METAPLASM; POLARITY 137 



relative rate of metabolism is the main criterion of the cell's physiological 



age, "young" cells having a high rate and "old' cells a relatively low 

 rate, and a gradual decline in this rate occurring throughout the life of 

 the cell. In embryonic (physiologically young) cells the cytoplasm ap- 

 pears to be comparatively homogeneous and undifferentiated. Older 

 cells, on the contrary, are ordinarily marked by the presence of products 

 of differentiation in the cytoplasm. The true measure of age is there- 

 fore not time, but physiological differentiation. 



In many cells a rejuvenating process may occur, whereby a high meta- 

 bolic rate is restored and the products of differentiation lost: this is 

 regarded as a "return to the embryonic state" — a real physiological 

 rejuvenescence. "Senescence is primarily a decrease in rate of the 

 dynamic processes conditioned by the accumulation, differentiation, and 

 other associated changes of the material of the colloid substratum. 

 Rejuvenescence is an increase in rate of dynamic processes conditioned 

 by changes in the colloid substratum in reduction and dedififerentiation " 

 (Child, p. 58). Such a rejuvenescence occurs in connection with 

 regeneration, vegetative and other asexual reproduction, and sexual 

 reproduction. In each case the cell which begins the new life cycl< — the 

 meristematic regenerating cell, the zoospore, or the zygote — has a high 

 metabolic rate and is comparatively free from the products of differentia- 

 tion. 



In the lower organisms cell differentiation in this sense is not so great 

 but that almost any cell may retain the power to "dedifferentiate" and 

 begin the development of a new individual vegetatively. In these forms 

 asexual reproduction may occur repeatedly and keep the organism as a 

 whole (in protozoa and protophyta) or the protoplasm of the race (in 

 lower metazoa and metaphyta) physiologically young. Only when the 

 metabolic rate falls very low does sexual reproduction, the most effective 

 of all the rejuvenating agencies, ensue. 



In the higher plants the retention of the power of dedifferentiatinn 

 is strikingly shown in the well known cases of Begonia and BryophyUum, 

 which can regenerate complete new individuals from a few leaf cells. 

 In the higher animals cell differentiation is usually so great thai the 

 somatic cells can no longer dedifferentiate and reproduce the organism 

 asexually. Here rejuvenation occurs only after the union of two gametes, 

 which are themselves, unlike the zoospores of algae, physiologically old. 

 Although local rejuvenescence may occur, as in secretory cells which are 

 "younger" after secretion, and also in wound tissue, the differentiation 

 of the body cells is carried so far that their metabolic rate falls low enough 

 to make a recovery or rejuvenescence no longer possible. Tim- ii i- 

 only the functioning reproductive ('('lis that endure: the ultimate cessation 

 of all life processes in the body cells is the price which is inevitably paid 

 by the complex multicellular organism for the advantages conferred by 

 its high degree of differentiation. 



