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 black. 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 was 

 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 cf X star 9 ; B. C., FI 

 star 9 X black plexus cf . 



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. 



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 



