F. M. Durham 167 



The F^ numbers obtained are as follows : 



Yellow Non-yelloir 



Cu^not(2) 263 100 



Castle (5) 800 435 



My own 448 232 



1511 767 



Expectation at 2 : 1 1518-6 759 3 



Expectation at 3 : 1 1708-5 5695 



From these figures there can I think be no longer any serious doubt 

 that the pure yellow zygotes are actually formed in fertilization, but that 

 for some unknown cause they are unable to develop. The case becomes 

 therefore exactly comparable with that observed by Baur(7) for the 

 varietcUes aureae, which form albino embryos incapable of existence. 



It has been argued that if this representation is correct the average 

 numbers per litter should be less for the mating yellow x yellow than 

 for yellow mated with some other colour, and Cuenot and Castle record 

 a difference of this kind, giving the following averages: 



From my experience I incline to doubt whether much importance 

 can be attached to differences of this order. 



The following averages have been compiled from an ample series, 

 75 litters being the lowest included. 



yellow X yellow 



yellow X other colour 



black X black 



black X other colour (not yellow) 



chocolate x chocolate 



chocolate x other colour (not yellow) 



agouti X agouti 



agouti X other colour (not yellow) 



albinos x other colour (not yellow) 



I have not mated albinos together often enough to make it worth 

 while to compare the results of mating albino x albino with the other 

 figures. 



Only mice which lived long enough to have their colours determined 

 are included in these averages, but Castle's figures evidently are based 

 on the numbers actually born. It is clear nevertheless that large 

 differences exist where no special disturbance, analogous to that we are 



