456 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
inhibited and female differentiation superimposed if the female gonad 
is introduced in the absence of the male gonad. 
A beautiful experiment conducted by nature herself helps to drive 
home the hormone theory of sex differentiation. In cattle, as shown 
recently by F. R. Lillie, twins occur in a small percentage of cases and 
involve the simultaneous fertilization of two eggs. These eggs lie as 
a rule in opposite horns of the forked uterus, but owing to the growth 
of their embryonic membranes the two individuals come to fuse cir- 
culations so that there is an admixture of blood (Fig. 95). The result 
is that if the twins are zygotically of the same sex, no untoward effect 
of blood admixture is apparent, but when the twins are zygotically a 
male and a female, the female individual is always stopped in its 
female differentiation and becomes more or less completely trans- 
formed in a male direction. It appears, however, that at the time 
when blood admixture occurs, the female individual has already 
differentiated so far with respect to the external genitalia and in other 
respects that, even though subsequent development be entirely male 
in character, the resultant individual is always a sterile creature, 
neither fully a female nor a complete male. Such individuals have 
long been known as “‘freemartins.” Asa rare exception to the general 
rule an occasional case has appeared in which a male and a female pair 
fail to undergo blood admixture. In such cases both develop into 
normal animals. It now appears that the reason why the female sex 
is the one to suffer is that the male gonads differentiate precociously, 
before the female, and inhibit the subsequent development of female 
gonads. Hence the only hormones in the blood of both twins are 
the male hormones. 
In conclusion we may say then that though chromosomes tend 
to determine the primary sex differences, they have no effect on the 
differentiation of secondary sexual characters. These are due to 
substances secreted by the gonads that has been called a hormones. 
