2 6o BONY FISHES ix. 10- 



after the use of ergotamine). We must assume, therefore, that this 

 form of stimulation has the opposite effect to section and only stimu- 

 lates the melanophore-contracting nerve-fibres. Since neither of these 

 propositions is adequately demonstrated, we must reject the hypo- 

 thesis and say that there is not sufficient evidence of melanophore- 

 expanding nerve-fibres. 



There is, however, another agent that causes expansion of melano- 

 phores in a wide variety of vertebrates, namely the posterior lobe of 

 the pituitary(see p. 206), and there is evidence that this works also in 

 teleosts. Hypophysectomized specimens of the Atlantic minnow 

 Fiindulns are nearly always lighter than normal individuals, especially 

 when on a dark background. Injection of posterior pituitary extracts, 

 or placing of isolated scales in the extract, causes expansion of the 

 chromatophores of any teleost. We may conclude that colour change 

 is produced by the nerve-fibres tending to make the animals pale and 

 secretion of the posterior pituitary to make them dark. Adrenaline 

 induces contraction of chromatophores, and is presumably similar to 

 the sympathetic transmitter. 



It is more difficult to decide how external influences are linked with 

 this internal mechanism. Fishes mostly become pale in colour on a 

 light background and vice versa, and the effect is produced pre- 

 dominantly through the eyes. There may also be a slight direct effect 

 of light on the chromatophores. The change in colour begins rapidly, 

 but its completion may take many days. Analysis of the rates of 

 change in normal fishes and in those with anterior and posterior 

 pituitary removal has led to the suggestion that the anterior lobe 

 produces a substance tending to make the fish lighter in colour, at 

 least in the eel. A similar hypothesis has been fully worked out by 

 Hogben and his colleagues in amphibia (p. 300). The colour is also 

 influenced by the pseudobranch, a secretory tissue in the first gill 

 arch. After removal of this a fish becomes dark and the choroid gland 

 of the eye, which receives blood from the pseudobranch, degener- 

 ates. It is suggested that the pseudobranch produces a hormone, 

 whose entry into the circulation is controlled by the choroid gland. 



The value of the colour change in bringing the animals to the same 

 tint as their surroundings is considerable. Fishes kept on a light back- 

 ground are very conspicuous for the first few minutes when trans- 

 ferred to a dark one. The fisherman acknowledges this by painting 

 the inside of his minnow-can white, to ipake the bait conspicuous. In 

 the flat fishes, living on the sand, the protection assured by the colour 

 change is of special importance. It has been suggested that it is pos- 



