An improved method has been developed for the preparation of autoradiographs for electron optical study. The refinement lies principally in the routine production of uniformly thin layers of photographic emulsion over the tracer-labeled cell sections. This is accomplished by means of a centrifugal spreading mechanism. Maize root tips grown in the presence of H3-thymidine were examined electron microscopically by this technique and were found to display nuclear labeling with impressive clarity. The new procedure utilized here yields objects in which the finest cell structures are easily resolvable without recourse to gelatin digestion techniques.

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