338 Mntatioii in Jlicro-Orgamsms 



There is one important paper to be considered in this connexion. 

 It is a record of some extensive experiments performed by Franz Wolf 

 (1909)', who has studied Bacillus jn-odigiosus, Staphylococcus pi/ogeiies, 

 Sarciiia lutea, and numerous Myxobacteria. The aim of the experi- 

 ments was to induce mutations, in respect of colour, by chemical or 

 physical means. It may be stated at once that *S'. lutea gave only 

 negative results, and that most of the changes observed in the other 

 species were modifications — that is, transitory changes, not mutations 

 (cf. p. 326). Many interesting observations were made concerning the 

 effects of temperature changes and of a large number of chemical 

 substances. But as we are now concei-ned with the mutations, further 

 mention of the modifications will be omitted^. It should also be men- 

 tioned that all the observations were carefully controlled by means of 

 plate cultures ^ 



The first series of experiments was made with the historically 

 interesting B. prodigiosus*. The initial colony on gelatine was 

 coloured bright red=. Its purity was carefully tested, and as a further 

 control the organisms were propagated under " normal " conditions for 

 a long time. The strain was many times transplanted (altogether 

 more than 50 times), being tested by a series of plate cultures each 



' It should be noted that Wolf's results are not always easy to comprehend from his 

 account of them. Certain statements, for example, in his summary contradict others in 

 the body of the paper. Dr Wolf has, however, kindly elucidated (in correspondence) all 

 the doubtful passages, so that I trust my statements in the following lines coincide 

 accurately with the facts. The discrepancies arose chiefly in the following way. The 

 paper originally contained a summary — in graphic form — of all his experiments. This 

 was eliminated by the editor, as he thought it superfluous. Without it, however, the 

 paper is not always easy to understand, as in the text details are not given of all the 

 cultures. 



- These modifications may, however, have some significance for the interpretation 

 of the mutations. The original should be consulted. 



■' No cultures were made from isolated individuals, as the organisms were found to be 

 too small for this to be done. The plating out was done so carefully and frequently, 

 however, that this is probably a matter of small moment. 



■* This is Ehrenberg's Monas prodiyiosa — the organism which is supposed to have 

 given rise to the legend of the Bloody Host. It occasionally appears in large masses 

 on bread, etc. 



^ The bright blood-red pigment has been named " prodigiosiu." Though it has been 

 much studied, its chemical composition is still practically unknown. It possesses several 

 remarkable properties. It is formed only in presence of oxygen. It is bleached by 

 sunlight — both from colonies of the bacteria and from solutions. Like most similar 

 pigments, it is not present in the organisms themselves, but is excreted into the 

 surrounding medium — thus giving the colonies their characteristic colour. It is not 

 known what part it plays in the economy of the bacteria. 



