A STUDY OF THE MECHANISM INVOLVED IN SHIFTING OF THE PHASES OF THE ENDOGENOUS DAILY RHYTHM BY LIGHT STIMULI

Abstract

The phase relations and form of the diurnal rhythm of color change in Uca pugnax were experimentally altered by subjecting the crabs to a higher illumination by night (5 P.M. to 5 A.M.) and a lower one or darkness by day (5 A.M. to 5 P.M.) for 5 consecutive days. A graded series of degrees of shift from complete reversal of phases (considered as a half-cycle forward shift), through no shift, to a shift of a quarter cycle backwards. The degree of backward shift was shown to be correlated with the brightness of the nighttime illumination and the degree of forward shift by the strength of the light stimulus due to illumination increase in passing from the lower daytime to the higher nighttime illumination at 5 P.M. An hypothesis is advanced as to the normal mechanism of shift in the phase relations of the persistent endogenous rhythm by altered conditions of illumination.