38 



Scientific Proceedings (55). 



This agrees with the previous observations of Loeb, who under 

 similar conditions, in which however rats with inoculated instead 

 of spontaneous tumors were used, never observed a transmission 

 of a sarcoma to another rat. It is however noteworthy that in 

 strain No. 8 the large majority of cancers appeared in groups, 

 inasmuch as several, in one case even as much as five mice, which 

 were kept in the same box were simultaneously affected by cancer. 

 Whether we have in this case which was not duplicated in the 

 case of other strains to deal with an accidental occurrence we are 

 unable to state at present. 



9. From our investigations we may conclude that hereditary 

 factors play a great part in the incidence of cancer among mice 

 and that hereditary transmission is to a great extent responsible 

 for the so-called endemic occurrence of cancer among animals. 

 Certain observations especially of Borrel and Fiebiger concerning 

 the occurrence of parasitic worms in certain kinds of cancer of 

 animals indicate that also other factors of an infectious character 

 may be responsible for this endemic occurrence. 1 



Miss Maud Slye, of Chicago, in experiments carried on simul- 

 taneously with our own, also came to the conclusion that the 

 incidence of cancer varies in the different strains of mice which 

 she had under observation according to a preliminary communica- 

 tion she made at the last meeting of the American Association for 

 Cancer Research in May, 1913. In the discussion to Miss Slye's 

 paper we mentioned some of the results of our work. 



22 (839) 



The influence of pregnancies on the incidence of cancer in mice. 



By A. E. C. Lathrop and Leo Loeb. 



[From the Department of Pathology, Barnard Free Skin and Cancer 



Hospital.} 



In order to analyze still further the various factors causing 

 the spontaneous development of cancer of the breast in mice we 



1 On a previous occasion (Cenlralblatt f. allg. Pathologic Bd. XII, N. 22, 1911, p. 

 994) we published already a tree of one of the families of mice under our observation 

 in which the hereditary transmission of tumor had been apparent. Cf. also Inter- 

 state Medical Journal, Vol. XX, No. 5, 1913- 



