CHAPTER XI 



The Cytological Basis of Sex-Determination 



IN the chapter (Chapter IX) on natural parthenogenesis 

 reference was made to several examples of eggs, the 

 development from which of males or females is dependent 

 either on the nature of the maturation process, or on whether 

 the egg is or is not fertilised. Such cases naturally lead to 

 the suspicion that the determination of sex, that is, the 

 production of a male or a female, is dependent on the cyto- 

 logical condition of the egg before development begins, and 

 a great amount of evidence pointing toward the same 

 conclusion has been accumulated in recent years. This is 

 not the place to discuss the question whether sex is in all 

 animals irrevocably determined from the beginning of 

 development 1 ; all that can be done is to describe the 

 evidence which shows that it is so in many cases. 



Perhaps the clearest instances of determination of sex 

 by a cytological difference in the egg before development 

 begins are provided by the Hymenoptera. In every example 

 of this order of insects which has been examined, the female 

 is derived from an egg containing the diploid number of 

 chromosomes and the male from one with only the haploid 

 number. Most frequently the difference depends on whether 

 the egg is fertilised or not, as was first discovered in the 

 Honey Bee, and subsequently shown to be true of a number 

 of other species. In all these cases fertilised eggs yield fe- 

 males and unfertilised eggs males, and it is remarkable that 



1 For a discussion of this problem reference may be made to the writer's 

 book The Determination of Sex (Camb. Univ. Press, 1915). 



