In this study the attempt has been made to follow the fate of tubercle bacilli in the lung, liver, spleen, kidney and bone marrow of rabbits infected intravenously with large and small doses of human and bovine tubercle bacilli by determining the number of colonies recoverable from similar quantities of tissue on egg media at varying intervals during the course of infection. This method offers certain possibili-ties for the elucidation of this problem precluded by the modes of attack used hitherto. Histological methods, while giving precise data in regard to tissue changes produced by the tubercle bacilli, are poor instruments for determining the fate of the bacilli in a given organ. Without stressing the notorious difficulties in staining the organism at all times, histological technique can give no definite answer to the question whether certain stained bacilli are living or dead, and it is the number of living bacilli that is of importance. Again animal inoculation, while an excellent index of the presence of living virulent bacilli, is a very inaccurate index of the number of living bacilli in a given specimen of tissue, for it is possible to infect guinea pigs with even a very few bacilli.

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