Chromatophores and Color Change 709 



have redarkened^'**^ or may be produced by electrical excitation^-^ of the 

 integumentary ner\'es. These bands follow the distribution of the cut nerves 

 and not necessarily of the blood vessels. Furthermore, light bands may be 

 produced by a similar transverse cut in a fin from which the blood supply 

 has been cut off. All these facts point to a nerve supply to the melanophore 

 whose function it is to induce pigment concentration. There is no indication 

 whatsoever that pigment-dispersing nerve fibers are present in this animal. 



Parker'-^^ postulated that the concentrating fibers influence the melano- 

 phores through the production of a chemical mediator which he called sela- 

 chine. If skin from a pale animal is extracted with ether or olive oil, but 

 not water, the material may be extracted. Injections of an olive-oil extract 

 of this substance into a dark-adapted animal will produce temporary light- 

 ening, which spreads very slowly from the point of injection. Olive oil by 

 itself produces no such effect. 



The other elasmobranch fishes which have been investigated, the skate, 

 Raja,'^'^^' the dogfish, Scyllium,-^~ and the lamprey, Lampetra,-^'-^ show no 

 evidence of direct innervation of their melanophores, and hence their melano- 

 phores may be said to be aneuronic. 



Blood-borne agents typically supplement the nervous system in the sec- 

 ondary responses of the melanophores of fishes to light stimuli. In the more 

 primitive fishes, such as the cyclostomes and many of the elasmobranchs 

 which possess aneuronic color cells, hormones alone are the agents in\'olved. 

 Among these hormones an important substance is a pigment-dispersing prin- 

 ciple from the pituitary, the B-substance of Hogben, or intermedin.-^"* This 

 substance is produced by the posterior lobe of the pituitary. Lundstrom and 

 gg^jios observed that hypophysectomized MiisteJus become and remain 

 pale. The fish may be darkened again by injection of extract of the posterior 

 lobe. A similar role of the posterior-lobe principle has since been demon- 

 strated for other elasmobranchs. Raja and Scyllmm,''- and the cyclostome 

 Lavipetra.-'^'-^ 



There is some evidence that in Scyllmm and Raja a second neurohumor 

 from the pituitary acts as a pigment-concentrating agent.^- Evidence for a 

 role of such a body-blanching principle has been derived frorn studies of 

 the characteristics of the melanophore responses to background and light- 

 intensity changes and to the influence of hypophysectomy on the state of 

 the pigments, and of pigmentary responses to injections of posterior-lobe ex- 

 tract. The general methods of experimentation and logic involved in these 

 experiments are the same as those developed in the studies of amphibian 

 melanophore control. According to the bihumoral concept of Hogben and 

 his associates the state of the melanophores in these fishes is determined by 

 the ratio of B-substance to W-substance present in the blood at any given 

 instant, and this ratio is in turn controlled by visual stimuli. The visual 

 stimuli, through differential dorso-ventral retinal stimulation, result in dif- 

 ferent rates of secretion of the two principles bv their two respective 

 sources. 



The chromatic pituitary hormone inxolved in melanin dispersion seems 

 also to be present and active in normal color change to a greater or lesser 

 degree in all teleost fishes. The eel, Anguilla, shows sluggish color changes 

 requiring days for completion.^'" In this fish, despite the apparent presence 



