SEX DETERMINATION IN THE WHITE-FLY 289 
erley is so clear cut and definite that it seems as if this interpre- 
tation is the only one that can be made at the present time. Har- 
rison and Doncaster (14), who crossed Biston zonaria with B. 
hirtaria, found that in the hybrids synapsis occurs to a limited 
extent. Pairing is also noticeable prior to the first maturation 
division, and such chromosome pairs undergo reduction in a 
normal manner. The unpaired chromosomes may either divide 
in each of the two divisions or else go undivided to either pole in 
one and divide equationally in the other division. The latter 
thus behaves, as might be expected, like an unpaired chromosome 
in the spermatogenesis of typical Hemiptera or in the lata type 
of Oenothera (Gates, 714). This, however, does not apply to 
those chromosomes which seemingly are subject to two equation 
divisions like those of the Pygaera hybrids. 
Doncaster, observing that the hirtaria chromosomes are about 
four times as large as those of zonaria, but that the latter has 56 
compared with 14 in hirtaria, suggests that the smaller number 
arises from the larger by a union of chromosomes. In other 
words, they are compound or multiple chromosomes. If such 
multiples be made up of unit chromosomes which have the same 
constitution (and they must bear some such relation to each 
other if the same ones always join to form the multiples), then 
in the 56 units of zonaria there must be 14 quartets of similar 
units. It is thus possible that the Pygaera chromosomes are 
really multiple chromosomes which in one of the divisions are ~ 
merely separated into their component parts. This would then 
not be a true equation division, for it involves the separation of 
joined whole chromosomes, neither would it be a true reduction 
division, as I have mentioned under Rhodites. Nevertheless, 
some such process must take place in all cases where the diploid 
number of a reduced egg is restored through a reunion of the 
equational polar body with the nucleus or by a secondary dou- 
bling of chromosomes within a cell. All such chromosome sets 
must be made up of pairs of homologous units. How such mul- 
tiple chromosomes manage to persist in the maturation of ordi- 
nary non-hybrid Pygaeras presents still another difficulty, but 
