During the early adolescent spurt of growth, sex differences in body shape become apparent. The pelvic region of a girl grows more than her shoulder girdle, becoming wider, roomier, and more shallow than the male pelvis. Fat is deposited over her hips, so that she begins to develop feminine contours. In boys, the reverse occurs. A boy’s shoulders become wider and heavier. These changes are due to the secretion of a pituitary hormone – the human growth hormone.
The secretion of the sex hormones into the blood circulation produces an even greater difference between the sexes than does the secretion of the growth hormone. In boys the main sex hormone secreted is testosterone, in girls it is oestrogen (but each sex secretes a quantity of the main sex hormone of the other sex). The hormones are secreted by the sex glands, or gonads. These are a boy’s testicles and a girl’s ovaries. They start producing hormones because they are stimulated by hormones produced in the pituitary gland, called gonadotrophins.
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