T. H. MORGAN AND C. B. BRIDGES 643 



of the Fi result, so that the curve of the observed F2 should have lain 

 between the calculated F2 curve and the Fi curve. Instead of this 

 the actual deviations were mainly in the opposite direction. 



Chromosome Carrying "With." 



That the gene for "with" is situated in the third chromosome was 

 determined by means of the F2 of the cross of pink by "with," which 

 produced no pinks that were surely "with" (higher grades) or, 

 conversely, no sure "withs" that were pink. The higher grades of 

 "with" are the homozygates and the ratio was thus comparable 

 with the 2: 1: 1:0 ratio ordinarily observed in "repulsion," crosses 

 between two recessives whose genes are carried by the same 

 chromosome. 



" Super-With:' 



The "with" stock used in the preceding experiments had just 

 undergone somewhat over a year of mass selection with little or no 

 observable progress. However, after a few months of selections, 

 decided progress had been obtained in a sister line. A certain culture 

 (E, November, 1910) was seen to contain individuals of still darker 

 grades (IX and X) and selection soon established a "super-with" 

 population. This E line then proved as immovable as before, al- 

 though selection was practiced continually. At the end of twenty-three 

 more generations a census of this line (September, 1911) showed the 

 condition in Table I. This stock was lost before tests were carried 

 out, but we can judge from its sudden manner of origin and subse- 

 quent stabiHty that a second mutation had occurred. Whether 

 this second mutation ("super-with") was a modifier of "with" or 

 an allelomorph of "with" was not determined. 



