202 



THE SECOND-CHROMOSOME GROUP 



now comprises 5 allelomorphs (wild- type, vestigial, strap, antlered, 

 and nick.) 



Tliat strap is entirely independent of the third chromosome in its 

 inheritance was demonstrated by an F 2 and a back-cross carried out 

 between strap and pink. The F 2 ratio approximated 9 :3 :3 :1, and 

 the straps that were pink were of the same type as those that were not. 



The back-cross test of FI males (table 47) gave 1,063 flies, of which 

 534 were recombinations. This is a percentage of 50.2, where 50.0 was 

 expected, with free assortment between different chromosomes. 



TABLE 47. PI, strap 9 X pink d"; FI wild-type 9 back-crossed 

 by strap pink cf . 



The character strap has never been used in new linkage experiments, 

 since vestigial answers as well in this regard, and the strap wing is not 

 quite large enough to permit the simultaneous use of such other wing 

 and venation characters as curved, arc, plexus, etc. 



ARC (a). 



(Plate 7, figure 4.) 



ORIGIN OF ARC. 



A stock of flies was being looked over by Bridges in search of indi- 

 viduals with black palpi, which were occasionally produced, when it 

 was noticed that roughly 10 per cent of the flies were showing a new 

 wing-character (culture B 30, May 24, 1912). 



DESCRIPTION OF ARC. 



This character was called "arc, " since the wing was bent downward 

 in an even curve from base to tip, and also from side to side. The 

 margins tend to roll slightly in diagonal lines, so that the wing ap- 

 proaches a diamond-shape. The wing is somewhat broader than 

 normal. The texture of the wing is only slightly thinner than normal. 

 Usually the wings diverge slightly, and occasionally tilt over to the 

 side, giving the appearance of a droop to the outer edge. As far as 

 can be seen, the character is restricted entirely to these wing changes. 



