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biology 0610printed 216 · PDF 216

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printed 216 · PDF 216

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216 ● Some unwanted substances can also cross the placenta. These include toxins such as alcohol, carbon monoxide and pathogens. Amniotic Sac ● The fetus is surrounded by a strong membrane called the amniotic sac. ● This makes a fluid called the amniotic fluid. ● Amniotic fluid helps to support the embryo and protect it from mechanical injury. The embryo floats in the fluid and is able to move its arms and legs which helps the muscles and bones to develop correctly. ● After nine months, when the fetus is ready to be born, contractions of muscles in the uterus wall often cause the amniotic sac to break, releasing the amniotic sac so that it flows into the vagina. This is often the first sign that the a woman is about to give birth. Testosterone and Oestrogen ● The main reproductive hormone produced in men is testosterone. While in women oestrogen and progesterone are secreted. ● The time when a person approaches sexual maturity is called adolescence. ● The point at which sexual maturity is reached is called puberty. ● In men during puberty, the hormones cause growth of facial hair and pubic hair, broadening of shoulders, general muscular development and a deepening voice. ● In women during puberty, the hormones cause the breasts to grow larger, pubic hair to grow and hips to become wider. The Menstrual Cycle ● Eggs develop in small structures called follicles. ● After a girl reaches puberty, one egg is usually released from a follicle into the oviduct every month. Before the egg is released the lining of the uterus becomes thick and spongy so that it is ready to receive a zygote. It is full of tiny blood vessels which will supply the embryo with nutrients and oxygen if it should arrive.