Sperm cell fertilising an ovum, illustration. Each sperm (spermatozoan, four shown) has a rounded head and a long tail which it uses to both swim and burrow into the egg. Women usually release one egg (ovum) per month, whereas men release millions of sperm in each ejaculation. Only one of these sperm will penetrate the egg's thick outer layer (zona pellucida) and fertilise it. Fertilisation occurs when the genetic material (deoxyribonucleic acid, DNA) in the sperm's head fuses with the egg's DNA forming a zygote. The egg then forms a barrier to other sperm.