Experiments are described in which it is shown that, in addition to the amount of virus injected, the chance of a lesion also depends on the tissue mass (number of cells) exposed to the virus shortly after injection, and that the larger the number of host cells per virus particle the greater the probability of a lesion. This point has been shown by three types of experiments: (1) varying the size of the inoculum showed that the smaller sizes were relatively more effective than the larger ones; (2) localizing the virus by the estrogenic hormone decreased the chance of a lesion occurring; (3) spreading the virus over a larger area increased the probability of a lesion.

It is also shown that because the ratio of virus particles to host cells varies and because this ratio partly determines whether a lesion occurs, the number of particles of virus cannot be predicted by the use of the Poisson law of small numbers.

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