OLD AGE AND NATURAL DEATH 



phenomenon of ageing, as it is reflected in structural changes 

 of tissues and cells and, more particularly, in the type and in- 

 tensity of tissue and cellular metabolism. Only scraps of such 

 information are now available: he would have to collect more. 

 (The physiologist might in any case become more fully aware 

 of the dimension of time in his experimental work. Nearly all 

 his work is done with mature animals; studies on youngsters 

 and animals past the reproductive period are far too few.) 



With an adequate background of purely descriptive evid- 

 ence, the physiologist could then bring the experimental 

 method to bear. The first problem he would seek to solve is 

 this: is the phenomenon of ageing something ''systemic*' in 

 nature — something manifested only by systems of the degree of 

 organization of whole animals — or is it intrinsically cellular? 

 Studies on tissue cultivation have given a partial answer to this 

 question, but there are grounds for supposing that in certain 

 critical respects it is misleading. One promising alternative that 

 has become available to him is the technique of tissue and 

 oi^an transplantation between animals of different ages. The 

 majority at least of the members of very highly inbred strains 

 of mice are from the standpoint of tissue-interchange genetic- 

 ally identical, for after many generations of repeated brother- 

 to-sister mating they come to resemble each other (sexual 

 differentiation apart) almost as closely as identical twins. One 

 may therefore interchange parts of their bodies on a scale 

 limited only by the exigencies of technique; one may make 

 time-chimeras of youth and old age.* How, then, does tissue 

 transplanted from a baby animal to a dotard develop in its 

 ''old'' environment? Does it rapidly mature and age, or does it 

 remain like a new patch on an old pair of socks? Conversely, 



* [The grafting of tissues between animals of different ages might be 

 described as 'heterochronic', and it does for age or time what 'heterotopic' 

 grafting does for place or space; see The Uniqueness of the Individual. 

 Professor P. L. Krohn is making a particular study of these problems.] 



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