http://www.dac.neu.edu/womens.studies/egg&sperm.htm 중에서... In this recent investigation, the researchers began to ask questions about the mechanical force of the sperm's tail. (The lab's goal was to develop a contraceptive that worked topically on sperm.) They discovered, to their great surprise, that the forward thrust of sperm is extremely weak, which contradicts the assumption that sperm are forceful penetrators. Rather than thrusting forward, the sperm's head was now seen to move mostly back and forth. The sideways motion of the sperm's tail makes the head move sideways with a force that is ten times stronger than its forward movement. So even if the overall force of the sperm were strong enough to Mechanically break the zona, most of its force would be directed side ways rather than forward. In fact, its strongest tendency, by tenfold, is to escape b.y attempting to pry itself off the egg. Sperm, then, must be exceptionally efficient at escaping from any cell surface they contact. And the surface of the egg must be designed to trap the sperm and prevent their escape. Otherwise, few if any sperm would reach the egg. The researchers at Johns Hopkins concluded that the sperm and the egg stick together because of adhesive molecules on the surfaces of each. The e gg traps the sperm and adheres to it so tightly that the sperm's head is forced to e flat against the surface of the zona, a little bit, they told me, "Like Br'er Rabbit getting more and more stuck to tar baby the more he wriggles." The trapped sperm continues to wiggle ineffectually side to side. The mechanical force of its tail is so weak that a sperm cannot break even one chemical bond. This is where the digestive enzymes released by the sperm come in. If they start to soften the zona just at the tip of the sperm and the sides remain stuck, then the weak, failing sperm can get oriented in the right direction and make it through the zona provided that its bonds to the zona dissolve as it moves in. ------------------- Unattractiveness or counterintuativeness of such methods become advantages, because they force one to accept new and better ways of thinking about the subject. -- From W. Siegel, "Fields".