SEX DETERMINATION IN THE WHITE-FLY Qed. 
The fact that males do sometimes occur in English collections 
may mean that a few representatives of the so-called American 
race exist here and there or else that there is at times an irregu- 
larity or reversion to normality in the maturation divisions. 
Should it be shown definitely that the English line does indeed 
produce a mixture of sexes from the mated females, we would 
have to take recourse to an explanation based on some such 
chromosomal behavior as is known in the Hemiptera and Orthop- 
tera. To explain it on the basis of the cytological evidence from 
the American line or on the basis of the chromosomal behavior 
as found in the Hymenoptera, we would have to resort to the 
somewhat far-fetched hypothesis that there are two kind; of 
spermatozoa, both of which, on entering eggs, cause them to un- 
dergo reduction, one of these spermatozoa being otherwise non- 
functional, so that the egg fertilized by it is left with the haploid 
number and produces a male. If the other kind of spermatozoon 
then be assumed to be normal and to restore the diploid condi- 
tion in the egg it has entered, such an egg would produce a fe- 
male. An analogous case, where the spermatozoon enters the 
egg, starts development, but plays no further part, and finally 
disintegrates, is found in Rhabditis aberrans (Kriiger, ’13). 
The origin of the English race—the American line representing 
the normal or parent stock (Williams, ’717)—may be due to 
environmental influence. It is conceivable that new physical 
influences of a hitherto unoccupied region may in some way act 
on the egg so as to modify or suppress the polar-body formation. 
But the question immediately arises why these same influences 
should not also act on the eggs of such a strain as was found at 
Merton and which was evidently not of the English type. 
A more acceptable hypothesis would be based on an origin by 
mutation. Reciprocal crosses between the English and Ameri- 
can lines should make it possible to determine whether the prob- 
lem rests on a Mendelian basis. Once a female-producing line 
had become established, it might, and probably would, displace 
a male-producing line, as has been pointed out by Williams (’17) 
in some mathematical considerations of the problem. This 
‘would be especially true in a new country where the distribution 
