Courtesy of Prof.H.Kentenich.
Fertilization is an active process which requires an interaction between the sperm and the eggs.
In the first instance the sperm must reach the eggs. In conception via intercourse the sperm is deposited against the cervix and after liquefaction of the semen the sperm swim through the cervical canal to begin their journey towards the egg. Only a few of the most active sperm will complete the journey up the uterus and fallopian tubes towards the egg, and usually only one will penetrate the zona pellucida to fertilize the egg. As a result of the action of the sperm, and during fertilization, these cells fall away from the egg. In IVF they are often dissected away by the embryologist.