No. 550] 



BREEDING MICE 



If the young escape the perils of the first few days they 

 usually grow rapidly and, at the end of a few months, 

 reach maturity. There is a disease, however, which at the 

 end of the second week may attack the young, leaving 

 them emaciated or, when more severe, killing them in 

 great numbers. 



Between birth and maturity four well-defined stages 

 occur. To know these is often of practical service to the 

 breeder for the determination of age, sex and the like. 



The first stage is that in which the newly-born young 

 have a peculiarly red and transparent skin through which 

 is seen the stomach white with milk. Following this at 

 the end of the sixth or seventh day a second stage is evi- 

 dent in which the body is covered with flaky scales of 

 dandruff— the forerunners of a coat of silky fur. A third 

 important stage, which I have designated as the early 

 stage for distinguishing sex 7 is usually shown on the 

 ninth or tenth day, at which time the mammae in the young 

 females appear. These can be observed for an interval 

 up to the thirteenth or fourteenth day, at which time the 

 fur usually obscures them. 



Determination of sex after the body is covered with fur, 

 for example at the time of weaning, is often difficult. Be- 

 cause of this I have found it advantageous at the end of 

 the third period to mark the young females by clipping a 

 tuft of fur at the root of the tail, so that later, when they 

 are to be mated, no difficulty is found in distinguishing 

 with certainty males from females. 



The fourth period, on the fourteenth day, is character- 

 ized by the advent of sight. This like all other periods 

 shows slight variation. While in a few cases I have found 

 the eyes to open as early as the thirteenth day, in others, 

 equally normal in other respects, I have found them to be 

 delayed until the fifteenth and even the sixteenth day. 

 The regularity with which this period occurs, however, 

 is a sufficiently exact criterion to make it an index of age. 



From the fourteenth day to the twenty-first, the date 



T This does not mean that sex can not be determined earlier than this 

 period. As a matter of fact, sex can be determined at birth, this, however, 

 m difficult and less certain than to determine it at the third period. 



