n the reptilian brain.

[Illustration: From _Cat. R.C.S. England_.

FIG. 23.--Lateral view of cerebral hemisphere of Gorilla
(_Anthropopithecus gorilla_).]

It will now be seen that the original neural canal, which is lined by
ciliated epithelium, forms the ventricles of the brain, while
superficial to this epithelium (_ependyma_) the grey and white matter
is subsequently formed. It has been shown by His that the whole neural
tube may be divided into _dorsal_ or _alar_, and _ventral_ or _basal_
laminae, and, as the cerebral hemispheres bud out from the dorsal part
of the anterior primary vesicle, they consist entirely of alar
laminae. The most characteristic feature of the human and anthropoid
brain is the rapid and great expansion of these hemispheres,
especially in a backward direction, so that the mesencephalon and
metencephalon are hidden by them from above at the seventh month of
intra-uterine life. At first the foramina of Munro form a
communication not only between the third and lateral ventricles, but
between the two lateral ventricles, so that the cavity of each
hemisphere is continuous with that of the other; soon, however, a
median longitudinal fissure forms, into which the mesoderm grows to
form the falx, and so the foramina of Munro are constricted into a
V-shaped canal. In the floor of the hemispheres the corpora striata
are developed at an early date by a multiplication of nerve cells, and
on the external surface a depression, called the _Sylvian fossa_,
marks the position of the future central lobe, which is afterwards
hidden as the lips of the fossa (_opercula_) gradually close in on it
to form the Sylvian fissure. The real fissures are complete infoldings
of the whole thickness of the vesicular wall and produce swellings in
the cavity. Some of them, like the choroidal on the mesial surface,
are developed very early, while the vesicle is little more than
epithelial, and contain between their walls an inpushing of mesoderm
to form the choroid plexus. Others, like the hippocampal and
calcarine, appear in the second and third months and correspond to
invaginations of the nervous tissue, the hippocampus major and minor.
The sulci appear later than the fissures and do not affect the
internal cavity; they are due to the rapid growth of the cortex in
certain areas. The corpus callosum and fornix appear about the third
month and their