336 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



cerebral nerves, derived from the basal plate, produce swellings in the floor of the 

 fourth ventricle which are bounded laterally by the sulcus limitans. The terminal 

 nuclei of the mixed and sensory cerebral nerves he 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 cerebrospinal tracts or pyramids. 

 The Metencephalon. — Crarual to the lateral recesses of the fourth ventricle 

 the cells of the alar plate prohferate ventrally and form the numerous rela- 

 tively 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. Cere- 

 bral fibers from the cerebral peduncles end about the cells of the pontine nuclei. 

 Others pass thrxaugh the pons as fascicles of the pyramidal tracts. 



A 



Mesencephalon ^j^:,; ■ ^'.'///^^^^^^ Mesencephalon 



Cerebellum— I ^^^^ Cerebellum 



Ependymal layer -^<2.s>^y^ 



Posterior medullary velum Anterior medullary velum 



Fig. 338.— Median sagittal section of the cerebellum and part of mid-brain. A, from a 24 mm. 



embryo; B, from a 150 mm. fetus. 



Cerebellum.— When the alar plates of the cranial end of the myelencephalon 

 are bent out laterally the caudal portions of their continuations into the meten- 

 cephahc region are carried laterally also. As a result, the alar plate of the meten- 

 cephalon 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 ^). Near the mid-line paired tliickenings indi- 

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

 aniages of the lateral lobes or cerebellar hemispheres (Figs. 337 B and 353). 



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

 surfaces are folded transversely. During the tliird month their walls bulge out- 

 ward 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 



