middle acrosomal attachment plasma p ece enzymes of filament membrane to receptor head sperm sperm and nucleus egg plasma ntry of jelly coat vitelline envelope plasma membrane fusion of nd egg nucle Figure 51.1 Fertilization of a sea star egg. A head of a sperm has a membrane-bounded acrosome filled with enzymes. When released, these enzymes digest away the jelly coat around the egg, and the acrosome extrudes a filament that attaches to a receptor on the vitelline envelope. Now the sperm nucleus enters and fuses with the egg nucleus, and the resulting zygote begins to divide. The vitelline envelope becomes the fertilization envelope, which prohibits any more sperm from entering the egg