132 RESPONSES OF CELLS ON THE BIOLOGICAL LEVEL 



that SH groups are of particular importance in mitosis. 

 It is not clear whether the SH groups concerned are to be 

 regarded as part of the chromosomes, part of the spindle 

 proteins or part of the general enzyme systems of the 

 cell. In support of the contention that SH groups are 

 particularly involved, it has been found that b.a.L. (dithio- 

 glycerol) is able to reverse the toxic effects produced by 

 sodium arsenite and phenyl arsenoxide, but not the toxic 

 effect of iodoacetamide. The position, however, is more 

 than a little complicated by the fact that b.a.l. is itself a 

 mitotic poison, and its toxic action can be reversed with 

 arsenoxide. 



The present position is that whilst it is clear that sub- 

 stances having a fairly high degree of specificity for SH 

 groups are mitotic poisons, it is not yet clear that the true 

 point of attack of these substances is in fact upon SH 

 groups. And beyond this, it is still less clear whether the 

 supposed SH groups are, as some contend, part of the 

 spindle proteins, or whether they may not even be so 

 distantly related to the actual processes of mitosis as to 

 be the SH groups of enzyme systems concerned in the 

 mobilisation of energy, such as the SH groups of phos- 

 phokinases. 



Substances which are generally regarded as having an 

 action upon respiratory mechanisms involved in the mo- 

 bilisation of energy, quite commonly have a very pro- 

 found effect upon mitosis. This is true of HCN, of 

 phenols such as hydroquinone, of urethan and of the 



