The Biology of Senescence 



limit of useful experiment, if 'growth energy' is itself a form of 

 energy in the physical sense, analogous to heat in a kettle or 

 electro-chemical energy in a battery — in the hypothetical case 

 where a population of cells was restricted in growth by exhaust- 

 ing a particular energy source, employed only for growth 

 and not for maintenance metabolism, such a system would 

 apply, and would not only depict but 'explain' the course of 

 events. 



The great value of the approach from probability, as Med- 

 awar points out, is in preventing a facile assumption that if a 

 growth-rate declines, this decline must result from the action of 

 a specific toxin or inhibitor. This does not mean, however, that 

 in a complex biological system we can avoid asking specifically 

 what declines, since a decline in rate implies real quantitative 

 and qualitative change in terms of chemical structure, and the 

 investigation of these changes is practicable. It appears mani- 

 fest that the reversion of explanted tissues to active growth is 

 in fact caused by removal from their previous environment. It 

 seems at least arguable whether the time lag in multiplication 

 which characterizes aged explants is inherent in the cell at all. 

 Simms found that the lag in cell-division of aortic explants 

 from old fowls can be reduced by a number of non-specific 

 procedures such as papain digestion, or washing with an ultra- 

 filtrate of serum (Simms, 1936; Simms and Stillman, 1937). 

 Such effects might even be purely mechanical. For most pur- 

 poses it is probably also desirable to regard growth energy less 

 as a 'store', since, to maintain the analogy, such a 'store' must 

 be almost immediately 'replenished' after hepatectomy or 

 explanation, than as a 'space', with walls defined by the con- 

 tinuously-varying properties of any individual cell in the grow- 

 ing tissue, and by the continuously-varying properties of the 

 'environment', in which are included all the adjacent cells of 

 the same tissue. Such a concept, and, in fact, any concept of 

 limiting size as an equilibrium process, would seem incidentally 

 to imply the continuous replacement of any deciduous cells. 

 The chief criticism of the humoral theories of growth limitation 

 is their readiness to assume that the limiting factors derived 

 from the 'environment' can (a) be treated in isolation and 

 (b) necessarily correspond to substances rather than to physico- 



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