posterior commissure, In front of these structures the roof of the fore-brain is epithelial, 
and remains so during life. It constitutes the epithelial roof of the third ventricle, and 
it becomes involuted along the middle line into the cavity by the choroid plexuses of the 
ventricle. The posterior commissure appears as a transverse thickening at the bottom of 
a transverse groove which appears in the roof of the early brain-tube behind the pineal 
diverticulum. 
* Cerebral Hemisphere.—The cerebral hemisphere is derived entirely from the alar 
section of the lateral wall of the telencephalon. From this it grows out and soon assumes 
very large dimensions. At first it grows forwards and upwards, and a distinct fissure, 
the early incisura longitudinalis cerebri, appears between the cerebral hemispheres of 
opposite sides. The separation of the two cerebral vesicles by the longitudinal fissure 
begins at the end of the first month. This fissure becomes occupied by mesoblastic tissue, 
554 THE NERVOUS SYSTEM. 
UPINEAL DIVERTICULUM 
| METATHALAMUS 
THALAMUS 
’ 
‘ 
‘ 
C We aD) 
4 / Mf \ 
OPTICN. 
LAMINA 
TERMINALIS 
a PITUITARY DIVERTICULIIM 
OPTIC 
RECESS CHIASMA 
Fic, 412.—Two Drawines By His, illustrating the development of the human brain. 
A, Median section through a foetal human brain in the third month of development. 
B, Schema showing the directions in which the cerebral hemisphere expands during its growth. 
P.M.H. Pars mammillaris hypothalami. M. Mammillary region. O. Occipital lobe. 
P.O.H. Pars optica hypothalami. F. Frontal lobe. T. Temporal lobe. 
P. Parietal lobe. 
which later on forms the falx cerebri. The cerebral hemisphere, in its further growth, is 
carried progressively backwards over the hinder parts of the developing brain. At the 
end of the third month it has covered the optic thalamus. A month later it reaches the 
corpora quadrigemina, and by the seventh month it has not only covered these but also 
the entire upper surface of the cerebellum. 
At the end of the first month the rhinencephalon or nasal brain begins to be marked 
off from the cerebral vesicle. It is situated below the cerebral vesicle, and is joined to the 
corresponding region of the opposite side by the lamina cinerea. Very soon the rhinen- 
cephalon is divided by a constriction or furrow, termed by His the jisswra prima, into an 
anterior and a posterior part. From the anterior part the olfactory tract and bulb grows 
out in the form of a hollow diverticulum, whilst the basal part of the same division 
of the rhinencephalon forms the olfactory trigone and the area of Broca (p. 528). The 
posterior part of the rhinencephalon furnishes the anterior perforated space and the gyrus 
subcallosus, 
In the floor of the hollow cerebral hemisphere a thickening takes origin, and this 
ultimately is developed into the corpus striatum. On the outer surface of the vesicle this 
thickening is seen to correspond to a depression which constitutes the early Sylvian fossa, 
the further development of which is described on p. 515, 
In the earlier stages of its development the cerebral hemisphere is a thin-walled vesicle 
with a relatively large cavity, which represents the primitive condition of the lateral 
ventricle. At first the vesicle is bean-shaped and the cavity is curved. At this stage the 
outline is very similar to that presented by the cerebral hemisphere of a quadruped, and 
there is little or no trace of an occipital lobe or of a posterior horn of the lateral ventricle. 
As development goes on, however, the occipital portion of the hemisphere grows backwards 
over the cerebellum in the shape of a hollow protrusion, and a distinct occipital lobe 
enclosing the posterior horn of the lateral ventricle is the result. This developmental 
stage, which is distinctive of man and the apes, begins about the fourth month. 
On the mesial aspect of the cerebral hemisphere, in the early stages of its development, 
an invagination of the wall of the vesicle takes place into the cavity immediately above 
