SEX DETERMINATION 451 
the male gametes have one X chromosome and the other half have 
no X, but sometimes Y and sometimes simply one chromosome less 
than the first type of male gametes. We can then speak of the two 
Fic. 94.—An armadillo egg about six weeks after fertilization, showing the 
quadruplet foetuses are derived from the single egg and all destined to be of the 
same sex. (From Newman.) 
sexes produced by union of male and female gametes simply in terms 
of the X chromosomes, females being characterized by XX (duplex) 
and males by X (simplex). 
SEX DETERMINATION IN PARTHENOGENETIC SPECIES 
Although it was at first thought that the facts of parthenogenesis 
(development of eggs without fertilization) was opposed to the 
chromosomal mechanism of sex determination, further study of this 
