324 WINGE, THE INTERACTION BETWEEN TWO CLOSELY LINKED LETHALS 
dual and a normal-winged one will be seen on the adjacent figures 
(resp. fig. l and 2). DEXTER only very shortly mentions the spread 
factor, and I suppose that it is due to a mistake when it is stated 
by him and later on by others in the literature that spread is indi- 
cated as recessive. ; 
THE CROSSING EXPERIMENTS. 
I shall here go over the experiments comprising about 13000 
individuals, which form the basis of the present work, and at 
each crossing I shall mention the genetic formulae. 
Thus we consider two pairs of allelomorphs, the dominant 
Spread factor. (Spr.), which at the same time is a recessive lethal 
(Spr 
(Spr 
linked to Spr and the allelomorph of which is a recessive lethal 
dies) and the dominant vital factor Vit 1), which is closely 
vit 
dies), The linkage is so strong that crossing-over rarely 
occurs, and for the present we completely waive crossing-over. 
The individuals used for pairing are constantly — at any rate 
as far as the females are concerned — born under control, in 
order to obtain absolute security that only virgin females are used. 
The nearly mature pupae were placed in moist filter-paper in 
small glass-tubes with cotton stoppers, and if desired they were 
assorted in female and male pupae. These are easily separated 
by means of a lens or binocular microscope, as the sex comb on 
the forelegs of the males is visible already in the pupe stage. 
As a rule the flies appeared the day after the pupae having been 
placed in the glasses. 

1) The reason why Iam not designating this factor leth III in accordance 
with the Morgan terminology is that I am opposed to call the wild type 
homozygotic Leth. On the whole I find it more natural to designate the 
factors than the mutants, and in this case as far as possible to choose the 
dominant characters as the basis of the factor designation. As Spr is a 
dominant factor, but at the same time is recessive lethal, the factor desig- 
nation in question ought to be either Spr, or Bean Consequently I only 
call it Spr. here. 


