INHERITANCE OF SEX 263 



sex and the other half do not, is based on the existence 

 in half of the germ- cells of things which can actually 

 be seen. 



It has been discovered that in certain insects 

 half the spermatozoa contain a supernumerary, or, 

 as it is called, accessory, chromosome. This chromo- 

 some is not merely an additional one, but it behaves 

 differently from the other chromosomes in the division 

 of the cells. In the insect Anasa, for instance, a 

 form allied to the Bug, the number of chromosomes 

 in one half of the spermatozoa is nine ; in the other half 

 ten ; the number in all the eggs is ten. The result 

 of the union between a spermatozoon with nine 

 chromosomes and an egg (with ten) is a male ; and 

 of the union between a spermatozoon with ten and 

 an e.gg (with ten) is a female. This is known to be the 

 case because the number of chromosomes in the 

 somatic cells of males is nineteen ; whilst that in 

 the somatic cells of females is twenty. 



If a case such as this is described in MendeHan 

 terms, the male, half of whose germ-cells contain an 

 element which the other half lack, must be regarded 

 as heterozygous (DR), whilst the female, all of whose 

 germ-cells are alike in respect of this element, must 

 be regarded as homozygous, but inasmuch as all her 

 germ -cells are alike in the possession of an element 

 she must be regarded as a dominant homozygote, 

 namely DD, not RR. 



It will be noted that the general conclusion as 

 to the nature of the difference between the sexes 

 based on the behaviour of the accessory chromosome is 



