256 SENESCENCE AND REJUVENESCENCE 



fragmentation into spores follows the union of the gametes and 

 may probably be regarded as corresponding to the period of cleavage 

 and rejuvenescence in the embryonic development of multicellular 

 forms. 



In many sponges new zooids arise as the result of growth, but 

 under depressing conditions and probably also in advanced senes- 

 cence, so far as it occurs, existing individuals may undergo more or 

 less extensive fragmentation into cell masses known as gemmules 

 which are capable of producing new sponge bodies. It was pointed 

 out in chap, vi that the medusa bud of the hydroids apparently 

 results from a decrease in dominance which is associated with a 

 decrease in rate of metabolism, while the hydroid bud usually 

 results from growth beyond the limits of individuation. In certain 

 of the bryozoa also budding occurs during growth and a partial 

 fragmentation into reproductive bodies, the statoblasts, under 

 depressing external conditions and apparently also in advanced 

 physiological age. On the other hand, in Tubularia (p. 220), in 

 Plan-aria (pp. 122-25), and in various other animals the same form 

 of reproduction may result either from growth or from decrease 

 in dominance. The evidence presented in chap, vi justifies the 

 conclusion that the regressive and reconstitutional changes involved 

 in all these reproductive processes bring about a greater or less 

 degree of physiological rejuvenescence. 



Reproduction, however, is not the only rejuvenating process in 

 the lower animals. Many forms undergo encystment or become 

 quiescent under conditions which do not permit active life and 

 become active again after a certain length of time, or when external 

 conditions permit. Usually there is at least some small amount of 

 metabolic activity during these quiescent periods, and a consider- 

 able degree of starvation and reduction may occur, as in the case 

 of Planaria velata (pp. 130-33), before resumption of active life. 

 The effectiveness of reduction as a rejuvenating factor in pla- 

 narians has been demonstrated in chap, vii, and it certainly plays 

 a similar role in many other forms. Moreover, in some cases the 

 increase in number of individuals or the decrease in supply of 

 nutrition with the change of seasons or other environmental changes 

 determines more or less regularly recurring periods of starvation 

 during active life, and these also play a part in rejuvenescence. 



