NATURAL RESISTANCE AND FATIGUE 161 



eventually pathological changes more or less similar 

 to those found in animals receiving large doses of 

 cytost over a shorter period of time. 



This result is of importance because it demonstrates 

 that the tissue cells of the intact animal behave towards 

 cytost as they do towards other alterations in their 

 immediate environment. As is well known, tissues in 

 general demand the presence of certain essential ele- 

 ments and compounds in their surrounding fluids. 

 In general, the mere presence of these essential sub- 

 stances does not fulfil the needs of living cells. In 

 order that the cells may live a normal life, the con- 

 centration of the essential elements and compounds 

 in the surrounding fluids must be within certain opti- 

 mal limits. When present in a concentration less than 

 or greater than the optimal, such essential substances 

 exert definitely toxic actions. This has been definitely 

 shown by the exhaustive investigations of Jacques 

 Loeb and others on the phenomena of salt antagonism. 



Similarly the writer's investigations concerning the 

 effects of cytost upon cells in tissue culture have shown 

 that while the presence of this substance appears neces- 

 sary for the growth of cells under such conditions, an 

 excess of cytost rapidly inhibits growth and may actu- 

 ally lead to the death of the cultures. 



From these results it may be reasoned that in the 

 intact animal the normal functioning of the body 

 cells is in part conditioned by the concentration of 

 cytost. Such being the case, it follows that as a result 

 of proper exercise the tissue growth which results 

 from the liberation of cytost must be due to the fact 

 either that cytost does not accumulate in the body 

 fluids to such an extent as to manifest toxic actions; 



