52 SENESCENCE AND REJUVENESCENCE 



rate decreases, new features appear as relatively stable com- 

 ponents of the structural substratum, and these become factors in 

 further changes. Probably also substances which were sufficiently 

 stable physiologically to become components of the structural sub- 

 stratum at the higher metabolic rate become more stable as the 

 metabolic rate decreases, not necessarily because of changes in 

 themselves, but because of the decrease in rate, or the conditions 

 which determine it. Thus the visible substratum of the cells 

 becomes more and more altered from its original condition, and 

 apparently the farther these changes go the less the ability of the 

 cell to synthesize protoplasm i.e., the general metabolic sub- 

 stratum of the organism and the less " protoplasmic ' does its 

 structure become. 



The non-protoplasmic substances which appear in the cell, 

 either in definite morphological form or as granules, droplets, or 

 inclosures in the protoplasm, have very commonly been grouped 

 together under the head of metaplasm. Kassowitz ('99), for ex- 

 ample, makes a sharp distinction between protoplasm and meta- 

 plasm and believes that only the accumulation of the latter is 

 responsible for decrease in metabolic rate in the cell. The distinc- 

 tion is doubtless of value theoretically, but practically it is impos- 

 sible to say what is protoplasm and what is metaplasm. And 

 there can be no doubt that the so-called metaplasmic substances 

 often take more or less part in the metabolic activity of the 

 cell instead of being inactive, as Kassowitz and others have 

 maintained. It seems therefore more in accord with the facts 

 to regard the cellular substratum as showing all gradations from 

 the purely protoplasmic condition of the embryonic cell to the 

 highly differentiated cell which may be loaded with substances 

 obviously non-protoplasmic in nature. 



Differentiation is very generally, though not necessarily, as- 

 sociated with growth. It is probable that growth cannot proceed 

 very far without bringing about some degree of differentiation, for 

 the accumulation in the metabolic substratum of substance, what- 

 ever its nature, must result sooner or later in altering metabolic 

 conditions. On the other hand, change in conditions external to a 

 cell or part may bring about differentiation without growth. 



