THE KILLIFISH 37 



This is pointed out by Abramowitz in work which is in 

 process of publication and from which T am permitted 

 to make the following excerpt. When new nerve-fibers 

 grow out from a proximal stump of a severed caudal 

 nerve they take, as already mentioned, a distal course 

 over the previously formed band and grow at an approx- 

 imate rate of half a millimeter a day. If during this 

 regeneration the progressing front of the growing nerve 

 as indicated by the melanophores under its control is 

 studied closely, it will often be found to vary in position, 

 depending upon the states of the melanophores used in 

 the test. If the fish is rendered dark by keeping it in a 

 black-walled vessel, the front of the regenerating band 

 of nerve as judged by the melanophores may be at one 

 place; if the fish is immediately thereafter blanched by 

 being put into a white-walled vessel the front as judged 

 in the same way may be measurably elsewhere. In cer- 

 tain fishes the developing front as shown by dispersed 

 melanophores may be in advance of that shown by the 

 concentrated ones; in other fishes the reverse may be 

 true. This disagreement in the position of the advanc- 

 ing front as shown by the two states of the melano- 

 phores, a disagreement that in a way is like that already 

 described on the edge of the fully formed band, is quite 

 consistent with the idea of double innervation, but very 

 difficult if not impossible to reconcile with single inner- 

 vation. Obviously the two sets of regenerative nerve- 

 fibers do not always grow forward at precisely the same 

 rate. In some instances the concentrating fibers are in 

 advance, in others the dispersing fibers. Here again the 

 idea of double innervation is a necessary part in the 

 explanation of a well-ascertained functional state, that 

 concerned with the regeneration of melanophore nerves. 

 In consequence of these two important lines of evi- 

 dence, one from the work of Mills and the other from 



