96 SCIENCE PROGRESS 



or pupae is an almost unfailing criterion of the sex. It seems 

 probable therefore that Nematus ribesii must be placed in the 

 same category with the bee. The gallflies (Cynipidae) offer 

 another anomalous instance ; for in them there are two genera- 

 tions in the year, one of which consists of both sexes, of 

 which all the eggs are fertilised and yield a parthenogenetic 

 generation consisting wholly of females. The eggs of the 

 latter yield both males and females but it is not known whether 

 there are any differences in the maturation or development 

 corresponding with the difference of sex. 



One more point must be mentioned here. In bees some 

 hives produce a large proportion of gynandromorphic indi- 

 viduals, which are irregular mixtures or mosaics of male and 

 female characters. Von Siebold l described such a case and 

 found that all the " Zwitterbienen " were in worker cells, the 

 drones being all pure. The hive was hybrid from Italian 

 stock crossed with black ; the drones were of the Italian type, 

 the workers mixed. Two possible explanations may be 

 hazarded ; first, that the egg which develops into a gynandro- 

 morph has begun to segment and that the male pronucleus con- 

 jugates with one of the segmenting nuclei ; or secondly, that 

 the male pronucleus conjugates with one (probably the second) 

 of the polar nuclei, and that both the zygote nucleus so 

 produced and the female pronucleus take part in the de- 

 velopment. Of these the second is perhaps the more likely 

 hypothesis. 



It has been mentioned above that Beard was one of 

 the first to suggest that the germ-cells bear the determinant 

 for one or the other sex ; it now remains to discuss the 

 evidence which has since accumulated in favour of that 

 hypothesis. It has received support on several very distinct 

 grounds. We may take first the cytological results with 

 which the names of several American investigators are chiefly 

 associated, although much similar work has been done in 

 Germany, France and elsewhere. To give an at all adequate 

 account of the numerous papers on spermatogenesis and 

 oogenesis, which have led to the hypothesis that the sex- 

 determinant is a visible chromosome-like body, would occupy 

 more space than is available ; so we will take the work of 

 E. B. Wilson as typical, although similar phenomena had been 



1 Zeit. IV/ss. Zoo. xiv. p. 72- 



