370 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



is first developed. In the higher Mammalia the lateral 

 parts constituting the hemispheres of the cerebellum 

 become formed as swellings at the sides at a consider- 

 ably later period; these 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 me- 

 dullse anterius (valve of Yieussens) (Fig. 119 vma) and the velum 

 medullse 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. It is represented in Birds by 

 a small number of transverse fibres on the floor of the hind-brain 

 immediately below the cerebellum. 



The mid-brain. The changes undergone by the 

 mid-brain are simpler than those of any other part of 

 the brain. It forms, on the appearance of the cranial 

 flexure, an unpaired vesicle with a vaulted roof and^ 

 curved floor, at the front end of the long axis of the 

 body (Fig. 67, MB). It is at this period in Mammalia 

 as well as in Aves relatively much larger than in the 

 adult: its cavity is known as the iter a tertio ad 

 quartum ventriculum or aqueductus Sylvii. 



The roof of the mid-brain is sharply constricted 

 off from the divisions of the brain in front of and 

 behind it, but these constrictions do not extend to the 

 floor. 



In Mammalia the roof and sides give rise to two 

 pairs of prominences, the corpora quadrigemina. 



These prominences, which are simply thickenings 

 not containing' any prolongations of the iter, become 



