Fertilization

              The first requires the process og sexual intercourse.When
a man becomes sexually excited,the penis becomes erect and stiff.This is due to an increase in blood pressure in the erectile tissues surrounding the urethra,because the arteries supplying them dilate as the veins constrict.The penis is inserted into the woman,and this cause ejaculation.
              Ejaculation is brought about by contraction of the muscles
surrounding the urethra.This expels semen through the urethra into the vagina.If fertilization occurs ,the corpus luteum secretes the hormone progesterone which prepares the wall of the uterus to receive the zygote.The uterus wall thickens.The blood sopply to the wall increases.The zygote divides into a ball of cells on its way down the oviduct and sinks into the wall of the uterus.This process is called implantation.
 

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