106 SEX INHERITANCE 
aphids, one stem-mother may produce both lines; 
z.e., some of her offspring may ultimately give rise 
to sexual females and others to males. 
THE SEx oF INDIVIDUALS PRODUCED BY 
ARTIFICIAL PARTHENOGENESIS 
In only two species have individuals that have 
been produced by artificial fertilization been raised 
to a stage when their sex can be determined. Delage 
reared two such sea urchins, that were probably 
both males. Loeb has reared a number of frogs 
that were mostly males, but a few were females. 
Both sexes in Loeb’s case appeared to have the 
diploid number of chromosomes; but until more is 
known of the chromosome situation in these cases 
it is not possible to offer any probable explanation 
of them. The work of Schmitt-Marcel and others 
also throws some doubt on the possibility of accu- 
rately determining the sex of frogs until long after 
metamorphosis. 
Sex AND SECONDARY SEXUAL CHARACTERS 
Males differ from females not only in the gonads 
and in the accessory organs of reproduction (ducts, 
glands, copulatory organs) but often show more 
superficial differences that are called secondary 
sexual characters. Genes concerned in the differ- 
entiation of these have been shown, in two races 
at least, not to lie in the sex-chromosomes. These 
