BREEDING MICE 

 TABLE Ila 



In the above table is given the record of one of the most 

 active males that I have yet had. The record, however, 

 shows that, although mated with vigorous females, only 

 40 per cent, of the copulations resulted in offspring. The 

 table further shows that after a rest from January to 

 April infertility is still shown. At this later period the 

 male had become inactive, so that it was difficult to secure 

 a copulation. 



A relatively high index of fertility is shown in the 

 record of a male designated as No. 5. 



TABLE 116 



Number 5 when mated with females equally as active as 

 those with which No. 6 was mated gave 80 per cent, of 

 fertility. Within this series were also two other copula- 

 tions by the same mouse, but since these were with female 

 No. 4, which always proved fertile, they were eliminated 

 and only those counted which were entirely comparable 

 with those of No. 6. 



It may be said that both males and females are found 

 which have a low index of fertility. Intensive breeding 

 requires that these be eliminated and that those be 

 selected the copulations of which result in a high per- 

 centage of fertility. 



When fertility does result from a copulation, the en- 

 suing period is of singular interest to the investigator. 

 Tins I have discussed in a former paper in which I have 



