1. The skeleton. A light, solid framework must be developed for the
body. 2. The appendages start as fins, and end as the legs and arms
of man. 3. The circulatory and respiratory systems developed so as
to carry with the utmost rapidity and certainty fuel and oxygen to
the muscular and nervous high-pressure engines. Or, to change the
figure, they are the roads along which supplies and munitions can be
carried to the army suddenly mobilized at any point on the frontier.
4. Above all, the brain, especially the cerebrum, the crown and goal
of vertebrate structure. The improvement is now practically
altogether in the animal organs of locomotion and thought. Still,
among these animal organs, the lower systems will lead in point of
time. The brain must to a certain extent wait for the skeleton.

1. The skeleton. The axial skeleton consists, in the lowest fish, of
the notochord, a cylindrical unsegmented rod of cartilage running
nearly the length of the body. This is surrounded by a sheath of
connective tissue, at first merely membranous, later becoming
cartilaginous or gristly. Pieces of cartilage extend upward over the
spinal marrow, and downward around the great aortic artery, forming
the neural and hæmal arches. These unite with the masses of
cartilage surrounding the notochord to form cartilaginous vertebræ,
which may be stiffened by an infiltration of carbonate of lime. The
vertebral column of sharks has reached this stage. Then the
cartilaginous vertebræ ossify and form a true backbone. I have
described the process as if it were very simple. But only the
student of comparative osteology can have any conception of the
number of experiments which were tried in different groups before
the definite mode of forming a bony vertebra was attained. At the
same time the skull was developing in a somewhat similar manner. But
the skull is far more complex in origin and undergoes far more