carpenter: development of the oculomotor nerve. 203 



(Balfour, '75). In this view lie was supported by Beard ('88). Dohrn 

 ('88 l , '9l) also saw the migration of medullary elements in selachians, 

 and C. L. Herrick ('93) affirms that in amphibians, reptiles and mam- 

 mals nerve-forming cells issue from the niduli of ventral nerve roots. 

 Piatt ("96) is of the opinion that in Necturus the motor nerves are 

 formed by emigrant bipolar cells derived from the neural tube. Among 

 recent observers who have likewise seen medullary cells migrating out 

 into the ventral roots of spinal nerves may he mentioned Harrison (:Ol) 

 and Neal (:03). The former made his observations upon Salmo salar, 

 the latter upon Squalus acanthias. 



The interpretations put upon these emigrant cells have been various. 

 They have been thought by some observers to be the cells which, as- 

 suming a moniliform arrangement, give rise to the neuraxons of the 

 nerves; they have been considered to be ectodermal additions to the 

 mesodermal cells which lie among the peripheral processes of centrally 

 located neuroblasts, and to be destined later to participate in the forma- 

 tion of the sheath of Schwann ; and, finally, they have been looked 

 upon, in part at least, as undeveloped ganglion cells. 



It is not unusual to meet with ganglion cells in the ventral roots of 

 the spinal nerves of adult animals. Freud ('78) describes such cells 

 in the ventral roots of the spinal nerves of Petromyzon. Schafer ('81) 

 and Kolliker ('94 a ) found ganglion cells similarly situated in the cat. 

 Thompsen ('87) figures what he believes to be ganglion cells in various 

 stages of degeneration in the roots of the third and fourth cranial nerves 

 in man, and his interpretation has been accepted by Gaskell ('89), and 

 more recently by Barratt (:Ol), to account for the presence of amorphous 

 fibrillar or granular masses observed by them in the human oculomotor 

 and trochlear nerves. Taking into consideration the observations which 

 have been made on the growth of the oculomotor nerve in the chick, the 

 suggestion is here offered that these degenerated ganglion cells may have 

 had their origin as indifferent cells migrating out from the mid-brain into 

 the roots of these nerves in an early embryonic stage. Becoming dif- 

 ferentiated here into ganglion cells, they may later have undergone a 

 more or less complete disintegration, owing to their failure to attain to a 

 condition of functional activity. 



The chief opponent of the view that medullary cells migrate out from 

 the neural tube has been W. His (see His, '89), and it is doubtless 

 largely owing to his prestige that the fact of migration, together with 

 the role played by the emigrant cells in nerve formation, has attracted 

 comparatively little attention. The followers of His have accounted for 



