THE BRAIN 



331 



mation 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. 318 and 319 A). The cavity of the myelen- 



Ganglion 

 jugulars 



V.Hypoq/ossus 

 \ N. accessor/us N- vagus ' N.HypoqlossusI 



FIG. 317. Transverse sections through the myelencephalon of a 10.6 mm. embryo (His). A, 

 through the nuclei of origin of the spinal accessory and hypoglossal nerves; B, through the vagus and 

 hypoglossal nerves (after His). 



Inner^ layer 



Roof plate 



Tractus sol'iiarius 



Formai'io 



Formatio retfcularis alba 



Rhombic lip 

 Restiform body 



Spinal V. 



Neuroblash from alar plate 



Marginal layer 



N. XII Septum medullae weuroblasts from a lor plate 



(.Rudimeni of accessory olive) 



FIG. 318. Transverse section through the myelencephalon of an eight weeks' human embryo (His). 



cephalon is thus widened from side to side and flattened dorso-ventrally. This 

 is most marked cranially where, between the alar plates of the myelencephalon 

 and metencephalon, are formed the lateral recesses of the fourth ventricle (Fig. 



