416 



THE AMERICAN NATURALIST 



[Vol. LV 



point prior to the beginning of this work, and the data 

 obtained will, therefore, have value. 



Eeciprocal matings were made. In the coupling series 

 agouti black-eyed-whites (AAWwss) were crossed with 

 non-agouti piebalds (aawwss). Since both parents were 

 homozygous for piebald (ss), that symbol may be omitted 

 hereafter. The agouti black-eyed-white young (AaWw) 

 were back-crossed to the double recessive (aaww). The 

 distribution obtained is shown in the coupling series of 

 Table II to the right of "new data." There is an ap- 

 parent excess of black-eyed-whites and agoutis, perhaps 

 due to random sampling and inexperienced grading of 

 some individuals. But this excess is distributed between 

 both the cross-over and the non-cross-over classes, which 

 consequently are not affected, the numbers being 176, 

 186. This gives a cross-over percentage of 48.61 ± 1.90. 



TABLE II 



Back Crosses between Agouti Black-Eyed-White Mice (AaWw) and 



Exp. 



combined Combined ..i Ohs. 



respectively. 



