THE LIFE CYCLE 45 



And it is these more continuously or more intensely active organs 

 which are more essential to life. But according to this view they 

 undergo less reduction in starvation, not because they are more 

 essential in life, but because they are more active. 



Reduction in an organ or part may also occur when conditions 

 change so that a decrease in the average rate of its metabolism below 

 a certain level occurs and synthesis of structural substances does not 

 compensate the gradual loss. The atrophy of organs from disuse 

 is a case in point. And, finally, reduction may occur in a part 

 when the correlative conditions which were an essential factor for 

 its continued existence as a part undergo change. In such cases 

 it is difficult to determine whether the change in metabolism is 

 primarily qualitative or quantitative. In the lower organisms 

 extensive reduction of this kind occurs when pieces are isolated 

 and undergo reconstitution. Previously existing organs may be 

 reduced and disappear and others be formed anew. In the higher 

 organisms such processes of reduction are narrowly limited. 



If we accept the general conception of growth and reduction 

 here outlined, then it is no longer necessary to assume the existence 

 of a mysterious growth-impulse which gradually decreases in inten- 

 sity during development, for growth is primarily the accumulation 

 of certain substances formed in the course of the metabolic reactions 

 which are physiologically more stable than other substances that 

 break down, furnish energy, and are eliminated. Reduction 

 occurs when the breakdown and elimination of the cell substance 

 is not balanced by the synthesis of new substance. Some such 

 conception of growth and reduction seems to be forced upon us by 

 the facts, for certainly there is every reason to believe that the 

 different constituent substances of the cell show very different 

 degrees of stability and that the stability of a given substance may 

 differ with different conditions. Organic growth remains a com- 

 plete mystery unless certain fundamental constituents of proto- 

 plasm are relatively stable under the conditions of their production 

 in the cell. 



DIFFERENTIATION AND DEDIFFERENTIATION 



Differentiation. The process of development in the organism 

 is also a process of differentiation, of apparent complication, but 



