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Dk. Cunningham—Surface Anatomy of the Primate Cerebrum. 51

ape is the equivalent of the "stem" or anterior calcarine fissure in man. In
all probability it is preceded by a precursor with which it presents a direct
continuity of existence. If this be the case this precursor does not diminish
in length as in man. Its hinder end maintains its position near the
posterior extremity of the hemisphere, or even turns round this as may be
seen in some of the anthropoids.

In the development of the parietooccipital fissure one or other of two
courses may be followed. The precursor may be completely wiped out,
and its place taken later on by a cortical sulcus which does not in any way
impress the ventricular wall so as to produce an internal elevation, or it
may be retained and modified so as to form the permanent fissure. In the
latter, but not in the former case, the parieto-occipital must be included
amongst the complete fissures.

An admirable example of the development of the parieto-occipital as a
complete fissure is seen in the cerebrum depicted in figs. 28 and 29, PL i.,
This specimen was obtained from a foetus which had probably reached
the seventh month of development. In fig. 29 the fissure is exhibited as
it is seen on the mesial aspect of the hemisphere. Its great depth is
apparent. In fig. 28 the outer surface of the same brain is shown. The
outer wall of the posterior ventricular cornu has been removed, and the
intra-ventricular elevation corresponding to the parieto-occipital infolding
is evident. This is so broad and conspicuous that it quite absorbs and
appears to be continuous with the "stem" or anterior calcarine fissure;
an examination of the bottom of the fissure from the opposite aspect,
however, shows that they are distinct.

In contrast to this specimen examine that which is represented in
figs. 27 and 30, PI. i., and which is from a foetus of about the same stage
of development. Here the parieto-occipital is shallow, and produces no
intra-ventricular elevation. In fig. 27, which gives a view of the interior
of the posterior cornu of the ventricular cavity of the same brain, the
bulging on the inner wall is seen to be of a totally different character from
that in fig. 28. In other words, it is entirely calcarine in its origin.

But we have mentioned that even in cases where in the early cerebrum
the parieto-occipital is a complete fissure, the bulging which it forms on the