Human fertilisation, abstract illustration. Each sperm (spermatozoan) has a rounded head and a tail which it uses to both swim and burrow into the egg (centre), which is shown here in a post-fertilisation state (the zygote) as it starts to divide. 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 and fertilise it. Fertilisation occurs when the genetic material (DNA, deoxyribonucleic acid) in the sperm's head fuses with the egg's DNA to form a zygote. The egg then forms a barrier to other sperm.