^ FREDERICK G. SMITH 119 



same mechanism as any of the respiratory inhibitors but only to indicate 

 that the various types of growth and respiratory effects shown by the 

 growth substances have parallels in the action of metabolic inhibitors 

 whose action is more or less well known. This is merely a further justi- 

 fication for the useful working principle that growth substances act, 

 either in a physiological or in a toxic way, by some effect on respiratory 

 metabolism. 



In conclusion, we are likely to make more progress toward understand- 

 ing the mechanism of growth substance action when we have a more 

 complete knowledge of the respiratory machinery of the tissues in which 

 the action of growth substances is studied, or possibly when we are able 

 to study their action in tissues whose respiratory systems are already 

 better known. 



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