Fertilisation. Illustration of sperm attempting to penetrate a human egg (round). Each sperm (spermatozoon) has a rounded head and a long tail with which it swims. 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 egg's thick outer layer (zona pellucida) and fertilise it. Fertilisation occurs when the sperm's genetic material, DNA (deoxyribonucleic acid), fuses with the egg's DNA. When this occurs the egg forms a barrier to other sperm.