SEX DETERMINATION AND SEX-LINKED HEREDITY 



397 



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

 quadruplet foetuses deriv^ed from the single egg and all destined to be of the same 

 sex. {Front 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 



