MITOTIC POISONS I33 



quinones. Whilst certain members of these groups, such 

 as urethan, may on some cells act in such low concen- 

 tration as to suggest that an action upon non-respiratory 

 mechanisms is involved, others, such as phenyl urethan, 

 act at a level of concentration which appears to be similar 

 to that involved in the inhibition of respiration. 



The ^-chloroethylamines. It is now well-known that many 

 compounds containing two /9-chloroethyl groups are po- 

 tent mitotic poisons. The first of these to be examined 

 was mustard gas itself. This was shown by Koller, in 

 studies on Tradescantia^ to produce chromosome breaks, 

 failure of division, the lagging of chromosomes and the 

 formation of bridges between the separating groups of 

 daughter chromosomes in anaphase and telephase. 

 Hughes and Fell have made a particular study of these 

 phenomena in tissue culture, and show that spindle ab- 

 normalities, such as tripolar spindles, are very common. 

 More recently attention has been focussed upon the 

 use of the so-called nitrogen mustard compounds of the 

 general type R.N:(CH2.CH2C1)2. These compounds in 

 some instances have a relatively selective effect upon the 

 growth of some types of tumours, and as Koller has 

 shown, this effect is probably largely produced by the 

 action of these compounds upon mitosis. Evidence has 

 recently been obtained by Revell that the hetero-chro- 

 matic regions of the resting nucleus are particularly sus- 

 ceptible to attack. On the chemical level, as with the 



