134 WILLIAM B. KIRKHAM 



CONCLUSION 



Three points stand out in a survey of the results attained in 

 this investigation. 



First, all mouse embryos encounter a crisis at the time of im- 

 plantation of the blastula in the wall of the uterus, and in un- 

 usually large sets of blastulas one or more appear always to 

 perish at this time without producing any uterine reaction. In 

 healthy mice other than yellows, however, those blastulas which 

 induce a swelling of the mucosa uniformly complete their im- 

 plantation, while the blastulas resulting from yellow matings al- 

 most always lose at least one of each set after the mucosa has 

 reacted. 



Second, apart from this loss of certain blastulas during im- 

 plantation, the embryonic and early postnatal history of yel- 

 low mice is exactly the same as that of mice of other coat colors. 



Third, the evidence that the blastulas lost in yellow females 

 during implantation are the missing homozygous yellow mice 

 consists on the one hand of the absence of any like phenomenon 

 in healthy white mice, and on the other hand of the statistical 

 correspondence of the percentage of embryos so lost with the 

 Mendelian expectation of homozygous yellows. 



