Cancer in Various Strains of Mice. 



37 



4. Although in several strains the incidence of cancer increased 

 with advancing age, in another strain (No. 8) a maximum was 

 reached between the age of 14 and 20 months, after which again 

 a decrease took place in the percentage of mice affected by cancer. 



5. In mating a strain rich in tumors (English) with a strain 

 very poor in tumors (cream) we received hybrids which have so 

 far been rich in tumors. In this case the male which is itself not 

 liable to have tumors transmits the liability to have tumors to the 

 daughters; the latter have tumors, although they are nursed by 

 mothers which are not liable to have tumors. We wish however 

 to collect still further data concerning this point before we regard 

 this conclusion as definite. 



6. It is not always probable to predict from the known cancer 

 rate of the parents the cancer rate of the hybrids. Thus No. 10 

 mated to a strain called "European" gave hybrids with a tumor 

 rate of 82 per cent., while No. 10 mated to No. 8 seems to give a 

 very low tumor rate, although pure Europeans had a lower rate 

 (13 per cent.) than No. 8 (30 per cent.). In this respect the 

 inheritance of the tumor rate may be compared to the inheritance 

 of color, in which also in many cases until a complete analysis of 

 the various factors has been made the color of the hybrids cannot 

 be predicted. 



7. There are certain other peculiarities observed in the case of 

 strains with different tumor rates. Thus the cream which have 

 the lowest tumor rate are mice which grow and mature more slowly 

 than any other strain of mice, although they reach ultimately a 

 large size, and they are very poor breeders. The hybrids between 

 No. 10 and European which have the highest tumor rate grow 

 rapidly and are very good breeders. The English strain also 

 grows well and breeds well. But the parallelism between these 

 characteristics and the incidence of tumors in the various strains 

 is not apparent in every case. Thus certain strains grow very 

 well and are good breeders without having a high tumor rate. 



8. Mice belonging to strains with a low tumor rate did not 

 develop tumors either by living in boxes in which mice with 

 spontaneous tumors had lived a long time, although the boxes 

 had been kept unaltered (not cleaned) during the course of the 

 experiment or by living in the same box with mice spontaneously 

 affected with cancer. 



