194 HEREDITARY CHARACTERS 



As long ago as 1891, Henking described two kinds of 

 sperms as occurring in a genus of insects. 1 The same 

 phenomenon has since been stated to occur in several 

 other orders of insects, and in certain centipedes (Myria- 

 poda) and Arachnids. 2 The difference between these two 

 classes of sperms depends upon the presence of one or 

 more additional chromosomes in some of them. This addi- 

 tional chromosome, the " accessory chromosome," is present 

 in the cells of the male organism, and when reduction takes 

 place, as it does not divide, half the sperms receive it, while 

 the other half do not. Thus when fertilisation takes place, 

 the chances that the fertilised ovum will contain this addi- 

 tional chromosome are exactly even. 



That this accessory chromosome might be a sex deter- 

 minant was first suggested by M'Clung. 3 



1 Heuking, H., " Uber Spermatogenese und deren Beziehung zur Entwickt,-- 

 lung bei Pyrrhocoris apterus," Zeit. Wiss. Zool., li., 1891. 



2 Montgomery, T. H., Jr., "The Spermatogenesis in Pehtatoma up to the 

 Formation of the Spermatid," Zool. Jakrb., xii., 1898 ; " The Spermatogenesis of 

 Peripatus (Peripatopsis) Balfouri up to the Formation of the Spermatid," Ibid., 

 vol. xiv., 1900; "Further Studies on the Chromosomes of the Hemiptera 

 heteroptera," Proc. Acad. Nat. Sci., Philadelphia, 1901; "A Study of the 

 Chromosomes of the Germ Cells of Metazoa," Trans. Amer. Phil. Soc., vol. xx., 

 1901 ; "Some Observations and Considerations upon the Maturation Phenomena 

 of the Germ Cells," Biol. Bull., vol. vi., 1904 ; " The Spermatogenesis of Syrbula 

 and Lyeosa, with General Considerations upon Chromosome Reduction and the 

 Heterochromosomes," Ibid., 1905 ; " Chromosomes in the Spermatogenesis of the 

 Hemiptera heteroptera," Trans. Amer. Phil. Soc., N.S., vol. xxi., Part III., 1906 ; 

 Wilson, E. B., Science, N.S., vol. xxix., No. 732, January 1909; Paulmier, F. C-, 

 "The Spermatogenesis of Anasa tristis," Journ. of Morph., xv., Supplement, 

 1899 ; Payne, F., " On the Sexual Differences of the Chromosome Groups in 

 Galgulus oculatus," Bid. Bull.., xiv. 5, 1908 ; Stevens, N. M., " On the 

 Ovogenesis and Spermatogenesis of Sagitta bipunctata," Zool. Jahrb. Anat. u. 

 Ontoff., xviii., 1903 ; "Studies in Spermatogenesis, with Especial Reference to 

 the ' Accessory Chromosome,' " Carnegie Institution, Washington, Pub. No. 36, 

 1905; "Studies in Spermatogenesis: II. A Comparative Study of the Hetero- 

 chromosomes in Certain Species of Coleoptera, Hemiptera, and Lepidoptera, with 

 Especial Reference to Sex Determination," Ibid , Pub. 36, ii., 1906; "A Study 

 of the Germ Cells of Certain Diptera, &c.," Journ. Exp. Zool., v. 3, 1908a ; " The 

 Chromosomes in Diabrotica, &c. ," Ibid., v. 4, 19086 ; Lefevre, G., and M'Gill, 

 C., "The Chromosomes of Anasa tristis and Anax. junius," Am. Journ. Anat., 

 viii. 4. 



3 M'Clung, C E., "The Accessory Chromosome Sex Determinant?" Biol. 

 Bulletin, vol. Hi., 1902. 



