THE NERVOUS SYSTEM 247 
have expanded enormously, particularly anteriorly and dorsally, 
and their median surfaces are flattened against one another in 
front of the lamina terminalis, which forms the anterior boundary 
of the telencephalon medium (Figs. 148, 149). Posteriorly the 
cerebral hemispheres extend to about the middle of the dien- 
cephalon and their lateral faces are rounded. The lateral walls 
of the hemispheres have become enormously thickened to form 
the corpora striata (Figs. 147 and 151 A), and the superior and 
lateral walls have remained relatively thin, forming the mantle 
of the cerebral hemispheres (pallium). Thus the cavity of the 
lateral ventricle is greatly narrowed. 
The dissection (Fig. 147) shows the corpus striatum of the 
right side forming the lateral wall of the hemisphere, and extend- 
ing past the aperture (foramen of Monro) between the lateral 
and third ventricles towards the recessus opticus, where it comes 
to an end. 
The olfactory part of the hemispheres is not well differen- 
tiated from the remainder in the chick embryo of eight days. 
There is, however, a slight constriction on the median and ventral 
face (Fig. 147) which may be interpreted as the boundary of the 
olfactory lobe. 
The telencephalon medium is crowded in between the hemi- 
spheres and the diencephalon; its cavity forms the anterior end 
of the third ventricle, and communicates anteriorly through two 
slits, the foramina of Monro, with the lateral ventricles in the 
hemisphere. In Fig. 147, the upper and lower boundaries of 
the foramen of Monro, are indicated by the grooves on either 
side of the posterior end of the corpus striatum. A hair intro- 
duced from the third ventricle into the lateral ventricle through 
the foramen of Monro in the position of the arrow in Fig. 147, 
‘an be moved up and down over the whole width of the striatum. 
The lateral walls of the telencephalon medium are formed by 
the posterior ends of the corpora striata and are therefore very 
thick. 
The lamina terminalis passes obliquely upwards and forwards 
cavity. Oes., Oesophagus. p. A., Pulmonary arch. par., Paraphysis. P.C., 
Pericardial cavity. Rec. op., Recessus opticus. R., Rectum. 3. Inf., Saccus 
infundibuli. T., Tongue. Tel., Med. Telencephalon medium. Tr., Trachea. 
V. 1, 10, 20, 30, First, tenth, twentieth and thirtieth vertebral centra.  r. A., 
right auricle. Vel. tr., Velum transversum.  V. 0. m., Omphalomesenteric 
vein. V.umb., Umbilical vein. 
