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30 Cunningham Memoirs.

We may assume that, although cranial and brain-growth, as a rule, go
on smoothly and evenly and in perfect harmony with each other, all steps
toward an advance of development must be initiated within the brain, and
that, for a time at least, the enclosing skull-capsule will resist these. This
being granted, we can readily understand that the tendency towards the
cerebral growth which gives rise to a well-mapped out occipital lobe is
more firmly impressed upon the brain than upon the skull. When the
primate head reaches in its development the quadruped stage, the
cerebrum goes on, without any intermission in its growth, towards the
higher development and the formation of a distinct occipital lobe. The
cranium, however, pauses in its growth. But this quadruped pause
marks only a stage in its evolution; it is merely temporary, although it
is of sufficient duration to produce the infoldings of the cerebral wall.

But against this view it may be argued that the temporary fissures appear
before the third month on the mesial face of the hemisphere. It must be
remembered, however, that the conditions of growth on the mesial face are
different from those on the outer surface of the hemisphere. In the first
place, the inner wall is distinctly thinner than the outer wall; and, as His
has pointed out: " Through the hemispheres being opposed to each other in
the mesial plane, and the space thus restricted, they exercise an influence
on each other. Instead of being able by bulging to push themselves out,
they are required to adapt themselves to the even and plain bounding
surface." *

I have already referred (p. 22) to the preservation of the temporary
fissures on the outer surface of a brain described by Dr. Hans Virchow. I
have reproduced a tracing from the photograph which the author gives of
this brain, and it is a matter of extreme interest to observe that the
cerebrum presents an outline similar to that of a brain between the third
and fourth months of intra-uterine life. The growth which would have
led to the mapping-out of a distinct occipital lobe has been arrested, and,
consequently, the temporary fissures have been retained. The general
similarity in outline to that of a quadruped brain is very marked.

Further evidence in support of the view which I have advanced might

* Unsere Korperfonn, Neunter Brief, p. 112.