Electron microscopy of the visual cells of the mudpuppy Necturus have revealed several new or hitherto neglected features of organization: (a) A system of deeply staining micelles in virtually crystalline array, probably located in the lamellae of the rod outer segments. These particles may contain the visual pigment, porphyropsin. Counts of the micelles, and microspectrophotometric measurements of porphyropsin in the retina and single rods yield the estimate that each lamellar micelle may contain about 50 molecules of porphyropsin. (b) Systems of about 30 cytoplasmic filaments (here called dendrites), continuous with the cytoplasm of the inner segment, and standing like a palisade about the outer segments of the rods and cones. In the rods, one such filament stands in the mouth of each of the approximately 30 deep fissures that carve the outer segment into a radial array of lobules. (c) A system of deeply staining particles in the membranes of the dendrites, and another in the membranes of the pigment epithelial processes. It is suggested that these may have a part in interchanges of material with the outer segments. The ciliary process is found to penetrate more deeply than is commonly supposed into the outer segments of the rods and cones. The edge of each double-membrane disc in rods forms a differentiated rim structure, both around the disc circumference and bordering the fissures. These anatomical arrangements are summarized in Figs. 13 and 14, and the relevant measurements in Table I. The dilution of visual pigment in Necturus rods and cones and a general consideration of their microstructures make it seem unlikely that such typically solid state processes as exciton migration or photoconduction can transport the effects of light far from the site of absorption. Excitation must, therefore, be conveyed to the receptor as a whole by some axial structure. Among axial structures, the plasma membrane is most likely to be the site of nervous excitation. The ciliary process probably plays its main role in the embryogenesis and regeneration of outer segments; and the dendrites and pigment epithelial processes in exchanges of material with the outer segments and perhaps with one another.

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