Introductory and Historical 



could be propagated indefinitely in vitro, and finally the theories 

 of Bidder (1932). 



Minot considered that senescence was the direct outcome of 

 cell differentiation, that differentiated cells, by reason of the 

 changes undergone, chiefly by their cytoplasm, in the course of 

 morphogenesis, had become largely incapable of growth or 

 repair. He believed that the negative acceleration of specific 

 growth, found in a very wide variety of organisms, and ultimate 

 senescence, were products of this process, and that the first was 

 a measure of the second. It followed from this that the rate of 

 senescence, so defined, must actually be highest in embryonic 

 life and in infancy, when the rate of differentiation is highest. 

 Many of Minot's concepts, such as the rigid irreversibility of 

 cell differentiation, echoed later by Warthin (1929), the in- 

 capacity of differentiated cells for growth, and the necessarily 

 increasing liability to senescence of successive cell-generations, 

 are now disproved or at least impugned. His work, however, 

 leaves with us the two important concepts of a gradual process 

 of senescence linked to morphogenesis, and of a relation between 

 it and the decline of growth-potential. By using negative growth 

 acceleration and rate of differentiation as a direct measure of 

 senescence, Minot arrived at the conclusion that the rate of 

 senescence is highest in foetal, and least in adult, life. This 

 concept has been widely adopted. Its validity depends upon 

 the acceptance of Minot's definition; if senescence be regarded, 

 as we shall regard it, in terms of deteriorative change in the 

 organism's power of resistance, the idea requires qualification. 



A far more important question, which had been latent in the 

 literature since Ray Lankester (1870) pointed to the apparent 

 non-senescence offish, was raised by Bidder (1932). With the 

 exception of Metchnikoff (1904, 1907) who was attempting to 

 relate longevity to the form of the digestive tract, very nearly 

 all biological theorists had assumed that senescence occurs in 

 all vertebrates. This may in fact be so, but if it is not, then 

 manifestly the general theories of senescence based on degree of 

 tissue differentiation, irreplaceability of neurones, and other 

 such systems fall to the ground. Bidder pointed out that there 

 were several lower vertebrates in which there was no ground 

 for suspecting that the mortality ever increased with increasing 



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