DETERMINATION OF SEX DONCASTER. 477 



the very fact of one kind of egg being larger and having more yolk 

 was the cause of its becoming a female. 



Probably the most convincing proof that the sex is irrevocably 

 determined from the beginning of development is obtained from the 

 study of cases in which the same eggs may be either parthenogenetic 

 or fertilized. The best-known example is the honeybee. In this in- 

 sect, as is well known, unfertilized eggs yield males and fertilized 

 eggs females, either queens or workers according to the treatment 

 to which the larva is subjected. This statement has several times 

 been denied, but the facts are overwhelmingly in favor of its truth 

 in the hive bee ; and numerous other examples are now known among 

 the Hymenoptera. As examples, we may quote the work mentioned 

 above by Silvestri on Litomastix and Ageniaspis, in which the de- 

 velopmental processes are precisely similar whether the egg be fer- 

 tilized or not ; but in the first case females are produced, in the sec- 

 ond, males. Similarly Wassiliew * found in the parasitic Hymenop- 

 teran Telenomus that all eggs laid by virgin females became males, 

 while those of fertilized females yielded about 80 per cent of females. 

 It may be assumed that the remaining 20 per cent received no sper- 

 matozoon. In instances of this kind it is perfectly clear that the sex 

 is definitely determined at fertilization; but it can not be supposed 

 that the egg bears irrevocably one sex or the other before the entrance 

 of the spermatozoon. As will be seen below, it has been assumed 

 by some writers on the subject that the egg before fertilization bears 

 maleness, and that the female element is introduced by the sper- 

 matozoon ; but it is at least conceivable that the unmatured egg poten- 

 tially bears both sexes and that the presence of the spermatozoon de- 

 termines which sex-determinant shall become effective. 



Before leaving this part of the subject it should be noticed that 

 there are a number of Hymenoptera which are anomalous in this? 

 respect. In ants and wasps workers sometimes lay eggs, said always 

 to yield males, 2 so falling into line with the bee ; but Reichenbach 3 

 states that the workers of a species of ant produced workers, except 

 at the time of year when males are normally produced in the nests, 

 when males appeared. It is, of course, possible that there was error 

 of observation, 4 but there is no doubt that in the sawflies some species 

 produce males, a few, mixed broods, some only females from unfer- 

 tilized eggs. The case of the common Nematus ribesii is remarkable; 

 males are usually yielded by virgin eggs, but a small proportion of 

 females (less than 1 per cent) is generally obtained. These may pos- 



1 Zoo. Anzeig., May. 1904. 



2 Field, Biol. Bull. vol. 9, 1905, p. 355; Marchal, Arch. Zoo. Exp. und Gen. (3), vol. 

 4, p. 1. 



3 Biol. Centralblatt, vol. 22, p. 461. 



4 [Reichenbach's observation has since been confirmed by Crawley and Donisthorpe, 

 Proc. Entom. Soc. London, 1910, vol. 5, p. 67.] 



