Longevity of Man and his Tissues 
organ culture; the published claims are contradictory. There 
is still doubt whether any cell which is kept dividing serially in 
culture is really unchanged from its original genetic status—and 
from what one knows of hidden selection and divergence even 
in slowly-reproducing creatures like mice, one might expect 
such effects in cell cultures to be very large indeed. Probably 
there will prove to be some somatic cells which form stable 
clones and others which do not. 
If somatic mutation, or any similar process based on copying 
faults, really does operate to time the rate of ageing, then we 
should find signs of it in stored and in cultured organs, unless 
(1) isolation leads to selective proliferation of unmutated cells, 
or (2) there are very big differences in the rate or the expression 
of the process between different organs. If Burnet is correct in 
his speculation about the réle in ageing of mutated lymphocytes 
which have lost their inhibitions about producing autoanti- 
bodies, we might find that organs are longer-lived if they can be 
cultured in the absence of lymphocytes, or in the presence of 
hand-picked lymphocytes only. But in spite of these possibilities, 
the information we really need about cell longevity, individual 
and clonal, in order to reassess ageing is that which deals with 
somatic cell behaviour in situ. If we had comprehensive 
information about the life-time changes in total and in functional 
cell number in a sufficient range of mammals alone, we could 
probably reduce the factors which may time ageing to a short 
list without further ado. 
Apart from these ideas, we are not really very much further on 
with the age problem than we were when the first Ciba Founda- 
tion conference on ageing* met here in 1955. We have collected 
a good many more data about ageing in vertebrates other than 
man. For example, it now appears likely that ageing occurs in 
lower vertebrates which display indeterminate growth as well as 
in creatures like ourselves, which have a fixed size—a point 
which is theoretically interesting in view of what I have been 
saying about the relative réles of fixed and of dividing cells. 
The most striking example of an experimental prolongation of 
active life in mammals is still the dietary experiment of McCay 
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