BRAIN REGENERATIOX IX AMBLYSTUMA 



207 



Hardesty ('04) has showii that the dividmg elements of the 

 primary ependyma, the germmal cells, give rise to a nuclear 

 layer just outside of the epend>ina from which later develop 

 the neuroblasts and the spongioblasts. It is c(mcei\able that 

 the differentiation of the nuclear layer into ner\e cells results 

 from the stimulus derived from the ingrowth of nerve fibers 



Fig. 3 TransvcM-so section of embryo without the right heiiiispluTc or nasal 

 placode three months after tlie operation, showing the thin ( urtain across tlie 

 interventricular foramcii. X 50. 



from othei- definiti\e ])arts of the nervous system. Evidence 

 of this is seen in the fact that the fiber tracts of the telencephalon 

 do not develop until the olfactory nerve fibers have grown into 

 the peripheral part of the hemis])here from the nasal placode. 

 The possibility of such a stimulus has been used by Kappers 

 ('14) in his neuro-biotaxis theory of the phylogenetic migration 

 of nuclear centers in the central nervous system. 



