Note the surface proteins on the plasma membrane and the vesicle membrane. When the vesicle fuses with the plasma membrane the proteins on the plasma membrane are displaced and proteins inside the vesicle are now exposed to the outside of the cell. The contents of the vesicle are expelled into the environment. In the case of the special "cortical granules" of the sea urchin egg, this expelling of contents is what raises the fertilization membrane, preventing further sperm entry.