The number, depth and elongation of disc incisures in the retinal rod of Rana catesbeiana.

The disc incisures in the bullfrog retinal rod were examined by electron microscopy. The number of disc incisures is proportional to the cross-sectional perimeter of the outer segment. While the diameter of the red rod outer segments varies with their location along the vertical meridian of the retina, the incisure number also changes similarly. It is estimated by regression analysis that a single disc lobule, bordered by two adjacent incisures, has on the average about 1 micron of arc-like rim edge. When red and green rod discs are compared at their middle outer segments in the ventro-central retina, the disc incisures are 1.8 times more numerous and 1.5 times deeper in the red rod (mean incisure number, 27; mean length, 2.7 microns) than in the green rod (15; 1.8 microns). These differences result in the total disc edge of the red rod (mean, 178 microns) being two times longer than that of the green rod (85 microns). The disc incisures emerge at the basal end of the outer segment and on the average become linearly longer toward the middle outer segment. During displacement of discs over the distance of 2.5 microns, which covers about 90 discs, the incisures reach more than 90% of their maximum length of 2.7 micron in the middle outer segment. In addition, the process of incisure elongation has a kind of fluctuation in that the incisure frequently shows a sharp decrease and increase in length accompanied by a change in course in the disc surface.