270 SENESCENCE AND REJUVENESCENCE 



is a question whether they can properly be called rejuvenescence, 

 or else they bring about death before any great degree of rejuvenes- 

 cence occurs, so that in such animals life after the early embryonic 

 stages is practically a continuous progression and senescence. Such 

 a continuous progressive development and senescence without 

 counterbalancing regression and rejuvenescence must inevitably 

 and necessarily terminate sooner or later in death in consequence 

 of decrease in rate of metabolism. From this point of view, then, 

 the increasing continuity of senescence and the appearance of death 

 as a natural termination of development in the course of evolution 

 from the lower to the higher animals are to a large degree the con- 

 sequence of the increasing fixity or stability of the structural sub- 

 stratum of the organism which determines on the one hand the 

 increasing degree of individuation and on the other the Umitation 

 of regression and reproduction. 



But in the course of this life history which ends in death, sexual 

 differentiation appears, and at a certain stage of development the 

 individuals of each sex or the organs of each sex in a hermaphroditic 

 individual give rise to the gametes which are highly specialized, 

 sexually differentiated cells, the egg and the spermatozoon. These 

 cells are cast off from the body which produced them like other cells 

 which have completed their developmental history and grown old, 

 and in most cases they do not react to the isolation by regression, 

 rejuvenescence, and reconstitution of a new individual, but sooner 

 or later die, unless union between two gametes of opposite sexes, 

 that is, fertilization, occurs. This union, when it does occur, initi- 

 ates in some way the process of regression and rejuvenescence in 

 the resulting cell, the zygote, and the reconstitution of a new indi- 

 vidual, or what we call embryonic development, occurs. The 

 gametes are the only cells in the higher animals which undergo 

 complete rejuvenescence and so escape death. This conception 

 of gametic reproduction will be considered more at length in Part IV. 



THE PROCESS OF SENESCENCE IN THE HIGHER FORMS 



The process of senescence in man and the higher animals is not 

 widely different in its general features from the age changes which 

 occur in the lower forms when agamic reproduction is absent. The 



