EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 377 



2. Data given for the A series of inbreds comprise 1752 litters 

 containing 13,116 individuals, or an average of 7.5 young per 

 litter (table 1) ; records for the B series of inbreds include 1656 

 litters having a total of 12,336 members, or an average of 7.4 

 young per litter (table 2). The two series combined comprise a 

 total of 3408 litters which contained 25,452 individuals. For the 

 entire strain the average size of the litter was 7.5 young. 



3. In any litter series of albino rats, whether the animals are 

 inbred or outbred, the first litter cast is the smallest of the series, 

 as a rule; the second litter is the largest ; while the third and fourth 

 litters are about the same size and a little smaller than the second 

 litter. 



4. The size of any litter cast depends chiefly on the age and 

 physical condition of the female, and is not affected by the related- 

 ness or the unrelatedness of the parents. 



5. A comparison of the data for the inbred strain with data for 

 litter size obtained from "a series of stock Albinos reared under the 

 same environmental conditions as the inbred strain shows that 

 each litter of the stock series was relatively smaller than the corre- 

 sponding litter in the inbred group. For the entire series of 424 

 stock htters the average size was 6.7 young per litter. This 

 average is 0.8 less than the average for litter size in the inbred 

 strain (table 7). 



6. In the A series of inbreds the range in litter size was from one 

 to seventeen; in the B series it was from two to fifteen. In both 

 series the most frequent litter size was seven (table 8) . 



7. In the early generations of these inbred rats malnutrition 

 greatly delayed the time of puberty in the animals. In the later 

 generations, under favorable nutritive conditions, the animals 

 bred at a relatively early age. 



8. While the records give no definite information regarding 

 the number of sterile animals in the inbred strain, they show 

 clearly that inbreeding did not decrease the productiveness of 

 the animals. Of the 954 females that were used for breeding, 

 653, or 68.5 per cent cast the required number of four litters. 

 "V\Tiere partial sterilit}' occurred in apparently healthy females it 

 was found to be due to a diseased condition of the reproductive 

 organs. 



