THE MECHANISM OF SEX-DETERMINATION 49 



a difference in size of X and Y. The sex chromosomes 

 must carry a host of factors other than those that de- 

 termine sex. Consequently it is not surprising that in 

 many species the sex chromosomes appear equal or 

 nearly equal in size. It is a fortunate circumstance for 

 us that in some species there is a difference in size or 



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an unpaired sex chromosome ; for, in consequence, we 

 are able to trace the history of each kind of sperm in 

 these cases ; but it is not essential to the theory that 

 X and Y, when present, should be visibly different. 



In the female of oncopeltus sixteen chromosomes 

 occur as in the male (Fig. 26, A). The reduced number 

 is eight double chromosomes (B). At one of the two 

 polar divisions eight chromosomes pass out, and eight 

 remain in the egg (C). At the second division also 

 eight pass out, and eight remain in the egg (D). 



