Progressively decreasing quantities of bacteria of some 20 strains were utilized in experiments upon the effect of dispersing the organisms in the rabbit skin through the agency of an extract of testicle or an invasive staphylococcus. The same was done with 6 strains of filterable viruses.

The bacterial lesions were enhanced by spreading when the organisms introduced were above a certain number or quantity (minimal effective concentration) and on the other hand were partially or totally suppressed when their number was less than this. Virulence and minimal effective concentration were observed to be in inverse relationship. The lesions due to the filterable viruses studied were, on the other hand, enhanced by the spreading factor even when the quantity of virus approached the minimal infective dose. This happened irrespective of whether the virus caused severe lesions or slight ones.

The highly virulent Pneumococcus Type I, injected into normal rabbits together with the spreading factor, yielded enhanced lesions even at practically its minimal infective dose; but when the resistance of the animal was raised with specific antiserum the lesions were totally suppressed by the experimental dispersion of the bacteria. When such an experiment was repeated on a filterable virus, vaccinia, no suppression took place as a result of the dispersion of the infective agent.

The significance of the differences in the bacterial and virus phenomena is discussed.

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