Fine Structure of the Retina in the Reptilian Third Eye

Recently Stebbins and Eakin (1) revived in modified form a concept of a photothermal function of the parietal eye of reptiles. Laboratory and field studies indicated that surgical removal of the "eye" from the lizard, Sceloporus occidentalis, leads to increased activity and exposure to light. They postulated that under stimulation the retina of the "eye" produces a hormone which, perhaps after accumulation in the cavity of the organ, passes into capillaries and exerts an inhib-itory effect upon the activity of the animal probably v/a the pineal, pituitary, thyroid, and other endocrine glands. Because of these findings it seemed desirable to examine the parietal eye with the electron microscope. "Eyes" of juvenile S. occidentalis were fixed in 1.25 per cent osmium tetroxide buffered to pit 7.4 for 24 hours at 2°C., sectioned at less than 0.1 #, and examined in a RCA EMU-2-E electron microscope. The electron photomicrographs (Figs. 2 and 3) add several structural details to the anatomical picture worked out by Nowikoff (2). Fig. 1, a photomicrograph of a thick section of the parietal eye taken with a light microscope, will orient the reader on the region of the retina included in Figs. 2 and 3. Receptor cells (r) possess at their outer ends complex processes which may extend more than 15 t* into the lumen (l) of the organ. The distal segment of each process, tapering and sometimes branched at the base, contains 100 to 200 transversely arranged discs or flattened sacs (rs) which are strikingly like those in the outer segments of the rods and cones of the lateral eyes of vertebrates (3). Each disc or sac, consisting of a dense membrane and a narrow cavity is about 200 A thick. The sacs are ordinarily slightly separated; the occasional wide gaps are interpreted as arti-facts. The surface membrane of the distal segment in this specimen has been damaged or lost altogether in the course of fixation and embedding. The rims of the sacs appear to be rounded or slightly tapered (insert, Fig. 2), in which respect the sacs resemble those of the cones in the rabbit's retina as described by De Robertis and Lasansky (3). In the basal part of the distal segment the periodicity of the flattened sacs is broken by vesicles (rv) which are like those designated by De Robertis and Lasansky as "unoriented cone sacs" in the rabbit. Clumps of granular material adhere to …