MITOTIC POISONS I31 



follow that the specificity represents an exceptional po- 

 tency for reaction with, say, a chromosome or spindle 

 constituent. The apparent specificity may in fact be due 

 to the operation of a process not even distantly related 

 to mitosis. Thus, those compounds which readily fit into 

 the secretory patterns of cells, and so are found in dis- 

 proportionately high concentrations inside cells by com- 

 parison with other appaiently similar compounds, would 

 be expected to display an apparently abnormally high 

 toxicity if these high concentrations of drug are effective 

 in those regions of the cell which are intimately con- 

 nected with mitosis. 



Among the substances having a disproportionately 

 high activity by comparison with their oil-water partition 

 coefficients are urethan and some members of the ni- 

 trogen mustard series. It has been suggested that ure- 

 than acts by interfering with nucleic acid metabolism, 

 and that the nitrogen mustards may also act in this way. 

 The evidence in support of this hypothesis is at present 

 very slender. 



An interesting example of the difficulties which may 

 be encountered is found in the action of arsenical com- 

 pounds on mitosis. Substances such as sodium arsenite 

 and phenyl arsenoxide are fairly strong mitotic poisons, 

 and share this property with iodoacetamide. One of the 

 few properties which these three substances have in 

 common is that of combining vigorously with SH 

 groups, and it has therefore been suggested by Rapkine 



