DEATH IN UTERO OF MOUSE 



TABLE VIII 

 »NATE Size of Litters from Yellow x Yello 

 X Non-yellow Matings 

 (Pigiires in parentheses represent number of lit 



Cu6not (1909) 



Castle and Little (l 

 Ibsen and Steigledei 



The most striking fact brought out in this table is that 

 as the average size of the litters increases the ratio tends 

 to decrease and therefore to approach 75 per cent. It 

 would seem from this that if one could secure a race of 

 mice having a high enough average per litter the the- 

 oretical percentage could be obtained. Using Table VIII 

 as a basis for computation, such a race should average 

 approximately 10 young per litter in the non-yellow 

 X yellow mating and consequently about 7.5 per litter for 

 the yellow X yellow mating. It is not probable that a 

 race of this sort exists. 



Various theories have been advanced to explain this 

 unexpectedly large litter size in the yellow X yellow 

 mating. Two suggested by Castle and Little will be con- 

 sidered here. Both take as their starting-point that "the 

 perishing of a pure [homozygous] yellow zygote makes 

 possible the development of a certain number of other 

 fertilized eggs." The explanation follows: ''Two ways 

 may be suggested in which this might come about. First, 

 more eggs may normally be liberated at an ovulation than 

 there are young born subsequently. In that case, failure 

 of some eggs to become attached to the uterus may make 

 the chances greater that the remainder will become at- 



