42 



HELEN DEAN KING AND HENEY H. DONALDSON 



birth, there is the possibility that litters were cast which were 

 not recorded. There is the same possible source of error in 

 the data for the last litters cast, as offspring of old females 

 are often born dead and are eaten or lost in the debris of 

 the cage. It has always been part of the routine work in the 

 colony to examine cages containing breeding animals at least 

 once a day in order to make the litter records as complete 

 as possible. 



TABLE 6 



Showing length of the reproductive period in days in different generations of 



captive gray rats 



A summary of the data for the reproductive period in 

 females of the first nine generations of captive Grays is given 

 in table 6. Reproduction in females of the tenth generation 

 will be discussed in a future paper. 



Only one of the thirty-seven fertile females in the first 

 generation born in the colony cast a litter before she was 

 five months of age; many of them did not bear young until 

 they were over a year old. The average age at which these 

 females began breeding was 266 days, or approximately nine 

 months. As the generations advanced, breeding began at an 

 earlier age, and in the later generations some of the females 

 cast litters when they were less than four months old. 



