350 THE HIND-BRAIK. 



place its roof becomes in front very much extended and thinned out. 

 At the raphe, where the two lateral halves of the brain originally 

 united, a separation, as it were, takes j)lace, and the two sides of the 

 brain become pushed apart, remaining united by only a very thin 

 layer of nervous matter, consisting of a single row of flattened cells 

 (fig. 249). As a result of this peculiar growth in the brain, the roots 

 of the nerves of the two sides, which were originally in contact at the 

 dorsal summit of the brain, become carried away from one another, 

 and appear to arise at the sides of the brain. 



The thin roof of the fourth ventricle is triangular, or, in Mammalia, 

 somewhat rliomboidal in shape. The apex of the triangle is directed 

 backwards. 



At a later period the blood-vessels of the pia mater form a rich 

 plexus over the anterior part of the thin roof of the medulla, which 

 becomes at the same time somewhat folded. The whole structure is 

 known as the tela vasculosa or choroid plexus of the fourth 

 ventricle (fig. 250, chd 4). The floor of the whole hind-brain becomes 

 thickened, and there very soon appears on its outer surface a layer of 

 non-medullated nerve-fibres, similar to those which first appear on 

 the spinal cord. They are continuous with a similar layer of fibres 

 on the floor of the mid-brain, where they constitute the crura cerebri. 

 On the ventral floor of the medulla is a shallow continuation of the 

 anterior fissure of the spinal cord. 



In Elasmobranchii and many Teleostei the restiform tracts are well 

 developed, and are anteriorly continued into the cei'ebellum, of which 

 they form the peduncles. Near their junction with the cerebellum they 

 form prominent bodies, which are i-egarded by Miklucho-Maclay as repre- 

 senting the true cerebellum of Elasmobranchii. 



In Elasmobranchii a dorsal pair of ridges projects into the cavity of the 

 fourth ventricle, corresponding apparently with the fasciculi teretes of the 

 Mammalia. 



In Mammalia there develop, subsequently to the longitudinal fibres 

 already spoken of, first the olivary bodies of the ventral side of the medulla, 

 and at a still later period the pyramids. The fasciculi teretes in the cavity 

 of the fourth ventricle are developed shortly before the pyramids. 



When the hind-brain becomes divided into two regions the roof 

 of the anterior part does not become thinned out like that of the 

 posterior, but on the contrary, becomes somewhat thickened and forms 

 a band-like structure roofing over the anterior part of the fourth 

 ventricle (fig. 247 and fig. 253, cb). 



This is a rudiment of the cerebellum, and in all Craniate Ver- 

 tebrates it at first presents this simple structure and insignificant 

 size. In Cyclostomata, Amphibia and many Reptilia this condition 

 is permanent. In Elasmobranchii, on the other hand, the cerebellum 

 assumes in the course of development a greater and greater pro- 

 minence (fig. 248, ch), and eventually overlaps both the optic lobes 

 in front and the medulla behind. In the later embryonic stages it 

 exhibits in surface-views the appearance of a median constriction, and 



