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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 199

as absolutely satisfactory. I do not agree with Ecker and Mihalkovics in
considering it usual for the two hinder portions of the sulcus to appear
earliest on the field. As a rule it is the sulcus postcentralis inferior which
first shows itself, and its development is clearly associated with that of the
fissure of Rolando and the prsecentral furrow, as well as with the three
primitive sulci on the surface of the island of Reil. The more usual order
of formation is the fissure of Rolando first, then the prsecentral furrow, and
lastly the inferior postcentral sulcus. In some cases the prsecentral sulcus
precedes the fissure of Rolando, but I have only seen one hemisphere in
which the sulcus postcentralis inferior was the earliest to appear on the
cerebral surface. These three furrows constitute a group of sulci which
radiate upwards from the Sylvian region, and, as a rule, they are all in
place towards the end of the sixth month.

In its earliest condition the sulcus postcentralis inferior appears in the
form of a broad shallow groove or depression, which is chiefly brought into
view by the rise of the ascending parietal convolution. It is very rarely
seen before the middle of the sixth month; in fact, it is usually developed
in the second half of that month, or in the early part of the seventh (PI. i.,
figs. 22, 25, and 24, p.; and PI. il, figs. 13, 17, 18, 19, and 20, p.1).

At the end of the sixth month, or more usually at the commencement
of the seventh month, the ramus horizontalis and the ramus occipitalis come
into view. It is difficult to decide which has the priority in point of development
. If there is any difference it is in favour of the ramus occipitalis.
Certainly it is this furrow which, as a general rule, assumes in the early
condition of both the greatest depth and forms the most decided impression
on the cerebral surface (PI. n., figs. 17, 19, and 20, p.3 and p.4). It is very
rare to find a cerebrum in which the one is developed before the other (PI.
ii., fig. 18, p.3).

The ramus occipitalis is closely associated with the appearance of the
parietooccipital fissure on the outer surface of the hemisphere. As the
upper end of the latter begins to show in the form of a broad groove on the
mesial border of the hemisphere, the ramus occipitalis comes into existence
and marks off the portion of the cerebral surface which ultimately forms
the first annectant gyrus. It takes the form, therefore, of a curved sulcus