68 H. V. NEAL 



His ('88), Dohrn ('91), Chiarugi ('97), Neal ('98), Carpenter 

 ('06), Gast ('09), Belogolowy ('10). Appearing at first as proto- 

 plasmic outgrowths from the base of the midbrain, the anlage 

 of the nerve elongates in later stages in the direction of their 

 terminal organ — the first somite — with which it quickly becomes 

 connected. 



In the second place, the nerve, in all stages of differentiation 

 in embryonic material, manifests a fibrillation in the form of 

 coarse, deeply-staining fibrils which appear connected with the 

 deeply staining processes of midbrain neuroblasts. On the basis 

 of analogy with phenomena seen in the histogenesis of Rohon- 

 Beard cells and in preparations of the living nerve fiber in cover- 

 glass preparations, such evidence points toward the neuroblastic 

 origin of the neurofibrils. In no sections prepared by any of 

 the neurological methods used, is evidence to be found of the 

 fusion of centripetally growing fibers with processes of medullary 

 cells in the manner suggested by Paton ('07) . 



Finally, the inference is further strengthened by the absence 

 of any positive evidence in favor of the cell-chain hypothesis. 

 It is true that cells appear along the nerve anlage from very 

 early stages in histogenesis, but none of these cells show the 

 familiar form and staining properties of neuroblasts. On the 

 basis of this evidence the conclusion appears warranted that 

 each of the neuraxons of the oculomotor is the product of one 

 or more medullary neuroblasts and not the product of the fusion 

 of a chain of cells. The analogy with the histogenesis of spinal 

 somatic motor nerves is clear. 



Burckhardt ('92) describes medial and lateral niduli of the 

 oculomotor in Protopterus, confirming Ahlborn's ('84) descrip- 

 tion of the central, relations of the nerve in Petromyzon. Ahl- 

 born's assertions, however, are not confirmed by Johnston ('05) 

 who finds the nidulus of the oculomotor to be that of a somatic 

 motor nerve. All investigators agree that the nidulus of the 

 oculomotor in selachians belongs in the somatic motor column. 



e. By what means does increase in the length of the neuraxon 

 take place? The oculomotor develops under conditions which 

 differ somewhat from those which have been described for spinal 



