118 O. MUHLBOCK 



strains the males begin to neglect themselves with increasing 

 age. As a result they are often infested with lice, which 

 results in marked anaemia. The usual disinfectants have 

 failed, in our experience. At our laboratory an excellent 

 means of controlling infestation with lice was found by keep- 

 ing old males with young females in one cage. The young 

 females take good care of the old males, as a result of which 

 the latter survive much longer. 



As has been discussed by Comfort (1956), the choice of 

 material in experimental gerontology depends on the nature of 

 the problem with which one is dealing. Investigations into the 

 genetic aspects of the ageing process can only be made with 

 inbred strains. For many purposes the F x hybrids between 

 two inbred strains may be most suitable. These F x hybrids 

 have a uniform genetic constitution. Their chief advantage is 

 the hybrid vigour. They are therefore admirably suited for 

 most experiments in which healthy vigorous animals are 

 required. Moreover, Gruneberg (1954) has recently drawn our 

 attention to the fact that in the case of some multifactorial 

 characters, at least, the homozygous state tends to be less 

 stable developmentally than the heterozygous one. That may 

 be another explanation for the fact that, for a number of 

 purposes, inter-strain F ± hybrids are to be preferred. The 

 disadvantage of F x hybrids is that they will not breed true. 

 The F 2 hybrids have a great genetic variation with the genes 

 provided by the parental strains. 



The point is stressed that in spite of an identical genetic 

 constitution and in spite of controlled environmental condi- 

 tions the variation between individuals may still be consider- 

 able. That point is illustrated in the following experiment 

 (Muhlbock, 1950): 



Sisters from the same litter in the genetically pure dilute 

 brown strain were invariably kept together in a cage. The 

 conditions of life were consequently completely identical. 

 The time of occurrence of the first mammary carcinoma in a 

 litter was noted. The experiment covered 88 animals from 22 

 different litters. The sister with the first tumour was made a 



