744 THE AMERICAN NATURALIST [Vol. LI 



X yellow are combined for this purpose, it is found that 

 the dead embryos in these combined matings constitute 

 8.9 per cent, of all embryos present. Subtracting this 

 from 24.5, the per cent, of dead embryos in the yellow 

 X yellow mating, the difference is only 15.6 per cent. 

 When all these results are combined with those of Kirk- 

 ham the difference is still only 19.4 per cent., which is con- 

 siderably lower than that obtained by using only the 

 classes considered by him. An attempt will be made 

 farther on to explain this deficiency. 



Tables II and III, representing the reciprocal crosses 

 of yellow X non-yellow, show marked contrasts in sev- 

 eral respects. In the yellow ? X non-yellow <^ mating 

 (Table II) 12.3 per cent, of the embryos were dead, while 

 in the non-yellow ? X yellow mating (Table III) the 

 percentage is only 4.5 per cent. For both matings the 

 percentage of dead embryos theoretically should be the 

 same as in the non-yellow X non-yellow mating (Table 

 IV), since in neither case is there the possibility of any 

 of the offspring being homozygous yellows. The per- 

 centage in Table III is approximately the same as in 

 Table IV, but in Table II it is much higher. It is well 

 known that yellow females tend to take on more fat than 

 females of other colors. There is a possibility that this 

 physiological difference may also in some way inflnence 

 the production of a greater number of dead embryos A. 

 This is offered merely as a suggestion, since there is no 

 direct evidence for or against it. 



In discussing the stages of pregnancy in which the dead 

 embryos are to be found, Kirkham makes a statement 

 which does not accord with our observations. He says: 

 "No degenerating embryos have been found in either 

 white [albino] or yellow mice pregnant more than sixteen 

 days." He believes complete resorption of the degen- 

 erating embryos has taken place by the end of the six- 

 teenth day. Table V, which is a summary of all our four 

 types of matings, indicates that in our material there is 

 no marked decrease in the percentage of dead embryos 

 toward the end of pregnancy. 



