M. M. SWANN 



two when the cell reached a certain size, but the idea has not met with much enthusi- 

 asm, perhaps because it seems to neglect the visible machinery of mitosis and 

 cleavage. 



If growth is responsible for starting off the processes of mitosis and cleavage, any 

 substances which interfere with the syntheses that underlie growth should appear 

 to be inhibitors of division. It is possible for instance that some of the so-called pre- 

 prophase inhibitors of mitosis are in fact inhibitors of growth. If so, it might be expec- 

 ted that the substances in question would not be effective in stopping mitosis in cells 

 where there was no growth, e.g. totally cleaving eggs. Inhibitors that are effective 

 on the one type of cell, however, seem in general to be equally effective in inhibiting 



Ceil 

 SL 3 e 



Tune, 



4 



Sye 



V 



Turrte 



Figure I. Possible types of cell growth curve; (a) growth at a 

 uniform rate; (b) growth most rapid immediately after division. 



the other type, though a search of the literature in this field might bring to light 

 some interesting exceptions. 



From what has been said above, it will be apparent that there are no very solid 

 grounds for supposing that division is triggered off when cell size reaches a certain 

 level. The fact that, for a given cell type, size may vary over a considerable range, 

 and that most if not all inhibitors of division act alike on cells whether or not they 

 are growing, might suggest, on the contrary, that growth and division are not very 

 closely linked together. At present, all too little is known of individual cell growth in 

 relation to division. But two new techniques, namely the measurement of the reduced 

 weight of cells, using the Cartesian diver, and the estimation of cell dry weight by 

 means of the interference microscope, now offer the possibility of great advances. 



1 86 



