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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 277

the sulcus under consideration is to be referred to the inferior frontal
furrow, and the sulcus prsecentralis inferior has not yet appeared.

The terminal bifurcation or transverse piece of the inferior frontal
furrow does not come into existence until the middle of the seventh month
(fig. 60, p. 268), and often it is later in making its appearance (figs. 54 and
55, pp. 250 and 251). It may be developed separately, or in direct continuity
with the main stem of the furrow. The two secondary transverse furrows
which lie in front of this (Eberstaller's sulcus radiatus and lateral part of
the sulcus fronto-marginalis) are seen for the first time towards the end of
the eighth month or the beginning of the ninth month (fig. 33, PI. I.).

The sulcus frontalis superior and the sulcus prsecentralis superior appear
in the early part of the seventh month. It is not unusual, however, to
meet with them before this period. Thus, in figure 13, PI. n., which
represents the cerebral hemisphere of a foetus in the early part of the sixth
month, the sulcus prsecentralis superior is already evident (p. c. s.), whilst
in figures 23, 25, and 26, which are drawings of hemispheres of about the
same stage of development, traces of the superior frontal sulcus are apparent.

The first steps in the development of both of these furrows may be
studied in figures 17 to 20, Plate il, and later stages are exhibited in figures
62 and 63, pp. 274 and 276. The sulcus frontalis superior appears in two or
three pieces, which are placed one in front of the other. The whole process
points in the most unmistakable manner to the extremely close connexion
which exists between the sulcus prsecentralis superior and the first frontal
furrow, and all the conditions which we have noted in the adult can be
explained by the different modes of development of these sulci in the
foetal brain. The sulcus prsecentralis superior is usually formed in two
pieces—an upper and a lower. The basal or hinder part of the frontal
sulcus is most commonly developed in direct continuity with the lower
piece of the superior prsecentral sulcus (fig. 19, PI. n.3 p. c. s.). Sometimes,
however, it is continuous from the first with the upper part of the sulcus
proecentralis superior (PI. H.3 fig. 17, p. c. s.). Again, it is not unusual
to find the superior praecentral furrow separate in its earliest stages,
and then it is the lower part which, as a rule, appears first (PI. il, figs. 18
and 20, p. c. s.).