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 tow^ards 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, 

 can 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. S. 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. o. m., Omphalomesenteric 

 vein. V. umb., Umbilical vein. 



