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 sulcus 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 habenulae. 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 lohi 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 foiu'th 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. Eiist., Eustachian tube. Gn. 1, 13, First and thirteenth spinal 

 ^angha. 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. pc-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. 



