34 PROBLEMS OF FERTILIZATION 



Following the union of the ovum and spermatozoon 

 in the act of fertilization, there is a rapid acceleration of 

 metabolism, which can be measured in various ways, 

 mounting, in stages succeeding cleavage, to a maximum 

 from which the curve of senescence, which has been 

 outlined above, takes its start. 



Fertilization thus initiates a period of rejuvenescence 

 which is brief and rapid and apparently complete. 

 However, in forms that reproduce asexually, rejuvenes- 

 cence may be effected in other ways without any partici- 

 pation of gametes; and in parthenogenesis a certain 

 amount of rejuvenescence at least occurs without ferti- 

 lization. In most such animals, however, sexual repro- 

 duction and fertilization occur either regularly or 

 irregularly in certain generations. 



It is thus readily seen how the idea of rejuvenescence 

 came to be associated with the fertilization problem. 

 The germ cells alone in most animals (and all the higher 

 ones) possess this capacity of resuming youthful con- 

 ditions after differentiation, and they do so (except in 

 case of parthenogenesis) only after union in fertilization. 

 The exceptions, however, show that in this respect 

 fertilization is not an absolutely unique process. 



Child (191 5, p. 58) defines senescence and rejuvenes- 

 cence as follows: ''Senescence is primarily a decrease 

 in rate of dynamic processes conditioned by the accumu- 

 lation, differentiation, and other associated changes of 

 the material of the colloid substratum. Rejuvenescence 

 is an increase in rate of dynamic processes conditioned 

 by the changes in the colloid substratum in reduction 

 and dedifferentiation. " If we accept this definition as a 

 reasonable physiological formulation of the processes 



