Growth and Senescence 



ture to be too complex for simple estimation of coefficients. 

 Morphogenesis depends upon a large number of simultaneous 

 and occasionally contrary processes. We should almost certainly 

 now be inclined to interpret the programme fulfilled by an 

 animal during its life-cycle in terms drawn from experimental 

 morphology and from the study of control systems, rather than 

 directly from physical chemistry. 



The postulation that senescence always accompanies, or 

 follows, the cessation of growth, which certainly appears to fit 

 many of the observed facts, we owe originally to the work of 

 Minot (1908). It is, in fact, no more than a postulation, since, 

 as we have seen, there may be organisms in which senescence 

 occurs hand-in-hand with growth, and there are certainly 

 organisms, such as terrapins, which have a virtual maximum 

 size but are not known to exhibit senescence. Senescence in 

 man, judged by the life-table, commences while active growth 

 is in progress: Minot himself considered that the rate of sen- 

 escence was actually greatest when growth rate was at its maxi- 

 mum. If the relationship between senescence and growth- 

 cessation is real, it might mean ( 1 ) that that which 'causes' the 

 cessation of growth also causes senescence — implying that 

 growth-cessation results from an active and inhibitory principle, 

 (2) that that which no longer grows, senesces, (3) that growth- 

 cessation and senescence are parallel phenomena, both arising 

 from the process of differentiation. 



The dissociability of growth from development was first 

 shown by Gudernatsch's researches upon the action of thyroid 

 in the developing tadpole (1912). Metabolism, measured by 

 respiration, is dissociable from both. 'The fundamental mechan- 

 isms are not separable only in thought: on the contrary, they 

 can be dissociated experimentally or thrown out of gear with 

 one another' (Needham) . The fundamental problem in relation 

 to the 'rate of living' lies, therefore, in determining which of 

 these processes, and in what proportions, make up the essential 

 sequence of operations through which the organism must pass 

 before senescence makes its appearance. In its crudest form the 

 question is: given that these processes, though dissociable, are 

 normally interdependent, does this organism undergo sen- 

 escence (1) when it reaches a particular stage of cellular 



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