332 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



319 A). Into the ependymal roof of the myelencephalon blood-vessels grow and, 

 invading the lateral recesses, form there the chorioid plexus of the fourth ventricle. 

 The plexus consists of small finger- like folds of the ependymal layer and its cover- 

 ing mesenchymal layer. The line of attachment of the ependymal layer to the 

 alar plate is known as the rhombic lip and later becomes the taenia and obex of the 

 fourth ventricle (Fig. 319 B). 



The further growth of the myelencephalon is due (1) to the rapid formation 

 of neuroblasts, derived from the ependymal and mantle layers; (2) to the de- 

 velopment of nerve fibers from these neuroblasts; (3) to the development and 

 growth into it of fibers from neuroblasts in the spinal cord and in other parts of 

 the brain. 



The neuroblasts of the basal plates early give rise chiefly to the efferent fibers 

 of the cerebral nerves (Fig. 317). They thus constitute motor nuclei of origin 

 of the trigeminal, abducens, facial, glossopharyngeal, vagus complex and hypo- 

 glossal nerves, nuclei corresponding to the ventral and lateral gray columns of the 

 spinal cord. The basal plate also produces part of the reticular formation which 

 is derived in part also from the neuroblasts of the alar plate (Fig. 318). The 

 axons partly cross as external and internal arcuate fibers and form a portion of 

 the median longitudinal bundle, a fasciculus corresponding to the ventral ground 

 bundles of the spinal cord. Other axons grow into the marginal zone of the same 

 side and form intersegmental fiber tracts. The reticular formation is thus differ- 

 entiated into a gray portion situated in the mantle zone and into a white portion 

 located in the marginal zone (Fig. 318). The marginal zone is further added to 

 by the ascending fiber tracts from the spinal cord and the descending pyramidal 

 tracts from the brain. As in the cord, the marginal layers of each side remain 

 distinct, being separated by the cells of the floor plate. The alar plates differ- 

 entiate later than the basal plates. The afferent fibers of the cerebral nerves 

 first enter the mantle layer of the alar plates and coursing upward and downward 

 form definite tracts (tractus solitarius, descending tract of fifth nerve). To these 

 are added tracts from the spinal cord so that an inner gray and an outer white 

 substance is formed. Soon, however, the cells of the mantle layer proliferate, 

 migrate into the marginal zone and surround the tracts. These neuroblasts of the 

 alar plate form groups of cells along the terminal tracts of the afferent cerebral 

 nerves (which correspond to the dorsal root fibers of the spinal nerves) and con- 

 stitute the receptive or terminal nuclei of the fifth, seventh, eighth, ninth and tenth 

 cerebral nerves. Caudally, the nucleus gracilis and nucleus cuneatus are developed 

 from the alar plates as the terminal nuclei for the afferent fibers which ascend from 



