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. This hypothesis is based upon the 

 discovery in 1908, by Clarence E. McClung, 

 that the spermatocytes of certain insects con- 

 tain an accessory or unpaired chromosome, 

 whose passage into the ovum during the process 

 of fertilization determines the sex of the future 

 product of the fertilization. E. B. Wilson quickly 

 realized the importance of this odd chromosome, 

 which has now been located in the spermatozoa 

 of more than a hundred different kinds of in- 

 sects, 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 sper- 

 matozoon. In addition to these, however, the 

 sperm mother-cell contains an unpaired element, 

 sometimes consisting of a large chromosome, 

 sometimes of a group of peculiar chromosomes, 

 which pass into one or the other of the sperma- 

 tozoa. Such elements Wilson calls the #-ele- 

 ments or heterochromosomes. 



According to the researches of McClung and 

 Wilson, now well confirmed by many others, it 



