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Dr. Cunningham—'Surface Anatomy of the Primate Cerebrum. 73

The mid-part of the collateral fissure .is thus occasionally a complete
fissure, and in the same sense as the fissura perpendicularis externa. It
may, like the latter, produce an infolding of the ventricular wall and an
internal elevation. These differ from the other complete fissures, both
transitory and permanent, in their time of origin. They do not appear
until, at least, two months later, when the primitive transitory furrows have
completely disappeared and the occipital lobe is thoroughly mapped out. It
is questionable, therefore, whether we should put them in the same category
as the others, because we can be by no means sure that after a lapse of
two months the same causes which produce the primitive complete fissures
are still operative. Still it should be noted that there appears to be
a tendency at this period to surround the base of the occipital lobe by a
circular infolding, the three different pieces of which are not in direct
union. On the outside we have the fissura perpendicularis externa, on the
inside the parietooccipital, and on the under surface the transverse mid-
collateral fissure. Further, as we have noticed, there appears to be a
tendency for the parietooccipital to run into the mid-collateral in the
early stages of both, although this connexion is afterwards obliterated by
the upheaval between them of the connecting piece of the gyrus
lingualis and the hippocampal convolution. There cannot be a doubt
but that the mid-collateral fissure, when present, belongs to this fissural
system.

XII. Summary.—1. At an early period in the development of the
cerebral hemispheres a series of deep infoldings of its thin walls make
their appearance. On the exterior of the hemisphere these show in the
form of sharply-cut linear fissures.

2. Certain of these fissures are permanent; the great majority are
transitory.

3. The transitory fissures, with two exceptions, have disappeared by the
time that the corpus callosum is fully formed.

4. A deficiency in the corpus callosum is associated with a persistence
of the temporary fissures.

5. The temporary fissures indicate an important stage in the growth of

BOYAL IEISH ACADEMY.-CUNNINGHAM MEMOIRS, NO. VII. [10]