1. References to the original papers can be found in the surveys by: Kamen, M. D., "Radioactive Tracers in Biology" (New York: Academic Press, Inc., 1947); Hevesy, G., "Radioactive Indicators" (New York: Interscienee Publishers, 1948): Süe, P., "Dix Ans d'Application de la Radioactivité Artificielle" (Paris: Société d'Éditions Scientitiques, 1948); Fourth and Fifth Semi-annual Report of the U.S. Atomic Energy Commission (Washington, D.C.: U.S. Government Printing Office, 1948 and 1949). The vast predominance of biological experiments in to-day's applications of the tracer technique may be contrasted with the situation twenty-two years ago; see Paneth, F. A., Nature, 120, 884 (1927).