626 
GENETICS: A. WEI N STEIN 
Proc. N. a. S. 
plausible as the identification of the yellow forked series in the two species, 
it would be impossible in the absence of further evidence to decide which 
interpretation, if either, is correct. 
The balance of proof has swung in favor of the identification of the 
two yellow forked series by the discovery of the mutant crossveinless in 
both D. melanogaster (by Bridges) and in D. virilis (by Weinstein). In 
crossveinless flies of virilis (fig. 2c) the posterior crossvein is entirely absent, 
except that very rarely a small segment in the middle of the crossvein 
persists; the anterior crossvein is very faint and may be partly missing. 
A 
yellow cross\'As vesiciilated Iiairy nia^nla forked rugose 
D.virilis 
17.6 18.8 
48.6 54.1 58.1 
78.7 
yellow crossveinless 
B 
' 13.7 ' 41. 
yellow crossveinless 
17.6 
40.5 
forked 
forked 
D.melanogasW 
D.virilis 
siitded 
C 
35.5 
forked (=sin^eJ?) 
in 
flatecl 
vesiciilated(=inflated?) 
39.5 
FIG. 1 
D. melanojast^r 
D.virilis 
fn crossveinless mutants of melanogaster (fig. 26), the posterior cross- 
vein is missing, the anterior crossvein is very faint and almost entirely 
absent, and in addition the second longitudinal vein is slightly thickened 
at its distal end (see Bridges' paper elsewhere in this issue of the Pro- 
ceedings, p. 660). Both factors are recessive and sex-linked, and occupy 
similar positions in the X chromosome with respect to yellow and forked 
(fig. IB). 
A map of the sex-linked factors of D. virilis is given in figure lA.^ 
Crossveinless in Drosophila virilis. — Crossveinless in Drosophila virilis 
was discovered (November, 1917) in the sons of a female containing the 
factor yellow in one X chromosome and the factors vesiculated, hairy 
