248 THE DEVELOPMENT OF THE CHICK 
from the recessus opticus to the region between the foramina of 
Monro. It is very thin, excepting near its center, where it is 
thickened to form the torus transversus, containing the anterior 
commissure. At its dorsal summit it 1s continuous with the 
roof of the telencephalon medium, which has formed a pouch- 
like evagination, the paraphysis. Just behind the paraphysis 
Fig. 149. — Median sagittal section of the brain of a chick embryo of 7 
days. (After v. Kupffer.) 
c., Cerebellum. ca., Anterior commissure. ed., Notochord. ch., Pro- 
jection of the optic chiasma. ep., Posterior commissure. e., Epiphysis. 
e’., Paraphysis. hy., Hypophysis. I., Infundibulum. It., Lamina termi- 
nalis. Lop., Optie lobe. M., Mesencephalon. Mt., Metencephalon. 
opt., Chiasma of the optic nerves. p., Parencephalon. ro., Recessus 
opticus. s., Saccus infundibuli. se., Synencephalon. tp., Mammillary 
tubercle. tp., Tuberculum posterius.  tr., Torus transversus. Tr., De- 
cussation of the trochlear nerves. Va., Velum medullare anterius. Vi., 
Ventriculus impar telencephali. vp., Velum medullare posterius. 
is the velum transversum, where the roof bends upwards sharply 
into the roof of the diencephalon. The epithelial wall around 
the bend is folded to form the choroid plexus of the third ven- 
tricle, which is continued forward into the lateral ventricle along 
