78 ELBERT CLARK 



a result the most varied experiments have been conducted and 

 a vast amount of evidence on all possible phases of the subject 

 has been presented. Unfortunately, however, the question re- 

 mains unsettled. Unprejudiced authors of text-books still include 

 both theories. 



Two distinct and opposite views relative to regeneration after 

 the embryonic nerve fiber stage are at present current. Accord- 

 ing to one, most vigorously and ably advocated by Bethe, the 

 embryonic nerve fiber is capable of producing per se a new medul- 

 lary sheath and new axis cylinder, which, later making connec- 

 tion with the central stump, results in a regenerated and func- 

 tioning medullated nerve fiber; in young animals at least, the 

 regenerated fiber is capable of conducting impulses regardless of 

 whether or not connection with the central stump is estabhshed. 

 The supporters of the contrarj^ theory claim that a new axis 

 cylinder for the peripheral stump is attained only by a down 

 growth of axis cylinders from the central stump. While there 

 is some difference of opinion as to minor points, this is the main 

 contention of those who advocate the 'outgrowth' theory. The 

 sequence of events as interpreted by the adherents of this theory 

 is briefly set forth by Halliburton ('07) as follows: 



From the microscopic study of the distal portions of divided nerve 

 trunks, we arrived at the conclusion that the activity of the neuri- 

 lemmal cells has some relation to the development of new nerve fibers. 

 At an early stage in degeneration their nuclei multiply; later they par- 

 ticipate with phagocytes in the removal of the broken up myelin drop- 

 lets; subsequently they elongate and, becoming connected end to end, 

 lead to the formation of what some have termed "embryonic" nerve 



fibers We arrived finally at the conclusion similar to 



that which Howell and Huber reached fifteen years ago, that, although 

 the peripheral structures are active in preparing the scaffolding, the 

 axis cylinder which • is the essential portion of the nerve fiber has an 

 exclusively central origin. 



Stroebe ('93), Huber ('95), Cajal ('05), Marienesco ('06), Ran- 

 son ('12) and others have described and illustrated microscopic 

 preparations of medullated nerves in regeneration after section 

 which seem to show beyond a doubt that outgrowth of the axis 

 cylinder from the central stump does take place. On the other 

 hand Bethe ('02), who bases his opinion mostly upon physio- 



