SUSCEPTIBILITY TO TRANSPLANTABLE TUMORS 71 



2. Factors underlying susceptibility to transplantable tumor tissue 



a. Race as a factor has already received sufficient preliminary 

 consideration. 



b. Sex as a factor underlying susceptibility to inoculable tumor 

 tissue is disputed. Certain investigators have been able to 

 determine a significant difference between the sexes in their 

 receptivity toward transplanted tumors. Others have been 

 unable to determine any significant difference between the sexes. 



c. Age is a recognized factor underlying susceptibility to 

 transplantable tumors as well as having some causative relation 

 to the origin of spontaneous neoplasms. 1) A very young individ- 

 ual from a susceptible race will sometimes fail to grow the 

 transplanted tissue, although the same individual will do so if 

 inoculated when it is one-half to three-fourths grown. At this 

 age it is more susceptible than at any other period in its life- 

 cycle. 2) Very young animals from a non-susceptible race 

 will sometimes grow a transplanted tumor when inoculated, 

 although no matured animal in that particular race grows the 

 same tissue. 



d. Pregnancy. Several investigators have concluded that preg- 

 nancy has some influence on the rate of growth of the neoplastic 

 tissue. 



Leo Loeb has studied several reactions with transplantable 

 tissues in relation to pregnancy. He determined that if carci- 

 noma of the mammary gland be inoculated into pregnant females, 

 the tissue would fail to grow, although no such behavior was 

 encountered in the controls. Later he discovered the interesting 

 fact that after autotransplantation, an adenofibroma of the 

 mammary gland survived, but showed progressive growth only 

 when the host became pregnant. Here we see that the factor 

 of pregnancy can apparently differentiate between malignant 

 and benign growths. 



According to Loeb, transplanted normal mammary-gland 

 tissue behaves in the same way as adenofibroma tissue during 

 pregnancy of the host. Further, he recognizes that the normal 

 embryonal tissue reacts like carcinoma in the mouse, but evi- 



