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De. Cunningham—Surface Anatomy of the Primate Cerebrum. 29

by its surroundings, and its very existence depends, as His has pointed out,
upon the strong u Briickenkrtimmung " of the embryonic primate brain.
This backward thrusting also of the posterior part of the hemisphere is
chiefly instrumental in preserving the precursory calcarine and parietooccipital
fissures.

The transitory fissures on the outer surface of the cerebral hemisphere
make their appearance, as we have noted, about the beginning of the third
month, and undergo obliteration toward the end of the fourth month, viz.
at the time when the occipital lobe becomes clearly mapped out as a distinct
portion of the cerebrum. This is a circumstance which is deeply suggestive
and significant. The majority of observers who have studied the transitory
fissures are agreed in ascribing their formation to a more rapid growth of
the hemisphere wall than of the skull-capsule within which it is enclosed.
The necessary result of such a growth-restraint is, that the thin wall of the
hemisphere becomes folded along lines which run at right angles to the
axis of growth-energy. But no one has attempted to explain why, at this
period, the growTth-rate of the skull-capsule and of the contained cerebrum
should be at variance with each other. We may, I think, take for granted
that the cerebral inf olclings occur only in the embryonic brain of primates.
Schmidt failed to observe them in sheep, oxen, or pigs, although he
thought he saw weak traces of them in the embryo of the cat.* This
latter statement I cannot verify. Although I have looked for the transitory
fissures in cat and dog embryos of different stages, I have never seen
any infolding of the cerebral wall that could be compared with those we
have described in connexion with the human brain. It is, however, a
question which requires further investigation.

The temporary fissures, therefore, are in all probability peculiar to the
primates; they occur at a stage of growth prior to the appearance of a
well-marked occipital lobe 5 the great majority of them are effaced when
this portion of the cerebrum is moulded into shape; and, lastly, their
formation appears to be clue to a want of harmony between the growth-
rate of the cerebrum and of the skull-capsule. How can these facts be
explained ?

* "Beitrage zur Entwicklungsgeschichte des Gehirns," Zeitsch. f. "Wiss. Zool., Bd. xi., 186.