F. M. Durham 167 



The F„ numbers obtained are as follows : 



Cuenot(2) 



Castle(5) 



My own 



Expectation at 2 : 1 

 Expectation at 3 : 1 



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

 that the pure yellow zygotes aie 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 

 varietates 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: 



Cuenot 



Castle 



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 3'90 young 



yellow X other colour 3'97 „ 



black X black 460 



black X other colour (not yellow) 3'99 „ 



chocolate x chocolate 3'96 „ 



chocolate x other colour (not yellow) 3'93 „ 



agouti X agouti 3"47 „ 



agouti X other colour (not yellow) 3'32 „ 



albinos x other colour (not yellow) 4'27 „ 



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 



