In a recent publication1 the writers described 521 spontaneous tumors borne by 489 members of a colony of rats used in the study of induced Cysticercus sarcoma of the liver, coming to autopsy from September 30, 1920, to May 4, 1929. The experiments on Cysticercus sarcoma, which have been in progress since October, 1919, and of which several reports have been published in this and other journals1 started with 548 purchased animals, to which 1,819 unpedigreed descendants were soon added. Breeders were selected both from the purchased stock and their unpedigreed descendants and a pedigreed colony was established. This gradually increased in size, but for the last four or five years the population has fluctuated between 7,000 and 10,000.

The purpose of this paper is to present a statistical analysis of the population data of this rat colony in respect to the occurrence of spontaneous neoplasms. For the most part these spontaneous tumors are the ones described in the paper referred to above, but the two sets do not entirely coincide since the former contains three groups of animals on which full population data are not available and in the latter are included spontaneous tumor bearers which came to autopsy at a somewhat later date.

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