E. R. Saunders 357 



Pea'. Though, however, the scheme of coupliug is based on the same 

 principle in the two cases, there is in the Stocks an additional com- 

 plexity owing to the limitation of the power of carrying these factors 

 to the gametes of the female sex. 



(6 a) The scheme of coupling is such that the combinations of the 

 allelomorphs XxYy carried by almost all the ovules in an eversporting 

 individual are the combinations borne respectively by the male and 

 female germs which united to produce that individual ; the rarer terms 

 in the series are those which represent recombinations of these factors, 

 one factor in the recombination being derived from the male parent, 

 the other from the female. The number of gametes required to exhibit 

 the whole series being taken as 2h, we may represent the gametic series 

 in the eversporting forms in general terms thus : 



Ovules Pollen 



n-\ X7 all xy 



1 Xy 



1 xY 

 n—lxy 



where XY represent factors, required for singleness, and where the 

 zygote has arisen from the union of an XY ovule with an xy pollen 

 grain. The value for n is probably 15 for the type forms though 

 in some cross-breds it may be 31 (or possibly some higher term in 

 the series). 



(6 b) The above formula holds good for all the eversporting strains 

 investigated, hence when no further complications arise the proportion 

 of doubles remains the same whether these strains are self-fertilised 

 or inter-crossed. Thus we ai-e able to understand how it is that these 

 eversporting strains produce a constant excess of the recessive (double) 

 form. The only other instance of the kind, at present known, that 

 seems in any way comparable, is that of one of de Vries' Oenothera 

 hybrids — a tall form which gave an excess of dwarfs^. 



(7) Singleness in the pure-bred, non-double-throwing single is due 

 to the presence of the same two factors (A'' and Y), but in these strains 

 these two factors are linked together (XY), so that when this type of 

 single is crossed with an eversporting form recombinations of the two 

 pairs of allelomorph do not occur. 



(8) In addition to the pair of factors referred to under (6) and (7), 

 1 See note, p. 322. - Ber. der Deut. Bot. Gesell. Bd. xxvi. a, 1908, p. 667. 



Journ. of Gen. i 24 



