Conclusion 



the factors which predominate in mammalian ageing, or which 

 can modify it, encounter a rather different obstacle. The choice 

 of experimental animals for such work obviously presents great 

 difficulty, since it is necessary either to work on forms whose 

 life-span is short compared with that of the investigator, or to 

 use elderly individuals whose early history has not been fol- 

 lowed. The complication which this time-factor introduces is 

 of great importance for the planning of research. Man is by far 

 the most numerous senile animal, and his life cycle is extremely 

 well known — even to the point at which we can estimate his 

 physiological age by inspection; some research on senile men 

 can be justified ethically, but the gerontological aspects of 

 laboratory animal-breeding cannot much longer be neglected, 

 since in many problems no further progress is possible until 

 mammals of known life cycle, heredity and physiology are 

 available in quantity. At present the choice lies between experi- 

 ment on patients, the basing of general conclusions upon the 

 behaviour of invertebrates and small rodents, and postponing 

 investigation for several years while a chosen population of 

 larger mammals completes its life cycle. Failure to deal with 

 the logistics of this problem now will hinder research in ten 

 or twenty years time, and that hindrance could be avoided by 

 forethought. 



Another extremely important source of information is likely 

 to be found in the creation of age chimaeras. In spite of the well- 

 merited disrepute incurred by much work upon organ grafting 

 in relation to old age, the resources of modern transplantation 

 techniques now offer a very tempting range of experimental 

 possibilities. The reimplantation of stored infant tissues into the 

 same animal, the cross- transplantation of organs such as ovaries, 

 or tissues such as portions of skin, and the observation of the 

 reciprocal influences of host and implant, are technically prac- 

 ticable and established procedures. Parabiosis between animals 

 of disparate age but identical genotype, or even between 

 retarded and unretarded littermates, is another tool of the same 

 kind. The emphasis of such research is likely to be chiefly 

 directed to dissecting tissue and somatic factors in the senile 

 process, but its possible use in experimental, and even later in 

 clinical, prosthetics is thoroughly well-justified. This is, of 



199 



