332 WINGE, THE INTERACTION BETWEEN TWO CLOSELY LINKED LETHALS 
New Spread X New Spread 
New Spread X New normal 
New normal X New normal 
2 New Spread: 
1 Old Spread: 
1 Old normal: 
1 Old normal: 
1 Old normal 
1 New Spread: 
1 New normal 
1 New normal 
Neither the New Spread type nor the New normal one thus is 
constant, but they may be propagated in mixed cultures. New 
Spread and Old normal ones may thus be cultivated together in mixed 
cultures, as together they would only reproduce the same two types. 
The normal one, however, will quickly predominate, in case we do 
not regulate, which agrees with what might be expected as Spread 
is a lethal factor. 
THE OCCURRENCE OF NORMAL-WINGED INDIVIDUALS IN 
SPREAD CULTURES. 
After thus having sufficiently substantiated that each single cross 
really gives the result that might be theoretically expected, it 
may be stated with certainty that the theory must be correct. It 
still remains to explain, how normal individuals occasionally arise 
in Spread cultures (of old type), a phenomenon which, as aforemen- 
tioned, caused the present investigation. 
It ought to be rather clear that crossing-over between the two 
III"¢ chromosomes in Spread individuals is the most likely and 
only probable explanation hereof, and at the same time we can 
possibly determine the distance between the locus of the factors 
Spread and Vital. 
In Old Spread cultures the individuals have as aforementioned 
the following formule ar and are thus double heterozygotes. 
In case a crossing-over in the Drosophila female takes place between 
Spr and Vzt, some of the eggs get the formula Spr vit and spr Vit 
respectively. When these eggs are fertilized by Old Spread males, 
the following 4 types arise: 

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) 

