D. PHILLIPS / SCIENCE PHOTO LIBRARY D. PHILLIPS / SCIENCE PHOTO LIBRARY
Fertilisation. Coloured scanning electron micrograph (SEM) of a group of human sperm (spermatozoa) trying to penetrate an egg (ovum). Only one sperm can penetrate the egg's thick outer layer (zona pellucida) and fertilise it. Fertilisation occurs when DNA (deoxyribonucleic acid) released by the sperm fuses with DNA contained in the egg. Women produce a single egg at a time, whilst men release around 500 million sperm. The sperm use their flexible tails to travel through the female reproductive tract. Only a few thousand sperm reach the egg, which is located in one of the two fallopian tubes.
Model release not required. Property release not required.