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This work was supported with funds from the Eugene Higgins Trust allocated to Princeton University.
†
Experiments on 24-hour rhythms involve great labor, since continuous hourly observation is demanded over a period of several days. The experiments discussed in this paper would have been impossible without careful and cheerful assistance from the following undergraduate students: J. Angelo, W. Schlaepfer, H. Zehner, L. Brower, L. Parry, D. Trend, and E. Merrifield. It is a pleasure to acknowledge their help and interest. I have also benefited very greatly from discussion with J. E. Burchard and R. Levey.