THE MECHANISM OF SEX-DETERMINATION 53 



zoa. The large cell divides again, all of the chromo- 

 somes dividing. Two functional spermatozoa are 

 produced, each carrying one sex chromosome. These 

 spermatozoa correspond to the female-producing sper- 

 matozoa of other insects. 



In the sexual female there is an even number of chro- 



«• 



-Mi, 



. /--Jj^''! 



Fig. 29a. — 20 and 21, oogonia (equatorial plate) ; 22, growth period ; 

 23, before fertilization; 24-25, entrance of sperm; 26-31, prophases of 

 first division ; 32-33, formation of first polar body ; 34-36, extrusion of 

 same and formation of second polar body; 37, two pronuclei ; 38-41, union 

 of pronuclei ; 42-45, cleavage. (After Mulsow.) 



mosomes — one more than in the male. They unite 

 in pairs. When the two polar bodies of the sexual 

 egg are formed, all the chromosomes divide twice, so 

 that each egg is left with one sex chromosome. 



It is now evident why only females are produced 

 after fertilization. The female-producing sperm alone 

 is functional. 



