NATURAL RESISTANCE 187 



these surviving animals, including the young, v^ere 

 sent to the writer's laboratory, w^here they vs^ere kept 

 under continued observation. By December, 1924, all 

 the mothers of the control series had died, w^hile all 

 the members of the immunized group were still alive. 



This result leaves no doubt but that the attempted 

 immunization was actually efiFective in protecting the 

 mothers against cytost liberated during pregnancy. 

 Further, this result is of interest in that it shows again 

 that animals may actually be immunized to cytost by 

 such a series of injections — perhaps because of the 

 elaboration of a specific anticytost, as previously sug- 

 gested. 



It may be suggested that these rather striking re- 

 sults are due to some factor other than the cytost in- 

 jections. Such a possibility would demand serious 

 consideration were it not for the fact that essentially 

 the same result was obtained time and time again on 

 repetition of the experiment. 



In these early experiments the cytost injections 

 were begun too late during the course of the preg- 

 nancies to warrant any conclusions as to the effect 

 upon the offspring; hence other experiments must be 

 considered. Before passing to these, we may interpo- 

 late an experiment made under the writer's direction 

 by Dr. Banzhof and his associate, Mr. A. M. Klein. 

 A number of guinea pigs was given a series of three 

 cytost injections spaced a week apart, and was then 

 set aside in the animal house for a period of fifteen 

 months. At the end of that time, they were large and 

 healthy, while, according to Dr. Banzhof, animals of 

 the same age and size usually show a mortality rate 



