746 



THE AMEBIC AN NATURALIST [Vol. LI 



During 1916-17, the period at wMcli the embryological 

 study was carried on, numerous living litters of mice 

 were born in the laboratory. Miss Sarah V. Jones, who 

 was also working with mice, has generously furnished 

 data from her own breeding experiments, and these have 

 been incorporated with ours. Some of the mice in her 

 experiments were black-and-tans, but as Dunn (1916) 

 has shown that these are a form of yellow and are also 

 always heterozygous, they have been classified as yellows. 

 Table VII shows the average size of litters for the various 

 types of matings. 



TABLE VII 

 Average Size of Litters Born Duking 1916-17 

 (Figures in parentheses indicate the number of litters.) 



1. Yellow $X yellow ^ 5.36(140) 4.55(121) 



2. Yellow $ X non-yellow 6.21(88) 5.63(71) 



3. Yellow X non-yellow ? 7.02(50) 5.95(46) 



(2 and 3 combined) 6.51(138) 5.76(117) 



4. Non-yellow ? X non-yellow ^ 6.88(42) 5.49(37) 



(2, 3 and 4 combined) 6.59(180) 5.69(154) 



It will be seen that there is a deficiency of litter size 

 from the yellow X yellow mating here as in the em- 

 bryological material. Theoretically the average size of 

 litter from yellow parents should be 75 per cent, of that 

 from any of the other matings. Both Cuenot, and Oastle 

 and Little, however, have found it to be above 80 per 

 cent. Our results are in accord with these findings.^ 

 Table VIII shows the percentages found by the various 

 investigators. In order to make them comparable with 

 the others our results as given in the table are for the 

 living litters only and do not include the data from the 

 embryological investigations. 



