380 THE BABBIT. 



but much more pronounced in the later stages, the posterior 

 lobes of the mid-brain on the eighteenth day (Fig. 151 ) com- 

 pletely overlapping the cerebellum, BL. 



The roof of the mid-brain gives rise to the corpora quadri- 

 gemina. A transverse furrow first appears, dividing it into a 

 larger anterior, and a smaller posterior portion ; the anterior 

 portion soon becomes divided, by a median longitudinal groove, 

 into the anterior lobes of the corpora quadrigemina, or nates ; 

 the posterior portion, overhanging the cerebellum, is not divided 

 until a much later stage. 



The floor of the mid-brain forms the very sharp bend at the 

 base of the brain which has already been noticed (Figs. 150 and 

 151) ; as in the chick, it becomes greatly thickened on the 

 formation of the longitudinal pillars of nerve fibres known as 

 the crura cerebri (Fig. 154, cc), which connect the optic 

 thalami and corpora striata with the hind brain. 



The cerebellum is developed from the roof of the anterior part 

 of the hind-brain, in much the same way as in the chick. On 

 the tenth day (Fig. 147) a slight thickening is formed across 

 the roof of the anterior end of the hind-brain ; by the twelfth 

 day this has become more conspicuous (Fig. 150, bl), but is 

 still only a slightly thickened transverse band. By the eigh- 

 teenth day (Fig. 151, bl) it has increased considerably in 

 thickness ; and, shortly after this stage, it becomes folded trans- 

 versely on itself, as in the chick (cf. Fig. 116, bl). Secondary 

 foldings appear on its surface, and the complicated structure of 

 the adult cerebellum is gradually acquired. 



Of the several parts of the adult cerebellum (cf. Fig. 152), 

 the median lobe, or vermis, is the first to be formed ; the lateral 

 lobes and floccular lobes appearing at a much later date. 



Immediately in front of the cerebellum, between it and the 

 mid-brain, the roof of the brain becomes extremely thin, forming 

 the velum medullaa anterius (Fig. 153, va), which is ultimately 

 reduced to a single layer of epithelial cells devoid of nervous 

 elements. 



The medulla oblongata is formed from the floor of the 

 hind-brain, and from the part posterior to the cerebellum. The 

 floor of the medulla oblongata remains thin in the actual mid- 



