360 H. B. ANDERVONT 



leukemia is greatly reduced. Can the agent be detected in the thymus before 

 the occurrence of the disease, in the same manner that the virus of mouse 

 mammary cancer is detectable in the milk? What influence will thymectomy 

 exert upon the production of the disease by the inoculation of filtrates from 

 leukemic tissue extracts? Is the agent in tissues of low-leukemia strains of 

 inbred mice developing the disease after exposure to chemical carcinogens, 

 ionizing radiations, or hormones? It is conceivable that all cases of leukemia 

 in mice are not caused by a virus. Here, experience gained with the mammary 

 tumor virus of mice could be of some assistance to those interested in 

 leukemia. The scope of the problem with this tumor virus is so large and the 

 practical implications so important that researchers engaged in the work are 

 obliged to utilize every assistance offered by their predecessors, and, more 

 important, to co-operate fully with their contemporaries 



All who have administered extracts of leukemic tissues to newborn mice 

 agree their test animals developed parotid gland tumors, and it would now 

 appear that investigations of this tumor are providing more interest than the 

 leukemia studies. Mice of the NCI and Bittner substrain of C3H mice 

 develop parotid tumors after receiving leukemic extracts but, thus far, 

 spontaneous tumors of this type have been observed only in the Bittner 

 substrain. If mice develop this tumor because they harbor the virus, then it 

 must be in both substrains. But when Gross (1957b) used cell-free extracts 

 from C3H parotid tumors as inocula for other C3H mice, only 7 % responded 

 with similar tumors and Stewart et al. (1957a) reported negative results in 

 their C3H mice after receiving similar extracts. Dulaney et al. (1957) casually 

 remark they have carried the agent through 2 serial passages by means of 

 nitrates. When Gross (1957b) used extracts of parotid tumors as inocula for 

 C3H mice, he observed a leukemia incidence of 16 % in the test animals, but 

 when Stewart et al. (1957a) used tissue cultures infected with extracts of 

 parotid tumors or leukemic tissues as inocula for (C3H X AKR)F a hybrids, 

 none developed leukemia. The variety of tumors and other lesions in Stewart's 

 mice, and their occurrence only in animals with parotid tumors, raises the 

 question whether the other tumors would arise in mice whose parotid glands 

 had been removed. 



Schmidt (1956) injected infant mice with a fraction from the cytoplasm of 

 cells of the transplantable Ehrlich carcinoma and observed the appearance of 

 malignant tumors of different types, including salivary gland tumors, in the 

 inoculated mice. It is obvious that the problems of this agent require 

 clarification. 



The appearance of leukemia, parotid tumors, and sarcomas in his test mice 

 posed a major problem for Gross. He discussed frequently the possibility of 

 the same virus or different viruses being responsible for the various lesions 

 and, while inclined to favor the view that different agents were responsible, 



