A distinct difference in acid agglutination optimum for Type D (bacillus of rabbit septicemia) and its mutant form, Type G, has been observed. The optimum for Type D lies between pH 3.5 and pH 3.0. This changes during mutation, the resulting Type G mutants having in general an optimum lying between pH 4.7 and pH 3.8.

The constancy of the optimum for Type D is very strict, while that for Type G is slightly less so. The variation is never so great as to cause an overlapping of optima and consequent failure of differentiation.

These acid agglutination optima are in the nature of physical constants for the two types and would imply a fundamental difference in the chemical constitution of the organisms.

Animal passage, far from causing a reversion of the mutant Type G to the primordial Type D form, brings about a still greater instability in the presence of H ions.

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