Physiological Approach in Study of Ageing 21 



The situation is quite different in lower vertebrates. The 

 fertility of the females does not generally lessen with age, and 

 the longer and older the female, the greater is its capacity to 

 produce eggs — at least up to a certain age. In snakes, for 

 instance, fertility increases with the age of the mother. In the 

 Matrix natrix we have studied in our laboratory, the clutch 



240 

 220 

 200 



180 

 160 

 140 

 120 



o 



q 100 

 o 



CL 



80 



o 



o 60 



40 



20 



10 II 



I 2 3 4 5 6 7 8 



AGE OF HEN (vears) 

 Fig. 1. The variation with age of egg production in three strains of 

 domestic fowl. (After Romanoff and Romanoff, 1949.) 



size varies with age from 16 to 51 eggs per clutch (Fig. 2). 

 Another species, Coronella laevis, shows the same trend 

 (from 4 to 13 young per brood in that viviparous snake). A 

 senile decrease in fertility can nevertheless be noted in the 

 oldest individuals, as shown by Klauber's (1936) data, but it 

 takes place at a much later age than in warm-blooded verte- 

 brates of similar size (Fig. 3). In fishes the situation appears 

 to be quite similar. To take a single example, the average egg 



