Illustration showing human sperm (blue) swimming towards an egg or ovum (yellow) in the oviduct, prior to fertilisation. In fertilisation, only a single sperm may successfully penetrate the ovum to fuse with the female nucleus. Barriers to be overcome include layers of follicular cells surrounding the ovum (corona radiata) and an underlying glycoprotein membrane, the zona pellucida. The membrane is digested by enzymes released from the acrosome, a cap on the head of the sperm: subsequent rapid chemical changes in the zona pellucida prevent competing sperm from entering.