336 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



nerves lie lateral to this sulcus. The enlarged cuneate and gracile nuclei 

 bound the ventricle caudally and laterally as the cuneus and clava. The 

 inferior olivary nuclei produce lateral, rounded prominences, and ventral 

 to these are the large cerebro -spinal tracts, or pyramids. 



The Metencephalon. Cranial to the lateral recesses of the fourth 

 ventricle, the cells of the alar plate proliferate ventrally and form the 

 numerous and relatively large nuclei of the pons. The axons from the cells of 

 these nuclei mostly cross to the opposite side and form the brachium pontis 

 of the cerebellum. Cerebral fibers from the cerebral peduncles end about 

 the cells of the pontine nuclei. Others pass through the pons as fascicles 

 of the pyramidal tracts. 



A 



Mesencephalon 



i 



Cerebellum 



ft : '^ 6U ^^^- 



Ependymal layer 



Posterior medullary velum Anterior medullary velum 



FIG. 338. Median sagittal sections of the metencephalon and adjoining parts. A, from a 24 

 mm. embryo; B, from a 150 mm. fetus. 



The Cerebellum. When the alar plates of the cranial end of the myel- 

 encephalon are bent out laterally, the caudal portions of their continua- 

 tions into the metencephalic region are carried laterally also. As a result, 

 the alar plate of the metencephalon takes up a transverse position and 

 forms the anlages of the cerebellum (Fig. 337 A). During the second 

 month the paired cerebellar plates thicken and bulge into the ventricle 

 (Fig. 338 A). Near the mid-line, paired thickenings indicate the anlages 

 of the vermis, while the remainder of the alar plates form the anlages of the 

 lateral lobes, or cerebellar hemispheres (Figs. 337-6 and 353). 



The cerebellar anlages grow rapidly both laterally and in length, so 

 that their surfaces are folded transversely. During the third month their 

 walls bulge outward and form on either side a convex lateral lobe, connected 

 with the pons by the brachium pontis (Fig. 337 C). In the meantime the 

 anlages of the vermis have fused in the mid-line, producing a single struc- 

 ture marked by transverse fissures. The rhombic lip gives rise to the 

 flocculus and nodulus. Between the third and fifth months the cortex 



