284 THE ACTION OF THE LIVING CELL 



to produce retrograde senescent changes by the fre- 

 quent injection of cytost. This opens the way to the 

 possibility of obtaining important information con- 

 cerning the nature of senility, a problem of immense 

 importance in biological science. 



It has been shown that by an appropriate series of 

 immunizing injections, one may raise an animal's re- 

 sistance to cytost, and we have deduced that this is 

 of importance in determining an animal's so-called 

 natural resistance. This is of importance from both 

 the theoretical and practical standpoints, and conse- 

 quently is deserving of further investigation. In this 

 connection we have postulated the existence of an 

 anticytost whose formation within the animal pro- 

 tects the latter from the action of cytost. Since anti- 

 cytost is an antibody which protects an animal from 

 a toxin developed by his own cells, a new concept 

 has been added to the science of immunology. 



The writer is confident that the various types of 

 experiments described in the text are capable of sub- 

 stantiation in the hands of other experimenters. It 

 must be remembered, however, that animals vary 

 markedly in their susceptibility to injury of any sort; 

 but as we have seen, the gross effects which result 

 from various types of injury have in common the 

 liberation of the tissue substance which we have 

 termed cytost. In consequence, the individual dif- 

 ference between animals may be largely explained 

 as due to their difference in susceptibility to cytost; 

 or, in other words, to put it crudely, to their available 

 anticytost, with which they are able to combat the 

 action of cytost itself. Bearing this in mind it is 

 obvious that the experimenter working with a limited 



