Ageing in Inbred Strains of Animals 117 



constant as possible. The systems used to ensure this are 

 dependent on the available resources, geographical situation, 

 etc. Prevention of infections is essential in prolonged experi- 

 ments such as are required for gerontological investigations. 



At the Amsterdam Cancer Institute the animals are kept 

 in cages made completely or partly of glass, which enables 

 direct observation to be made at all times. 



Special attention has been given in recent years to the 

 number of animals kept together in one cage. The starting 

 point of a series of investigations was the observation that in 

 cages containing a larger number of animals the incidence of 

 mammary carcinoma was clearly lower than that in the cages 

 containing a smaller number (Muhlbock, 1950, 1951). Investi- 

 gations subsequently made confirmed this observation. The 

 percentage of mammary carcinoma was highest among the 

 animals kept separately in one-animal cages. Since the other 

 conditions were exactly identical, it is as yet impossible to 

 provide a satisfactory explanation of this phenomenon. 



That psychic factors can be involved is demonstrated by the 

 investigations made at our Institute by van der Lee and Boot 

 (1955). Regular check-ups on the oestrus cycle of female 

 mice repeatedly revealed prolongation of the dioestrus period. 

 Detailed analysis of this phenomenon showed that these 

 dioestrus periods indicate a state of spontaneous pseudo- 

 pregnancies. A striking feature was found in the fact that the 

 phenomenon was only observed regularly if the cages con- 

 tained more than one female. 



It was demonstrated that these pseudopregnancies were 

 not the result of attempts at copulation among the females. 

 By resection of the olfactory bulbs it was shown that the 

 cause of the pseudopregnancies should be sought in the ol- 

 factory sense. These pseudopregnancies, therefore, are caused 

 by stimulation via the central nervous system of the pituitary. 

 This example is given in order to demonstrate that the number 

 of animals kept in a cage is a factor to be taken into account. 



In old males, another phenomenon depending on the 

 housing condition is frequently encountered. In some 



