THE KILLIFISH 



29 



Precisely what induces the excitation at the cut and 

 maintains it cannot now be stated definitely. Since the 

 tail of the fish is in almost continuous lateral movement 

 during the life of the animal it might be supposed that 

 after the cut is made the continual rubbing of the sev- 

 ered ends of the nerve-fibers in the wound is the occasion 

 of the prolonged activity of these fibers. That such, 

 however, is not the case can be shown by the simple 

 experiment of cutting a window 

 in the tail instead of making 

 merely a transverse slit (Fig. 

 23). Under such circumstances 

 the nerve-fibers are no longer 

 rubbed on the rough surfaces of 

 the wound, and yet from a 

 window a caudal band is formed 

 and maintained precisely as it 

 is from a simple transverse cut. 

 Mechanical stimulation prob- 

 ably has nothing to do with 

 the maintenance of the band. 

 There are, however, enough 

 other disturbances in the cut to 

 account for continuous stimula- 

 tion, but precisely what these disturbances are is at 

 present unknown. 



Nerves that are cut as these caudal nerves have been 

 would naturally be expected to undergo degeneration, 

 and it is reasonable to ask whether this process plays 

 any part in the activities under consideration. After a 

 denervated darkened band in the tail of a Fundulushas 

 blanched, the activity of its severed nerve-fibers can be 

 tested by recutting them. If the band darkens a second 

 time, the fibers must be regarded as still functionally 

 active; if not, they may reasonably be suspected of de- 



Fig. 23. Diagram of 

 a caudal fin of a killifish 

 in which a small window 

 was cut resulting in the 

 formation of a dark 

 band. Parker, Pro. Nat. 

 Acad. Set., 1934, 20, 308, 

 % 4- 



