The ultrastructure of photoreceptor cells of the crayfish (P. clarkii) has been examined by means of thin sections and freeze-fracturing. The study reveals that in the photoreceptor membranes there are particles associated primarily with the A faces of freeze-fracture preparations which have a mean diameter of 80-84 A and a density of 6,600 per per micrometer2. Treatment of the retina with digitonin (a substance capable of extracting visual photopigments) in Ringer's causes marked disruption of the hexagonal arrangement of the microvilli, breakdown of the microvilli into smaller segments, and gradual removal of the particles. The estimated photopigment concentration in the microvillus is 4,000 per micrometer. It is suggested that the observed particles represent the photopigment in situ.

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