A systematic electron microscopic study was made of the structure of foveal cones of Macaca spp. Transverse sections of inner (IS) and outer segments (OS) were made in sequence, from the pigment epithelial zone (PEZ) to the outer limiting membrane (OLM). The smallest diameters of hundreds of cone sections were measured from electron micrographs with a Zeiss particle-size analyzer, and analyzed statistically. Some details are also included about Cebus photoreceptors. It is claimed in the literature that foveal cones are rod-like (cylindrical) and untapered. Our study shows the foveolar cone to be a tapered structure. There has been some confusion between the foveola, which is rod-free, and the fovea, which has a high concentration of cones, but is not rod-free. Within the fovea, as the ratio of cones to rods falls from infinity to 1, with distance from the central bouquet of cones, the cone center-to-center distances increase, the inner segment diameters increase, and the number of cones/sq mm decreases. The tapered calycal processes are more massive in M. irus than M. mulatta, and the lateral fins are better developed. Lateral fins are not present in the foveola. The cones are arranged in straight lines.

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cause of the selection rules for Aj. The finestructure composition of a beam component in the 2 3 P states thus changes as the beam travels.