NEUROHUMORS 45 



served in Fundulus, therefore, is not to be attributed to 

 some action of the subjacent blood and lymph. It must 

 be due to some influence that enters the band from 

 the side. 



It is a noteworthy fact, as Smith (1931^) pointed out, 

 that bands or like areas do not fade when fishes are 

 kept permanently on a black background. Fading oc- 

 curs only when the adjacent region in the tail or other 

 part of the body is pale, as though some agent from the 

 neighboring pale area made its way into the band. 

 Such an agent might well be a neurohumor produced 

 in the pale area by the concentrating nerve terminals of 

 that region and transmitted laterally into the band. 



Such a neurohumor may be conceived of as spreading 

 slowly from the region of its production into that of the 

 band and of inducing there the same kind of change that 

 it is capable of accomplishing over the rest of the body. 

 That the action is in all probability a slow diffusion of 

 this kind is not only shown in the beginning of these 

 changes on the periphery of the band or other dark 

 areas, but by the fact that bands of different widths take 

 different amounts of time in which to disappear (Parker, 

 1934^). A band of a width of one millimeter on a pale 

 Fundulus will require on the average a little over twenty- 

 six hours in which to disappear; another two millimeters 

 in width calls for nearly fifty-two hours in which to 

 blanch. These determinations support the hypothesis 

 that a neurohumor produced in the pale region of the 

 tail and responsible for the tint of that region diffuses 

 from the pale region into the band and thus causes it in 

 time to blanch. 



Not only is there reason for believing that concen- 

 trating fibers act on melanophores through concentrat- 

 ing neurohumors, but also that dispersing fibers act on 

 melanophores by corresponding means. After a caudal 



