The frog's rod outer segment observed by the negative staining technique.

Rod outer segments of frogs (Rana esculenta) were isolated by shaking the retina in R i n g e r solution. Before spreading each drop was examined by light microscopy and checked for purity. The outer segments were then either spread on the surface of a phosphotungstic acid solution (1.8 . . . 2.0%) containing 0 . 0 5 . . . 0.1% bovine serum albumin (pH 7.3) or negatively stained as drop preparations; instead of bovine serum albumin a 0.07% glycerol solution was also used. In contrast to micrographs of ultrathin sections (e.g. F E R N Ä N D E Z M O R Ä N 3 ; N I L S S O N 5 ) and previous negative staining experiments (BLASIE et al.2) which reveal "dense granules", "globular substructures" or "particles ~ 4 0 Ä in diameter" we found fibrillar elements (Fäden) in the outer segments as already postulated before on the basis of freeze-etching experiments (ROSENKRANZ 6 ) . In surface-spread and drop preparations we observed fibrils of different structure and diameter as well as partially preserved membranes. In preparations exposed to surface spreading we found predominantly well-spread, smooth fibrils (da). Their diameter is 49 Ä < da ^ 58 Ä, see Fig. 1 *. This wide range is due to the fact that several observers in our department made a total of 100 measurements from the same electron micrograph. Some micrographs indicate that each da-fibril might in turn be composed of fibrillar substructures.