THE BEAIN 



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- 



Alar plate 



Sulcus 

 limitans 



Basal 

 plate 



Ganglion 

 jugulars 



I.Hypoglossus 

 \ N. accessor! us N. Vac/ us I Mrlypoa/o. 



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 plaie Tractus . soliiarius 



forwatio reticularis 

 grisea. 



Formdtio reticularis alba 



•Rhombic lip 

 Restiform body 



Spinal V. 



Neuroblasts from alar plate 

 Marginal layer 



N. XII Septum medullae neuroblasts from alar plats 



CRudiment 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. 



