568 PHOTOPERIODISM IN INVERTEBRATES 



concerning the nature of the clock system are consistent with the idea, 

 outhned and discussed in considerable detail (Bruce and Pittendrigh, 

 1957; Pittendrigh and Bruce, 1957; Biinning, 1956) that clocks result 

 from an endogenous self-sustained oscillation (ESSO) of cellular com- 

 ponents. 



Four overt manifestations of rhythmicity have been observed in 

 Gonyaulax, and the question arises as to whether the organism pos- 

 sesses a single "master oscillating system" by which the overt rhythms 

 are governed, or whether these are several independent, or quasi inde- 

 pendent, oscillating systems. Evidence from earlier studies (Hastings 

 and Sweeney, 1957b) led us to suggest the possibility of several inde- 

 pendent oscillating systems. Our more recent evidence, presented here, 

 makes the "master clock" proposal (Pittendrigh, 1958) seem more 

 likely. The question still awaits a more definitive study, however. 



THE RHYTHM OF LUMINESCENCE 



This is a rhythm of induced luminescent flashing. Gonyaulax char- 

 acteristically emits light (as a discrete flash; see Fig. 1) only when it is 



Fig. 1. Flash of Gonyaulax in response to stimulation. Ordinate, light 

 intensity; abscissa, time. Sweep time, 0.25 sec. Cells in the phase of 

 maximum luminescence were placed in a test tube in front of the photo- 

 tube and the table was tapped lightly. Temperature, 24°C. 



