The control of cell division 



The results are complicated, and the details must be left to the original paper. 

 It is sufficient to say here that ether appears to have two quite distinct effects on the 

 division process. At a concentration of i per cent, it prevents the building up of the 

 mitotic figure, but does not affect the filling of the reservoir. That is to say if it is 

 applied, and then removed, before the critical point, it causes no delay in cleavage. 

 If on the other hand it is removed after the critical point, it causes a delay in cleavage, 

 not equal to the total duration of the inhibition, but only to the duration of the inhi- 

 bition beyond the critical point. The egg in other words remains blocked at the criti- 

 cal point. At higher concentrations not only does ether block the building up of the 

 mitotic figure, but it reduces the rate of filling of the reservoir as well. At a concentra- 

 tion of 2 per cent, for instance, the rate of filling is halved. 



The first effect of ether can therefore be likened to preventing the siphoning out 

 of the reservoir, or to preventing the normal action of the reservoir contents once 

 they have siphoned out. The second effect resembles that of carbon monoxide in 

 reducing and ultimately stopping the filling of the reservoir. It is of some interest to 

 note that i per cent, ether, by blocking the cell at the critical point, offers a means of 

 bringing batches of eggs fertilized at different times into synchronous division. 



THE NATURE OF THE RESERVOIR 



Having arrived, by two quite different routes, at the same hypothesis for the control 

 of division in sea-urchin eggs, it may perhaps be permissible to speculate a little about 

 the proposed mechanism. The compelling question, of course, is the physical and 

 chemical nature of the reservoir. Is it to be regarded as a store of energy, or of some 

 other non-energy-carrying compound ? If the latter, it must follow from the carbon 

 monoxide experiments that division itself can function on glycolytic energy alone. 

 The critical experiment must be to test the effect of a range of inhibitors of glycolysis, 

 when mitosis is already under way. The experiments of Hughes (1950), however, 

 indicate that fluoride, which is amongst other things a glycolytic inhibitor, applied 

 under these circumstances to tissue cells, may slow down division but does not actually 

 stop it. More work is needed, however, to be certain on the point. 



The evidence would seem to point therefore to the reservoir being a store of 

 energy. This energy might of course be built up in a physical form, such as the ori- 

 ented protoplasmic structure of the sperm aster. The results of the ether experiments, 

 however, make this seem unlikely, for whereas the lower concentrations of ether can 

 be seen under the polarizing microscope to solate the oriented structures of the cell, 

 they are, as was pointed out earlier, without effect on the filling of the reservoir. 



It is natural then to turn to the idea of a chemical store of energy, perhaps of some 

 organic phosphorus compound. There are difficulties, however, in thinking in terms 

 of the best-known of these substances, namely ATP and phosphagen. The amount 

 of ATP, it is generally agreed, does not seem to vary during the division cycle. The 

 amount of phosphagen does, it is true, appear to increase shortly after fertilization, 

 but the time relations do not fit in at all with the reservoir hypothesis (Chambers and 

 Mende, 1953). Moreover both these compounds appear to be freely available to the 

 cell as a whole, as they decrease during anaerobiosis (Barth and Jaeger, 1947; 

 Cleland, 1953). But the postulated store of energy in the reservoir does not get used 



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