308 OUTLINES OF CHORDATE DEVELOPMENT 



remains thin and non-nervous, and ultimately forms the cho- 

 roid plexus of the fourth ventricle. The ventral and ventro- 

 lateral walls become greatly thickened as the medulla oblongata 

 or spinal bulb. This region becomes flexed ventrally forming 

 the pontine flexure (Fig. 124). The cervical flexure, previously 

 mentioned, is gradually disappearing, but the original cranial 

 flexure of the mesencephalic region remains as a permanent 

 feature of the brain. 



Posteriorly the elongated myelencephalon passes into the 

 spinal cord. This is of lesser diameter than the medulla, and 

 at first is vertically elongated with somewhat thickened lateral 

 walls. Its cavity, the central canal, continuous with the fourth 

 ventricle, is narrow but deep. The lateral walls continue to 

 thicken and finally the cord becomes approximately circular 

 in section, and its cavity is reduced to a very small tube by the 

 fusion of its dorsal walls. The median dorsal and ventral 

 walls of the cord remain thin. As the lateral walls begin to 

 thicken their constituent cells become highly differentiated. 

 The original epithelial elements of the neural tube become dif- 

 ferentiated as the ependymal cells; these are non-nervous, sup- 

 porting cells. The free ends of the ependymal cells, bordering 

 the central canal, become ciliated. The embryonic nerve cells, 

 or germinal cells, originally scattered through the epithelial 

 cells, multiply rapidly and form both neuroblasts, which give 

 rise to the gray matter of the cord, and non-nervous supporting 

 cells or glia cells. From the neuroblasts various processes grow 

 out, for the most part remaining within the central nervous 

 system and forming in part the white matter. Some of the 

 neuroblasts of the ventro-lateral regions (ventral cornua) send 

 out processes which leave the central system as the components 

 of the ventral or motor (efferent) roots of the spinal nerves. 



From the functional standpoint the most important facts 

 concerning the development of the central nervous system 

 are the histogenetic processes going on in the different regions 

 of the walls of the brain and cord, especially the establishment 

 of the various centers (nuclei) and tracts. Although of the 

 greatest importance and interest, all such details lie without 



