1909.] 



Gametogenesis of the Oall-Fly, etc. 



107 



factor, so that ? eggs are without that character ; ^ eggs have it, but in the 

 male, since the ? determinant is absent, there is no such spurious allelo- 

 morphism, and the grossulariata factor is indiscriminately distributed among 

 the spermatozoa. In insects with heterochromosomes these may be regarded 

 as bearing tlie ^ and ? factors as suggested above. In colour-blindness, the 

 factor for the disease can only be borne by a gamete which contains a sex- 

 determinant, but the latter may be either or ? ; in the male, the factor 

 is borne by the S -bearing spermatozoa, but not by those which have no 

 sex-determinant (O) ; since the latter fertilise J" -bearing ova and the former 

 ? -bearing, an affected male transmits the factor for the disease only to his 

 daughters. But in the heterozygous female the colour-blind factor may be 

 associated with either or ? eggs, and thus some of her children of each sex 

 receive it. 



The suggestion may here be made in passing that this hypothesis of 

 sex-transmission may explain the cases not rarely met with, especially in the 

 offspring of hybrids, in which all or nearly all the offspring are of one sex. 

 If a ? egg were occasionally fertilised by a O spermatozoon, the resulting 

 individual would be a female containing no male determinant, and either all 

 its eggs would be ? -bearing, or if any contained no sex-determinant they 

 might be sterile or unfertilised. In crosses which give only male offspring 

 (as in the case of the moths, Tciyhrosia crejmscularia and T. histortata described 

 by Tutt*), the ? -bearing eggs may fail to attract the spermatozoa of the 

 other species, and so only male offspring would result. 



After this digression, we must now return to the case of the Bee, Wasp, 

 and other parthenogenetic cases. In a previous paperf I have made the 

 suggestion (which I hnd has also been made by Morgan) that the presence or 

 absence of a spermatozoon in the egg might cause the maturation to take 

 place differently. It is possible that when a spermatozoon is present the 

 egg of the Bee undergoes normal " reduction " divisions, halving the chromo- 

 somes both quantitatively and qualitatively, and removing the ^ determinant. 

 But when the egg develops unfertilised the polar divisions may both be 

 equational, leaving the haploid (reduced) number of bivalent chromosomes in 

 the egg, and in this case expelling the ? determinant. This would be com- 

 parable with the supposed male eggs in the spring brood of Neurotcrus, and 

 in each case in the germ-cells of the male the reduced number would remain 

 throughout, but in the body-cells the bivalent chromosomes would separate 



* Tutt, 'Trans. Ent. Soc.,' 1898, Part I, p. 17. Quoted by Castle, "Heredity of Sex," 

 ' Bull. Mus. Comp. Zoo. Harvard,' vol. 40, 1903, p. 206. 



t " On the Maturation of the Egg in the Teuthredinida;," ' Q. J.M.S.,' vol. 49, 1906, 

 p. 586. 



