ANIMAL BREEDING 207 



In the case of the colonies of mice investigated by 

 the author, no special facilities for exercise were pro- 

 vided. Injections of cytost, however, proved an effec- 

 tive substitute, since our colonies v^ere carried on for 

 a period of three years without the introduction of 

 any new stock. Prior to the application of cytost to 

 these colonies it had been found necessary to clear 

 out all the stock, old and young, every year or two 

 and begin again with fresh breeding stock in order to 

 insure the production of healthy, vigorous young. 



It has been stated previously that experiments indi- 

 cated that in both cats and guinea pigs the immunity 

 to cytost developed in the mothers was transmitted to 

 the offspring. This was also observed in the case of 

 mice, as noted above. With the latter animals we have 

 been able to follow the effects of cytost immunization 

 through several generations. This was made pos- 

 sible by the fact that young mice mature rapidly. Thus 

 by observing the mortality, appearance, activity, and 

 fecundity of succeeding generations of mice, it was 

 found that the beneficial effects of cytost immuniza- 

 tion appear to be transmitted through at least three 

 generations. 



Perhaps it is erroneous to speak of the transmis- 

 sion of cytost immunity, for this seems to imply the 

 alteration of a genetic factor. What we mean to con- 

 vey is simply the experimental fact that succeeding 

 generations of animals having an increased natural 

 resistance result from the immunization of mothers. 

 This may be due to the fact that, as stated above, the 

 first generation born of immunized mothers shows 

 a greater activity than do the offspring of those not 

 immunized. Such an increased activity must of neces- 



