208 THE ACTION OF THE LIVING CELL 



sity result in effects akin to those resulting from any 

 other form of exercise. In consequence we may as- 

 sume that the increased activity of such young animals 

 suffices to stimulate anticytost production to a degree 

 comparable to that which results from active immuni- 

 zation. 



Some exceedingly interesting results have been ob- 

 tained by the immunization of old guinea pigs. As is 

 well known, with the onset of age females tend to 

 become non-fertile; or, if fertile, they frequently 

 abort, or give birth to young which die shortly after 

 birth. When such animals, which are normally dis- 

 carded by breeders, were subjected to a series of cytost 

 injections identical with those used to immunize 

 younger animals against the toxemia of pregnancy, it 

 was found that they became fertile and proved useful 

 as breeding stock for another year. This apparent 

 rejuvenation suggests that their lack of fertility prior 

 to immunization was due to the toxic effects of cytost 

 which had accumulated in their bodies during the 

 course of their lives. On the other hand, if such an 

 assumption is true, it is difficult to see why the addi- 

 tional cytost administered in the immunizing pro- 

 cedure should not result in a distinct toxicity. 



This difficulty may be circumvented by assuming 

 that the apparent poor fecundity of such animals is 

 due not to a persistent accumulation of cytost, but 

 rather to an anticytost deficiency. Indeed, this is made 

 manifest by the extreme susceptibility of such animals 

 to the toxemia of pregnancy and abortion. In other 

 words, prior to insemination they do not suffer from 

 an accumulation of cytost, but when the embryo be- 

 gins to attack the uterine tissues, the resulting libera- 



