-epithehal, and undergoes 
GENERAL OUTLINE OF DEVELOPMENT OF THE BRAIN. 443 
those structures which occupy the interpeduncular space in the base of the brain 
(viz. the posterior perforated spot, the corpora mammillaria, and the tubercinereum); 
whilst the roof or dorsal 
wall remains thin and 
no nervous development. 
The hollow cerebral 
hemispheres soon outstrip 
all the other parts of the 
brain in their development. 
They expand not only in 
an upward and_ forward 
direction, but chiefly in a 
backward direction; and 
by their excessive growth 
backwards they gradually 
come to overlie the dien- 
cephalon, the mesenceph- 
alon, and at last the parts 
derived from the rhomb- 
encephalon. It thus comes 
about that, when the adult 
brain is viewed from above 
nothing but the cerebral 
hemispheres are visible—all 
the other parts of the brain 
he under cover of them. 
At first the cavity of 
each cerebral hemisphere 
is connected with the cavity 
of the front portion of the 
fore-brain by an exceedingly 
short but relatively wide 
passage. This is the early 
condition of the foramen of 
(3 
Re, 
~N 
Ey 
Sd 
Fic. 329.—TuHe BRAIN OF A HUMAN EMBRYO IN THE FIFTH WEEK 
(from His). 
A, Brain as seen in profile. B, Mesial section through the same brain. 
M, Mammillary eminence; Tc, Tuber cinereum; Hp, Hypophysis 
Monro. The fore-part of the (pituitary diverticulum from buccal cavity); Opt, Optic stalk ; 
fore-brain 1s: now seen to be TH, Optic thalamus; Tg, Tegmental part of mesencephalon ; Ps, 
bounded in front between Pars subthalamica ; Os, Corpus striatum ; FM, Foramen of Monro ; 
L, Lamina terminalis ; RO, Recessus opticus ; Ri, Recessus infun- 
the two hollow cerebral,  gipui. 
hemisphere-pouches by a 
narrow thin strip, which represents the extreme anterior wall of the neural tube, 
and consequently it receives the name of lamina terminalis. The cavity of the 
fore-brain not only in its hinder diencephalic part, but also in its anterior part (7.e. 
the part from which the cerebral hemispheres bud out), persists as the third 
ventricle of the brain, whilst the cavities of the primitive cerebral hemispheres 
are represented in the adult by the lateral ventricles of the brain. The foramina 
of Monro, relatively much reduced in size, are preserved as narrow throats of 
communication between the lateral ventricles and the third ventricle. The 
olfactory lobes are formed as hollow outgrowths from the cerebral hemispheres. 
Flexures of the Brain-tube.—At a very early period, and while the changes 
detailed above are being carried on, the cerebral portion of the neural tube becomes 
sharply bent upon itself at certain points. The first flexure which occurs is the 
primary cephalic flexure. It occurs in the region of the mesencephalon, and involves 
the entire head. The fore-brain becomes bent in a ventral direction round the 
fore-end of the notochord and the fore-gut, until the long axis of the fore-brain 
forms an acute angle with the long axis of the hind-brain and the ventral wall 
of the one comes to lie nearly parallel with the corresponding wall of the other. 
Through this curvature the mid-brain is considerably modified in form, and for a 
time it comes to occupy the most prominent and foremost part of the embryonic head. 
