540 ADAPTATIONS TO PHOTIC QUALITY 



they go fast enough to make nervous control seem reasonable — much 

 more so than in most amphibians. The consensus, however, is that in 

 lizards the principal controlling mechanism is an antagonism between 

 adrenalin and a dispersing hormone produced by the pars intermedia 

 of the pituitary. 



The response to excitement is particularly prompt, but it is unques- 

 tionably entirely endocrine. This response is given by lizards to any noxi- 

 ous stimulus, a number of which — electrical stimulation of the mucous 

 membranes, for example — are used experimentally to induce the so- 

 called excitement pallor. Its appearance is very regular, but may be sup- 

 pressed by low temperatures. The blanching has been abundantly proven 

 to be brought about by the adrenals. It occurs even in denervated areas, 

 indicating that the influence of adrenalin is direct, and that the adrenalin 

 or adrenalin-like substance involved is not a neurohumor, secreted in tiny 

 amounts by nerve fibers ending in the chromatophores. Hadley, however, 

 doubts the direct action, on the basis of his 1931 experiments on bits of 

 excised Anolis skin. While the melanophores of such bits would respond 

 directly to illumination, showing them to be apparently normal despite 

 their isolation, direct applications of strong adrenalin expanded them — 

 whereas the same solution injected into an intact animal produced the 

 usual wholesale contraction and pallor. Pituitrin expanded the pigment 

 cells both in intact lizards and bits of skin. 



Local effects have also been produced in the intact animal. Redfield, 

 working with the horned lizard Phrynosoma, found that local heating of 

 the skin (light being excluded) would contract the melanophores without 

 affecting those elsewhere. Local illumination is also effective as a stim- 

 ulus, but produces expansion — which cannot here be due, like so many 

 supposed biological effects of light, to a heating action of the light. But 

 this apparently paradoxical expansive effect of heatless light does tie 

 in with some of the findings of Sarah Atsatt, whose recent paper on 

 desert lizards may sometime be called a classic : 



Miss Atsatt so designed her apparatus as to divorce temperature from 

 light, and make each independently variable. Her findings tend to ex- 

 plode some hitherto well-rooted ideas, but they were so different for dif- 

 ferent species that only further work along the same line will show just 

 which of our smug generalizations (some of them stated above) must be 

 discarded. In thirteen iguanid species and one gecko, the response to high 

 temperature (35-43 °C) was the light phase; and to low temperature, the 

 dark phase. One species, Callosaurus rentralis, became partly pale again 



