228 CONTROL MECHANISMS IN CELLULAR PROCESSES 



stages in the life cycle. This is spectacularly illustrated in the course 

 of embryological development (McElroy and Glass, 1958). Here 

 also a possible presumption is that an essential aspect of the control 

 mechanism is that certain events A, B, C, D, E, etc., occur in a fixed 

 sequence, but it is not known how a given event relates to the others 

 in the series. In these cases also we are largely concerned at the 

 present time with gaining descriptive information but may certainly 

 hope that information concerning the nature of the chemical control 

 will be forthcoming. 



Distinctly set apart from mitotic and life cycles are the daily 

 tidal, lunar, and annual cycles exhibited by cells and organisms. 

 These also involve cyclical changes, but their frequencies match the 

 corresponding cycles of the physical environment (Symposium on 

 Biological Clocks, 1961; Brown, 1959; Biinning, 1958a; Hastings, 

 1959; Pittendrigh and Bruce, 1959). A possible relationship— inso- 

 far as mechanism is concerned— between these two major kinds of 

 cycles is not yet definitely ruled out. 



In the present paper, we shall limit ourselves to a consideration 

 of daily or diurnal rhythms. The persistence of diurnal rhythms 

 with a period of approximately 24 hours indicates that an endoge- 

 nous control mechanism is responsible for the cyclically recurring 

 processes (Pittendrigh, 1958). In this respect the nature of the 

 mechanism may be similar to mitotic and life cycles. But in a sec- 

 ond respect diurnal rhythms exhibit a major difference. The period 

 of the cycle is always close to 24 hours (but rarely if ever exactly 

 that), and it is not readily subject to change as a consequence of 

 different environmental conditions. Whereas the period of the mito- 

 tic cycle is generally found to be shorter at higher temperatures, and 

 vice versa, with Qio values in the range of 2 or 3, the period of 

 diurnal rhythms has been found to be relatively unchanged by tem- 

 perature (Pittendrigh and Bruce, 1957). 



The important implication of the temperature-independent period 

 of biological rhythms is that physiological and biochemical processes 

 may be regulated in time by means of a cellular or subcellular mecha- 

 nism, a kind of chemical clock, with the important features neces- 

 sary for relatively accurate timekeeping. 



A large number of animals and plants have been shown to exhibit 

 diurnal rhythms. In every case studied, the free-running period 

 (i.e., the period of the rhythm under conditions of constant tern- 



