DISSOCIATION AND TRANSFORMATION 155 



by acquisition of properties of the typical S form. In the experi- 

 ments, reversion was invariably toward the specific type from 

 which the R form was originally derived.* 



REVERSION BY MEANS OF PNEUMOCOCCAL VACCINE 



In Griffith's experiments on reversion he introduced a new prin- 

 ciple, which later was to effect still more surprising and momentous 

 changes in the biological character of Pneumococcus. He reported 

 that the most certain method of producing reversion was to add to 

 the R culture before subcutaneous injection into the mouse a dose 

 of a heat-killed culture of a virulent strain of the same type. Re- 

 version from R to the S form could occasionally be brought about 

 by the simultaneous inoculation of a virulent culture of another 

 type when the culture had been heated for only a short period to 

 60°, that is, a Type IIR strain reverted to its original condition 

 when inoculated with a heated, virulent Type I culture. The Type 

 I antigen appeared to lose the power to cause reversion more easily 

 than the Type II antigen, the former becoming inactive after heat- 

 ing to 80°, whereas the latter was still effective after steaming at 

 100°. Griffith found, moreover, that the antigen of certain Group 

 IV strains appeared to be closely related to that of Type II. Both 

 were equally resistant to heat, and stimulated the reversion of R 

 forms derived from Type II, but failed to bring about the rever- 

 sion of the RI strain to its S form. 



Transformation of Type 



More surprising and important was the successful transforma- 

 tion by the method of an R strain derived from one specific type 

 into the S form of the same type as that of the heated culture. 

 The S form of Type I was evolved from the R form of Type II 

 Pneumococcus, and the S form of Type II from an RI organism. 



* Alloway (1932)8 cited Kelley as having discovered that normal hog serum 

 was rich in these anti-R bodies and could be substituted for anti-R serum in 

 activating the reversion process. 



