METENCEPHALON. 



419 



limitans of the rhomboid fossa, containing the }ovea superior and 

 the locus caruleus, hence, the pons comprises the greater portion 

 of the metencephalon, including the entire ventral zone and a 

 part of the dorsal zone. 



TABLE IX. 



DERIVATIVES OF METENCEPHALON (McMurrich, modified). 



Roof -plate 



Metencephalon Proper 



j Inferior velum 

 ) Cerebellar vermis 



Isthmus 



{ Superior velum, or valve of 

 Vieussens contains de- 

 cussation of trochlear 

 nerves. 



Dorsal Zones 



Ventral Zones ~{ 



Floor-plate 



f Lobes of cerebellum 

 | Terminal nuclei, of sensory 

 nerves (part of fifth and 

 eighth) 

 Ganglia of cerebellum ( ?) 

 Nucleus pontis ( ?) 

 Genetic nuclei of motor 

 nerves (fifth, sixth, sev- 

 enth) 



Nuclei centrales and nucleus 

 lateralis medius of the re- 

 I ticular formation 



Median raphe 



Brachium conjunctivum of 

 cerebellum grows through 

 it. 



i 



{ Inferior part of tegmen- 

 tum and of substantia 



\ nigra. 



The basis pedunculi grows 

 down through it. 



Median raphe. 



Cerebellum. It forms the roof of the upper part of the fourth 

 ventricle, that is as far down as the transverse fold of invaginated 

 roof-plate, called the plica chorioidea, produced by the meten- 

 cephalic flexure (Fig. 118, between II and I). Above the plica 

 chorioidea the dorsal laminae of the metencephalon thicken 

 rapidly and form a prominent transverse ridge on either side of 

 the median line. The two thickenings are partially separated 

 from each other in the median line by a deep ventricular sulcus 

 but joined dorsally by a thin bridge of tissue, the metencephalic 

 roof-plate. At this early stage there is no representative of the 

 vermis cerebelli, the ridges represent the hemispheres. But 

 very soon, cells from the dorsal laminae invade and thicken the 



