Illustration of a human sperm fertilising an egg. The picture shows the size difference between the egg, or ovum, & the spermatozoon. Human spermatozoa are extremely elongated single cells about 65 micrometres long, divided into 3 main regions: a head, neck, & tail. The head, which contains the male nucleus, is about 7 micrometres long. Here, the head appears buried in the follicular cells, which form the corona radia that surrounds the ovum. Beneath the corona radiata is a glycoprotein membrane, the zona pellucida, which the sperm must penetrate to reach the female nucleus. This version on black background. Version on white background is P648 010.