CHAPTER XIII 



THE DETERMINATION OF SEX 



1. PREVALENT IDEAS 



THE mechanism of sex determination has been a 

 matter of speculation since time immemorial and many 

 erroneous as well as impossible ideas remain even to-day 

 in the mind of the layman. These speculations fall 

 into three categories, according to whether the belief 

 is held (1) that the sex of the offspring is predeter- 

 mined in the egg; (2) that it is determined at the time 

 of fertilization; or (3) that it is not determined until 

 after the zygote has been formed. 



All the older experiments on sex were based on the 

 last of these suppositions. It was believed that by 

 varying the nutrition of the developing embryo either 

 sex, as desired, could be obtained. This belief was ap- 

 plied even to human beings. Experiments on tadpoles 

 seemed to give definite positive results, but we now 

 know that the death rate in these experiments was so 

 large that the results may be more truly explained as 

 due to differential mortality. 



Others held that the age or vigor of the parent de- 

 termines the sex, the older or more vigorous of the two 

 parents tending to impress its sex upon the offspring. 



Yet another belief, and one still held by many, re- 

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