THE NERVOUS SYSTEM. 



491 



(olivo-cerebellar fibers) pass across the median line (seventh or eighth week) to 

 the opposite dorsal border where they, together with axones from the lateral 

 nuclei and the continuation from the cord of Flechsig's tract, form (end of 

 the second month) the bulk of the restiform body (Fig. 417). At three months 

 the olives have acquired their characteristic folded appearance. 



Owing to the later development and ventral migration of the alar plate 

 neuroblasts, there are thus formed the various nuclei which lie external to the 

 reticular formation in the adult. The continuations of ascending spinal cord 



Outer part of rhombic 

 lip migration 



Inner part of r. 1. mig. 

 Inner layer 

 Tractus solitarius 

 Marginal layer 

 Mantle layer 



Ext. arcuate fibers 

 Int. arcuate fibers 





Septum Beginning white N. XII Gray reticular 

 medullae reticular formation formation 



FIG. 415. Half of a transverse section of the medulla of a 10.5 mm. human embryo 

 (end of fifth week). His. 



tracts (Flechsig and Gowers) occupy the most external position on the lateral sur- 

 face, and other cord continuations (medial fillets) the most external mesial 

 positions. Later, however (fifth month), there is added ventral to the fillets 

 the descending cortico-spinal fibers (pyramids). Their decussation takes 

 place at the cervical flexure. 



By the external accessions from the alar plate above described, forming 

 terminal nuclei of overlapping tracts from above (especially the nucleus of 

 the spinal V) , the tractus solitarius becomes buried, as it were, hence its deep 

 position in the adult. The great development of the reticular formation 

 may contribute to this result. As the trigeminus is the most cephalic rhombic 



