160 THE ACTION OF THE LIVING CELL 



the growth stimulus induced by functional activity. 

 Continuation of this line of thought seems to indi- 

 cate logically that aside from mere muscular develop- 

 ment, the general benefits of exercise upon the body as 

 a whole may be traced to the action of cytost ; for even 

 though exercise brings about the liberation of cytost 

 in the muscles alone, the substance may be carried to 

 the other tissues of the body by means of the circula- 

 tion; and if, as postulated above, the concentration of 

 cytost in the body fluids is of such an order of mag- 

 nitude as to cause an increase of metabolism of the 

 cells bathed by these fluids, there will result a general 

 increase of the animal's metabolism and growth, for 

 each is an integration of the chemical activities of 

 the constituent body cells. 



If there is any truth in this concept, one would ex- 

 pect that the introduction of small quantities of cytost 

 into the circulation of animals must result in an in- 

 creased metabolic activity and general salutary ef- 

 fects akin to those resulting from regular exercise. 

 Now this is precisely what has been observed in the 

 author's laboratory when very low concentrations of 

 cytost in the form of extracts of autolyzed tissue are 

 injected into cats for a protracted period of time. If 

 the reader will refer to the results tabulated on page 

 1 12 and summarized in the graph on page 1 14, he will 

 note that as the result of such a series of injections 

 the experimental animals underwent an increase in 

 weight considerably greater than did the control ani- 

 mals of the same age group on a similar diet. When, 

 as shown in the experiments just referred to, the cytost 

 injections were continued for a much longer period 

 of time, the animals suffered a decline in weight, and 



