THE NERVOUS SYSTEM. 



529 



segment, its descending fibers are not overlapped by fibers from above and 

 therefore occupy the most external position of all these descending peripheral 

 systems. 



Inner layer Gray ret. form 

 I 



F.r.a. 



Rhombic lip migration 



Ext. arc. fib. in marg. layer N. XII F.r.a. v. Septum medullae 



FIG. 454. Half of a transverse section through the medulla of a 13.6 mm. human embryo 



(beginning of sixth week). His. 



F. r. a., Beginning of white reticular formation in dorsal part of septal marginal layer. 

 Another bundle has termed more ventrally (F. r. a. v.) . 



Inner layer 



Roof plate 



Tractus solitarius 



Formatio reticularis 

 grisea 



Formatio reticularis alba 



N. XII Septum 



medullae 



Spinal V 



Neuroblasts from alar plate 

 Marginal layer 



Neuroblasts from alar plate 

 (Rudiment of accessory olive) 



FIG. 455. Transverse section through the medulla of an 8 weeks' human embryo. His. 



The terminal nuclei belonging to the auditory (acustico-facialis-abducens) 

 segment are those of the vestibular and cochlear portions of the VIII nerve. 



