Think of an immense army composed of millions of soldiers. They are advancing toward a common target, they encounter obstacles on the way and find themselves in mortal danger, but in spite of all these difficulties the army does not give up. The space these soldiers must traverse in order to reach their target is one hundred thousand times larger than they are themselves. Of course, such a crowded army setting out on such a difficult road will need guides and equipment to help them reach their goal. This huge army with 300 million soldiers is inside the male body. The soldiers are the sperm. Each one is about one hundredth of a millimetre in size. Their target is to reach the egg cell, and to do this, they must travel a very long road.
Reference lists:
2. Arthur C. Guyton, John E. Hall, Textbook of Medical Physiology, 10th ed., Harcourt International Ed., PA, 2000, p. 916
3. Arthur C. Guyton, John E. Hall, Textbook of Medical Physiology, 10th ed., Harcourt International Ed., PA, 2000, Arthur C. Guyton, John E. Hall, Human Physiology and Mechanisms of Disease, 6th edition, p. 649
4. Arthur C. Guyton, John E. Hall, Textbook of Medical Physiology, 10th ed., Harcourt International Ed., PA, 2000, p. 916
5. Arthur C. Guyton, John E. Hall, Textbook of Medical Physiology, 10th ed., Harcourt International Ed., PA, 2000, p. 1005 
6. Vander, Sherman, Luciano, Human Physiology, p. 654
7. Arthur C. Guyton, John E. Hall, Textbook of Medical Physiology, 10th ed., Harcourt International Ed., PA, 2000, p. 919
8. Dr. Gerald E. Poesnecker, One Flesh, A New Look at Human Sexuality, chapter 3. (http://www.oneflesh.org/OF-Chap%203.html)
9. Arthur C. Guyton, John E. Hall, Textbook of Medical Physiology, 10th ed., Harcourt International Ed., PA, 2000, p. 917
10. Arthur C. Guyton, John E. Hall, Human Physiology and Mechanisms of Disease, p. 12; Gerard J. Tortora, Introduction to the Human Body: Essentials of Anatomy & Physiology, p. 527.
11. Arthur C. Guyton, John E. Hall, Textbook of Medical Physiology, 10th ed., Harcourt International Ed., PA, 2000, p. 919
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