98 



THE EYE IN EVOLUTION 



Rana 



Xenopus 



Anal is 



Phrynosoma 



blanching, mediated by the pars tuberahs, and a melanin-dispersing 

 B-stibstance (intermedin) mediated by the posterior or intermediate 

 lobe ; their relative concentration in the blood is determined by 

 environmental stimuli operating through the eyes and their activity is 

 usually abolished when these or the optic nerves are destroyed, although 

 some residual responses remain after removal of both eyes which may 

 be due to the direct action of light on the hypothalamic region 

 (Rowlands, 1952-54). These conclusions have been confirmed by the 

 effects of excision of the whole or parts of the pituitary and by the 

 induction of colour changes by the injection of extracts of the gland 

 both in the frog, Raiia} and in the African clawed toad, Xenopus 

 Icevis - (Fig. 73). It is also interesting that injection of pineal extract 

 produces a contraction of the melanophores of Xenopus (Bors and 

 Ralston, 1951) while pigmentary changes are constantly produced in 

 tadpoles by feeding on pineal tissue (McCord and Allen, 1917). 



REPTILES, like Aniijhibians, show less elaborate responses to light 

 than teleostean Fishes ; the only conspicuous changes occur among 

 lizards and the only active cells are the melanophores which send up 

 pigment into their branches entwined among the variegated chromato- 

 phores, thus varying the colour scheme ; the gaiety of their various 

 costumes is due to individual variation in these static cells. The only 

 phdtic response is to light entering the eye, and the colour of the 

 background is without primary significance. Within these limitations, 

 however, the large family of lizards shows every possible variation 

 in control. On the one hand, the iguanid, Anolis (the " Florida 

 chameleon "), shows little evidence of nervous intervention ; it 

 becomes dark brown in bright illumination, pale green in darkness ; a 

 hypophysectomized or a blinded animal becomes light green and 

 thereafter loses all colour responses except a peculiar mottling on 

 electrical stimulation or on the injection of adrenalin ; while denervated 

 areas of the skin respond as do normal areas (Kleinholz, 1938) (Figs. 62 

 and 63). An intermediate position is occupied by such iguanids as the 

 American horned "toad," Phrynosoma. The chromatophores of this 

 animal are under the influence of pigment-concentrating nerve fibres 

 and react to adrenalin ; stimulation or section of these fibres results 

 respectively in blanching or abolition of responses. At the same time 

 the injection of pituitrin or of the blood of a darkened specimen induces 

 darkening, while hypophysectomy or the injection of adrenalin, 

 adrenal extract or of blood from a pale animal induces blanching 

 (Redfield, 1918 ; Parker, 1938). It would seem that the two antago- 

 nistic hormones act directly on the chromatophores since they are 



1 Hogben and Winton (1922-23), Steggerda and Soderwall (1939), Parker and 

 Scatterty'(1937). 



2 Hogben and Slome (1931-36), Atwell and Holley (1936). 



