The visual cells of the chicken as revealed by phase contrast microscopy

Phase microscopic investigations of Kolmer-fixed, depigmented sections of the adult chicken retina have provided photomicrographic evidence of the existence of three different photoreceptors: single rods, single cones, and double cones. The rod extends the entire thickness of the visual cell layer and is characterized by a uniformly thick outer segment and a hyperboloid-containing inner segment which is devoid of an oil droplet. The single cone is the shortest element; it contains a red oil droplet. The double cone consists of two unequal members, a tall, slender chief cone and a broad accessory cone. The chief component contains a large yellow oil droplet, whereas the accessory cone houses a small, oval, yellowish-green droplet and a characteristically large, oval paraboloid. The rod hyperboloid and the accessory cone paraboloid contain glycogen. No colorless droplets have been observed. Owing to the close association between oil droplet color and cone type, three colored layers of oil droplets are formed within the thickness of the retina: a proximal row of red droplets (the short, single cones), an intermediate layer of yellowish-green droplets (the accessory cones), and a distal row of yellow droplets (the tall chief cones).