708 Comparative Animal Physiology 



The presence of a second set of fibers, pigment-dispersing ones, has been 

 demonstrated for many teleosts through experimentation with melanophores 

 of the tail fins. If, in a teleost such as Fiindulus, kept on an' illuminated 

 white background, a group of the radiating caudal nerx'es is cut by a trans- 

 verse incision, the band of fin innervated by these fibers darkens quickly 

 and then over the course of a few days fades again^-^ (Fig. 272). If now 

 a second cut is made parallel to the first cut, and somewhat distal to it, the 

 faded band will repeat the transitory darkening behavior. This latter be- 

 havior leads to the hypothesis that the melanophore response observed must 

 be due to a restimulation of the dispersing nerve fibers that had been tran^ 

 sected and stimulated by the first incision. It cannot be explained simply in 

 terms of a transection of the concentrating fibers, as possibly the response to 

 the first cut could be. Interpolation of a cold-block between the point of the 

 nerve transection and the melanophores abolishes the response.^^^ Such re- 

 darkening of faded bands after a second incision has been observed by a 

 number of investigators for a number of species of teleost fishes. These in- 

 clude Holocentrtis,^'^- Parasiliiriis,^'^^ Pterophylhim,^'*'-^' ^-'^ Ameiiinis,^'-^^' and 

 Gohiiis.''^ An activation of melanin-dispersing fibers in the catfish tail has 

 also been produced by electrical stimulation.^^" 



Other lines of evidence, both morphological and phvsiological, have given 

 further support to the hypothesis of a dual innervation of teleost melano- 

 phores. Ballowitz^'^ many years ago clearly demonstrated that the melano- 

 phores of the perch receive nerve terminations from more than one fiber, thus 

 providing an anatomical basis for the conclusions reached by more recent 

 physiological experimentation. Furthermore, a critical examination has been 

 made of the responses of chromatophores at the edges of denervated caudal 

 bands of Fimdulivs,^^^ and of those near the regenerating front of nerve 

 fibers in the course of re-innervation of denervated bands,^ as the animals 

 darken on black backgrounds and lighten on white ones. These observa- 

 tions pro\'ide strong evidence that many of the melanophores located in 

 these regenerating fronts possess only one type of fiber, either concentrating 

 or dispersing, but not both as under normal circumstances. Some of these 

 melanophores show rapid pigment concentration and very slow dispersion; 

 others show the reverse. Studies of the influence of drugs on chromato- 

 phores of Phoxinus^'^ and Fundidus^"'^ also support the concept of a dual in- 

 nervation. 



It now appears that the dispersing fibers exert their action on the melano- 

 phores through the mediation of acetvlcholine. Acetylcholine is known to 

 cause dispersion of the melanin of fishes^"^' when the latter are eserinized to 

 prevent rapid destruction of the material. In fact, a bioassay of the acetyl- 

 choline content of the skin of a dark-adapted catfish Anieiuriis'^'^^ or snake- 

 fish Ophiocephahis showed its presence in a concentration of about 0.078 

 gamma per gram of skin. This is approximatelv the concentration of acetyl- 

 choline which, when injected into the body fluids of eserinized fish, was in 

 general non-toxic and at the same time quite effective upon melanophores. 



Of the fishes thus far investigated the dogfish Miistelus and Sqnahis ap- 

 pear to possess mononeuronic melanophores. If a transverse cut is made in 

 the pectoral fin of a dogfish of intermediate tint a light band is produced 

 distal to the point of the cut. Such light bands may be revived after they 



