Growth and Senescence 



too extensive for simple life-table experiments, but its use as a 

 tool in work on the specific size-specific age relationship deserves 

 further thought. The difficulty of such a direct application may 

 be less than it appears. Moon and his co-workers (Moon et al. 9 

 1952) found that massive administration of growth hormone 

 (2 mg./day) to mice evoked tumours in only one of the tested 

 strains. It appears, moreover, that growth hormone alone fails 

 to induce tumours in hypophysectomized animals (Asling et aL, 

 1952a, b). The resistance which develops to the heterologous 

 (ox) hormone used in such experiments may perhaps be sur- 

 mountable. The idea underlying this kind of investigation was 

 already present in the work of Robertson (Robertson and Ray, 

 1919; Robertson, 1923) at a time when endocrinology was 

 insufficiently advanced to enable it to be realized. The results 

 they obtained in retarding growth with 'tethelin' were almost 

 certainly non-specific. Work upon growth hormone in mammals 

 whose epiphyses do not unite would appear to be one of the 

 critical experiments in finding out how far growth and develop- 

 ment are an integrating system tending to senescence at a fixed 

 point, and how far mere growth, induced by one of many 

 anabolism-stimulating factors, is capable of reversing or pre- 

 venting senile change. 



Attempts to accelerate mammalian senescence have been sur- 

 prisingly unsuccessful. While laboratory animals can be pre- 

 maturely killed by a number of drugs or deficiencies, these do 

 not in general affect the process of senescence. Experimental 

 efforts to accelerate ageing in rats with dinitrophenol (Tainter, 

 1936, 1938) and thyroid (Robertson et al. 1933) or retard it 

 with thiouracil (Hartzell, 1945) have been uniformly unsuc- 

 cessful in bringing about any change in the specific age. Petrova 

 (1946) obtained evidence that induced neurosis at least shortens 

 the life, if it does not affect the specific age, in dogs: it is signi- 

 ficant that in man the most effective means of reducing the 

 apparent rate of senile change, ceteris paribus, are psychological, 

 social and occupational. 



5-3 Growth-cessation and Mammalian Senescence 

 Mammals in captivity under 'optimal' conditions exhibit 

 both specific size and specific age, and these vary widely 



153 



