30 Edmund B. Wilson 



nant only by spermatozoa containing the large idiochromosome. 

 In this type, accordingly, it is clear that the large idiochromosome 

 (like the heterotropic chromosome to which it corresponds) passes 

 alternately from one sex to the other, while the small one never 

 enters the female; and this would remain true even did selective fer- 

 tilization not occur (Fig. 6). The same interpretation may finally 

 be extended to Nezara, where the idiochromosomes are of equal size 

 in both sexes, the relations of dominance being the same as before. 

 The two vital points in this result are first, the assumption of 

 selective fertilization, and second the relations of dominance and 

 recession in the two sexes. As regards the first point, until the 

 appearance of Cuenot's paper, referred to above, almost no 

 definite evidence had been produced of an infertility between 

 particular classes of gametes in the same species; though it has 

 long been known that many plants are in a greater or less degree 

 infertile to their own pollen, and an analogous fact has been more 

 recently demonstrated in Ciona by Castle ('96) and Morgan ('04). 

 Correns ('02), in his study of hybrid maize, was led to suggest 

 that in this case there might be a somewhat diminished fertility 

 between the gametes bearing the recessive character (thus account- 

 ing for a relative deficiency of extracted recessives in the second 

 generation of crosses, F 2 ). In studying the breeds of mice 

 Cuenot has found it impossible to obtain pure or homozygous yellow 

 forms. Yellow mice are invariably heterozygotes (the yellow 

 being dominant over gray, black or brown) and when crossed 

 with a pure race of a different color (e. g., gray) give the typical 

 Mendelian result, yellow and gray offspring appearing in equal 

 numbers. This proves that a complete Mendelian disjunction 

 of the yellow and gray determinants takes place in maturation. 

 When yellow mice of known constitution (e. g., Y(G)) are paired 

 with like forms, the first offspring include pure gray forms (ex- 

 tracted recessives) slightly in excess of the normal ratio of 25 per 

 cent., and yellow forms; but contrary to the Mendelian expec- 

 tation the latter, when paired with one another, never give pure 

 dominants (YY), but again produce pure grays (GG) and 

 heterozygous yellows (Y(G)). Cuenot therefore concludes that 

 although complete segregation of both the gray and yellow 



