The following are the events that take place from the time of entry of the human sperm into the ovum till fertilization is complete. :

1. The sperm penetrates the ovum and enters into it by dissolving the outer membrane of the ovum. The sperm acrosome produces enzymes which are known as sperm lysins. These dissolve the egg membranes in the local area and clear the path for spermatozoan to reach the surface of egg.

2. This follows dissolution of the plasma membrane of the ovum and sperm head at the point of contact and the nucleus and cytoplasm of the sperm are drawn inside the ovum.

3. The entry of sperm invokes a chemical signal in the egg cell. The signal is transmitted to the egg surface incapacitating hundreds of sperms in the vicinity to enter the egg cell. The rest of the follicle cells around the fertilized ovum disperse before the implantation of the fertilized ovum on the inner wall of uterus. Thus with the penetration of sperm into the egg cell, a fertilization membrane is formed around the egg. This prevents the entry of other sperms in the egg cell.

4. The penetration of sperm activates the egg and initiates a series of changes in the egg cortex and in egg cytoplasm. The secondary oocyte undergoes meiotic II division or the second maturation division producing the ovum and second polar body. The second polar body immediately degenerates. Inside the ovum, the sperm nucleus moves towards egg nucleus along a definite path called copulation path. The fusion of male and female pronuclei results in the formation of zygote i.e. called fertilization.