334 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



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 through the pons as fascicles of the pyramidal tracts. 



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- 

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



cephalon takes up a transverse 



JVleseh cephalon 



Cerebellum 



Ependymal 

 layer 



B 



Mesencephalon 



Cerebellum 



position and forms the anlages 

 of the cerebellum (Fig. 319 A). 

 During the second month the 

 paired cerebellar plates thicken 

 and bulge into the ventricle 

 (Fig. 320 A). Near the mid- 

 line a thickening indicates the 

 anlage of the vermis, while the 

 remainder of the alar plates 

 form the anlages of the lateral 

 lobes or cerebellar hemispheres. 



The cerebellar anlages grow 

 rapidly laterally and also 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. 319 C). In the 

 meantime, the anlages of the 

 vermis have fused in the mid- 

 line producing a single structure 

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

 nodulus. Between the third and fifth months the cortex cerebelli grows more 

 rapidly than the deeper layers of the cerebellum and its principal lobes, folds 

 and fissures are formed (Fig. 319 C, D). The hemispheres derived from the 

 lateral lobes are the last to be differentiated. Their fissures do not appear until 

 the fifth month. 



Cranial to the cerebellum the wall of the neural tube remains thin dorsally 



Post. med. velum Am)med. velum 



Fig. 320. — Median sagittal section of the cerebellum 

 and part of mid-brain. A, from a 24 mm. embryo; B, 

 from a 150 mm. embryo. Ant. med. velum, anterior 

 medullary velum; Post. med. velum, posterior medullary 

 velum. 



