422 AisnsnjAL report Smithsonian institution, 19 63 



More often, however, the stimulus is received through the sense 

 organs and the response is mediated via the central nervous system. 

 The most complete and detailed control of color and pattern is found 

 in the octopus. Here the chromatophores are never at rest. Blushes 

 of color pass over the surface and the individual chromatophores 

 expand and contract all the time. The muscles which operate the 

 chromatophores in these animals are under direct nervous control 

 and change in degree of expansion of each single chromatophore is 

 a matter of fractions of a second. Such is the complexity of the control 

 of the color and pattern that an entire discrete lobe of the brain of 

 the octopus is set aside for its operation, and the color can express 

 the mood of the animal and reflect its activity. 



In the lower vertebrates, too, the chromatophores of the skin are 

 under nervous control, but here this is supplemented by hormonal con- 

 trol. In an unstimulated condition the pigment in the chromato- 

 phores is more or less completely dispersed, producing a dark skin, 

 and nervous stimulation tends to cause the pigment granules to mi- 

 grate toward the center of the chromatophore, leading to paling. 

 This migration is a much slower process than muscular contraction 

 and may take two or three minutes. An opposing innervation will 

 serve to reverse the process if this is to take place relatively rapidly, 

 but if the initial nervous stimulation merely ceases, the pigment starts 

 to disperse to fine branches of the chromorrhizae — a slow process 

 which may take several hours. Naturally, the different areas of the 

 skin and the different colors of chromatophores are under separate 

 nervous control, so that pattern and color changes may be brought 

 about. The nervous control is reinforced by hormones from the pitui- 

 tary gland, which, slower to act in the first place, may maintain the 

 response for far longer. Though the hormones in sufficient dose may 

 serve to initiate a color change independently of the nerves, it is more 

 likely that their main action is exerted in lower dose by maintaining 

 the status quo once a pattern has been set by nervous action. That is 

 to say a small dose of hormone may act to prevent reexpansion of 

 the pigment Avhich has been concentrated by nervous action and so 

 fix a pattern and shade of color which has been established by nervous 

 control. 



Finally, many pigmentary effectors are under hormonal control 

 alone. This is especially true of the sexual colors of so many animals. 

 The breeding plumage of many male birds is under the control of the 

 endocrine system — the system of ductless glands which secrete the 

 hormones. The same is true of many fish. The male cuckoo wrasse 

 produces a white blotch across his shoulders by concentration of the 

 pigment within the red chromatophores. This seems to be under 

 endocrine control, though whether the hormone concerned derives 

 from the testis or from the pituitary gland we do not know. It is 



