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. 



Mantle layer 



Inner layer Gray ret. form 

 1 _ 





% N A- i 



JX_F.r.a. 



Rhombic lip migration 



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



Fig. 434. — 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 formed more ventrally (F. r. a. v.) . 



Inner layer 



Roof plate 



Tractus solitarius 



iomic lip 



Formatio reticularis 

 grisea 



Formatio reticularis alba 



J^Spinal V 



Neuroblasts from alar plate 



Marginal layer 



Neuroblasts from alar plate 



N. XII Septum (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. 



