Fertilisation. Illustration of a sperm cell that has penetrated a human egg. Women usually release one egg (ovum) per month, whereas men release millions of sperm in each ejaculation. Only one of these sperm can penetrate the eggs thick outer layer (zona pellucida) and fertilise it. Fertilisation occurs when the sperms genetic material, DNA (deoxyribonucleic acid), fuses with the eggs DNA. When this occurs the egg forms a barrier to other sperm.