120 O. MUHLBOCK 



age of 16 months. Vaginal smears showed that cyclic produc- 

 tion of oestrogenic hormones is nearly always continued after 

 the termination of the period of fertility, and in many cases 

 until death. 



Histological investigation of the ovaries showed that 

 follicles may still be formed in old age, while ovulation is only 

 possible in exceptional cases. However, the ova show signs 

 of reduction both in number and in quality. Transplantation 

 of fertilized ova obtained from young animals into the uterus 

 of old pseudopregnant mice was not followed by pregnancy. 

 This shows that the loss of fertility is not caused exclusively 

 by anomalies in the production of ova (Boot and Muhlbock, 

 1954). 



Causes of Death 



Inbred strains will often show a strain-specific pattern of 

 senile disease. The high incidence of such diseases within the 

 strain concerned enables us to study specific causes of death. 

 In females of several strains, which are specially kept in 

 Cancer Institutes, a high incidence of mammary carcinoma as 

 a cause of death is observed. In the well known DBA strain, 

 for instance, the mammary-tumour incidence was 59 per cent 

 in virgin females at an average age of 546 days (Muhlbock 

 et al., 1952). The non-tumour bearing mice died at an aver- 

 age age of 537 days. That means that most of these non- 

 tumour bearing mice died before they developed mammary 

 cancer. 



We can easily eliminate the factor which is responsible for 

 the high incidence of mammary cancer in this strain by means 

 of appropriate foster nursing by a mother without the 

 mammary-tumour agent. The incidence of mammary cancer 

 then drops to 4 per cent in virgin females. The lifespan in the 

 females without tumours is prolonged to 598 days as against 

 546 days. As shown in Table I, 87 per cent of these females 

 die without any gross lesions, besides being very atrophic. 

 In other strains too, the percentage of deaths without easily 

 recognizable cause is of the same order. 



