558 THE EYE IN EVOLUTION 



requires further elucidation, as also does the mechanism, if any, whereby the 

 hypothalamus may control the anterior lobe of the pituitary body (see 

 Zuckerman, 1954). 



The activities of the neuro -endocrine system in Vertebrates as centred in 

 the hypothalamo -hypophyseal complex are extraordinarily extensive and 

 varied, for the hypothalamic nuclei exercise a supervisory control over most 

 of the other endocrine organs. They take direct control of the pituitary 

 body itself with its immense influence on the processes of pigmentation, 

 growth, diuresis and intermediary metabolism and its vasopressor and 

 oxytocic effects ; in addition, they exercise a quick-working stimulation of 

 the adrenal medulla through the sympathetic, and through the medium of 

 the 23ituitary they exert a slow-working control over a host of endocrine 

 activities, stimulating the thyroid by means of the thyrotropic hormone, the 

 steroids of the adrenal cortex through adrenocorticotropic hormones, as 

 well as controlling the development of the sex organs and the reproductive 

 rhythms through the gonadotropic hormones. The holistic nature of 

 endocrine balance is seen in the feed-back from these peripheral organs to 

 the hypothalamus by hormones of opposing nature which inhibit the exces- 

 sive production of those stimulatory agents by the neuro -endocrine system. 

 Most of these activities do not affect the eye ; but some do. 



The role of hormonal control over the integume^itary pigment of Fishes, 

 Amphibians and Reptiles has already been discussed at length. ^ It will be 

 remembered that environmental changes in many species are effected solely 

 by the control of mutually antagonistic hormones associated with the 

 pituitary, the release of which is determined by stimuli operating through 

 the eyes (Cyclostomes, Selachians, Amphibians ^ and some Reptiles) ; 

 in other species a nervous control is partially (Teleosts) or entirely (chameleon) 

 responsible. Similarly, the cyclic diurnal variation of the coloration which 

 occurs in many of these tjrpes — Cyclostomes such as the lampern. Amphibians 

 such as salamander larvse and frogs, and Reptiles such as the lizard, Anolis, 

 the chameleon and the American horned " toad," Phrynosoma — is due to 

 the rhythmic release of the appropriate hormones by the pituitary under 

 the control of its associated hypothalamic centres. Hypophysectomy 

 abolishes the darkening and lightening of the skin of the frog (Hogben, 1924), 

 and in the lizard, Anolis, suppresses the rhythmic change from brown 

 during the night to green during the day (Rahn and Rosendale, 1941). 



In a similar manner there is evidence that the pigmentation of the iris in the frog 

 is influenced by the hypophysis (del Castillo, 1955). 



The migration of the retinal pigment of Vertebrates ^ is essentially a direct response 

 to light and, unlike that of Crustaceans, shows little indication of hormonal control. 



1 p. 06. 



- After removal of the eyes from the toad, Bujo, some of the responses of the melanophores 

 to illmninati'd backgrounds persist, perhaps due to the direct action of light on the hypo- 

 thalamus (}. '8). 



» p. 61. 



