586 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



gradually, so that intermediate forms develop, which are able to produce a 

 trace of soluble specific toxin and cause immunity in the mouse. Subsequently, 

 it was found by Avery and Dawson that these R pneumococci may regain 

 their specific properties and again become typical S forms if they are grown 

 in anti-R serum, that is, in serum of a rabbit that has received repeated 

 injections of the heated R organisms and has developed immune bodies 

 against the latter; even normal hog serum may act similarly to anti-R 

 rabbit serum. The S pneumococci which were recovered under these cir- 

 cumstances were always of the same type as those from which the R forms 

 were originally derived. This indicates that the R forms with which one had 

 to deal in these experiments had retained their type specificity, although they 

 had lost their capsules, and with the latter, the type-specific carbohydrates. 

 Thus, a type I pneumococcus which had been converted into the non-virulent 

 R pneumococcus again became a fully developed type I pneumococcus after 

 reversion to the original S form. In this case, therefore, the R forms still 

 possessed, potentially, their type specificity, which under these conditions was 

 presumably localized in the central bacterial body. 



However, the, experiments of Griffith and Dawson seem to indicate that a 

 still furthergoing change, one of type, is possible. They observed that if a few 

 living R pneumococci are injected subcutaneously into mice, together with 

 very large numbers of killed virulent S pneumococci of a type other than 

 that from which the R forms were originally derived, there may be cultivated 

 in many instances from the injected mice after death, pneumococci of the 

 type to which the killed S organisms belonged, which had been used for 

 injection. 



A similar change can be produced even in vitro. Thus when very small 

 particles of R pneumococci cultures were added to suitable culture media 

 containing killed S pneumococci, of a type other than that to which the R 

 cells belonged, S forms developed, which were of the same type as that of the 

 killed S pneumococci. This result is obtained with special readiness if a 

 little anti-R serum is added to the culture media. According to Alloway, the 

 same effect can be noted when instead of adding killed S bacteria, as such, 

 a heated cell-free extract of the S bacteria is used. We have here, apparently, 

 results analogous to those which Reynolds obtained in protozoa. The nature 

 of the substance in the extract, which stimulates the R forms to synthesize the 

 particular polysaccharides involved, has not yet been determined. However, 

 it cannot be the soluble type-specific carbohydrate itself, because the addition 

 of this substance in a purified state does not produce such a transformation. 



Of a related character are the experiments of Veblen, who grew micro- 

 organisms such as streptococcus viridans and bacillus typhosus for several 

 generations in dilute horse serum and then was able to demonstrate agglutina- 

 tion of these bacteria by an anti-horse precipitating rabbit serum in high 

 dilution, the microorganisms losing at the same time their ability to agglu- 

 tinate on addition of their own specific agglutinating sera. In this case, a 

 radical change in the organismal differentials of the bacteria, which their 

 agglutination of the latter by horse serum suggests, can be excluded ; but we 



