34 Discussion 



the mother. We did that with the C57 black strain. We considered 

 the Hfespan only of female animals living longer than one year, i.e. 

 half the usual lifetime of a mouse. The fertility age of this strain is 

 up to twelve months. We divided the offspring according to the age 

 of the mother, first three months, then four to six months, and then 

 six to twelve months. There was no difference in the lifespan of 

 these offspring. The environmental factors were the same and there 

 is no difference in genetic make-up. 



Berg: We have been able to extend the age of fertility considerably 

 in the rat, by regulating the food intake. For example, 700-day-old 

 female rats fed ad libitum have a fertility rate of 12 per cent. On a 

 controlled food intake fertility is increased to 67 per cent. 



Jalavisto : One feature of these family records is that the families 

 are very big. There are great age differences between the members 

 of one family, although the environment is fairly consistent. 



Hinton: Your main maternal effect may be restricted to the first 

 two years of the life of the offspring partly because the older mothers 

 have more experience. For example, we have children 17 years 

 apart, and we took care of our later children much better than we 

 did of the first ones. 



Comfort: Even so I think there is a higher early death rate among 

 first babies born to prosperous mothers over 40 than among those 

 of underprivileged mothers of 18. I doubt if this is entirely a matter 

 of maternal experience. 



Jalavisto: I have divided the material into first and second 

 children and there is no difference. The second and subsequent 

 children show the same effect as the first children. 



Danielli: It may just be that the mothers do not bother so much 

 with the later children. 



Benjamin: The present discussion does seem to tie in with what 

 Dr. Berg said earlier about deaths of children being due to acute 

 infections or other endogenous diseases. This factor of the experience 

 of the mother helping her to deal with infections would seem to be 

 important. You would find great difficulty in making such researches 

 in this country now, because there seems to be a strong tendency for 

 women to have their children very young and very early in married 

 life. 



Jalavisto: In Finland we have on record what is probably the 

 highest maternal age — a woman nearly 60 years old ! 



