SEX DETERMINATION IN THE WHITE-FLY 287 
heterozygous for sex. While reproduction is sexual in the great 
majority of the order, there are some well-established cases of 
parthenogenesis on record. ‘Thus Saga serrata is represented by 
both males and females in Asia Minor, but in southern Spain 
only females are ever found (Bolivar, ’97). Hebard (718) has 
not seen a single male among some thousand immature and adult 
specimens of the blattid, Pycnoscelus surinamensis, although a 
male has recently been reported by another entomologist. Mac- 
Bride and Jackson (715) raised three thousand offspring of the 
phasmid, Carausius morosus, all of which were parthenogenetic- 
ally produced, and among this number observed only six males 
and one gynandromorph. ‘These and many similar cases dem- 
onstrate that parthenogenesis occurs, that in some species it is 
practically obligatory, and that with very rare exceptions it 
gives rise to females. A few cases are, however, known in which 
asexual reproduction is facultative, and eggs will develop whether 
fertilized or not. As in the obligatory type, parthenogenesis 
here produces females, while fertilized eggs give a mixture of 
males and females. I need mention only the grouse locust, 
Apotettix, reported by Nabours (’19), and the phasmid, Clitum- 
nus sp?, which was studied by Fryer (13). Experimental gene- 
tic work was done in both cases, and this showed that segregation 
occurred in the first generation of parthenogenetically produced 
offspring. This is good evidence for the conclusion that reduc- 
tion occurs in these unfertilized eggs as well as in the fertilized 
ones. A further confirmation of this was obtained when Nabours 
found that the F, parthenogenetic generation was homozygous for 
certain characters under observation. 
Cytologically, the maturation phenomena in parthenogenetic 
eggs have been studied only in Bacillus rossii, where von Baehr 
(07) found two maturation divisions. He seems to believe that 
both of these are equation divisions (Buchner, (’16), has also 
gathered this impression from von Baehr’s account), and that 
therefore no reduction takes place. However, his figures of 
equational plates of the first division show tetrad-like structures, 
and since he is unable to count or even estimate the oogonial or 
somatic chromosomes, it is probable that here also reduction 
occurs as in sexual species. 
