M. M. SWANN 



up during inhibition; if it did, the delays induced by inhibition would always be 

 greater than the duration of the inhibition, in order that the depletion of the reser- 

 voir could be made good. Such objections, however, do not rule out the possibility of 

 other more specific organic phosphorus compounds constituting the postulated store 

 of energy. 



THE APPLICATION OF THE RESERVOIR HYPOTHESIS 

 TO DIVISION WITH GROWTH 



We are now in a position to consider the reservoir hypothesis in relation to the vast 

 majority of cells where division is accompanied by growth. Since the reservoir fills 

 up not only throughout interphase, but during the preceding division as well, it 

 must be presumed, unless the reservoir mechanism is peculiar to egg cells, that a 

 similar mechanism also operates during interphase and the preceding division in 

 ordinary cells. How might such a mechanism fit in with the growth process; and is 

 it possible that the two mechanisms could run side by side and independently as 

 suggested in the first section? 



The issue can perhaps be clarified a little by considering the apportioning of the 

 cell's energy supplies between the various cellular activities. It has long been realized 

 that all activities do not have an equal call on these energy supplies; movement, 

 division and irritability, to mention only three, can all be brought to a halt by 

 partial respiratory inhibition, without affecting the cell's maintenance activities, or 

 at least without affecting them irreparably. There is thus no a priori reason to suppose 

 that division and growth will have an equal call on the energy supplies. 



In the case of the dividing egg cell, two activities alone presumably absorb all the 

 available supply of energy. The first of these is maintenance : making good the wear 

 and tear on the whole protoplasmic structure, and preserving the salt and water 

 balance of the cell; the second activity is division. It is not difficult to see why the 

 normal processes of selection will lead to maintenance having a prior call over divi- 

 sion on the energy supply. Division can be postponed without the organism perish- 

 ing; maintenance cannot. It is not surprising therefore to find that division is sup- 

 pressed in any tissue by a measure of respiratory inhibition which leaves maintenance 

 more or less unaffected. In the case of the sea-urchin egg, for instance, division is 

 suppressed by a degree of anaerobiosis, which reduces the oxygen consumption to 

 about 30 per cent, of normal (Krahl, 1950). Under these conditions, however, the 

 cell can survive for long periods. It is conceivable, in the light of this figure, that 

 division requires as much as 70 per cent, of the sea-urchin egg's total energy output, 

 i.e. about twice as much as maintenance. 



When the cell is both dividing and growing, the situation is more complex. 

 Growth must be added to the two activities mentioned above and, in some cases, 

 movement and perhaps other activities as well. For simplicity, however, maintenance, 

 growth and division can be considered alone. Once again it seems likely that main- 

 tenance will have a first call on the energy supply, and as with eggs, there is evidence 

 that cells can withstand for considerable periods a degree of anaerobiosis or respira- 

 tory inhibition that prevents division, and in all probability, growth as well. The 

 difficulty arises over the priority for energy as between growth and division. From 

 the point of view of selection, it would seem that a cell which maintained its growth 



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