CHAPTER IV 



THE SEX CHROMOSOMES 



(i) The Sex Chromosomes in Insects 



In 1891 Henking, working on the spermatogenesis of the Hemipteran 

 insect Pyrrochoris, discovered that, contrary to the general rule in other 

 groups of the animal and vegetable kingdoms, the diploid number of 

 chromosomes as determined in the spermatogonia is an uneven one, 

 namely, twenty-three. 



Consequently, instead of all the spermatozoa having eleven chromo- 

 somes as if the diploid number were twenty-two, or all having twelve 

 as happens when it is twenty-four, half the spermatozoa have eleven 

 chromosomes and half have twelve. Taking a large stride in the 

 historical development of the case, we now know that the female Pyrro- 

 choris has twenty-four as its diploid number, and hence all its eggs 

 have twelve (Wilson, 1909 a). 



Now it is obvious that if an e^g is fertilized by one of the 

 spermatozoa containing eleven chromosomes, the resulting zygote will 

 have twenty-three, which as we have seen is the diploid number of the 

 male. If on the other hand an ^gg is fertilized by a spermatozoon with 

 twelve chromosomes, the resulting zygote will have twenty-four, the 

 number of the female. Thus it appears that in these insects the sex of 

 the individual is determined by the nature of the spermatozoon which 

 fertilizes the o^gg — the eggs being all alike, or indifferent, and the sperma- 

 tozoa of two kinds in equal numbers, namely, male-producers with 

 eleven chromosomes, and female-producers with twelve chromosomes. 



As the odd chromosome of the male was first supposed to be an 

 additional one, it was for a time known as the " accessory chromosome " 

 (M'Clung). It is now known, however, that the female has one more 

 chromosome (in these forms) than the male, so that the unevenness of 

 the number in the male is not due to the addition, but to the subtraction 

 of a chromosome. Hence the term accessory is obviously unsuitable. 

 Other terms have been proposed, such as heterochromosomes (Montgomery) 

 and heterotropic chromosomes (Wilson). The now generally accepted 



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