THE BRAIN 



333 



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 (Figs. 334 B and 335). In the 



\ N. hypoglossus 

 N. accessorius 



N. vagus 



Alar plate 



Sulcus 

 limitans 



Ganglion 

 jugulare 



FIG. 335. Transverse sections through the myelencephalon of a 10.2 mm. embryo (His). 

 X 37. A, Through the nuclei of origin of the spinal accessory and hypoglossal nerves; B, 

 through the vagus and hypoglossal nerves. 



Inner layer 



Roof plate 



Tractus soliiarius 



Fortnatio reticularis grisea 



Formatio reticularis alba 



Rhombic lip 

 Restiform body 



Spinal tract of 

 N. trigeminus 



Ncuroblasts from 

 alar plate 



Marginal layer 



N. hypoglossus Septum medulla Neuroblasts from alar plate 



(Rudiment of accessory olive) 



FIG. 336. Transverse section through the myelencephalon of a 22 mm. embryo (His). X 10. 



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

 ependymal zones are differentiated as in the spinal cord (Fig. 335). 

 Owing to the formation of the pontine flexure at the beginning of the 

 second month, the roof plate is broadened, especially in the cranial portion 

 of the myelencephalon, and the alar plates bulge laterally (Figs. 336 and 



