THE DETERMINATION OF SEX 



579 



(lacking X), the female combination upon fertilisation by a 

 spermatozoon of the X-class. Fig. 3 shows this in diagram, 

 and the result may be formulated as follows : 



Egg X + Spermatozoon no X = Zygote X (male), 

 Egg X + Spermatozoon X = Zygote XX (female) ; 



the nuclei being otherwise of identical composition. 



In the type represented by Lygceus or Euschistus, where 

 a Y-element (" small idiochromosome ") accompanies the X 

 (" large idiochromosome ") the same conclusion is shown 

 even more clearly; for in many species the Y-element is the 



Zygote, o 



Spermutozoco 



Maturation 

 Dirision 



Sperm atogonium 



mum 



Fig. 3. — Diagram to show the relation of the two classes of spermatozoa to fertilisation. 



The formation of gametes in the male is shown at the left, in the female at the right ; fertilisation, 

 producing the male or female zygote, in the middle. 



smallest of all the chromosomes and is therefore readily 

 recognisable. In all such cases — as was first shown by Miss 

 Stevens in the Coleoptera and the writer in the Hemiptera — 

 this chromosome is confined to the male, while in the female 

 its place is taken by a second X-chromosome (" large idiochromo- 

 some "). The female diploid groups contain accordingly XX, 

 the male XY (being otherwise identical) ; and upon reduction 

 each mature egg contains one X, while half the spermatozoa 

 contain X and half Y. Obviously, therefore, the same fertilisa- 

 tion-formulas apply as in the former case, if Y be inserted in 

 its proper place, i.e., 



Egg X + Spermatozoon Y = XY, Male, 

 Egg X + Spermatozoon X = XX, Female. 



