THE BRAIN 



333 



is broad, thin, and flattened to form the ependymal layer (Figs. 334 B and 335) . 

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



\ N. hypoglossus 

 N. accessorius 



N. vagus 



Alar plate 



Sulcus 

 limitans 



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. 



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 



Inner layer 



Roof plate 



Traclus solilarins 



Formatio rettciilari ^i isi i 



Formalw nluiiliii!, ilhi 



Rhombic lip 

 ■stiform body 



n it tract of 

 t) igeminus 



I isls from 

 jte 



]I II II j/ / i\ir 



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. 



plate is broadened, especially in the cranial portion of the myelencephalon, and 

 the alar plates bulge laterally (Figs. 336 and 337 A). The cavity of the myelen- 



