334 Mutation in Micro- Organisms 



both in the ordinary way and also from individuals isolated by his 

 Indian ink method (p. 330, footnote). Each non-saccharose-splitting 

 initial organism was found to give rise to only " a very low percentage " 

 of saccharose-splitting colonies. The latter, however, had acquired the 

 power permanently, and always bred true to this character — after " any 

 number " of sub-cultures in many different media. 



Burri then obtained Massini's strain from Neisser, and tested it by 

 shake cultures. He made, conversely, test cultures of B. imperfectum 

 on a modified Endo medium (containing saccharose instead of lactose). 

 He concluded from all his experiments that B. imperfectum behaves 

 towards saccharose exactly as B. coli mutahile behaves towards lactose^ 

 Experiments were then made to determine what percentage of 

 the offspring of B. imperfectum (non-saccharose-splitting) underwent 

 mutation into B. perfectum (saccharose-splitting). The results of some 

 ingenious work were surprising. An initial culture begun with some 

 10,000,000 germs of imperfectum, produced about 50 colonies of per- 

 fectum, but the majority were of the imperfectum type. Beginning 

 with about 100,000 imperfectum (only ji^ of the first number), how- 

 ever, he again obtained about 50 colonies of perfectum. And by 

 beginning with a very few germs, he found that practically every 

 colony was of the mutated form (perfectum). " In a series of cultures 

 with a diminishing number of initial germs, of which the greatest is 

 a million times greater than the least, each member of the series 

 shows approximately the same number of mutated colonies^" Burri 

 was thus able to demonstrate that all the individuals of the imper- 

 fectum race are — under suitable cultural conditions — able to mutate 

 completely into perfectum forms. In other words, all imperfectum 

 individuals are, as regards their power of mutation into 'perfectum 

 races, equipotential. The apparent partial mutation of the race is 

 dependent upon the different environmental conditions to which the 

 different individuals of a large colony are subjected. When the indi- 

 viduals are sufficiently separated so that every one is offered the same 

 favourable opportunities, they all behave in the same way — that is, all 

 mutate. To borrow a term from Driesch, every individual has the same 

 prospective potency. 



^ Massini, p. 328 supra. 



2 In other words, suppose we begin by sowing 100 million individuals — we get then 

 nearly all the colonies like the initial germs, and only x colonies with the new character. 

 If we sow only x individuals of the initial germs, however, we obtain x colonies with the 

 new character. Similarly, any number of initial germs between 100 million and x, always 

 produced x mutations. 



