GUANIN AND THE ARGENTEA 235 



apparently compounded of still smaller elements. Those of the seals 

 have been found to resist weak (but not strong) acetic acid, and are 

 blackened by osmic acid, suggesting a lipoid nature which their double 

 refraction confirms. 



Guanin and the Argentea — The best known of the retinal tapeta 

 lucida — called pseudo-tapeta by Briicke, who published the first exten- 

 sive description of tapeta lucida in 1845 — are those in which the pigment 

 epithelial cells contain masses of particles, or crystals, of guanin. This 

 substance is also employed in chorioidal tapeta, which otherwise resemble 

 the mammalian tapetum fibrosum. Guanin plays the essential role in the 

 amazing tapetum of the elasmobranchs, and it is employed in an alto- 

 gether different kind of mirror located on the outside of the eyes (and 

 bodies) of fishes. It deserves more than a few words on its own account : 



Guanin is chemically a purine, and is closely related to uric acid. In 

 extracted form it is an uninteresting, pale yellow, chalky powder; but 

 when deposited, either as simple guanin or as the calcium salt, in the 

 right places and in the right way, it can endow living tissues with the 

 metallic lustre of silver or gold. Guanin has long been employed, wher- 

 ever a mirror was needed, by fishes and a few higher vertebrates. Before 

 them, invertebrates had used salts of uric acid to form concentrating 

 mirrors in light-producing organs, which are often built much like an eye. 

 The silvery sides of a minnow are plated with guanin-laden scales. 

 Indeed, the name of the substance comes from 'guano', the term for the 

 excrement (of Peruvian cormorants) which is mined for fertilizer on the 

 sea islands where the piscivorous guanay-birds of millennia once piled 

 it a hundred feet deep. Before it has been through the alimentary canal 

 of a bird, the guanin of fish scales is known commercially as argentine, 

 and under the name of 'essence d'orient' it was formerly used in the man- 

 ufacture of artificial pearls. 



The entire uveal tract of a fish eye becomes jacketed, in the larva, 

 with a guanin-laden outer layer called the argentea. Just as the silver 

 reflections from an adult fish's sides blend with the bright water surface 

 when seen from below by a predator, so does the argentea of a larval 

 fish eye render that eye inconspicuous within the glassy body, by con- 

 cealing the black pigment of the uvea which has already developed so 

 that the little eye can function. This interpretation of the argentea as 

 an embryonic adaptation to light is confirmed by the fact that it is 

 seldom found in fishes which live in the darkness below 400 meters. 



