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Cunningham Memoirs.

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one which belongs to the same system as the intraparietal. The ontogenetic
development of the sulcus furnishes evidence of a still more
satisfactory nature.

In a previous chapter (Chapter I., pp. 67-69) we have seen: 1, that there
is little room for doubt that the fissura perpendicularis externa of Bischoff
is the representative of the " Affenspalte" in the apes; 2, that this is a complete
fissure, and produces a distinct bulging into the interior of the posterior
horn of the lateral ventricle; 3, that it is evanescent, and in almost
every case disappears before the sulcus transversus occipitalis shows on the
surface of the hemisphere; 4, that the " Affenspalte " of the apes is a complete
fissure, whilst the sulcus transversus occipitalis is not. These facts
of themselves are sufficient to show that there are very great difficulties
in the way of regarding these two fissures as homologous with each other.
But a study of the manner in which the terminal bifurcation of the ramus
occipitalis developes places the matter beyond a doubt. This is well
seen in figure 33, Plate I. The bifurcation (i. e. the sulcus transversus)
first makes its appearance towards the close of the seventh month or the
beginning of the eighth month.

In the specimen referred to (fig. 33, Plate I.), the lower limb of the
bifurcation is formed by a downward inclination of the terminal part of
the main stem of the ramus occipitalis, whilst the upper limb is present in
the form of a shallow depression behind the arcus parieto-occipitalis (first
annectant gyrus) which runs into the ramus occipitalis a short distance
from its extremity. Sometimes this upper limb remains separate for a
considerable time, and even when it joins the ramus occipitalis the point
of junction may be marked by a deep annectant gyrus. In twenty-eight
adult hemispheres I noticed such an annectant in three specimens.

But the terminal bifurcation of the ramus occipitalis is extremely
variable in its mode of development, and that exhibited in figure 33, Plate i.,
cannot be considered as being the typical or more usual mode. In its early
condition the ramus occipitalis commonly describes a U-shaped curve
around the arcus parieto-occipitalis (first annectant gyrus), and in many
instances the hinder limb of the U forms the upper limb of the later
terminal bifurcation (sulcus transversus), whilst the lower limb is a new