2.  Your life began as a single cell.  That single cell was produced by the joining of two other cells  Egg: female sex cell with 23 chromosomes  Sperm: male sex cell with 23 chromosomes  The joining of an egg and a sperm is called fertilization.

3.  During fertilization, the 23 chromosomes in a sperm join the 23 chromosomes in an egg.  The result is a zygote with 46 chromosomes.  The zygote contains all the information needed to produce a new human being.

4. Male Reproductive System  The male reproductive system is specialized to produce sperm and the hormone testosterone.  The structure of the male reproductive system include the testes, scrotum and penis.

5. The Testes  This is where sperm is produced.  They are located in an external pouch of skin called the scrotum. This placement is key since sperm needs to be a couple of degrees cooler than regular body temperature to develop normally.  The testes also produce testosterone. Testosterone is a hormone that controls the development of physical characteristics in mature men (facial hair, deepening voice, broadening of shoulders and ability to produce sperm).

6. The Path of Sperm Cells Sperm cells are produced in the testes. Once sperm cells form, they move to the epididymis. From there, sperm travel through other structures in the male reproductive system. As they move, they mix with fluids forming semen. These fluids provide an environment in which sperm cells are able to swim, as well as the nutrients they use as a source of energy. Semen leaves the body through the urethra located in the penis.

7. Female Reproductive System  The role of the female reproductive system is to produce eggs and, if an egg is fertilized, to nourish a developing baby until birth.  The organs of the female reproductive system include the ovaries, fallopian tubes, uterus and vagina.

8. The Ovaries  The ovaries are the female reproductive structures that produce and store eggs.  Like the testes in males, the ovaries produce hormones:  Estrogen: triggers the development of some adult female characteristics (hips widen and breast development)  Progesterone: maintains the uterus during pregnancy.

9. Path of the egg cell The ovary releases a mature egg, called oocyte. The egg enters the nearest fallopian tube. Fertilization usually occurs within a fallopian tube. The egg enters the uterus. If it was fertilized it attaches to the wall of the uterus and pregnancy begins If the egg was not fertilized it starts to break down, passes through the cervix and exits the body through the vagina.

10. The Menstrual Cycle  When the female reproductive system becomes mature, there are about 400,000 undeveloped eggs in the ovaries. However, only about 500 of those eggs will actually leave the ovaries and reach the uterus.  An egg is released about once a month.  The monthly cycle changes that occur in the female reproductive system is called menstrual cycle.

11.  During the menstrual cycle, an egg matures in an ovary.  At the same time, the lining of the uterus becomes thicker.

12. Days 1-4: Menstrual discharge Days 5-13: Egg develops in follicle Days 14-15: Ovulation occurs. Days 16-22: Egg moves through the fallopian tube. Uterus lining becomes thicker. Days 23-28: Unfertilized egg enters the uterus