148 BIOLOGY OF PNEUMOCOCCUS 



on the presence of an admixture of both forms within the culture, 

 but rather that each individual R strain might or might not pos- 

 sess the ability to revert. This varying tendency of R strains was 

 exemplified in one experiment in which one of four other strains of 

 Type IR, obtained by growing a freshly isolated Type IS strain 

 in 25 per cent Type I anti-R serum, reverted to Type IS after 

 forty transfers. 



The finer details of colony appearance of R and S forms inter- 

 ested Paul (1927), 1067 who chose a small number of standard R 

 and S strains and studied their growth on agar under a limited 

 number of cultural conditions. Paul described the R colonies as 

 having a rough surface, with a gradual and progressive increase in 

 size over a period of several days and a tendency to remain dis- 

 crete. The colonies failed to undergo rapid autolysis in early gen- 

 erations and exhibited limited secondary colony formation. Methe- 

 moglobin formation was present but might be replaced by slight 

 hemolysis. Paul's S strains grew in rapidly developing disc-shaped 

 colonies with a smooth surface which later showed irregularities. 

 The colonies tended to become confluent and exhibited marked au- 

 tolysis in thirty-six to ninety-five-hour cultures. In the same pe- 

 riod, secondary colony formation took place. Methemoglobin for- 

 mation was a constant feature. 



In a second paper, Paul 1068 gave further information concern- 

 ing the conditions which affected colony formation. Under extreme 

 crowding, the individual S colonies gave way to irregular, amor- 

 phous, slightly elevated masses with myriads of tiny structures 

 having irregular and roughened surfaces, comparable to R colo- 

 nies, but on transfer to less crowded conditions they developed as 

 typical S colonies. The true R colonies tended to remain discrete, 

 but in dense growth resembled the S colonies under similar condi- 

 tions. The effect of age on the S colonies was to increase autolysis 

 and papilla formation. With the R colonies there was no autolysis, 

 but roughness, opacity, and compactness became emphasized, with 



