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EVOLUTION, GENETICS, AND EUGENICS 



low, more males than females are born. This is probably a case of 

 differential prenatal mortality. By that we mean that more females 

 die unborn than males, because the latter are hardier and stand pre- 

 natal malnutrition better. 



e) Sex is determined at the time of fertilization. — Perhaps the best 

 evidence that sex is determined at the very beginning of development 



Fig. 52. — An armadillo egg about six weeks after fertilization, showing the 

 quadruplet fetuses derived from the single egg and all destined to be of the same 

 sex. (From Newman.) 



is derived from one-egg twins and quadruplets. In the nine-banded 

 armadillo practically every female gives birth to quadruplets, four 

 essentially identical young being produced in each litter. All in a 

 given set of quadruplets are invariably of the same sex, either four 

 males or four females. Newman and Patterson have shown that each 

 set of quadruplets comes from a single egg which at a very early stage 



