The control of cell division 



The other references to variation in cell size almost all relate to the growth of 

 populations of one sort of micro-organism or another. The most detailed work on 

 Protozoa is that of Ormsbee (1942) who worked with Tetrahymena and found that 

 while oxygen consumption was high during the logarithmic growth phase and 

 decreased in the final stationary phase, cell dry weight behaved in the opposite 

 way. This is what would be expected in the light of the argument above. Such 

 studies have not however always given consistent results (see, for instance, references 

 to earlier work given by Ormsbee, 1942; Calkins and Summers, 1941; Adolph, 

 1931). But this is not surprising, for although there is general agreement that 

 oxygen consumption and hence, no doubt, energy production falls in the stationary 

 phase (Hall, 1953), presumably as the result of a shortage of energy-producing 

 foodstuffs, numerous other substances that are essential specifically for growth may 

 also run short. More detailed knowledge of the factors underlying the onset of the 

 stationary phase would be needed to get much further along these lines. 



The same is true of the other studies on the growth of micro-organisms. There 

 does, however, seem to be good evidence that cell size in yeast is at a minimum during 

 the logarithmic phase and increases during the stationary phase (Richards, 1934). 

 In bacteria too there is wide agreement that cell size varies, though in a different 

 way, being at a maximum in the late lag and early logarithmic phases (Dubos, 1949). 

 But since Robinow (1949) has shown that the so-called cells of this phase are in 

 reality multiple, it seems more likely that cell dry weight is no greater, and perhaps 

 less, at this stage than in the later phases. 



The complex conditions of the stationary phase of a culture of micro-organisms 

 are clearly not suited to an investigation of the effect of energy supply on division 

 and growth. If the limiting factor is the energy-supplying substrates, all may be 

 well; but if, as is very likely, other compounds are also lacking, the results will 

 necessarily be confused. Information on the effect of anaerobiosis or respiratory in- 

 hibition on both growth and division is therefore required during the logarithmic 

 phase. Such studies should present no special difficulties given the new techniques 

 mentioned earlier. 



CONCLUSION AND SUMMARY 



There is no direct evidence about the way in which division is controlled in cells 

 that are also growing. The tendency has been to think in terms of division being 

 triggered off in some way when the cell has grown to a certain size, but there are a 

 number of objections to such an idea. 



It would appear from the experiments on sea-urchin eggs with carbon monoxide 

 and ether that division in cells without growth is controlled by a continuously oper- 

 ating reservoir mechanism. The preparations for a division are in fact going ahead 

 not only during the previous interphase, but during the previous division as well. 



Such a mechanism may of course be peculiar to cells that are not growing. But it 

 is possible at least that a similar mechanism exists in cells that are growing, in which 

 case it would seem that growth and division must be two separate processes function- 

 ing simultaneously and more or less independently. If this were so, the relative 

 constancy of cell size would be due simply to the fact that the two mechanisms oper- 

 ated at more or less constant rates. Size however does not remain completely constant 



193 



