200 PHYSICAL BASIS OF HEEBDITY 



lengtli of the oviduct to reach, the egg as it enters the 

 tube, the greater size or weight of the female-producing 

 sperm may give a slight advantage to the male-produc- 

 ing sperm in the long trip up the tube. This would lead 

 to an excess of males. There are still other possibilities, 

 which if realized, would suffice to slisrhtly change the equal- 

 ity of the output of the machine. 



Non-disjunction 



Females of Drosophila are occasionally found that 

 give exceptional breeding results which have been 

 explained by Bridges on the view that these females are 



FEMILE MtlE 



11*. -.^ ' * 





FiQ. 92. — Normal female and male groups of chromoeomes of the vinegar fly, with the 

 XXY female group below. 



XXY individuals (Fig. 92). It has been shown by cyto- 

 logical examination that such females do actually contain 

 an additional Z-ehromosome. The four possible ways in 

 which these three chromosomes might be expected to 

 behave at the reduction division when the polar bodies 



