backward from the brain. The nerve-centres are therefore here
all dorsal, and the notochord or skeleton lies between these and the
digestive or alimentary canal. The skeleton of the clam or snail is
purely protective and a hindrance to locomotion. That of the insect
is almost purely locomotive, but external, that of the vertebrate
purely locomotive and internal. It does not lie outside even of the
nervous system, although this system especially required, and was
worthy of, protection. It does not protect even the brain; the skull
of vertebrates is an after-thought. It is almost the deepest seated
of all organs. But lying in the central axis of the body it
furnishes the very best possible attachment for muscles. Around this
primitive notochord was a layer of connectile tissue which later
gave rise to the vertebrae forming our backbone.

[Illustration: 10. CROSS-SECTION OF AXIAL SKELETON OF PETROMYZON.
HERTWIG, FROM HIEDERSHEIM.
_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