THE BREEDING OF MICE IN LABORATORIES 55 



three numerals, 1, 2, and 3, are denoted by holes in the ear 

 at top, side, and bottom respectively. The second three, 

 4, 5, and 6, are represented by notches in the edge of the 

 ear at top, side, and bottom. The next three numerals, 7, 8, 

 and 9, are indicated by combinations of two notches, 7 being 

 notches at both top and side, 8 being notches at both side and 

 bottom, and 9 being notches at both top and bottom. 



With this system, employing the right ear for units and 

 the left ear for tens, one may number animals from 1 to 99. 

 One hundred has a hole in the center of each ear, but may be 

 distinguished from 200, 500, 700 etc., by age, color, parents, 

 cage number, and other records. By these data one may also 

 identify mice whose individual record numbers bear the 

 same last two digits. 



Temperature. Mice are quite sensitive to changes in 

 temperature. A cold draft from a window may prove fatal 

 to a high percentage of a colony within a single night. Even 

 the opening and shutting of outside doors on cold winter 

 days may chill the mice considerably. The optimum condi- 

 tion is a room regulated between 70° and 80° F. both day 

 and night. Animals suffering slightly from exposure may 

 wheeze chronically, but appear in fair health and even breed. 

 This condition is known to the fanciers as "singing" or 

 "asthma." 



Food. Food should be readily available to mice at all 

 times. A mouse that spends more than twenty-four hours 

 at one time without food is in grave danger of starving to 

 death. If only a meager amount of food is present, life may 

 be sustained, but the animals will not reproduce. A suitable 

 ration for mice measured by weight consists of: 



240 parts rolled oats 

 30 parts powdered skim milk 

 8 parts cod-liver oil 

 1 part salt 



A formula for rat feed, which is probably satisfactory also 

 for mice, has been recently prepared by Maynard (131) 

 consisting in parts by weight as follows. 



