VISUAL SYSTEM: STATE OF THE ART 27 



produced an unsaturated spectrum. For example, the tapetum of Squalus 

 is bluish green, reflecting very efficiently (90%) in the middle wavelengths 

 (500-515 nm) but falling off to 50% in the blue (420 nm) and red (650 

 nm). In a clear presentation, Best and Nicol explained the origin of indi- 

 vidual crystal colors as well as the general coloration of the tapetum. As 

 suspected, the high reflectivity in color is due to constructive interference in 

 the multicrystal layer "thin-film" system of the tapetum. Thin-film systems 

 are found elsewhere, such as in fish scales, wings of beetles, squid eyes, 

 butterfly wings, etc. (Denton and Land 1967). Interference phenomena are 

 also used extensively in optical instruments when relatively pure colors are 

 required (i.e., interference filters, diffraction grating). Best and Nicol con- 

 clude with a detailed discussion on fine structure, orientation, and alignment 

 of reflecting cells and crystals in the tapetum. 



Biochemistry— Biochemistry of the tapetum was reported by Nicol 

 and van Baalen (1968), extending the earlier work on composition of tapetal 

 crystals. The authors eventually prepared a pure sample of crystal for anal- 

 ysis. An enzyme assay was used in which the tapetum was first treated with 

 xanthine oxidase to convert residual xanthine to uric acid. Guanase was 

 then added, converting guanine to xanthine, which goes over to uric acid 

 from the previous enzyme addition. Increase in optical density at 290 nm 

 indicated the presence and amount of guanine in the sample. The authors 

 reported that choroid contains "astonishingly large amounts of guanine, 

 nearly 1 mg/cm 2 in Dasyatis" (p. 76). Clean crystals from the tapetum of 

 Dasyatis were secured by digesting the choroid in trypsin. The resultant 

 enzyme analysis unequivocally demonstrated that these crystals were chem- 

 ically identical to guanine. At least four fluorescent substances were isolated 

 from the choroid but could not be absolutely identified. A fluorescent ma- 

 terial though to be xanthopterin had previously been reported from the eye 

 of Squalus (Pirie and Simpson 1946). The importance to vision of fluo- 

 rescent materials in the tapetum has been discussed by Dartnall et al. (1965). 

 Another biochemically important material in the tapetum is the light- 

 screening pigment responsible for occlusion of the light-adapted tapetal 

 plates. While melanin has been associated with tapeta of other animals, Fox 

 and Kuchnow (1965) point out that other dark pigments such as chromo- 

 lipids could be substituted in the elasmobranch choroid and thus be con- 

 fused with melanin. For positive chemical identification, they extracted the 

 opaque choroidal pigment of Prionace, Heterodontus, and Platy rhino idis. 

 Melanin, however, is a rather complex and inert polymeric molecule and thus 

 difficult to characterize. Their strategy was to try a number of different 

 chemical tests (at least 14) to determine whether the extracted pigment had 

 properties similar to those of melanin. They concluded that the screening 

 pigment in all tests behaved quite like melanin and was thus chemically 

 identical to that substance. 



Kinetics of Occlusion— Kuchnow then turned his attention to the 

 physiology of tapetal pigments (Kuchnow and Gilbert 1967, Kuchnow 



