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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 49

is it evident in figs. 7,10, 14, 15, and 17 (c1). In these the process which
I have described may be observed at different stages.

The ground which was previously occupied by the portion of the
calcarine precursor which has disappeared is afterwards taken up by
another fissure—quite distinct and altogether different in its mode of
origin. I refer to that part of the -^-shaped fissural arrangement of the
adult which I have named the posterior calcarine sulcus. This is very
constant in its mode of origin, and is a secondary sulcus in every sense of
the word. It follows the interrupted mode of fissural development. In
other words, two punctiform depressions, or dimples, appear—one on the
occipital pole, and the other midway between this and the hinder end of
the anterior calcarine fissure or "stem" (PI. i., fig. 26, c2 and c3; also
PI. in., figs. 14, 15, 17, 21, &c, c2 and c3). These deepen, and then
show a tendency to run towards each other in the horizontal direction.
The anterior depression extends more actively than the posterior one, and
soon it approaches the "stem" in front, and its fellow-depression behind.
The two small portions of cerebral surface which intervene between the
three parts of the fissure represent the early condition of the two annectant
gyri, which we have observed concealed within the adult calcarine fissure,
viz. the gyrus cuneo-lingualis anterior and the gyrus cuneo-lingualis
posterior. In the course of time the three parts of the calcarine fissure
join over these intervening portions of cortex, which become pressed
down into the bottom of the fissure. The union which first takes place
is that between the intermediate piece and the stem ; the junction
between the intermediate piece and the hinder piece is of much later
occurrence.

The time at which these two punctiform depressions appear to form
the secondary posterior calcarine sulcus is very variable. In figs. 14
and 15, PI. in., they are well seen in a brain midway between the fifth
and sixth months. In figs. 12, 13, and 18, PI. in., older specimens
arc exhibited in which not a trace of the developing sulcus is to be seen.
Fig. 26, PI. in., represents the inner surface of a foetal brain close upon
the seventh month, and yet on it the two punctiform depressions are
merely seen in their initial stages; whilst in fig. 29 a brain is depicted very

ROYAL IRISH ACADEMY.-CUXXINGHA1I MEMOIRS, NO. YII. [? ]