Part of the sperm's cell membrane then fuses with the egg cell's membrane, and the contents of the head sink into the egg. In the mouse it has been demonstrated that ZP3, one of the proteins that make up the zona pellucida, binds to a partner molecule (to the β1,4-galactosyl transferase receptors) on the sperm. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of different species from fusing.