BIOGRAPHICAL MEMORANDA 



this conclusion appears modern even to-day, and it is striking that 

 Mayer carried out some experiments in order to test the possibil- 

 ity that the formation of dextran might be induced in the strains that 

 had lost the capacity to produce capsules in sucrose media, by in- 

 cubating the latter with killed cells of dextran-producing strains, i.e., 

 by using the approach that several years later gave rise to the spectac- 

 ular experiments of Avery and associates on the transformation of 

 pneumococci under the influence of desoxyribonucleic acids from dif- 

 ferent types of these organisms. 



That the attitude which yielded the final conclusion was based on 

 a careful appraisal of all the known facts, and was anything but dog- 

 matic is evident from the formulation of the verdict: 



'The old idea of Beijerinck's that bacterial variation is the result of 

 gene mutations possesses a degree of probability that is unmatched 

 by any other current interpretation.' (p. 182). 



As is well known, this concept came into prominence during the 

 next decade, and it is now universally accepted as the most satisfactory 

 explanation of many forms of microbial variability. The term 'dissoci- 

 ation' has been relegated to limbo, largely because of Braun's [1947] 

 masterly analysis of that phenomenon on the basis of the mutational 

 concept. Braun approached this problem by studying the kinetic 

 aspects of the gradual increase of mutant types in bacterial cultures, 

 which he could attribute to a strong selective influence of the pro- 

 gressively changing medium on the growth of the original organism and 

 of the different variants. In this connexion it is interesting to recall 

 Mayer's observation that cultures of Betab. vermiforme, grown in yeast 

 extract-sucrose solutions, show a much lower incidence of mutants 

 than do comparable cultures in glucose solutions or on solid media. 

 This, of course, suggests a similar selective effect, and it accounts for 

 the occurrence of but one colony type of lactic acid bacteria on plates 

 prepared from fresh 'Tibi grains'. 



EPILOGUE 



In 1939 the 'Delft School', by which here is meant the many asso- 

 ciates and students of Albert Jan Kluyver, celebrated the twenty-fifth 

 anniversary of the date on which he had received his Doctor's diploma. 

 On this occasion the Dutch chemical weekly, 'Chemisch Weekblad', 



152 



