460 SENESCENCE AND REJUVENESCENCE 



When the organism is adding to its structural substance, and trans- 

 formation from more active to less active physical and chemical 

 conditions takes place, senescence occurs. When conditions change 

 so that previously formed structure is wholly or in part broken 

 down and replaced by a new structural substratum, rejuvenescence 

 occurs. 



Senescence occurs chiefly during the vegetative life of the indi- 

 vidual, while rejuvenescence is usually associated with reproduc- 

 tion, although various other conditions, such as starvation in which 

 extensive breakdown of previously formed structure occurs, may 

 bring it about. Reproduction may be defined as the regression or 

 dediflerentiation and reconstitution into a new individual of a 

 physiologically or physically isolated part of a pre-existing indi- 

 vidual. In agamic reproduction the changes result from the isola- 

 tion of the part without further external action, but in gametic 

 reproduction speciaKzation of the part concerned, i.e., the gamete, 

 has proceeded so far that the union of the two widely different cells 

 is necessary — except in parthenogenic eggs — to initiate the regres- 

 sive and reconstitutional changes. 



The occurrence of reproduction of one kind or another depends 

 on various physiological conditions, the degree of individuation, 

 physiological age, etc. In general the vegetative forms of agamic 

 reproduction occur in relatively young organisms, the more spe- 

 ciahzed agamic reproductions, such as formation of spores, gem- 

 mules, etc., are characteristic of somewhat later stages with a 

 lower metabolic rate, and finally gametic reproduction is a feature 

 of relatively advanced age and the gametes are cells which have 

 reached the end of their progressive developmental history, have 

 no further function in the parent organism, and are cast off as 

 waste products or remain as physiologically isolated quiescent cells. 

 Before their isolation they were integral physiological parts of the 

 organism, and they represent a more highly specialized, physio- 

 logically older condition than those parts which when isolated 

 develop agamically. 



The degree of physiological integration or individuation in- 

 creases in general and up to a certain Hmit with increasing stability 

 of the structural substratum. In general, also, the greater the 



