Since the ovules are protected by carpels and integuments, it takes something special for fertilisation to happen. Angiosperms have pollen grains comprising just three cells. One cell is responsible for drilling down through the integuments, and creating a passage for the two sperm cells to flow down. The megagametophyte has just seven cells; of these, one fuses with a sperm cell, forming the nucleus of the egg itself, and another other joins with the other sperm, and dedicates itself to forming a nutrient-rich endosperm. The other cells take auxiliary roles. This process of ""double fertilisation"" is unique and common to all angiosperms.