368 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



The thin roof of the fourth ventricle thus formed 

 is somewhat rhomboidal in shape. 



At a later period the blood-vessels of the pia 

 mater form a rich plexus over the anterior part of 

 this thin roof which becomes at the same time some- 

 what folded. The whole structure is known as the 

 tela vasculosa or choroid plexus of the fourth ventricle 

 (Fig. 119, chd 4). The floor of the whole hind-brain 

 becomes thickened, and there very soon appears on its 

 outer surface a layer of longitudinal non-medullated 

 nerve-fibres, similar to those which first appear on the 

 spinal cord (p. 252). 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 fourth ventricle is a shallow continuation of the 

 anterior fissure of the spinal cord. 



Subsequently to the longitudinal fibres already spoken of, 

 there develope 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 con- 

 trary, becomes somewhat thickened and forms a band- 

 like structure roofing over the anterior part of the 

 fourth ventricle (Fig. 39 cb). 



This is a rudiment of the cerebellum, and in all 

 Craniate Vertebrates it at first presents this simple 

 structure and insignificant size. 



In Birds the cerebellum attains a very considerable 

 development (Fig. 119 cbl), consisting of a folded central 



