NERVOUS SYSTEM OF THE VERTEBRATA. 427 



ment. The median lobe or vermiform process is first developed. 

 In the higher Mammalia the lateral parts forming the hemi- 



crls 



FIG. 250. LONGITUDINAL SECTION THROUGH THE BRAIN OF A CHICK OF TEN 



DAYS. (After Mihalkovics.) 



hms. cerebral hemispheres; alf. olfactory lobe; alf v olfactory nerve; ggt. corpus 

 striatum ; oia. anterior commissure; chd 3. choroid plexus of the third ventricle; 

 pin. pineal gland; cmp. posterior commissure; trm. lamina terminalis ; chin, optic 

 chiasma ; inf. infundibulum ; hph. pituitary body ; bgm. commissure of Sylvius (roof 

 of iter a tertio ad quartum ventriculum) ; vma. velum medulla: anterius (valve of 

 Vieussens); cbl. cerebellum; chd 4. choroid plexus of the fourth ventricle; obt 4. roof 

 of fourth ventricle; obi. medulla oblongata; pus. commissural part of medulla; inv. 

 sheath of brain; bis. basilar artery; crts. internal carotid. 



spheres of the cerebellum become formed as swellings at the 

 sides at a considerably later period, and are hardly developed in 

 the Monotremata and Marsupialia. 



The cerebellum is connected with the roof of the mid-brain in front and 

 with the choroid plexus of the fourth ventricle behind by delicate membranous 

 structures, known as the velum medullas anterius (valve of Vieussens) and 

 the velum medullas posterius. 



The pons Varolii is formed on the ventral side of the floor of the 

 cerebellar region as a bundle of transverse fibres at about the same time as 

 the olivary bodies. 



The mid-brain. The changes undergone by the mid-brain 

 are simpler than those of any other part of the brain. We 

 have already seen that the mid-brain, on the appearance of the 

 cranial flexure, forms an unpaired vesicle with a vaulted roof and 

 curved floor, at the front end of the long axis of the body (fig. 

 1 1 8, MB}. It is at this period in most Vertebrates relatively 

 much larger than in the adult ; and it is only in the Teleostei 

 that it more or less retains in the adult its embryonic pro- 

 portions. 



