66 



Gay (16) found a difference in the susceptibility of white rats 

 from different sources. In his work with carcinoma in rats he found 

 metastases regularly in the lungs and rarely in the lymph nodes. 

 He was able to raise the virulence of the tumor by transplantation 

 of the lung metastases. This increase of virulence was shown by 

 increase in the rapidity in growth, increase in metastases, and the 

 increase of the epithelial elements over the stroma. 



Brooks (17) in considering the subject of tumors in animals con- 

 cludes that true neoplasms are very rare in wild animals living under 

 natural conditions. It should be stated, however, that Brooks 

 refers especially to higher mammals such as are found in zoological 

 collections. 



TUMORS OF WILD RATS. 



A new growth is found approximately once in every thousand rats 

 examined in San Francisco. Ninety-two tumors have been examined 

 microscopically. Time has been available for the study of but one 

 or at the most two sections from each tumor and while in some cases 

 the diagnosis was easily made in others there was room for con- 

 siderable difference of opininon as to the nature of the growth. It 

 is obvious that it is hardly fair to expect to make a final diagnosis 

 in every case from one or two sections taken from one part of the 

 growth, and it is possible that further study will throw more hght 

 upon the histological nature of some of them. 



Location. — The largest number of the tumors have been found in 

 the subcutaneous tissue of either the thorax or of the abdomen, 

 and as the majority of these have been found in female rats we have 

 assumed that they were probably of mammary origin. The growths 

 were occasionally located directly under the nipple, but in such 

 cases the nipple was not retracted, and it was exceptional to find any 

 ulceration. The tumors were very rarely adherent to the surround- 

 ing tissue. After the subcutaneous tissue tumors were found most 

 frequently in the liver. Histologically, the most of these growths 

 were sarcomas and the majority of them had a parasite, the Cysti- 

 cercus fasciolaris, in some part of the tumor. This parasite, as is 

 well known, is the larval stage of a tapeworm found in the cat. 

 These tumors of the liver were frequently associated with an enor- 

 mous number of secondary growths varying in size from a millet 

 seed to 1 centimeter in diameter scattered through the omentum, 

 mesentery, and other abdominal structures. 



Several growths have been found in the kidney, mostly of an 

 epithelial nature, one being a particularly well-marked example of 

 a cystic papilloma. A few have been found in connection with other 

 parts of the genito-urinary tract. A large bloody tumor, which 

 upon microscopical examination was found to be an angiosarcoma, 



