The foramen of Monro becomes very much narrowed
and reduced to a mere slit.
The walls are at first nearly uniformly thick, but


378 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
Fie. 122,


Zo


mo


Brain oF A THREE Montus’ Human EMBRYO: NATURAL SIZE.
(From Kélliker.)


1. From above with the dorsal part of hemispheres and mid-
brain removed ; 2. From below. f anterior part of cut wall
of the hemisphere ; f’. cornu ammonis ; tho. optic thalamus ;
est. corpus striatum; to. optic tract; em. corpora mammil-
laria ; p. pons Varolii.
the floor becomes thickened on each side, and gives rise
to the corpus striatum (Figs. 124 and 125 sé). The
corpus striatum projects upwards into each lateral ven-
tricle, and gives to this a somewhat semilunar form, the
two horns of which constitute the permanent anterior
and descending cornua of the lateral ventricles (Fig. 126
st).
With the further growth of the hemisphere the cor-
pus striatum loses its primitive relations to the de-
scending cornu. The reduction in size of the foramen
of Monro above mentioned is, to a large extent, caused
by the growth of the corpora striata.
The corpora striata are united at their posterior
border with the optic thalami. In the later stages of
development the area of contact between these two
pairs of ganglia increases to a large extent (Fig. 125),


XI1.] THE CORPORA STRIATA. 379


and the boundary between them becomes somewhat
obscure, so that the sharp distinction which exists
in the embryo between the thalamencephalon and
cerebral hemispheres becomes lost.
Fic, 123.
