§ 3.224 EFFECTORS WITH MOVABLE PIGMENT GRANULES 107 



complicated and varied than in the other groups considered, 

 because there appear to be at least two distinct types of response 

 to hormonal stimulation, as well as the differences in reaction 

 introduced by mixed control by both hormones and nerves. In 

 some of the more primitive forms, such as the pike, Esox, and the 

 carps, eels and Ameiiirus, the control appears to be purely hor- 

 monal. The melanophores disperse in response to the melano- 

 PHORE-STIMULATING HORMONE, MSH, from the pars intermedia, 

 when it is injected into pale specimens, whether the source of 

 the hormone is from the same fish, from amphibia or even from 

 mammals. The majority of other teleosts like FunduluSy are in- 

 sensitive to MSH, at least until the melanophores are denervated. 

 This second group is, however, found to be more sensitive to a 



MELANOPHORE-CONCENTRATING HORMONE, MCH, which is present 



in most extracts of fish adenohypophyses. It is present in Antui- 

 trin and has no action on Amphibia. 



A few teleosts, like Phoxinus, although having their melano- 

 phores normally under nerve control, can be induced to respond 

 appropriately to both MSH and MCH under suitable conditions. 

 Hormonal control in Phoxinus apparently serves to maintain the 

 melanophore reactions over long periods when the nerve control 

 becomes fatigued (Healey, 1948). Many other teleost fish have 

 melanophores which are controlled only by nerves (for further 

 details see Pickford and Atz, 1957). 



Reptilia. Hormones, especially intermedin, B, play a part in 

 the dispersal of the melanophores of certain of the older genera of 

 lizards, such as Phrynosojna, Hemidactylus and Anolis; but con- 

 centration appears to be controlled by adrenaline and not by a 

 pituitary secretion. Dual nerve control replaces this in the more 

 highly evolved chameleons and others, producing varied patterns 

 of colour change. 



3.224 Discussion of the hormonal control of pigmentary effectors 



Looking back over the pigmentary effectors just considered 

 (Tables 9, 10, 11), it seems possible to make some generaliza- 

 tions, although the more complex cases have been left out, and the 

 evidence is not always complete for those that have been included. 

 Two hormones are concerned in the control of nearly all types of 



