THE DETERMINATION OF SEX 95 



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 fertilisation ; but it cannot 

 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 fertilisation bears maleness, and that the 

 female element is introduced by the spermatozoon ; but it is 

 at least conceivable that the unmatured egg potentially bears 

 both sexes and that the presence of the spermatozoon determines 

 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, 1 so falling into line 

 with the bee; but Reichenbach 2 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 ; but there is no doubt that in the sawflies 

 some species produce males, a few mixed broods, some only 

 females from unfertilised 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 one per 

 cent.) is generally obtained. These may possibly be intro- 

 duced by accident, since the species is so abundant ; but the 

 writer 3 concluded on cytological grounds that there were two 

 kinds of eggs : one requiring fertilisation and yielding females, the 

 other developing into males without being fertilised. But more 

 recent experiments (made with the help of Mr. A. C. Tunstall, 

 and not yet sufficiently extensive) do not support this idea. 

 Out of two broods — one of 58 eggs, the other of 102 — 53 and 67 

 male pupae or adult larvae were reared, which clearly indicates 

 that the absence of females was not caused by their dying off; 

 for among the eggs of fertilised females at least 50 per cent, 

 commonly yield females. It was not possible in these experi- 

 ments to hatch out the flies ; but the size of the adult larvae 



1 Field, Biol. Bull. ix. 1905, p. 355 ; Marchal, Arch. Zoo. Exp. und Gen. (3), 

 iv p. 1. 



1 Biol. Centralblatt, xxii. p. 461. 



3 Doncaster, Q./.M.S. vol. li. 1907, p. 101. 



