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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 47

these infoldings as the precursors of the parieto-occipital and calcarine
fissures.

One or other, or perhaps in some cases both, of these precursors may
be retained in part or in whole, and ultimately enter into the formation of
their respective fissure or fissures. Most frequently, however, one disappears,
whilst the whole or a part of the other is retained. That which is obliterated
is replaced later on (in the fifth month or in the beginning of the
sixth month) by the permanent furrow; and this takes up very much the
same ground as its forerunner, although it does not show an unbroken
continuity of existence with it. I am satisfied that under no circumstances
do both of the precursors entirely disappear again to be replaced by permanent
successors. In the very large collection of foetal brains which I
possess, representing as they do every stage in the development of the
cerebral surface from the third month upwards, there is not one in which
there is not some part of this fissural system present.

The cerebral hemispheres which are represented in Plate in., figs. 3, 4,
5, and 6, afford us examples of the manner in which the precursor.of the
parieto-occipital fissure [p. o.) disappears, whilst the precursor of the calcarine
fissure maintains its position. In these the parieto-occipital precursor
is seen in various stages of obliteration. About the fifth month
of intra-uterine life, therefore, it is not uncommon to find hemispheres in
which the parieto-occipital fissure is totally absent, although the calcarine
is present (fig. 15, PI. i.). Sometimes, however, at this stage a faint
trace of the parieto-occipital may be visible (figs. 4, 5, and 6, PI. in.,
and fig. 16, PI. i.). It is often hard to determine in these cases whether
the faintly-marked parieto-occipital fissure is in course of obliteration or in
process of formation. As a rule, a prominence on the inner surface of the
ventricular wall corresponds with the weakly marked fissure, and this, taken
in conjunction with the fact that, in the reappearance of the calcarine
fissure in cases where a portion of its precursor has been obliterated, I
have failed to detect such a projection, makes me incline to the view
that the parieto-occipital in this instance is really in process of obliteration.
His* has called attention to the fact that in certain foetal brains of the

* Unsere Korpcrform, Neunter Brief, p. 114.