The characteristic appearances exhibited by the surface colonies of both S and R pneumococci in 24 and 48 hour cultures upon rabbit blood agar have been reviewed. Emphasis has been laid upon the behavior and structure of the colonies formed by R pneumococci, their frequent similarity to the colonies formed by certain strains of Streptococcus viridans, and their failure to undergo rapid autolysis in the first 48–96 hours, a phenomenon which is highly characteristic of the S pneumococcus colonies. With the S pneumococci it has been shown that "pseudo rough" colonies may be immediately produced under certain unfavorable cultural conditions but such changes in colony morphology as these do not indicate that complete dissociation has taken place and that the organisms may be classified as true R pneumococci.

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