THE GONYAULAX CLOCK 571 



the concentrations of the chemical components of the luminescent 

 system — the flashes are much brighter during the night phase. The 

 latter notion is also supported by the fact that the activities of the 

 components of the luminescent system {Gonyaiilax luciferin and luci- 

 ferase) extractable from the cells during the night phase have much 

 higher activities than those extractable during the day phase (Hastings 

 and Sweeney, 1957a). 



The gradual rise and fall of the baseline on the record (Fig. 3) re- 

 sults from a steady light emission, or glow. It is so dim that we have 

 not been able to see it even after several hours of dark adaptation. This 

 glow also recurs at approximately 24-hr intervals. Since the cells are 

 kept in darkness during the recording, the glow rhythm does not per- 

 sist for a long time, owing to the necessity of light for nutrition, as 

 previously mentioned. 



The glow rhythm appears to have the characteristics of an endoge- 

 nous rhythm even though it is not possible to demonstrate its persist- 

 ence for more than three days (Sweeney and Hastings, 1958). It has, 

 however, provided a useful technique for the study of phase shifting. 



THE RHYTHM OF CELL DIVISION 



When Gonyaulax cultures were observed under the microscope, it 

 was found that a large proportion of the cells were in the process of 

 cell division at about the time when the glow was taking place, and 

 that there was very little cell division at other times during the day 

 (Sweeney and Hastings, 1957b; Sweeney and Hastings, 1958). Since 

 recently divided daughter cells remain attached as "pairs" for about 30 

 min after cell division, the most convenient index of cell division is the 

 number of these pairs. In LD cultures, a very sharp maximum in the 

 number of pairs occurs every 24 hr, just at the end of the dark period. 

 This peak occurs an hour or so after the maximum of the luminescent 

 glow, so it is thought that the glow occurs during, and perhaps results 

 from, a stage or stages of mitosis. If cell division occurs at only one 

 time of day, then the growth curve should have a staircase shape. Di- 

 rect cell counts made on LD cultures confirmed that this was indeed 

 the case (Fig. 4). 



