CATALYZERS OF GROWTH 503 



tion of Child that starving planarians undergo retrogression to a 

 relatively embryonic eharaeter. Child accounts for this Rejuvenescence 

 by the sweeping out from the cell of accumulations of colloidal sub- 

 stances which impede the cell-activities and are consumed in starvation 

 for purposes of furnishing energy. The nature of the impediment 

 constituted by these substances is, however, by no means clear; but 

 it may very conceivably be possible that a high proportion of water is 

 essential to the production of growth-catalyzers in abundance. The 

 relative rejuvenescence of metazoa by starvation is, however, more 

 probably to be attributed to the ascendency in mass and numbers 

 acquired by the tissues which are normally possessed of a high metabolic 

 rate, which enables them, when food is readmitted, to push forward all 

 of the processes of growth, including the growth of slowly metabolizing 

 and water-poor tissues, with unusual energy. 



CATALYZERS OF GROWTH. 



If a catalyzer is of the "typical" variety and is not in any degree 

 consumed during the reaction which it accelerates, then it necessarily 

 follows that it cannot alter the final Equilibrium of the reaction, for a 

 shift in chemical equilibrium means, generally speaking, that heat is 

 either produced or absorbed and the equivalent in work or heat must 

 be supplied by agencies external to the reaction itself, or by spme 

 other collateral chemical reaction. Since the catalyzer introduces no 

 condition not implied in its presence, the energy-change involved in 

 a shift of equilibrium would of necessity be equated by a change in the 

 energy-content of the catalyzer which could only be supplied by its 

 chemical transformation, i. e., by consuming it. It follows, of course, 

 that a catalyzer cannot initiate a chemical reaction which is not already 

 proceeding, however slowly, in its absence. 



If Endogenous Catalyzers of growth really exist, therefore, we should 

 expect them to display the following characteristics, distinguishing them 

 more or less clearly from the growth-substrates : 



1. Since these catalyzers are not the only, nor necessarily quan- 

 titatively important constituents of the tissues which are the sum 

 of the products of growth, it follows that the effect of catalyzers of 

 growth may be totally disproportionate to their nutritive (i. e., calorific) 

 value. 



2. The ultimate growth attained by two groups of animals under the 

 influence of unequal amounts of the catalyzer may be expected to tend 

 toward equality, since the ultimate station of equilibrium of a reaction 

 is unaffected by a catalyzer, although the velocity with which equi- 

 librium is attained may be profoundly affected. This tendency is, 

 however, limited by three groups of factors, namely (a) the mechanical 

 delay or prevention of growth which may be imposed upon an animal 

 by the formation of a skeleton or of a circulatory or respiratory system 

 of limited dimensions, (6) By the unequal effect of catalyzers upon 



