DEVELOPMENT OF PARTS DERIVED FROM FORE-BRAIN. 553 
Basal Lamina.—The part of the diencephalon and telencephalon which lies below 
the level of the sulcus of Monro retains its primitive form, and undergoes only slight change. 
Consequently, when this region in the adult brain is compared with the corresponding 
region in the embryonic brain, 
the resemblance between the 
two is very striking. 
In the fore-brain, therefore, 
it is the alar lamina which 
plays the predominant part in 
the formation of the cerebrum. 
The value, also, of the basal 
part of the wall of this portion 
of the neural tube is. still 
further reduced by the fact 
that it no longer contains the 
nuclei of origin of efferent 
nerves. The highest of these 
nuclei (the oculo-motor) is 
placed in the mesencephalon. 
The region of the fore-brain 
which lies below the sulcus of 
Monro is termed the hypo- 
thalamus. The part of this 
which corresponds to the 
diencephalon is called the 
pars mammillaris hypothalami, 
whilst the part in front, 
which belongs to the telen- 
cephalon, receives the name of 
pars optica hypothalami. 
From the pars mammillaris 
hypothalami are derived the 
corpus mammillare and a 
portion of the tuber cinereum. 
With the pars optica hypo- 
thalami are associated the 
following parts, viz. the tuber A, Reconstruction of the fore-brain and mid-brain of His’s embryo KO ; 
profile view. B, Same brain as A, divided along the mesial plane 
and viewed upon its inner aspect. 
M, Mammillary eminence; T.c, Tuber cinereum; Hp, Hypophysis 
Fig. 411.—Two DrawiINcs OF THE EMBRYONIC BRAIN (by His). 
cinereum, with the infundi- 
bulum and the cerebral part 
of the pituitary body, the optic 
chiasma, the optic recess, and 
the lamina cinerea. 
The corpora mammillaria 
(pituitary diverticulum from buccal cavity); Opt, Optic stalk ; 
TH, Optic thalamus ; Tg, Tegmental part of mesencephalon ; P.s, 
Pars subthalamica ; C.s, Corpus striatum ; F.M, Foramen of Monro ; 
L, Lamina terminalis ; R.O, Recessus opticus ; R.i, Recessus infun- 
dibuli. 
form, in the first instance, a 
relatively large downward bulging of the floor of the brain-tube. As development goes 
on this bulging becomes relatively small, and about the fourth month the single pro- 
jection becomes divided into the two tubercles. 
The infundibulum and posterior or cerebral lobe of the pituitary body are developed 
as a hollow downward diverticulum of the floor of the telencephalon (p. 508). A portion 
of the original cavity is retained in the upper part of the infundibulum, and constitutes 
the infundibular recess in the floor of the third ventricle. 
The optic nerve is chiefly formed by the passage of fibres backwards from the retina 
in the wall of the original optic stalk, whilst the chiasma takes form by the transit of 
fibres across the middle line in front of the infundibulum and behind the optic recess. To 
a large extent these fibres are derived from the optic nerve. The optic recess of the third 
ventricle marks the spot where the hollow optic vesicle originally bulged out from the 
lower and lateral part of the fore-brain, and in the adult it therefore represents a portion 
of the primitive cavity of the tubular stalk of the optic vesicle. In the course of develop- 
ment the optic nerve fibres, which appear in the stalk of the optic vesicle to form the 
optic nerve, seek an attachment much further back, and through the optic tract they are 
even carried as far as the mesencephalon. 
The roof of the fore-brain remains thin, and does not proceed to the development 
of nervous elements, except in its posterior part. Here it forms the pineal body and the 
