CONCLUSIONS AND SUGGESTIONS 199 



Except in the case of parthenogenic eggs, which 

 develop without fertilization, neither of the gametes 

 undergoes dedifferentiation and a new development by 

 itself, but in some way their union, or conditions asso- 

 ciated with it, or in various cases certain experimental 

 conditions ("artificial parthenogenesis"), initiates the 

 process of dedifferentiation and rejuvenescence which 

 makes possible the development of a new individual and 

 a new period of differentiation and senescence. The 

 increasing metabolic rate and the loss of differentiation 

 in the early stages of embryonic development indicate 

 clearly that rejuvenescence is occurring-, but sooner or 

 later the intake of nutrition results in renewed accumula- 

 tion of substratal substance and senescence begins again. 

 The period of dedifferentiation and rejuvenescence is 

 short, and during most of its development the sexually 

 produced organism is growing old. 



As I have endeavored to show, the development of 

 the individual in gametic reproduction is fundamentally 

 the same process as in agamic and experimental repro- 

 duction. In most cases the polarity, i.e., the major 

 axial gradient, and in some cases the minor gradients, 

 are determined in the eggs before embryonic develop- 

 ment begins, usually, so far as observation permits 

 definite conclusions, by their relations to the parent 

 body, but in some of the lower plants the major axis is 

 apparently determined after the egg leaves the plant- 

 body by the direction or differential action of light or 

 other external factors. The point of entrance of the 

 sperm seems in many cases among animals to be a 

 factor in determining the symmetry gradients, where 

 they are not already determined. In at least many 



