COLOUR CHANGES 



525 



mic" drugs. When peripheral nerves are cut {Plewonectes, Pseudopleuro- 

 nectes, Fundulus), dark dermal bands appear in a few minutes due to 

 expansion of the paralysed melanophores. The extent of the region affected 

 corresponds to the area innervated by the interrupted sympathetic nerve 

 fibres. Electrical stimulation of the nerves proximal to the injury is without 

 effect, but stimulation distally causes the denervated region to blanch. 

 Moreover, when such an experimental animal is transferred to white sur- 

 roundings, the denervated region no longer participates in the general 

 blanching which ensues in the remainder of the body. The affected area 

 remains dark for several days (two to three days in Fundulus heteroclitus) 

 and then gradually fades. It is concluded that in a normal animal the 

 sympathetic nerve fibres exert a tonic contracting effect on the melano- 

 phores. When the peripheral tonus is diminished or removed, as the result 



Fig. 12.13 (a). Caudal Fin of a Killifish Fundulus, Showing a Dark band 

 Produced by a Transverse Cut near the Base of the Tail. (b). A Faded 

 Band in the Tail of a Killifish within which a New Short Cut has been 

 Made. This Cut has Induced the Formation of a New Dark Band Distal 

 to the Injury. (From Parker, 1934.) 



of stimulation of the eye by a dark background, or artificially by nerve 

 section, the chromatophores expand. On a light background, however, the 

 resultant retinal stimulation leads to an inhibition of the original inhibition, 

 the tonic state reasserts itself and the melanophores become concentrated 

 (60, 70). 



An alternative theory has been advanced by Parker who believes that 

 the chromatophores of teleosts are subject to dual nervous control by 

 antagonistic fibres, one category causing contraction of the colour cells, 

 the other expansion. If a small transverse cut is made near the base of the 

 caudal fin of the killifish Fundulus so as to sever the caudal nerves but still 

 avoid the major blood vessels, a dark band appears in the fin distal to the 

 incision (Fig. 12.13). When this fish is kept on a light background for 

 several days the band gradually fades away, but a new cut peripheral to 

 the former injury brings about a revival of the original dark band. 

 Interruption of the chromatic fibres at the spinal level in a light-adapted 

 fish with a faded tail band results in dispersion of the normal 

 melanophores; this is followed by dispersion of the melanophores in the 



