50 LABORATORY MOUSE 



selves at the slightest noise or motion. The degree of this 

 activity also has a genetic basis, because members of certain 

 strains are much more active than members of other strains. 



By the thirteenth day the young mouse is almost an adult 

 save in size and sex differentiation. Sex maturity is usually 

 reached between the second and third months, although 

 mice increase a small amount in size during the several 

 succeeding months. 



Fostering. Fostering is often desirable where the female 

 is of a feeble strain, is a poor mother, or is one from which 

 a maximum number of offspring is desired. 



In fostering it is a good practice to employ as nurses 

 females which are first-generation offspring of a cross between 

 two inbred varieties which show great vigor, although any 

 docile vigorous female will do. A number of these prospective 

 nurse mothers are mated simultaneously with those whose 

 young are to be fostered so that nurses will be available 

 when the desirable young are born. 



Some foster mothers object to an exchange of young, espe- 

 cially if the mice to be fostered are younger than her own 

 litter. The foster litter and the foster mother's own litter 

 are shaken together gently by some investigators in order 

 that the foster mice may obtain the odor of the foster moth- 

 er's nest and be more acceptable. The mice to be fostered 

 are picked out and given to the foster mother, while the 

 foster mother's own litter are killed. 



If a female mouse has not nursed much, she will usually 

 breed within twenty-four hours after parturition. 



Killing. Some investigators prefer to drop all discard mice 

 into a covered jar containing a piece of cotton saturated with 

 ether, although simple mechanical methods of terminating 

 their existence are both adequate and painless. 



Parasites. Even with the best of care a mouse colony may 

 occasionally be infested with fleas, mites, or lice to the ex- 

 tent that they prevent breeding, although these parasites 

 seldom prove fatal. Pyrethrum powder or pulverized tobacco 

 dusted upon the animals from time to time are good preven- 



