202 THE ACTION OF THE LIVING CELL 



their offspring. This would seem particularly im- 

 portant when the animals are of necessity kept in 

 highly artificial surroundings, such as is usually the 

 case with laboratory animals. 



In the preceding chapter, we have presented ex- 

 perimental evidence that immunization to cytost is 

 of considerable value in the maintenance of a guinea 

 pig colony, since it appears to insure the birth of young 

 animals having a higher natural resistance than is 

 normally the case under similar conditions of breed- 

 ing. 



Similar experiments have been conducted with 

 white mice. These experiments are especially in- 

 structive, since the mouse is an animal with a span 

 of life shorter than that of most other laboratory ani- 

 mals; and since they reach maturity more rapidly 

 than do other species, it is comparatively easy to fol- 

 low through several generations under fairly constant 

 environmental conditions. Further, it is well known 

 that white mice show a very high mortality even under 

 apparently ideal conditions of temperature, feeding, 

 etc. This fact, which frequently proves troublesome 

 to the breeder, is ideal for our purposes, since a com- 

 parison of the mortality rates of mice immunized to 

 cytost and those of non-immune animals offers a ready 

 means of testing the validity of our conclusions. 



In the experiments to be discussed presently, the 

 mice were kept in clean, spacious cages in a well venti- 

 lated, sunny room, and all of them were given the 

 same food. They were in charge of an experienced 

 breeder of mice who for a period of seven years had 

 kept careful records of the food requirements, mor- 

 bidity, and mortality. At times the latter had reached 



