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The fine structure of the extrafoveal receptors of the Rhesus monkey was studied by electron microscopy. In addition, some observations were made on the relations of the receptors to such other cells as bipolar neurons, Muller cells and the pigment epithelium. In the outer segments of both rods and cones the cell membrane encloses a column of flattened sacs. In the basal third of cone outer segments and in the basal twentieth of rod outer segments, the membrane of the sacs was frequently seen to be continuous with the cell membrane, but the points of continuance fell off sharply above these levels. This might represent a narrowing of the infolding sites with resultant diminution of the chance of encountering them in sections. However, cross-sections of distal rod outer segments have never shown sac and cell membrane confluences and suggest independence of the sacs from tbe cell membrane. Sacs also lack continuities with one another, unlike the situation in the guinea-pig as described by Sjostrand. Receptor inner segments were rich in mitochondria. In one area studied, cones exhibited about fifteen times the number of cross-sections of this organelle seen in rods. Descriptions are also provided of the basal bodies, rootlets, Golgi areas, external limiting membrane, nuclear areas, and synaptic terminations. Observations pertinent to the question of retinal attachment, the site of the so-called “R” membrane, and general receptor function are discussed.

The term "internal limiting membrane of the retina (membrana limitans interna)" has been applied in the past to both the surface created by the inner expansions of Miiller's "fibers" 1 and to the cuticular layer that lies upon them. 2,3 A second cuticular layer or "membrane" is present on the outermost surface of the retina, applied to the bases of the retinal pigment epithelial cells (the inner lamella of the glass membrane of the choroid ). 2 The homology between these two cuticular layers was pointed out by Arey, 4 who also drew attention to a similar homology between the "external limiting membrane of the retina" and a less widely appreciated "membrane" in the pigment epithelium. This latter "membrane" has been known variously as the "membrana reticularis retinae," 5 the "fenestrated membrane," 6 or the "cement substance" 2 of the retinal pigment epithelium. The cuticular membranes (i.e., the internal limiting membrane