54 COLOR CHANGES IN ANIMALS 



region concerned, disappeared spontaneously after a few 

 days. When they were first formed they could be tem- 

 porarily obliterated by an injection of adrenalin. Ex- 

 traction of the skin of Ameiurus by ether, hot or cold, 

 yielded residues that were slightly active in darkening 

 the skin, but they were by no means so effective as were 

 the ether extracts in the case of Mustelus. However, 

 the evidence from the catfish supports the view that in 

 Ameiurus a dispersing neurohumor is present which is 

 soluble in oil and in this respect resembles the concen- 

 trating neurohumor of Mustelus. 



The survey that has just been made of the means by 

 which the melanophores of Fundulus> of Mustelus, and 

 of other related species of fishes are activated indicates 

 with reasonable certainty that the distinction between 

 excitation by nerves and excitation by hormones is not 

 a fundamental one. In what is ordinarily called direct 

 stimulation of melanophores by nerves, as occurs for 

 instance in Fundulus, there is sufficient ground to as- 

 sume that of the two sets of nerve-fibers present each 

 one when active produces a substance, a neurohumor, 

 that can excite in a melanophore an appropriate re- 

 sponse. This neurohumor is believed to be produced 

 by the numerous nerve-terminals that surround the 

 color-cell. It must pass from its region of origin, the 

 terminal organ, over the almost submicroscopic space 

 to the responding cell. In the sense that it passes from 

 one place to another it is a hormone, but it is a hormone 

 that ordinarily travels over only a very short distance. 

 However, as already demonstrated in Fundulus, it may 

 pass over as much as a millimeter or so of intervening 

 space. It is therefore in all essential respects as much 

 a hormone for the activation of melanophores, as the 

 pituitary secretion is. So far as transmission is con- 



