A membranous sac at or near the front of a sperm that assists in penetration of the egg. The acrosome contains enzymes, which are released when the sperm contacts the egg prior to fertilization. The enzymes break down the outer layers of the egg to permit entry of the sperm. In some invertebrate sperms the acrosome contains actin filaments, which elongate to help gain entrance to the egg. In sea urchins molecules of the protein bindin on the surface of the acrosome attach to receptors on the vitelline envelope surrounding the egg. Binding occurs only between bindin proteins and receptors of the same species, thereby preventing attachment of sperm from a different species.