OTHER PERIODICITIES IN ORGANISMS. 335 



in all cases thus far examined. In short, both morphological and 

 physiological evidence force us to the conclusion that the mature sex 

 cells or gametes are highly differentiated, physiologically old cells. 



It has been shown by Loeb, Warburg and others, that an in- 

 crease in the rate of oxidation follows fertilization and Warburg 

 has shown further that an acceleration of the oxidation-rate con- 

 tinues through the cleavage stages and at least to the swimming 

 larval stage in the sea urchin. I have found that in all forms 

 examined thus far, including echinoderms, annelids, fishes and am- 

 phibia, the increase in susceptibility during early development indi- 

 cates that the rate of metabolic reaction increases up to a certain 

 stage of development and then begins to decrease. All the facts at 

 hand then indicate that the early period of embryonic development 

 is a period of physiological rejuvenescence. Morphologically- also 

 it is a period of dedififerentiation. The highly specialized structure 

 of the egg is lost and the cells gradually become undifferentiated 

 or embryonic in appearance. 



In fact the evidence indicates very clearly that the sexual repro- 

 ductive cells or gametes represent the final stages in a period of 

 differentiation and senescence, that their union initiates a process 

 of rejuvenescence which continues through the early stages of em- 

 bryonic development and is followed again, as the differentiation 

 and accumulation of structural substance overbalance dedifferen- 

 tiation and reduction, by a period of senescence which may be prac- 

 tically continuous throughout the life of the individual as in the 

 higher animals and man, or which may be interrupted or balanced 

 by periods of rejuvenescence connected with asexual reproduction 

 and other processes, as in many lower forms. The stage of most 

 extreme youth is not then at the beginning of embryonic develop- 

 ment but at some later stage, varying in different animals and in 

 different tissues of the same body. 



If these conclusions are correct there is no germ plasm in the 

 sense of a perpetually young protoplasm handed on from genera- 

 tion to generation, but the organism undergoes growth, differentia- 

 tion and senescence as a whole and in both asexual and sexual re- 

 production rejuvenescence occurs. In the higher animals and man 



