4 HELEN DEAN KING 



the animal was born; if no index number is used the rat was a 

 member of its mother's first Utter. The subscript following the 

 serial letter is the number which serves to distinguish each 

 particular rat from the other rats belonging to the same genera- 

 tion and litter group. When it is desired to indicate the sex of the 

 individual its number is inclosed by the sex symbol. An illustra- 

 tion will, perhaps, render the scheme clearer. 



This symbol denotes a female rat belonging in the seventh 

 generation of the A series of inbreds. She was a member of the 

 second litter cast by her mother, and her individual number in 

 the series of rats belonging to the second litters of the seventh 

 generation was twenty. 



In the early generations of both inbred series the animals 

 suffered severely from malnutrition which produced a marked 

 effect on their growth, fertility, and longevity, as previous papers 

 in this series have shown (King, '18, '18 a). During this period 

 a considerable proportion of the individuals were sterile, and it 

 was not possible to select animals for breeding; any rats that 

 would breed at all were used to continue the strain. Nutritive 

 conditions were improved at the time that the rats of the fourth 

 inbred generation were approaching maturity, and a decided 

 improvement in the condition of the animals was noted in a very 

 short time: they gained rapidlj^ in weight, the litters cast became 

 larger and sterility almost disappeared. At this stage of the 

 investigation it became possible to attempt to alter the sex 

 ratio by selection within the inbred strain. From the seventh 

 generation on, every female in the A series of inbreds that was 

 used for breeding was taken from a litter that contained an excess 

 of males; breeding females in the B series of inbreds were all 

 taken from litters containing an excess of females. The plan 



