202 SENESCENCE AND REJUVENESCENCE 



this behavior and the axes of polarity and symmetry. Here lies 

 the problem of organic individuation. 



From time to time parts of the individual give rise to new indi- 

 viduals, in which either the original axiation may persist or a new 

 axiation arise. This is reproduction. In the case of gametic or 

 sexual reproduction the process is further complicated by the union 

 of two nuclei, usually the nuclei of two highly specialized cells, pre- 

 ceding the development of the new individual. The problem of 

 how and why these new individuals arise is the problem of repro- 

 duction. And, finally, it is at once evident that the problems of 

 senescence and rejuvenescence are closely associated with these 

 problems of individuation and reproduction. 



During some fifteen years' study of reproductive processes in 

 the lower animals under experimental conditions I have been 

 brought face to face with these problems and have attempted to 

 gain some insight into the nature of the factors concerned in indi- 

 viduation and reproduction. In the remainder of the present 

 chapter the theory of individuation and reproduction which has 

 grown out of this investigation is outlined, and some of the more 

 important experimental evidence upon which it is based is briefly 

 stated. 



THE AXIAL GRADIENT 



By means of the susceptibility method described in chap, iii, 

 controlled in certain cases by estimations of carbon-dioxide pro- 

 duction by means of the Tashiro biometer (Tashiro, '136), it has 

 been possible to demonstrate the existence of a distinct gradient 

 in rate of metabolic reactions along the chief or so-called polar 

 axis of axiate animals, so far as they have been investigated. 1 In 

 its simple, primary form this axial gradient consists in a more or 

 less uniform decrease in rate of metabolism from the apical or 

 anterior region along the main axis. The point of importance is 

 that the apical region, or the head-region in cases where a head is 

 formed, is primarily the region of highest rate of metabolism and 

 that in general regions nearer to it have a higher rate than regions 

 farther away. In some animals, as for example in Planaria, this 

 gradient persists throughout life in the single individual, except 



1 Child, '12, '130, '136, '140, '146, 'i4c. 



