THE NERVOUS SYSTEM 251 
short spur runs forward, a still shorter one ventrally, and the 
longest arm extends backwards, gradually fading out beyond 
the middle of the diencephalon. This fissure is not a continuation 
of the sulcws Monroi, or backward prolongation of the foramen 
of Monro, but is, on the contrary, entirely independent. 
The lateral thickenings of the diencephalon constitute the 
thalami optici, each of which may be divided into epithalamic, 
mesothalamic, and hypothalamic subdivisions. In the chick at 
eight days there is a deep fissure between the epi- and meso- 
thalamic divisions (the thalamic fissure, Fig. 147). The substance 
of the epithalamus forms the ganglion habenule. The meso- 
thalamic and hypothalamic divisions are not clearly separated. 
The transition zone between the diencephalon and mesencephalon 
is sometimes called the metathalamus. 
The mesencephalon has undergone considerable changes since 
the third day. The dorso-lateral zones have grown greatly in 
extent, at the same time becoming thicker, and have evaginated 
in the form of the two large optic lobes. Hence the median 
portion of the roof is sunk in between the lobes (Fig. 147), and is 
much thinner than the walls of the lobes. The dorso-lateral 
zones and roof thus form a very distinct division of the mesen- 
cephalon, known as the tectum lobi optici. The ventro-lateral 
zones and floor have thickened greatly and form the basal divi- 
sion of the mesencephalon. The ventricle of the mesencephalon 
thus becomes converted into a canal (aqueduct of Sylvius), from 
which the cavities of the optic lobes open off. 
In the metencephalon likewise there is a sharp distinction 
between the development of the dorso-lateral zones and roof, 
on the one hand, and the ventro-lateral zones and floor on the 
other. From the former the cerebellum develops in the form 
of a thickening overhanging the fourth ventricle. This thick- 
ening is relatively inconsiderable in the middle line (cf. Figs. 148 
and 150). Thus the future hemispheres of the cerebellum are 
tooth. Eust., Eustachian tube. Gn. 1, 13, First and thirteenth spinal 
ganglia. Gon.,Gonad. Hem., Hemisphere. Lag., Lagena. Lg., Lung. M., 
Mantle of Hemisphere. Msn., Mesonephros. Olf. L., Olfactory lobe. Olf. 
N., Olfactory nerve. P. C., Pericardial cavity. Pz. 5, The fifth post-zyga- 
pophysis. R.C. 1, 2, Last two cervical ribs. R. th. 1,5, First and fifth tho- 
racic ribs. S. pe-per., Septum pericardiaco-peritoneale. S’r., Suprarenal. 
Symp., Main trunk of the sympathetic. Str., Corpus striatum. V. 1, 10, 
20, 30, First, tenth, twentieth and thirtieth vertebral arches. V.C. I., Vena 
cava inferior. V. L. L., Ventral ligament of the liver. 
