82 



Discussion 



Norway and Sweden — countries which have an exceptionally low 

 mortality amongst the males. 



While your rats were in captivity and being observed, Dr. Berg, 

 were they allowed their normal reproductive functions? 



Berg: No. The experimental animals were kept separate from the 

 breeders and w^re never mated. 



Verzdr: The rat colony kept in our laboratory has a 50 per cent 

 survival rate at 23-5 months, even under very varying conditions. 

 (This is, of course, not counting early deaths, since each mother is 

 allowed to feed only four or five young ones.) Spontaneous death 

 appears more frequently after the tenth month of age. The ages of 



2 4 6 8 lb 12 14 16 18 20 22 24 26 28 30 32 34 36 



Age in months (876 cf) 



Fig. 1 (Verzar). 



death of 1,602 rats (876 ^ and 726 ?) are shown in Figs. 1 and 2, 

 which also show the number of tumours present in these animals. 

 Tumours first appeared in 8-month-old animals, in both males and 

 females. The number is much higher in females than in males but the 

 occurrence is fairly constant during life. It decreases only between 

 the 29th and 37th months, because so few animals survive. This 

 means that the relative quantity of tumours in all deaths is smallest 

 when the death rate is highest, i.e. between the 18th and 24th 

 months. 



The main cause of death was lung disease (bronchiectasis), in 

 which the lung becomes infected and has large amounts of white pus. 

 This is found in 24 per cent of females and 35 • 5 per cent of males. 

 There are more tumours in females than in males (21 per cent against 

 11 per cent) and this difference thus counterbalances the greater 



