As an adjunct to the study of a transplantable malignant tumor of the rabbit an index of malignancy has been developed which has proven useful in evaluating the severity of the disease for the purpose of statistical investigations.

The index consists of two figures, one of which expresses the growth capacity of the primary lesion on a volume basis, and the other the extent of the metastatic lesions according to the distribution of peripheral tumors. In the present communication these two values for each animal are utilized for the study of the relationship existing between the size of the primary tumor and the distribution of metastases in this experimental tumor. The data from a group of 128 rabbits form the basis of the analysis. These animals represent three entire series that had been used for the purposes of other experiments at various times, all, however, conducted under carefully controlled uniform conditions. The indices of malignancy of these animals were analyzed by means of graphs, in which individual values are preserved while at the same time investigation of mass behavior is permitted.

From this analysis it is concluded (1) that the severity of the disease as a whole is very irregular among individuals of a series and from series to series, and (2) that the relationship between the extent of the primary lesions and that of the secondary lesions is inconstant among individuals but assumes definite proportions when the data are viewed in the mass. For the mass proportion between the two phases of the disease is predominatingly a direct one, but in certain small groups of seriously affected rabbits the proportion shows a tendency to reversal, while, in addition, there occurs a reduction in the rate of growth of the primary tumor, which reduction, together with the reversal in relationship, appears to be more pronounced from series to series as malignancy increases.

An attempt has been made to account for these results in the light of what has been ascertained of the modes of defense of the rabbit host against the inroads of the cells of this transplanted tumor.

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