OF MUTANT CHARACTERS. 



253 



crossing-over with black and 46.7 per cent with star, from which it 

 was known that the locus of plexus is to the right of bhick. A com- 

 parison of the combined black plexus cross-over value of 41.8 with 

 other values involving black indicated that the locus of plexus wa^ 

 closest to arc, but probably not quite as far to the right, since black and 

 arc gave 42.6 per cent of observed crossing-over. 



Table 100.— Pi, black plexus cT X star 9 ; B. C, F, 

 star 9 X black plexus d^ . 



The loci of plexus and arc were found to be so close together that it 

 was too hard a task to obtain the plexus arc double forms by any 

 simple method. The plexus speck double was obtained fairly easily. 



Table 101. — Summary of all crossing-over data on plexus. 



Loci. 



Star plexus. . . 

 Squat plexus . 



Black plexus . 



Purple plexus, 

 Plexus speck.. 



Date. 



July 20, 1915 

 AprU 3, 1916 



Jan. 1, 1915 

 July 20, 1915 



Apr. 3, 1916 



Feb. 29, 1916 

 Feb. 29. 1916 



By- 



Bridges 

 Do. 



Do. 

 Do. 



Do. 



Do. 

 Do. 



Reference. 



PX' 



So', 



S' 



b Px 



B. C; 1921-24. 



b Px 



B. C; 4044. 



Px- b PxB. C: 1084-'99. 



S' 



Vx\ 



Sq\ 



b Px 



B. C; 1921-24 



h Px 



B.C.; 4044. 



Ula: Pt Px «p Fi: S-'iaS-'SS. 

 II la; Pr Px »p Fj; 3535-'53. 



Only one linkage experiment involving the plexus speck cross-over 

 value is available, and this gave 8.9 per cent of crossing-over (table 

 135). The locus of plexus is therefore about 3 units to the left of arc, 

 since arc gave 5.9 per cent of crossing-over with speck. 



This position makes plexus of great importance, since it can serve 

 as a new base of reference for speck itself. At present speck is located 

 by reference to curved, which is too remote to give an accurate measure 



