GUANIN IN RETINAL TAPETA 237 



cell. Along with it, partway down the length of the cell-processes, is a 

 cloud of guanin crystals (Fig. 94) . The pigment migrates in the usual 

 way, retracting into the body of the cell in the dark and moving far 

 down into the processes in the light (pp. 146, 149). As the fuscin granules 

 surge on their way in either direction, they infiltrate among the guanin 

 crystals, leaving the latter relatively undisturbed in position — indeed, the 

 guanin may migrate to some extent in the opposite direction. Contract- 

 ing behind the guanin layer in dim light, the pigment exposes the crystals 



Fig. 94 — The occlusible retinal guanin tapetum of certain teleost fishes, as exemplified by 

 the European pikeperch, Lucioperca sandra. x 500. Redrawn, modified, after Wunder. 



a, visual-cell layer of light-adapted retina, showing cones contrarted to limitans, rods elon- 

 gated, and retinal pigment (black granules) expanded into the heavy pigment-cell processes 

 to mingle with the guanin (silver), destroying its effectiveness as a mirror (c/. Figs. 62, 63, 

 64, pp. 146-8). 



b, dark-adapted situation, showing rods contracted toward limitans, cones elongated, and 

 retinal pigment retracted into pigment-cell bodies to expose a guanin mirror distal to the 

 mass of tiny rods. 



pecb- pigment epithelial cell bodies; r- rods; c- cones; onl- outer nuclear layer. 



to serve as a reflective backing for the mass of rod visual cells. Migrating 

 past the guanin in bright light, into a position between it and the light, 

 the fuscin covers up the guanin layer. No light is then returned through 

 the visual cells, after having once traversed them. 



This type of tapetum may be said to be occlusible — that is, capable 

 of being occluded or covered up in bright light when it is not wanted. 

 It is thus fundamentally different (physiologically) from the tapeta of 

 Evermannella and other dim-light fishes in which the pigment epithel- 

 ium is crammed with guanin but contains no migratory dark pigment 



