26 VISION 



fewer spurious signals (i.e., noise). At the same time, the tapetum would in 

 effect increase sensitivity as if there were twice the amount of pigment, 



2) The lower pigment concentration would permit more rapid dark 

 adaptation while maintaining high sensitivity. Dark adaptation depends 

 partly on regeneration of visual pigment and so the absolute amount of 

 pigment could set the final limit on kinetics of dark adaption. However, 

 Gruber (1967) and Hamasaki et al. (1967) have shown that the rate of dark 

 adaption in several elasmobranchs is relatively slow. 



Nicol (1965) turned his attention to physiological mechanisms subserving 

 migration of the choroidal tapetal pigment. He stressed that pigment moves 

 through fixed channels. The experimental work was designed to determine 

 whether control of the tapetal pigments was achieved through direct action 

 of light on an independent effector or whether neuronal or hormonal factors 

 were involved. Methods included nerve sectioning, ablation, and pharma- 

 ceutical assays. Results of at least 14 separate experiments apparently pre- 

 cluded extraocular neuronal or hormonal control of the tapetal system. 

 However, as we have already seen, a tapetum devoid of retina darkens perma- 

 nently. Thus the pigment cells cannot be labeled as exclusively independent 

 effectors. In addition, Nicol found that physically replacing the retina on a 

 dark-adapting tapetum inhibited the expansion of pigment. Because of the 

 anatomical separation between retinal receptors and tapetal pigment cells, 

 Nicol rejected the notion of direct retinal control of pigment migration. 

 Thus, although many possibilities were ruled out, the actual mechanism was 

 not revealed. 



Kuchnow (1969a, 1969b), already measuring kinetics of tapetal light and 

 dark adaptation, realized that some type of interactive mechanism was indi- 

 cated with retinal control over pigment aggregation. Kuchnow and Martin 

 (1970a) thus looked at the fine structure of the melanocytes in Mustelus to 

 determine whether neuronal elements were present. Previous workers 

 (Bernstein 1961, Best and Nichol 1967) had failed to locate any nerve end- 

 ings in the pigment cells. However, Kuchnow and Martin (1970a) were able 

 to identify structures at the basal portion of the melanocytes that had the 

 characteristics of nerve endings. Synapses between neurons and melanocytes 

 with cleft distances of 30-40 nm were also identified. These findings pro- 

 vide strong evidence for neuronal control of pigment migration, but the 

 mode of action is still an open question. The most difficult point to rec- 

 oncile is how, by merely replacing the retina on a piece of "stripped" 

 tapetum, pigment migration was halted. 



Best and Nicol (1967) investigated ultrastructure, orientation, chemical 

 composition, and reflection properties of the tapetal reflecting cells in 

 Squalus and Scyliorhinus. The internal margins of the reflecting cells, i.e., 

 the surface which reflects light back to the photoreceptors, appear in gross 

 aspect to be irregularly overlapping rounded ellipsoids, somewhat like fish 

 scales. The faces of these reflecting cells are 100 X 60 jum and crystals ob- 

 tained from them are very thin, elongate hexagons. While light reflected from 

 individual crystals varied in color, the overall effect of tapetal reflection 



