Studies on the eyes of gars (Lepisosteidae) with special reference to the tapetum lucidum

Eyes of gars (Lepisosteus) possess a yellow tapetum lucidum which is located in processes of the pigment epithelium. A yellow reflecting pigment is involved, enclosed in tapetal spheres each about 240 nm in diameter. Retinomotor movements take place: in darkness (or dim light) the black retinal pigment retracts, exposing the tapetum, and the rods shorten; in the light the pigment migrates inwards, obscuring the tapetum, and the rods elongate. The tapetal pigment is soluble in water, dilute acid, and alkali, and insoluble in organic solvents; it is rendered insoluble (to water) by heavy metals; and it stains with reagents used to visualize phenolic compounds (ferric–ferricyanide; ammoniacal silver nitrate). A method of extracting the pigment is described, and the ultraviolet/visible spectrum shown. The refractive index is high (n 23 1.59) and that, in conjunction with the size and arrangement of the tapetal spheres, is conducive to backscatter. A second noteworthy feature of the pigment epithelial cells is the presence of an extensive Golgi apparatus consisting of many dictyosomes interconnected by a complicated tubular system of smooth membranes. The tubules exhibit periodic swellings or vesiculations in which the tapetal spheres individually are formed. Comparisons are made with a similar Golgi complex in Styela, and with membrane-bound vesicles, derived from Golgi and concerned with formation of ommochrome granules, in Drosophila.