THE NERVOUS SYSTEM 549 



Along the ventral wall of the mesencephalon develop two parallel fiber 

 tracts, the cerebral peduncles, visible on the ventral side of the brain stem, 

 which contain most of the ascending and descending fibers connecting the 

 hemispheres with the lower parts of the body. Just dorsal to these, 

 the tegmentum is differentiated as a region of interwoven fibers inter- 

 spersed with masses of nerve cells. 



Metencephalon. The chief modifications of the vesicle of the meten- 

 cephalon are the cerebellum which develops from the alar plates, and the 

 pons which is differentiated from the basal plates. 



Four anlagen are involved in the cerebellum. From a pair of small 

 thickenings near the median line the vermis is developed, while a larger 

 pair of lateral masses form the cerebellar hemispheres. The formation 

 of the cerebellar cortex resembles that of the cerebral. The relatively 

 rapid growth of the cortex results in the formation of lobules and fissures 

 of the adult cerebellum. The dentate nucleus is a cellular mass which 

 develops within the white matter of the cerebellum. Three large bundles 

 of fibers grow from the cerebellum, the brachium pontis, brachium con- 

 junctivum, and the restiform body. 



The cavity of the metencephalon forms the anterior part of the fourth 

 ventricle. 



Myelencephalon. The fifth and last of the brain vesicles forms the 

 medulla oblongata. Its ventricle enlarges to form the posterior part of 

 the fourth ventricle, which is partly obliterated by a chorioid plexus 

 developed from the thin dorsal roof of the medulla. Early in develop- 

 ment, the alar and basal plates of the medulla show a series of five expan- 

 sions, the rhombomeres or neuromeres, which disappear in ontogenesis 

 and have been interpreted as evidence of a primitive metamerism corre- 

 sponding to that of the trunk and possibly, it is suggested, inherited from 

 the metameric ganglia of invertebrate ancestors. The motor fibers of the 

 trigeminal, abducens, facial, glossopharyngeal, vagus, accessory, and 

 hypoglossal nerves arise from nuclei located in the medulla. The growth 

 of nerve fibers in all directions within the wall of the medulla results in the 

 reticular formation which is strikingly shown in cross section. Among 

 the nuclei which develop in the medulla are the nucleus cuneatus and 

 nucleus gracilis, the sensory centers of the fibers which pass into the 

 medulla from the spinal cord by way of the fasciculus cuneatus and 

 fasciculus gracilis. 



DEVELOPMENT OF THE SPINAL CORD 



When the neural plate becomes the neural tube, its wall is a simple 

 columnar epithelium. In consequence of more rapid cell proliferation, 

 limited to a layer of germinal cells which lie near the lumen of the cord, 



