340 



Prof. E. B. Wilson. 



of the egg by the X class of spermatozoa will produce the female combination 

 XX, by the no-X class the male combination X (or XO) (fig. 2). From these- 

 relations, and those found in a second type, described below, Miss Stevens 

 and I concluded that the determination of sex in these animals depends upon 

 which class of spermatozoon enters the egg, and that the X-cbromosome plays 

 some special role in the process. In a general way this substantiated 

 McClung's earlier suggestion, though he reversed the actual significance of the 

 two classes of spermatozoa. 



I discovered in my first researches on Hemiptera a second type, inde- 



Fr-otenor Type 



LygaeusType 



Diploid Nuclei XX XO 



Gametes 

 Fertilization 

 Zygotes 



etc 



Fio. 2. — Diagram of the relations of the sex-ehroniosom.es to sex-production, showing the- 

 two main types represented by the Hemiptera Protenor (Y-chromosome absent) and 

 Lygceus (Y-chromosome present). In either case random union of the maternal and 

 paternal gametes reproduces the original forms (males and females) in equal 

 numbers. 



pendently found and fully worked out in the Coleoptera by Miss Stevens, in 

 which the X-chromosome of the male is accompanied by a mate of 

 different type, often much smaller than X, which I called the Y-chromosome. 

 This was the first discovery of a heterogeneous chromosome-pair in any 

 animal or plant. In this case X and Y conjugate and disjoin like any other 

 chromosome pair — a fact here shown with incomparable clearness — so that 

 half the spermatozoa receive X and half Y. The X-class are, as before, 

 female-producing, while the Y-class are male-producing ; and the sexual 

 formulas become XX for the female and XY for the male (fig. 2). Owing to 

 the small size of Y, these differences are in many species conspicuously visible 

 in the diploid nuclei, despite the fact that the sexes here possess the same 

 number of chromosomes. The two types are connected by a series of inter- 



