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The fact is that the individual, though its genotypic sex is fixed at fertilisation, can be profoundly affected by the environment in which it develops. In the ants, bees, and termites the larval nutrition determines whether the genotypic female individual will become a fully developed female (‘queen’) or a sexually retarded worker. In these cases the whole organism is affected; but the gonads do not play a part in establishing the sexual differences of the body, or soma. In the vertebrates, however, the hormones secreted by the gonads are the essential regulators. Numerous experiments show that by varying the hormonal (endocrine) situation, sex can be profoundly affected. Grafting and castration experiments on adult animals and man have contributed to the modern theory of sexuality, according to which the soma is in a way identical in male and female vertebrates. It may be regarded as a kind of neutral element upon which the influence of the gonad imposes the sexual characteristics. Some of the hormones secreted by the gonad act as stimulators, others as inhibitors. Even the genital tract itself is somatic, and embryological investigations show that it develops in the male or female direction from an indifferent and in some respects hermaphroditic condition under the hormonal influence. Intersexuality may result when the hormones are abnormal and hence neither one of the two sexual potentialities is exclusively realised.
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