No. 596] SHORTER ARTICLES AND DISCUSSION 487 



constant excess production anions the functional gametes of those 

 carrying the double character. 4 Were this excess merely acci- 

 dental we should expect to obtain deviations from equality in the 

 direction of deficiency as well as of excess, but such deviations 

 have not been found to occur. The excess of doubles sug- 

 gests a ratio of 9D : 7S (56| per cent, of doubles) or possibly 

 8-5D:7-5S (53| per cent, of doubles). 



3. That the results of cross-breeding are such as to show that 

 in the pure-breeding single all the functional egg cells and male 

 germs carry singleness, whereas in the double-throwing strain all 

 the male germs and rather more than half the eug cells carry 

 doubleness, somewhat less than half the egg cells and none of the 

 male germs carry singleness. For if the cross is made in the form 

 no-d single $ X d-single $ all the F x singles give a mixture of 

 singles and doubles in F 2 , whereas in the reciprocal cross only a 

 proportion — generally rather more than half— of the F 1 cross- 

 breds behave in this way, the remainder give only singles in F 2 . 



To explain the above facts I have suggested : 



1. That singleness is due to the presence of two factors (X and 



Y). 



2. That in the present-day true-breeding single these two factors 



