THE NERVOUS SYSTEM. 



527 



entiating formatio reticularis, until they are arrested at the septal marginal layer 

 (Figs. 454 and 455). 



From these neuroblasts which remain in situ near the dorsal border are de- 

 veloped the nucleus gracilis and nucleus cuneatus. The axones of these nuclei 

 form internal arcuate fibers which decussate and form a bundle of longitudinal 

 fibers in the opposite septal marginal layer ventral to the reticularis alba. 

 This tract is the medial fillet whose fibers appear during the second month 

 and is one of the afferent paths to suprasegmental structures (mid-brain roof 



Inner rhombic furrow 



Rhombic lip 

 Outer rhombic furrow- 

 Alar plate \ 

 Sulcus limitans 



Tractus solitarius 

 Inner layer 



N. X (medullary XI) 

 Mantle layer 

 Marginal layer 



Basal plate 



Beginning of gray 

 reticular formation 



Floor plate F.r.a. N. XII Internal arcuate fibers 



(forming septum medullae) 



Fig. 452. — Half of a transverse section of the medulla of a 9.1 mm. human embryo 



(during the fifth week). His. 



The arrow is in the inner median sulcus. F. r. a., beginning of white reticular formation. 



and pallium). Other neuroblasts, which probably migrate further, form the 

 substantia gelatinosa of Rolando. Axones of this group also form tracts repre- 

 senting afferent paths to suprasegmental structures (pallium). Neuroblasts 

 which migrate further form, as already mentioned, afferent cerebellar con- 

 nections. Those migrating to the septal marginal layer form there an 

 L-shaped mass mesial to the root fibers of the XII cranial nerve (Fig. 455). 

 This is the medial accessory olive. Fresh groups of neuroblasts, added laterally 

 to these in streaks, form the inferior olivary nucleus, while others which have not 

 advanced so far form the lateral nucleus. Axones of the olivary neuroblasts 



34 



