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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 3

of an embryo which has barely reached the fifth week in which they are
present.

The choroid fissure occupies the lowest limit of the mesial wall, and
pursues a semilunar course. In front it reaches forward to the foramen of
Monro, whilst behind it curves downward, and extends towards the
extremity of the temporal lobe. It is formed by a deep infolding of the
mantle wall; and the fissure formed between the two layers of this fold
contains in the first instance no blood-vessels, and no extension from the
connective tissue which occupies the primitive longitudinal fissure and
represents the early condition of the falx major. As His remarks:
" The formation of the fold cannot therefore, as frequently happens, be
explained as a sinking in of the hemisphere wall through the ingrowing of
blood-vessels. The fold arises through an independent surface extension
of the epithelial layer." Later on it becomes occupied by the choroid
plexus. The two layers which form the fold remain epithelial, and do
not develope into nervous tissue.

The fissura arcuata {Bogenfurche of Arnold, or Ammonsfurche of Mihal-
kovics) makes its appearance on the mesial face of the hemisphere at a
higher level than the choroid fissure. Like the latter it pursues a curved
course, and between the two fissures a semicircular portion of the inner
surface of the hemisphere is marked off, which receives the name of arcus
marginalis (Randbogen of Schmidt). In front the fissura arcuata extends
into the frontal region, whilst behind it is carried round towards the
extremity of the temporal lobe.

His has recently shown that the fissura arcuata in the early stages of
its development is composed of two portions which he terms the " vordere
Bogenfurche," and the " hintere Bogenfurche." The former begins as a deep
furrow on the mesial face of the olfactory lobe, and takes an upward and
backward course. Becoming gradually shallower it ends above the forepart
of the choroid fissure. The hinder portion of the fissura arcuata is

* Die Fornientwickelung des menscliliclicn Vorderhirns vom ende des ersten bis zum Beginn
des dritten Monats—Des xv. Bandes der Abhandlungen der niatbematisch-pbysischen Classe
der KonigL Sachsischcn Gesfcllschaft der Wissenschaften. No. Tin. Leipzig, 1889, pp. 694 and
712.

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