334 WiNGE, THE INTERACTION BETWEEN TWO CLOSELY LINKED LETHALS 
furthermore exceedingly few cross-over males are found in my 
material in relation to the females; most often it is impossible to 
use the latter for further investigations, as they are rarely discovered 
so early that they may still with certainty be considered as virgin 
females. 
The normal-winged flies produced by mating cross-over indivi- 
duals to Old Spread individuals are — as will be understood — 
again New normals, and such individuals have been used for the 
pairings mentioned in table VII. 
MEASURING OF THE DISTANCE BETWEEN SPREAD AND VITAL 
IN THE IIIRD CHROMOSOME. 
I have not been able to determine the absolute position of the 
lethal factor, as I have not been in possession of mutants of 
Drosophila fit hereto. Although it must therefore be left to 
future investigators to decide whether the locus of Vit is located 
to the right or to the left of Spr, I am nevertheless able to state 
the distance between Spr and Vzt, as I have made a number of 
single pairings between Spread individuals born under control in 
pupe glasses. Nearly 3000 offspring individuals of such single 
_ pairings have in all been counted. The result hereof will be seen 
from table IX. 
IX. Old Spread X Old Spread 
Spr Vit En, Spr Vit 
spr vit 


— 
spr|Vit ‘spr 
Theoretically: | Spr ni 
Non-cross-overs 








Spr Vit | Spr Vit spr vit | spr vit 
Spr Vit spr vit Spr Vit spr vit 
Phenotype: . . . dies Spread | Spread dies 
(old type) | (old type) 
Cross-overs 
Spr vit | Spr vit | spr Vit | spr Vit 
| Spr Vit spr vit Spr Vit spr vit 
Phenotype: . . . dies | dies | Spread | normal 
| | (new type) (new type) 

