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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 53

and 29). This notcli appears at a variable period, and is the earliest indication
of the external parieto-occipital fissure. I have found it already
well-marked in a hemisphere from a foetus of about six and a-half months,
but often it is well on in the seventh month before it is formed.

We have seen that in the adult and nearly mature foetus a deep annec-
tant gyrus frequently intervenes between the upper and lower parts of the
parieto-occipital fissure. In the foetus, although we remark that the lower
part is narrow and fissure-like, and the upper part broad and open, we have
no evidence that the fissure is developed in two pieces, and I have not been
able to trace the manner in which the annectant gyrus intercuneatus is
formed. Still in the adult we know that the fissure sometimes occurs in
two pieces separated by this gyrus, which has risen to the surface. I have
seen this condition once in the 128 hemispheres I have examined. Sernoff,
of Moscow, has figured it. This would seem to indicate some tendency on
the part of the parieto-occipital fissure to adopt the interrupted form of
development.

I shall now proceed to take note of some variations in the development
of the parieto-occipital and calcarine fissures which I have observed in the
numerous specimens in my possession. Of course, as might be expected,
great differences are seen in different hemispheres in the relative lengths of
the " stem " or anterior calcarine fissure and the posterior calcarine sulcus.
This, I take it, indicates that the extent to which the hinder end of the
calcarine precursor is obliterated varies considerably within certain limits.
But there are many cases in which the obliteration only proceeds to a very
small extent, and then the greater length of the calcarine fissure is lineally
descended from its precursor. This may be considered to be an approach
to the condition present in the ape. It differs, however, in so far as
that under no circumstances whatever is the posterior calcarine sulcus
altogether absent, although it may in some cases be reduced to a small
portion on the occipital pole, and be developed from a single punctiform
depression.

But we occasionally meet with foetal hemispheres in which we have a
long deep fissure carried downwards from the upper margin of the hemisphere
to the gyrus fornicatus (PL in., figs. 18 and 19.) In such cases there
may not be a trace of any separation into a parieto-occipital part proper,