36 THE NERVOUS SYSTEM 



The Rhombencephalon. The ventral part of the rhombencephalon, includ- 

 ing both alar and basal plates, thickens to form the pans and medulla oblongata 

 (Fig. 17). Most of the roof of this division remains thin and forms the epithelial 

 lining of the tela chorioidea of the fourth ventricle. But in the caudal portion 

 of the myelencephalon the lumen of the neural tube becomes completely sur- 

 rounded by thickened walls, forming the central canal of the closed portion of 

 the medulla. The posterior edge of the alar plate in the metencephalon becomes 

 greatly thickened and, fusing across the median line with the similar structure 

 of the opposite side, forms the anlage of the cerebellum (Figs. 17, 137). Later 

 we shall see that, in general, motor structures develop from the basal, and sen- 

 sory parts from the alar, plate. 



The table on page 35 gives in brief the principal derivatives of the 

 neural tube. 



