454 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
sexes other than the primary difference of egg or sperm production are 
called secondary sexual characters. Usually very young animals show 
only slight differences in secondary sexual characters and the differ- 
ences increase markedly at sexual maturity. We speak of the gradual 
divergent development of the two sex types as sex differentiation. 
The question arises as to whether or not the chromosomal differences 
are the causes of the differentiation of secondary sexual characters. 
These secondary sexual characters are all somatic, and, since the soma 
is the product of cell division of the zygote, the soma cells must have 
either the male or the female chromosomal character. That the 
chromosomal mechanism in the somatic cells is not sufficient of itself 
to bring about, unaided, the differentiation of secondary sexual charac- 
ters can be shown readily in at least many animals. 
In the mammals, for example, it is known that the early removal 
of the testes or ovaries results in a retention of the juvenile or undif- 
ferentiated condition of secondary sexual characters. Evidently some 
influence is exerted by the tissues of the gonad that is necessary for the 
full differentiation of sex characters. The current theory is that 
certain glandular cells that form part of the body of ovary or testes 
excrete materials into the blood that stimulate various tissues in 
different ways and produce dimorphic results. The specific sub- 
stances produced by these glands are called “hormones,” for want 
of a better name. To test the efficiency of these hormones the crucial 
experiment of taking out the gonads of a young rat or guinea pig and 
implanting the gonad of an individual of the opposite sex has been 
many times performed. For example, Steinach castrated young male 
rats and then successfully grafted into them ovaries from young 
female rats. The result was that these young rats which started to 
be males became much altered in a female direction, the mammary 
glands becoming greatly enlarged, their instincts more feminine than 
masculine, and in a number of other particulars they showed more 
or less pronounced evidences of feminization. Conversely, spayed 
females with engrafted testes showed a tendency toward male differ- 
entiation, especially in instincts. These experiments have been 
largely confirmed by C. R. Moore. 
Similar experiments with similar results have been performed 
with fowls and ducks. All indicate that the glandular part of the 
~ gonads has a determinative effect on sex differentiation, and this in 
spite of the chromosome make-up of the somatic cells; for the male 
differentiation in cells with the male chromosome characters can be 
