Fig. 4. Hypothetical cone cycle in the retina and rod cycle in the retina/retinal pigment epithelium (RPE) of the chicken eye. Chicken retina stores 11-cis retinyl esters and chicken RPE stores all-trans retinyl esters. Light bleaches photopigments in the retina, leading to the accumulation of 11-cis retinyl esters in the retina and all-trans retinyl ester in the RPE. In the dark, 11-cis and all-trans retinyl ester pools deplete while visual pigments regenerate. The amount and the rates of accumulation/depletion of 11-cis retinyl esters in the retina correspond to the bleaching and regeneration of cone pigments, providing support for a cone visual cycle in the chicken retina. The amount and the rates of accumulation/depletion of all-trans retinyl esters in the RPE correspond to the bleaching and regeneration of rod pigments, thus supporting a rod visual cycle in the chicken RPE/retina. The types of retinal cell where 11-cis retinoids are synthesized and stored, as well as the biochemical mechanism of isomerization, are not known. The method to partition these two visual cycles remains to be studied.