No. 612] DEATH IN UTERO OF MOUSE 



749 



large death rate due to overcrowding, and this would 

 tend to increase the percentage relation between "yellow 

 X yellow" and the "non-yellow X yellow" instead of de- 

 creasing it, which actually is the case. 



We know that dead embryos B do not materially de- 

 crease the litter size in the non-yellow X yellow mating, 

 but on the other hand we have clear evidence that dead 

 embryos A have a decided effect in this respect. The 

 only manner we could postulate in which this could affect 

 the litter-size percentage relation for the two contrasted 

 types of matings would be to assume that in the yellow 

 X yellow mating dead embryos A are due almost entirely 

 to the fact of their being homozygous yellows, while in 

 the non-yellow X yellow mating separate agencies are at 

 work producing an appreciable number of deaths. If a 

 more careful examination of dead embryos A should re- 

 veal rather easily distinguishable differences this ex- 

 planation could be tested. 



There is still another explanation which so far as we 

 know has never been suggested, and which has more evi- 

 dence in its favor than any of the others proposed. When 

 yellows are mated to yellows it is to be expected that some 

 of the litters, especially if they are small, will consist en- 

 tirely of homozygous yellows. Since these do not com- 

 plete development, the entire litter will be composed of 

 dead embryos A and consequently there will be no living 

 embryos born at parturition. Such a litter will there- 

 fore be of size. In our embryological investigation two 

 litters consisting entirely of dead embryos A were found 

 in the yellow X yellow mating and none in the other types 

 of matings. Even with these two litters of size in the 

 yellow X yellow mating the average size of litter as com- 

 pared to the litters of the non-yellow X yellow mating 

 was not sufficiently low to bring the percentage relation 

 down to 75 per cent. It is, however, 80.18 per cent., which 

 is appreciably lower than the percentage obtained from 

 litters of animals allowed to give birth to their young. 

 (See Table VIII.) In the latter case it would naturally 

 be impossible to detect the litters of size. 



