In week 6 of the Embryonic Stage, the brain starts to produce patterns of electrical activity. It moves in response to stimuli, and the gonads, or sex glands (ovaries in females and testes in males), develop. At first, the gonads of male and female embryos are identical. However, between the 4th and 6th weeks, genes on the Y chromosome cause the male embryo to produce the male hormone testosterone. The testosterone causes the gonads to become testes. In the absence of testosterone, the gonads develop into ovaries.
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