THE LIFE CYCLE 59 



Senescence is then a necessary and inevitable feature of growth 

 and differentiation, while rejuvenescence is associated with reduc- 

 tion and with the various reproductive processes in which more 

 or less differentiated parts of the organism undergo dedifferentia- 

 tion. Even as regards gametic or sexual reproduction, the facts 

 indicate that the gametes or sex cells are very highly specialized 

 and differentiated cells and that early embryonic development is 

 essentially a period of dedifferentiation and rejuvenescence. 



Viewed from this standpoint, life is then really a cycHcal pro- 

 cess as it appears to be. The organism grows, differentiates, and 

 ages, and these processes lead, usually in nature through reproduc- 

 tion of one kind or another, to reduction, dedifferentiation, and 

 rejuvenescence. No part of the organism remains perpetually 

 undifferentiated and perpetually young. The young organism 

 arises from the old, not from a self-perpetuating source of youth, 

 which is itself always young, and the young becomes old again. 



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Child, C. M. 



1911. "Experimental Control of Morphogenesis in the Regulation of 

 Planaria," Biol. Bull., XX. 



Davenport, C. B. 



1897. Experimental Morphology, New York. 



Huxley, T. H. 



1853. "Review of the Cell Theory," British and Foreign Med. Chir. 

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Kassowitz, M. 



1899. Allgemeine Biologic. Wien. 



Lepeschkin, W. W. 



191 2. "Zur Kenntnis der Einwirkung suppramaximaler Temperaturen 

 auf die Pflanze," Berichte d. deutsch. hot. Ges., XXX. 



LoEB, J., and Wasteneys, H. 



191 1. "Sind die Oxydationsvorgange die unabhangige Variable in den 

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