230 SENESCENCE AND REJUVENESCENCE 



The analysis of the simple forms of agamic reproduction in 

 connection with the experimental reproductions in artificially iso- 

 lated pieces leaves no room for doubt that the formation of a new 

 individual from a part of a pre-existing individual results from the 

 removal of an inhibiting factor rather than from a positive stimu- 

 lation. According to the conception of the individual developed 

 above, a more or less complete physiological isolation of the region 

 or part concerned is a necessary condition for reproduction, or, 

 more specifically, this part must in some way escape from the con- 

 trol of the dominant region before it can lose its characteristics as 

 a part and so serve as the basis for a new individuation. 1 



In the simpler organisms, where isolated parts are capable of 

 reconstitution into new individuals, the effect of physiological 

 isolation of a part is essentially the same as that of physical isola- 

 tion by section, except that physiological isolation is a less violent 

 and injurious procedure. The isolated part undergoes dediffer- 

 entiation to a greater or less extent and begins a new development, 

 an agamic reproduction occurs. But in the higher forms, where 

 isolated parts are incapable of reconstitution, physiological isolation 

 may lead to death of the part isolated, or if nutrition is available 

 the part may continue to exist in its original form or to grow 

 and differentiate along the lines previously determined by its rela- 

 tions with other parts. 



It is evident that the final size of the individual is determined by 

 the limit of dominance only in the lower, simpler organisms. It 

 was pointed out above that in the higher animals other factors 

 such as the rapid differentiation and loss of capacity for growth and 

 division of cells and perhaps the increasing disproportion between 

 surface and volume -limit the individual to a size far below that 

 which the limit of dominance alone would determine. If, for ex- 

 ample, the size of man and mammals were limited only by the limit 

 of effective transmission of nerve impulses in fully developed nerve 

 fibers, they would certainly be very much larger than they are. 

 In early embryonic stages, however, the limit of dominance is 



1 For experimental data see Child, '070, '076, '10, 'nc, and for a general considera- 

 tion of physiological isolation of parts, the ways in which it is brought about, and 

 its significance, see Child, 'na. 



