The Biology of Senescence 



in this system causes the bias to be overcome at the correct 

 moment. The homoeostasis achieved by the juvenile hormone 

 is not absolute, otherwise metamorphosis would never take 

 place; the metamorphosis-producing hormone ultimately carries 

 the day. But occasional nymphs of Rhodnius devoid of the 

 thoracic gland cannot metamorphose, and appear to live for 

 long periods without senescence. This mechanism offers an 

 opportunity for the dissection of just such a system of partial 

 homoeostasis, directed to act as a delay-mechanism, as appears 

 to underlie so many life-cycles which end in senescence. Work 

 on insect senescence is in many respects unpromising as a source 

 of principles which can be extended to the biology of vertebrate 

 old age; such research is frequently confined to the very special 

 circumstances which exist in the imago — in other words, to a 

 system which is already in a time-limited equilibrium. For 

 measures of interference with the growing organism, however, 

 and attempts to stabilize the system in its earlier stages, insect 

 material may prove the most manageable. Any example of 

 indefinite stabilisation at an immature stage, in any organism, 

 would be of great biological interest. The degree of drift towards 

 the unstable state probably varies throughout development in 

 different insects — Bodenstein has shown (1943a, b) that in 

 Drosophila early salivary glands implanted in late larvae are not 

 immediately capable of metamorphosis: 'Whether the organ 

 discs respond with growth or differentiation depends on a 

 definite relationship between hormone concentration and organ 

 responsiveness' (Bodenstein, 1943b). 



5-2-3 VERTEBRATES 



The possibility of producing a long-lasting but recoverable 

 delay in mammalian growth and development by underfeeding 

 first arose from the studies of Osborne and Mendel, (1915, 

 1916). The work of McCay on rats, which extended the results 

 obtained by underfeeding upon arthropod growth directly to 

 mammals, is well known, but still very remarkable. It also still 

 represents the only successful assault which has ever been made 

 on the problem of mammalian specific age, which is itself the 

 key problem of medical gerontology; and the rather exceptional 

 growth-pattern of rats in no way diminishes its interest. The 



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