232 BIOLOGY: GENERAL AND MEDICAL 



of parthenogenesis showed that unfertilized 

 eggs always develop into organisms of one sex, 

 while fertilized eggs might develop into those of 

 either sex. 



Further significance arose from the discovery 

 by Henking that certain insects like Pyrorchis 

 produce two kinds of spermatozoa in equal 

 numbers. Fertilization by spermatozoa of one 

 kind gave rise to organisms of one sex, fertiliza- 

 tion by spermatozoa of the other kind to the 

 opposite sex. These facts were confirmed by 

 Paulinier, and are now known to be true of more 

 than one hundred different kinds of insects. 



G. That the sex of the offspring is an accident of 

 fertilization. E. B. Wilson, in 1909, discovered 

 an essential difference between the spermatozoa 

 of certain insects to be the presence or absence 

 of an odd chromosome to which he gave the 

 name ^-element. 



This ^-element has now been located in the sperma- 

 tozoa of more than a hundred different kinds of 

 insects, arachnids, myriapods, etc. In all of 

 these cases the spermatozoa are formed in pairs, 

 the sperm mother-cell that gives rise to each 

 pair manifesting the ordinary mode of nuclear 

 division with paired chromosomes, one member 

 of each pair passing into each spermatozoon. 



In addition to these, however, the sperm 

 mother-cell contains an unpaired element, some- 

 times consisting of a large chromosome, some- 

 tunes of a group of peculiar chromosomes, which 

 pass into one or the other of the spermatozoa. 

 Such elements Wilson calls the z-elements or 

 heterochromosom.es. 



According to Wilson's researches, now well 

 confirmed by many others, it is this z-element 

 that determines the sex of the offspring. Eggs 

 fertilized by spermatozoa containing the X* 



