The Incidence of Cancer in Mice of Known Age. 311 



laboratory by reference to an index, which at once gives the ancestry, the 

 date of birth, and the age of the animal in question. The mice have been 

 systematically examined daily. 



Tbere is difficulty in obtaining offspring from mice suffering naturally 

 from cancer, and the breeding experiments began to be regularly successful 

 in January, 1907. The first case of cancer was found in a female mouse in 

 March, 1908, the animal being nine months old. Since that date 18 

 additional spontaneous malignant new growths have been obtained. Every 

 case has been subjected to careful microscopical examination, and only 

 undoubted cases of malignant new growth are reckoned. So far all have 

 occurred in female mice, and with two exceptions have affected the mamma. 

 The exceptional cases were one of generalised malignant lymphoma, and one 

 of melanoma or melanotic sarcoma of the external ear. The remainder were 

 carcinomata of the mamma in which the adenomatous character was present 

 in varying degree ; in two of them small areas of keratinisation were found 

 in the sections examined. 



It was of interest to determine at what age the tumours wevejirst observed, 

 and to determine the number of animals of the same age under observation. 

 As no cases of new growth have yet occurred in the males* bred, these are 

 excluded from the present purview. The females were distributed by means 

 of a card index into five groups differing in age from each other by intervals 

 of three months. Animals under six months old have been excluded 

 because of the high mortality in the first six months of life from infectious 

 diseases of all kinds (pneumonia, enteritis, septicaemia), and because the 

 youngest mouse in which a true malignant new growth occurred was 

 exactly six months old. It will be noted from the table given below that 

 no female animal attained the age of two years. On April 26, 1909, 

 a census of the females was taken, and all those which had died over 

 six months old, since the beginning of the experiment, were added to the 

 corresponding age-groups. The mice which were then still liviug, after 

 developing cancer, and those which had died from the disease, appear in the 

 age-group corresponding to the age at which the disease was first discovered 

 in them. The percentages "of the following table are therefore not strictly 

 comparable with death-rates, but are to be read as giving the liability to 

 cancer at different age-periods. 



The progressive increase shown in the table presents a remarkable corre- 

 spondence with the facts long familiar to students of the incidence of 

 cancer in the human subject. It furnishes a striking confirmation in the 



* Of the first 1145 mice bred, 588 were males and 557 females. The preponderance 

 of cancer in the female is due to the great liability of the female to cancer of the mamma. 



