When the sperm reaches the outer layer of the egg, its outer membrane binds tightly to the surface receptors on the egg. When this binding occurs, the sperm sheds its outer covering (acrosome). At the same time, the membrane of the egg secretes a substance called "fertilizin," which is required to attract the sperm. This molecule makes the sperm able to move more quickly, allowing them to react with the egg membrane more easily. In addition, fertilizin facilitates the reaction of the acrosome found in the head of the sperm.
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When the sperm touches the egg membrane, new substances come into play and new reactions take place. When the sperm touches the egg, it secretes a substance called "anti-fertilizin" which neutralizes the effect of the fertilizin secreted by the egg. In this way, the first sperm to reach the egg will stop other sperm from approaching the egg.
The membrane which surrounds the egg cell begins to renew itself about two seconds after the sperm cell enters and never allows another sperm cell to enter. Experiments have been done in which a few sperm have been observed entering the egg when the membrane has been destroyed. For this reason it is necessary that the fertilisation membrane be formed as quickly as possible. After the formation of the fertilisation membrane, no sperm can enter the egg. In this condition, it is possible to compare the egg cell to a building protected by security. The outer membrane of the egg cell really acts like the security control system of a building which contains very important information; access is denied to the inside of the cell.
Once a sperm enters the ovum, its head swells and it wanders very slowly toward the centre of the egg. Later, within 30 minutes, the egg completely unites with the sperm inside it. As a result of all these processes, the genetic information contained in the sperm is transferred to the egg. 19
But here there is an important point: if the receptors on the sperm and the egg accept one another, they bind to each other; if not, binding is not possible. The reason for this is as follows: the egg of every living being secretes a substance called fertilizin, which has a particular chemical composition. This is a precaution which prevents sperm cells of other species (non-human species) from approaching the egg and causing the degeneration of the human species. Thus, a cat cannot mate with a horse and a human being cannot mate with any other living thing.20
The electrical charge carried by the sperm and the egg also has an effect on fertilisation. The egg always carries a negative charge and the sperm carries a positive one. Because opposite charges attract each other, the egg draws all the sperm towards itself. But with the first sperm that is able to enter the egg, the charge changes immediately. Now the egg assumes a positive charge like the sperm's. Because like charges repel each other, at the moment of union the egg begins to repel all other sperm.