ONTOGENESIS 



233 



is this z-element that determines tjie 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 z-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 z-element is the sex deter- 

 minant, or at least is the sex index. The differ- 

 ence between the male and female organism is 



Spermatocyte Division 



Zygotet 



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 = 9- Un- 

 equally = c? . x-chromosome black. 



that the male comes from an egg which develop- 

 ing either parthenogenetically or after fertiliza- 

 tion contains only a single z-element, while the 

 female starts from one which developing either 

 parthenogenetically or after fertilization contains 

 two ^-elements. 



The ovum of a sexual egg in the process of 

 maturation discards half of its normal comple- 

 ment of the z-element. If it be subsequently 

 fertilized by a spermatozoon containing an x- 

 element, these elements being paired, the result- 

 ing organism is a female; if by a spermatozoon 



