330 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



THE DERIVATIVES OF THE NEURAL TUBE 



THE LATER DIFFERENTIATION OF THE SUBDIVISIONS OF THE BRAIN 

 Myelencephalon. We have seen that the wall of the spinal cord differen- 

 tiates dorsally and ventrally into roof plate and floor plate, laterally into the basal 

 plate and alar plate. The boundary line between the basal and alar plates is the 



Hoof plate 



Mantle 

 layer 



Roof plat 



Sulcus limitans 



Ependymal 

 layer 



Alar plate 

 S. limitans 



Basal plate 



mm vf irai^ 1 ' - /- / 

 vllir x /J 



Spina.1 



qana/ion 



Ventral spinal root 



FIG. 316. Transverse sections. A, through the upper cervical region of the spinal cord in a 10 mm. 

 human embryo; B, through the caudal end of the myelencephalon of the same. X 44. 



sulcus limitans (Fig. 316 A.). The same subdivisions may be recognized in the 

 myelencephalon. It differs from the spinal cord, however, in that the roof plate 

 is broad, thin and flattened to form the ependymal layer (Fig. 316 B.). In the 

 alar and basal plates of the myelencephalon the marginal, mantle and ependymal 

 zones are differentiated as in the spinal cord (Fig. 317 A, B). Owing to the for- 



