ONTOGENESIS 245 



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 x-ele- 

 ments or heterochromosomes. 



According to the researches of McClung and 

 Wilson, now well confirmed by many others, it 



Spermatocyte Division Zygotes 



FIG. 99. Accessory or z-chromosome in Anosa. E. B. Wilson Recent Re- 

 searches on the Determination and Heredity of Sex-Science, Jan. 8, 1909. 



Sex determined by the x-element. Equally paired chromosome = ?. Un- 

 equally = cf. i-chromosome black. 



is this z-element that determines the sex of the 

 offspring. Eggs fertilized by spermatozoa con- 

 taining the z-element or heterochromosome, be- 

 come females; those fertilized by spermatozoa 

 not containing these elements, males. 



In certain cases bees the spermatozoa not 

 containing the ^-elements all die and degenerate, 

 hence in these insects all fertilized eggs must be 

 females, while eggs not fertilized become males. 



The trend of modern cytologists is in favor 

 of the view that the x-element is the sex deter- 



