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74

Cunningham Memoirs.

the cerebrum, and are apparently associated with the mapping-out of the
occipital lobe.

6. A quadrupedal pause in the growth of the cranium brings the skull-
capsule into antagonism with the growth of the hemispheres, and in
consequence the wall of the cerebrum is thrown into folds. These folds
disappear as the occipital lobe assumes shape, owing to the expansion of
the cranial cavity, and a restoration of growth-harmony between the skull
and brain.

7. Consequently it is only in Primates, which alone possess well-
developed occipital lobes, that transitory infoldings of the cerebral wall
in all probability exist.

8. The two transitory fissures which do not disappear before the full
development of the corpus callosum are :—(a) the external calcarine, and
(b) the external perpendicular fissure of Bischoff. In point of fact, the
latter fissure does not appear until after the full development of the corpus
callosum.

9. The external calcarine fissure produces an infolding of the outer
wall of the posterior horn of the lateral ventricle, which presents the same
direction, and lies immediately opposite to the true calcarine infolding, or
calcar avis.

10. The fissure corresponding to this in the apes is, as a rule, permanent,
and in some species its anterior end forms, in the adult, a bulging on the
outer wall of the ventricle.

11. The external calcarine fissure disappears before the sixth month of
foetal life in man.

12. The external perpendicular fissure is present on the outer surface of
the foetal cerebrum from the beginning of the fifth month to the end of
the sixth month.

13. It is a complete fissure and corresponds to the " Affenspalte " on
the ape's brain.

14. The " Affenspalte " on the ape's brain is also a complete fissure,
and presents a well-marked bulging on the outer wall of the posterior horn
of the ventricle; but, unlike the external perpendicular fissure of the
human foetus, it is permanent.