336 WINGE, THE INTERACTION BETWEEN TWO CLOSELY LINKED LETHALS 
14 normal-winged cross-overs are thus found among 2947 indivi- 
duals in all, but for the calculation of the cross-over percentage it 
must be remembered that the spread-winged cross-overs escape 
observation; therefore in reality about 28 of the 2947 individuals 
must be supposed to be cross-over individuals, so that the distance 
between the locus of Spr and that of Vzt may be calculated at 
hardly 1 centimorgan. It is peculiar to note the comparatively great 
number of females among the cross-over individuals. 
Only once I have been able to get hold of a New Spread cross- 
over individual by paring two Spread individuals from a pure 
Spread culture. Instead of giving nothing but Spread offspring, 
23 Spread and 18 normal-winged individuals were produced, 
a number which, it is true, only is in poor accordance with the 
theoretically-expected relation 2:1, but which of course absolutely 
excluded the possibility that the normal-winged flies were direct 
cross-over individuals. 
DISCUSSION. 
The theory of the two closely linked lethal factors, Spread and 
vital, gives a satisfactory explanation of all the phenomena as to 
the examined Spread mutant. 
However, also other theories might perhaps explain the condi- 
tions, especially as long as the cytology of the Spread mutant 
has not been more closely examined. Thus the supposition that 
1) Spread is a dominant factor in the IIlrd chromosome which 
is lethal in double dose (as presumed in the. first theory) and 
2) that the nearly constant Spread culture consisted of indivi- 
duals which wanted one of its IlIrd chromosomes, would explain 
the results of the crossings, always supposing that also lack of 
both the Illrd chromosomes has a lethal effect. In this case it 
would suffice to consider a single pair of factors: Spr Spr 
Old Spread should in this case have the formula 
Spr 
spr 
Old normal: PRES 
n LE „ ” » n „ spr 
spr 
New Spread „ An x SIEH £ ae 
sp 

New normal 5 RE » „ „ n 

