35 



Protection Test v. Pn. 9. 



Pn. 26 and Pn. 9 were of the same agglutination types. Pn. 9 was 

 highly virulent in this experiment, killing mice in a dose of • 000 000 01 c.c. 

 of broth culture in one day ; this strain was subject to unexpected variation 

 in virulence. 



Varieties of Type yielded by the same Patient. 



Reference has already been made to the observation of the 

 American workers on pneumococci, that Types I and II disappear 

 from the sputum during or soon after convalescence from lobar 

 pneumonia, and are often replaced by the types of pneumococci 

 which have been found of common occurrence in the normal 

 mouth. The American hypothesis is that the type strains die 

 out. An alternative theory is that the virulence of Types I and 

 II becomes attenuated during convalescence, and that this 

 change is accompanied by mutation of type characters, which 

 now become degraded into those of the heterogenous and less 

 virulent group termed IV. There are two experimental diffi- 

 culties about this view. Some of the IV strains are quite as 

 virulent for mice as any of the I and II strains ; so the charac- 

 teristics of IV cannot be taken as evidence of loss of virulence, 

 at any rate for the mouse. Again, I and II strains do not lose 

 their agglutinability towards the homologous sera after pro- 

 longed subculture, though they may have become completely 

 avirulent : it is not found, at any stage of subculture, that 

 mutation of type has occurred in any of the colonies. Still, it 

 is theoretically possible that mutation may occur in nature, 



B 2 



