No. 612] DEATH IN VTERO OF MOUSE 



751 



however, does not seem to lead to any very definite con- 

 clusions as to the special fitness of any of the various 

 theories discussed in the preceding paragraphs. It will 

 be seen that in the yellow X yellow mating there are pro- 

 portionately more litters of small size and fewer of large 

 size than are to be found in the other matings. This is 

 what one should theoretically expect on all the theories. 



TABLE IX 



Litters Born During 1916-17, Classified According to Xumber op 

 Young in Litter 



Summing up, we may say that all of our evidence tends 

 to confirm the conclusion of Castle and Little that in 

 mice homozygous yellow zygotes are produced in the 

 yellow X yellow mating, but that these zygotes fail to de- 

 velop normally after implantation in the uterus. Why 

 this should be so is not evident and our investigation has 

 not thrown any light on this point. It is possible a careful 

 microscopical study of the embryos which die early might 

 reveal some abnormality of development which would 

 account for their failure to survive, but it is not probable 

 that it would be of such a simple nature as the analogous 

 cases of death of homozygous recessives lacking chlo- 

 rophyll in corn and some other plants. It seems more 

 probable that in mice there may be a "lethal factor," 

 similar to those so well known in Drosophila, which is so 

 closely linked to the factor for yellow that they are prac- 



