180 B- BRINKMAN 



metabolically increased probability of not completely exact replication, 

 irradiation might have an analogous effect. In small macromolecules hke the 

 human haemoglobins it is surprising to see how, during an individual life- 

 time, its composition will change: foetal Hb is replaced by adult Hb, in at 

 least two possible forms and pathologically many variations may occur, 

 many genetically determined (Muller, 1962). French authors speak of 

 "vieiUissement moleculaire de I'hemoglobine". 



Koughly the macromolecule here is a well-defined mixture which changes 

 with age. It is not known to me if radiation can influence this process; in my 

 opinion it would be a good subject for research on animals, as the techniques 

 for "recognizing" the various haemoglobins in an animal are excellent. A 

 splendid example of chemically induced haemoglobin alteration is the pro- 

 duction of sickle-ceU haemoglobin in immature human red ceUs, caused by 

 incubation with ribonucleoproteins, isolated from a genetically different 

 marrow (Weisberger, 1962). 



With regard to individual "cell life", distinction must be made between 

 cells in the various phases of their cycle and between the duration of these 

 phases for various cells. 



It may be useful to remember Cowdry's grading of cells with their in- 

 creasing differentiation: vegetative intermitotics, producing differentiating 

 intermitotics ending up m the definite goal, post-mitotics. Many of these, 

 hke liver cells, capillary endothelium, fibroblasts and others still retain 

 mitotic potentiality (reverting post-mitotics) but others, on the highest plane 

 of differentiation have lost the mitotic way of rejuvenation (fixed post- 

 mitotics) and it may also be among those, which cannot be replaced that we 

 will have to look for delayed vital radiation damage (neuronic cells, cardiac 

 muscle cells) (Korenchevslcy, 1961). 



For a short survey of the theory which considers ageing chiefly 

 as a "nuclear" phenomenon, I can do no better than quote Sinex 

 (1961): 



Adherents of the theory that aging is centered in the nucleus generally 

 believe either that aging is an extension of normal differentiation or that 

 it is due to accidental genetic noise. 



The first group points out that, while we as individuals may view 

 aging as a catastrophe, it probably serves a useful evolutionary purpose 

 in insuring succession of generations. Insect physiologists and plant 

 physiologists are particularly likely to hold this view. Many insects, in 

 the adult form, have relatively short life-expectancy and may even be 

 born without mouth parts. In such insects differentiation produces a 

 phenotyjDc with a limited life-expectancy. The death of an annual plant 

 often appears to be the final step in an orderly development. One may 

 therefore argue that aging is a deliberate event, consisting of differen- 



