THE GONYAULAX CLOCK 



573 



-I 

 _j 



UJ 



o 

 o 



(r 



UJ 

 Q. 



100 BO MO 160 



HOURS IN CONTINUOUS DIM LIGHT 



220 240 



Fig. 5. The curves illustrate the diurnal rhythm of cell division at 

 18.5°C (lower curve) and at 25°C (upper curve), as measured by the 

 percent of paired cells present in a cell suspension. The average generation 

 time was 4.2 days at 18.5°C and 3.8 days at 25°C. Period as related to 

 temperature, see Table I. 



CHARACTERISTICS OF THE RHYTHM 



The period of each of the rhythms under constant conditions ( = 

 the natural period) is always close to, but not necessarily exactly, 24 

 hr. Moreover the phase, under constant conditions, may bear any de- 

 sired relation to solar time. As pointed out by Pittendrigh (1958) 

 and Pittendrigh and Bruce (1957), these are compelling reasons for 

 the assertion that diurnal rhythms are endogenous self-sustaining oscil- 

 lations. The phase and period are clearly not related to phase and 

 period of uncontrolled periodic variables in the environment. 



The period of each of the three overt rhythms which we have studied 



