THE FACTORS WHICH DETERMINE SEX 629 



hypothesis, but he subsequently accepted the interpretation of 

 von Siebold, who suggested that the queen which had given 

 rise to the apparently bastard drone was herseK of impure 

 descent, and that in reahty the egg had not been fertiUsed. A 

 further exceptional case has been recorded by Perez,^ who 

 found that a considerable number of male bees produced by an 

 Itahan queen which had been fertiKsed by a French drone 

 appeared to be of mixed blood. This result, which is admittedly 

 unusual, has been explained by Sanson ^ as due to " reversion," 

 while it has also been pointed out that the hybrid drones may 

 conceivably have arisen from hybrid workers which sometimes 

 lay eggs, and further that male bees are often very variable in 

 their characters.^ Either of these explanations would appear 

 to be possible. 



Moreover, the later observations of Petrunkewitsch,* showing 

 that sperm nuclei are not found in drone eggs whereas they 

 are commonly met with in worker eggs, supply an important 

 confirmation of Dzierzon's hypothesis. 



Attempts to extend this hypothesis to other hymenopterous 

 insects have not been so satisfactory, though it seems, as a 

 general rule, to hold good for ants. There are instances en 

 record, however, in which worker ants have developed from 

 parthenogenetic ova, and other exceptional cases have been 

 stated to occur.^ 



Among the Tenthredinidse or sawflies also the unfertihsed 

 eggs commonly develop into males, but this is by no means 

 invariable. Thus in some forms fertile parthenogenetic females 

 only have been known to arise for many generations in suc- 

 cession without the appearance of males.® 



' Perez, " Memoire sur la Ponte de I'Abeille reine et la Theorie de 

 Dzierzon," Annales des Sciences Nat., vol. v., 1878. 



^ Sanson, " Note sur la Parthfeogenese chez les Abeilles," Annales des 

 Sciences Nat., vol. v., 1878. ' Morgan, loc. cit. 



* Petrunkewitscli, "Die Eiohtungskorper und ihr Schicksal im befruoh- 

 teten und unbefruchteten Bienenei," Zool. Jahrb., vol. xiv., 1901. "Das 

 Schicksal der Eiohtungskorper im Drohnenei," Zool. Jahrb., vol. xvii., 1902. 



* Wheeler, " The Origin of Female and Worker Ants from the Eggs of 

 Parthenogenetic Workers," Science, vol. xviii., 1903. 



' Doncaster, " On the Maturation of the Unfertilised Egg and the Fate 

 of the Polar Bodies in the Tenthrcdinidm," Qiiar. Jour. Micr. Science, 

 vol. xlix., 1906. 



