IX. IO 



COLOUR CHANGE 



259 



moving inwards. Parker, who has made a careful study of these 

 phenomena, believes that they indicate the presence of melanophore- 

 expanding nerve-fibres (said to be of 'parasympathetic' nature). Fol- 

 lowing the cut these fibres are supposed to be stimulated to repetitive 

 discharge and hence to make the dark band. Later the band pales 



Fig. 154. Nerves of the melanophores of a perch. (From Ballowitz.) 



Fig. 155. Diagrams showing, left, a cut in the tail of the fish Funduhis producing 



a band of dark melanophores; right, when the dark band has faded a second cut 



makes the melanophores again dark. (After Parker, Quart. Rev. Biol. 13.) 



because the stimulating substances ('neurohumors') produced at the 

 nerve-endings of the melanophore-contracting fibres in the neigh- 

 bouring areas diffuse in gradually. 



This hypothesis involves two physiological propositions which are 

 so novel that they would require detailed evidence for acceptance. 

 Firstly the act of cutting is presumed to set up a discharge of impulses 

 lasting for several days, which is unlikely. Moreover, the discharge is 

 presumed to be only in the melanophore-expanding fibres and not in 

 those that produce contraction. Secondly, electrical stimulation of 

 nerves in teleosts always produces paling and never darkening (except 



