FROM TWELVE ‘TO THIRTY-SIX SOMITES 149 
two lateral walls, is a useful one. Each lateral wall may also 
be divided into a dorsal and ventral zone, the former of which 
is related to the sensory nerve roots and the latter to the motor. 
Cerebral Flexures. The cerebral flexures correspond to the 
cranial and cervical flexures of the entire head already described. 
Their form and rate of progress may be more readily learned 
from the figures (Figs. 67, 73, 83, etc.) than from any verbal 
description. Only the cranial flexure is permanent, and the angle 
thus formed ventrally in the floor of the mid-brain is known as 
the plica encephali ventralis. A third flexure is formed later in 
the anterior portion of the hind-brain, by a ventral bending of 
the floor which is barely indicated in the period now under de- 
scription, but becomes much more pronounced later; this is known 
as the pontine flexure. 
We may now take up separately the changes in each of the 
primary cerebral vesicles. 
The Prosencephalon. The principal events in the early de- 
velopment of the prosencephalon are: (a) the separation of the 
optic vesicles; (b) the delimitation of the tel- and diencephalon; 
(c) special differentiation of the walls. 
(a) A section across the optic vesicles of the 12s chick shows 
the prosencephalon as a central division with its cavity widely 
confluent with the cavities of the optic vesicles. This wide com- 
munication is rapidly narrowed by a ventrally directed fold of the 
roof at the line of junction of the optic vesicles and prosencephalon 
proper (Fig. 84); the fold also involves to a certain extent the 
anterior and posterior line of junction. In the 20s embryo the 
connection of the optic vesicles and prosencephalon has been re- 
duced in this way to about one third of its original diameter 
(from actual measurements), forming a narrow tubular stalk, the 
optic stalk, attached to the ventral portion of the fore-brain 
(Figs. 73 and 74); the cavities of the optic vesicles are still con- 
tinuous through the stalk with the cavity of the prosencephalon, 
dipping into the recessus opticus; the ventral wall of the optic 
stalk thus becomes continuous with the floor, and the dorsal wall 
with the lateral wall of the prosencephalon (Fig. 84). Growth 
of the mesenchyme situated above the original optic stalk appears 
to be an active factor in the separation; at least it grows at a 
rate sufficient to fill in the space produced by the constriction. 
At the same time there is a slight increase in the dorso-ventral 
