498 
Katharine Foot and E. C. Strobell 
female hybrid, as she presumably has only one dose of spot factors ; but 
this would involve inhibiting the spot in all the Fi niales also, which is 
opposed to the facts, as the spot is not inhibited in all the Fi hybrids. 
The facts show that the spot factors — -whatever they are — are latent 
in the female and if they are suppressed by an inhibiting factor, it would 
seeni that this cannot logically be located in a definite chromosonie.” 
“If we assume that eacli member of one diploid pair of chromo- 
somes contains the factors for the genital spot, in addition to the factors 
for inaleness, then the ripe egg of E. varioJamis contains one such chro- 
mosomc and it follows when the egg is fertilized by E. servus it will have 
one diploid pair of chromosomes that is homozygous for the factors pro- 
ducing a male, but heterozygous for the factors producing the spot. 
Such a diploid pair should be present in all the offspring — both males 
and females, and if we cancel it in the females by assuming an inhibitor 
some^Yhere, w^e still have it in the males and therefore the spot should 
be quite as pronounced in the Fi hybrid males as in E. variolarius." The 
facts, however, are as follows ; None of the 11 male hybrids have a spot 
as strong as E. variolarnis, 2 have no spot whatever, 4 have a spot so 
faint and small that it is barely visible, and 5 have a spot about one- 
third as pronounced as that of E. variolarius. 
“Assuming that the diploid pair of chromosomes in the egg of E. 
variolarms which was fertilized by E. servus is heterozygous for the spot 
factors (that they are in chromosonie A, for example, of the pair AB), then 
it must be asked why those in chromosonie A are completely suppressed 
in sonie of the Fi hybrids, while part of theni find expression in other 
hybrids. 
AVe might siniply assume that sonie of the factors of chromosonie A 
have dropped out, but the facts show this to be untenable for the spot 
reappears in the F 2 generation — in sonie cases quite as pronounced 
as in the pure E. variolarius. AA"e must thus assume that the female va- 
riolarius has at least half the spot factors which she transmitted to the 
male hybrids, although these Fj hybrids show either 110 spot at all or an 
incomplete spot. To account for these facts we must assume that the male 
hybrids differ froni the pure males in having an inhibitor that inhibits 
those spot factors which are present but which are not expressed. AVe 
have seen that the facts will not allow placing the inhibiting factors in 
either the X chromosomes or the ordinary chromosomes and we are thus 
forced to admit that inhibiting factors — whatever they are — must be 
located outside the chromosomes — in the region of pure hypothesis. 
The facts force us to consign to these hypothetical inhibitors, not only 
