46 SENESCENCE AND REJUVENESCENCE 



we find that differences in reaction or in capacity to react very 

 commonly exist in different parts even before visible differentiation 

 occurs, or in cases where it never occurs. The term " specifica- 

 tion" is often used for these differences which appear only in 

 physiological activity, and "differentiation" for the visible struc- 

 tural differences. The distinction is of course arbitrary, for the 

 visible differences result from differences in physiological activity. 

 An orderly sequence of differentiation during development is 

 characteristic of at least all except the very simplest organisms and 

 probably in these also some degree of differentiation exists. 



In its biological sense the term "differentiation" is purely 

 descriptive: broadly speaking, differentiation includes all per- 

 ceptible changes in structure or behavior from the primitive embry- 

 onic or "undifferentiated" condition, which occur either in the 

 cells or parts of an organism during its developmental history, or 

 in different organisms in the course of evolution. It is, in short, 

 a becoming different, but since the process of becoming different 

 in cells and organisms is a change from a generalized to a specialized 

 condition a progressive development of particular kinds of struc- 

 ture and activity in different parts of the whole differentiation in 

 organisms is a process of specialization. 



The problem of differentiation has long been one of the great 

 biological problems. Biological thought has always been divided 

 \/upon the question of prefbrmation versus epigenesis. To what 

 extent does the differentiation of the fully developed organism 

 actually exist as something preformed in the germ, so that develop- 

 ment is strictly an unfolding, a becoming visible, of what already 

 exists, and to what extent is there a real increase in complexity 

 during development? The corpuscular theories are an attempt 

 to answer the question from the point of view of preformation, but 

 they, like the vitalistic theories, succeed merely in placing the prob- 

 lem beyond the reach of investigation. It is evident that if the 

 organism is a physico-chemical system, at least some differentiations 

 must arise in the course of development. The adult organism is 

 represented, not in the morphological structure nor in the physical 

 and chemical changes of the reproductive cell or cell-mass, but 

 rather in its capacities. The experimental investigation of recent 



