94 Inside the Living Cell 



according to the theory, occasionally a new type of cell is formed 

 which can escape from the normal body controls, and thus will be 

 able to invade and replace other tissues of the body. 



The most cogent fact in favour of this theory is the discovery that 

 a number of chemical substances cause cancer and these substances 

 have usually also been found to be mutagens, they are capable of 

 producing definite mutations when appUed, for example, to the sperm 

 of flies or to fungi. 



This discovery is undoubtedly one of the great milestones in cancer 

 research. It was known in the nineteenth century that cancer was 

 common among certain types of workers, such as chimney sweeps 

 and cotton spinners, who used a particular kind of oil for lubricating 

 the spindles, and workers in the shale oil fields. However, it was not 

 till after 1920 that attempts were made to find the chemical sub- 

 stances responsible. In 1921, Block and Dreifuss of Zurich showed 

 that the active substance in coal tar was concentrated in the higher 

 boiling fractions. The first identification of active compounds was 

 achieved in 1932 by Kennaway, Hieger and Mayneord. It had been 

 found in the study of tars that the activity seemed to be associated 

 with a particular kind of fluorescence spectrum. This was followed 

 up and eventually it was discovered by Hieger that the pure hydro- 

 carbon substance, 1-2 benzathracene, possessed this kind of spectrum. 

 This led to the synthesis of a large number of similar hydrocarbons 

 by Cook and eventually pure hydrocarbons, which produce skin 

 cancers in mice and rats, were isolated from tar and also made syn- 

 thetically. A fair number of such hydrocarbons have since been 

 identified. 



More recently, it has been found that many other types of chemical 

 compounds are capable of causing cancer — sometimes in one organ, 

 sometimes in another. The list of known carcinogens has become 

 quite extensive (see Appendix, Fig. 8). It includes certain dye 

 stuffs (e.g. butter yellow) and also intermediates used in the dye 

 industry, like naphthylamine, which were found to be responsible for 

 bladder cancer which has been common among workers in the dye 

 industry. 



Most of these substances have also been found to be mutagens. 

 This is the case with compounds of the 'mustard* series, which have 

 already been mentioned as the first chemicals to be shown to have 

 the ability to produce mutations (see p. 63). But numerous other 

 types of carcinogens have since been shown to be mutagens, so that 

 there is at least a close connection between carcinogenicity and the 

 ability to cause mutations. One characteristic of mutagens is that they 

 are capable of reaching the genetic material in the nucleus of the 



