714 Comparative Animal Physiology 



ated one, despite the fact that the melanophores in the denervated area still 

 show pigment concentration in response to an injection of adrenalin. No 

 indication of pigment-dispersing fibers has been uncovered in this species. 



Phrynosoma, like Anolis, after hypophysectomy becomes pale and re- 

 mains so indefinitely. Injection of pituitrin or of extract of Phrynosoma pi- 

 tuitary induces strong darkening either in normal pale or in hypophysecto- 

 mized individuals. Furthermore, injection of defibrinated blood from a dark 

 specimen into the leg of a pale one produces darkening in the latter. These 

 results provide strong evidence that a hormone arising in the pituitary is 

 normally concerned in body-darkening. An action of a pigment-concentrating 

 hormone is also seen in that adrenalin or extract of Phrynosoma adrenals 

 strongly blanches dark specimens. The presence of a similarly acting agent in 

 the blood of white-adapted animals is seen in that their defibrinated blood 

 will, on injection into a leg of a dark specimen, fighten the latter. It has 

 been known for a long time that animals held on their back or otherwise 

 stimulated to struggle vigorously will lighten rapidly. By comparison of the 

 influences of denervation and occlusion of blood supply on the production 

 of this type of lightening it can be clearly demonstrated that a blood-borne 

 agent is involved. These observations lead to the obvious conclusion that the 

 paling is brought about by two methods, nervous and hormonal, either one 

 alone capable of producing the response. 



Both the dispersing hormone from the pituitary and the concentrating 

 hormone from the adrenals operate directly on the melanophores; each is ca- 

 pable of exhibiting its complete action after nerve transection and degenera- 

 tion of the nerve fibers. 



The influence of temperature and of light and darkness on the melano- 

 phores of Phrynosoma is a direct one, in which the melanophores act as in- 

 dependent effectors. The responses may be obtained locally by application 

 of the stimulus to the specific region in completely denervated portions of 

 the body. Since even degeneration of the innervating fibers does not result 

 in termination of the response, axon reflexes cannot be responsible. 



The coordination of the melanophores of chameleons, Chamaeleo or Lo-pho- 

 saura, unlike that in the iguanids, appears to be exclusively nervous.^""' Nerve 

 transection is followed by a darkening of the area normally innervated by the 

 nerves. These nerves are of the autonomic nervous system. The melanophores 

 are readily caused to concentrate their pigment by electrical stimulation of 

 the nerves. The results of nerve transection have been interpreted to be due 

 to the absence of tonic impulses reaching the pigment cells.^^*'- ^"^ There is 

 no clear evidence as yet that the pigment is actively dispersed by a second 

 set of nerve fibers, as appears to be true of many teleosts. 



There is little or nothing known for the most part as to possible roles of 

 hormones in the color changes in the chameleon, but the fact that color pat- 

 terns can be produced on the body by alternate light and shadow-*^ argues 

 against any considerable importance of such factors. 



The melanophores of intact animals respond to light and darkness by 

 pigment dispersion and concentration, respectively. Dark regions produced 

 by denervation show no such responses. These results were interpreted by 

 Zoond and Eyre^"^ to prove that the responses of the melanophores can oc- 

 cur only by way of reflexes involving the central nervous system. But Park- 



