22 C. M. CHILD. 



olism. From this stage on the process of senescence once more 

 becomes apparent in the decreasing rate of metabolism. This 

 stage where the process of accumulation of new structural 

 substance begins to overbalance the process of removal of the 

 old substance is undoubtedly different in different organisms, but 

 in all cases it occurs relatively early in development and during 

 most of its developmental history the organism is growing old. 



Incidentally it may be noted that if we accept these conclusions 

 it is quite unnecessary to regard growth as an autokatalytic 

 process as various authors have done. The period of acceleration 

 in growth is simply the period during which the removal of 

 obstacles to metabolism overbalances the development of new 

 obstacles, in other words it is the period of rejuvenescence, and 

 the period of retardation begins when the obstacles resulting 

 from continued metabolism become sufficient to retard the re- 

 actions; it is the period of senescence. 



In certain forms w r ith larval stages there is a second period of 

 rejuvenescence at the time of metamorphosis, though in many 

 cases the larval structures or some of them are too far advanced 

 in senescence to undergo dedifferentiation and are resorbed or 

 cast off, the further development being taken up by cells whose 

 metabolic activity has been previously more or less completely 

 inhibited. I believe that in general metamorphosis is the con- 

 sequence of senescence of the larval structures. 



To sum up: fertilization initiates the process of rejuvenescence 

 in the egg cell and the sperm nucleus which have previously 

 become so highly differentiated and so old in the organism of 

 which they formed a part that physiological or physical isolation 

 from the organism is insufficient to initiate the process of re- 

 juvenescence. According to this view then sexual reproduction 

 does not differ in its essential physiological characteristics from 

 asexual reproduction or from the regulation of artificially isolated 

 pieces. In all these cases the differentiation of the part which 

 forms the reproductive element or system is determined by its 

 correlation with other parts of the parent organism and repro- 

 duction is initiated by a process of dedifferentiation and re- 

 juvenescence. In asexual and experimental reproduction the 

 isolation of the part is usually a sufficient stimulus to initiate the 



