M. M. SWANN 



By taking advantage of the fact that the carbon monoxide-cytochrome oxidase com- 

 plex is only stable in wavelengths of light outside its absorption bands, it is possible, 

 merely by altering the wavelength of the illuminating beam, to switch on and off the 

 inhibition of respiration while actually observing the eggs. In this way, the eggs were 

 inhibited for varying lengths of time at different points of the division cycle, and 

 photographed by time-lapse, so that their average time of cleavage could be worked 

 out. 



The results show that if inhibition is applied before a certain critical point, which 

 occurs at normal temperatures between about 35 and 40 minutes after fertilization, 

 the first cleavage is delayed by a time roughly equal to the duration of the inhibition. 

 If however the inhibition is applied after this critical point, but before cleavage is 

 complete, then the first cleavage is unaffected, but the ensuing cleavage is delayed, 

 by a period again equal to the duration of the inhibition. 



The best explanation seems to be in terms of a reservoir mechanism. We may sup- 

 pose this reservoir to be filling steadily as a result of respiration, and to siphon out 

 when it is full, at about 35-40 minutes after fertilization. This starts off the division 

 process, which continues regardless whether the reservoir is then filling or not. In 

 the normal course of events, however, the reservoir would begin refilling at once, and 

 continue filling during division and the next interphase. At about 35-40 minutes 

 after the first emptying we might expect the reservoir to be full once more, and to 

 siphon out again, so starting off the second division. The time relations of the second 

 division suggest that this is what happens. 



Besides accounting for the observed facts, this scheme explains why the first divi- 

 sion takes longer than the subsequent ones. The reservoir has first to fill up, and the 

 division has then to run through. Subsequent divisions, however, can occur at 35-40 

 minute intervals, namely the length of time taken for the reservoir to fill up, and in 

 fact they do. The scheme also offers an explanation of why there are no major 

 fluctuations in respiration during the division cycle. The energy from respiration is 

 utilized continuously by the reservoir mechanism, and it is only the siphonings out 

 that are discontinuous. It is perfectly possible therefore for the proportion of the 

 cell's energy supply required for division to be a substantial fraction of the whole. 

 Finally the hypothesis explains such experiments as those of Jacoby, Trowell and 

 Willmer (1937) where it was found that tissue culture cells on a maintenance medium 

 could be brought into division by an application of embryo juice, but could only be 

 made to divide a second time by further applications of embryo juice, if these were 

 made during or after the first mitosis. 



EXPERIMENTS WITH ETHER ON DIVISION IN SEA-URCHIN EGGS 



With a view to finding out more about the postulated reservoir from a different angle, 

 a further set of experiments was carried out, in which a narcotic, namely ether, was 

 applied as before to synchronously dividing sea-urchin eggs, at different points in 

 the division cycle, and for varying lengths of time (Swann, 1954). These experiments 

 confirm the conclusions drawn from the carbon monoxide work. They indicate the 

 existence of the same sort of reservoir mechanism, and the same critical point in the 

 division cycle at which the reservoir siphons out. 



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