174 AVERY, MACLEOD, MCCARTY 



followed the sequence of papers in this volume will find hijuself 

 plunged into an entirely new vocabulary and terminology by this 

 work. The genetic facts and phenomena he has become familiar with 

 in the other papers are all here, but their setting is unfamiliar. It 

 is like encoimtering a traffic policeman in a foreigji country. His 

 duties are similar to those at hojne, but the details of his performance 

 are ?iovel, and slightly cofifusing. One must understand the local 

 situation before he drives successftdly, and so it is here. The effort 

 necessary to follow the intricacies of bacteriological detail is re- 

 warded by a new i?isight into ge?ietic ?nechanis?m and structure. 

 This will be foimd equally true of the paper by Benzer o?i p. 211. 

 This paper is of outstaTidijig importance to ge?ietics, of course, for 

 the precise way m which it shows DNA to be a primary hereditary 

 material. For the first time a definable chemical co?npoimd plays a 

 role i?i changing the hereditary makeup of an organisjn, and, once 

 it has played this role, shows also that it is capable of reproducing 

 itself precisely, ^Hn amounts far in excess of that originally added" 

 (p. 190). The experi?ne?2tal data and evidence the authors have mar- 

 shalled to prove this poi?it are quite impressive, and the reader should 

 note the multitude of techniques a?id apparatus involved. Genetic 

 investigations on micro-organisms are exceedingly complex, but are 

 also exceedingly rewardifig. 



Biologists have long attempted by 

 chemical means to induce in higher 

 organisms predictable and specific 

 changes which thereafter could be 

 transmitted in series as hereditary 

 characters. Among microorganisms the 

 most striking example of inheritable 

 and specific alterations in cell structure 

 and function that can be experimen- 

 tally induced and are reproducible un- 

 der well defined and adequately con- 

 trolled conditions is the transformation 

 of specific types of Pneumococcus. 

 This phenomenon was first described 

 by Griffith (1) who succeeded in 

 transforming an attenuated and non- 

 encapsulated (R) variant derived from 

 one specific type into fully encap- 

 sulated and virulent (S) cells of a 

 heterologous specific type. A typical 

 instance will suffice to illustrate the 

 techniques originally used and serve to 

 indicate the wide variety of transfor- 

 mations that are possible within the 

 limits of this bacterial species. 



Griffith found that mice injected 

 subcutaneously with a small amount of 

 a living R culture derived from Pneu- 

 mococcus Type II together with a 

 large inoculum of heat-killed Type III 

 (S) cells frequently succumbed to in- 

 fection, and that the heart's blood of 

 these animals yielded Type III pneu- 

 mococci in pure culture. The fact that 

 the R strain was avirulent and incapa- 

 ble by itself of causing fatal bacteremia 

 and the additional fact that the heated 

 suspension of Type III cells contained 

 no viable organisms brought convinc- 

 ing evidence that the R forms growing 

 under these conditions had newly ac- 

 quired the capsular structure and bio- 

 logical specificity of Type III pneu- 

 mococci. 



The original observations of Griffith 

 were later confirmed by Neufeld and 

 Levinthal (2), and by Baurhenn (3) 

 abroad, and by Dawson (4) in this 

 laboratory. Subsequently Dawson and 

 Sia (5) succeeded in inducing transfor- 



