56 SENESCENCE AND REJUVENESCENCE 



character that substances previously accumulated as structural 

 components of the part are now broken down and eliminated, and 

 this is dedifferentiation. 



If the cell is a physico-chemical system and not an entity sui 

 generis, the occurrence of dedifferentiation is no more difficult to 

 account for than the reappearance of a certain kind of chemical 

 reaction in a non-living chemical system when conditions which 

 altered the character of the reaction have ceased to act. The 

 occurrence of both differentiation and dedifferentiation is exactly 

 what we should expect from the physico-chemical point of view. 

 The assumptions of the germ-plasm theory merely complicate and 

 befog the whole problem, and not only that, but, as pointed out in 

 the preceding chapter, the theory is essentially "vitalistic"' and 

 even pluralistic in its logical implications. 



Within the last few years, however, many cases of dedifferen- 

 tiation have been recorded and various authors, among them 

 Lillie, Loeb, Driesch, Schultz, and others, have suggested that 

 development in animals is a reversible process. But reversibility 

 of development, so called, is not necessarily reversibility in the 

 chemical sense. Dedifferentiation may conceivably result from 

 the breakdown and elimination of the differentiated substratum 

 or certain components of it, and the synthesis of new undifferen- 

 tiated substances from nutritive material, as well as by the reversal 

 of the reactions which occurred in the differentiation. As in the 

 case of growth and reduction, it would certainly simplify our con- 

 ception of the process of development if we could regard it as a 

 reversible chemical reaction, but such a conception can only lead 

 us astray. Undoubtedly many reversible reactions are concerned 

 in development, but development itself is not a reversible reaction. 

 In fact, it is not simply a chemical reaction of any kind, but an 

 exceedingly complex series of interrelated physical and chemical 

 changes. Reversal of development may result from relative 

 changes in the rate of certain reaction components of the meta- 

 bolic complex as well as from reversal of reaction. In fact, it is 

 probable that reversal of development occurs at least as frequently 

 in this way as by reversal of reaction. A change in metabolism, 

 for example, such that a substance which has previously been 



