SEX DETERMINANTS 195 



Since this suggestion was made, the theory has been 

 elaborated in detail by Professor E. B. Wilson, and a great 

 deal more evidence has been produced. It has been shown 

 that in several animals the body cells of the female possess 

 at least one more chromosome than those of the male 

 individual. It is claimed that in nearly a hundred species 

 of insects, two classes of sperms have been demonstrated, 

 equal in number, that differ in respect to one or more of 

 the chromosomes that enter into the formation of their 

 nuclei. It is supposed that eggs fertilised by one class of 

 sperm produce males, those fertilised by the other class 

 females. 1 In the simplest cases the course of events is as 

 follows. The cells of the female organism contain two 

 accessory chromosomes, and so all the ova before matura- 

 tion, that is, before reduction takes place, contain them both. 

 When reduction has taken place, all the ova will contain 

 one accessory chromosome. On the other hand, the cells 

 in the male organism only contain one accessory chromo- 

 some, including those cells destined to produce sperms. 

 When reduction \takes place, half the resulting cells will 

 contain the accessory chromosome, half will not. Only half 

 the sperms, therefore, will contain the accessory chromo- 

 some. If a sperm containing the accessory chromosome 

 fuses with an ovum, the result will be a cell with two 

 accessory chromosomes, and a female will be produced. 

 If a sperm without an accessory chromosome fertilises an 

 ovum, a male will be produced. 



The difference between the sperms are by no means 

 always as simple as this, but the general idea of the theory 

 is adequately conveyed, and it would be out of place to 

 enter into the details in a general work of this nature. 



Observations upon bees and certain other animals are 



1 Wilson, E. B., "Studies on Chromosomes," I., Journal of Expcr. ZooL, 

 vol. ii., No. 2, 1905 ; " Studies on Chromosomes," II., vol. ii., No. 4 ; " Studies 

 on Chromosomes," III., vol. iii., No. 4, 1906 ; " Studies on Chromosomes," IV., 

 vol. vi., No. 1, 1909; "Studies on Chromosomes," V., vol. vi., No. 2, 1909; 

 " Recent Researches on the Determination and Heredity of Sex," Science, N.S., 

 vol. xxix., No. 732, January 1909. 



