Summary of events leading to the fusion of egg and sperm plasma membranes in the sea urchin (A) and the mouse (B). (A) Sea urchin fertilization is external. (1) The sperm is activated by and chemotactically attracted to the egg. (2, 3) The egg jelly causes the acrosomal reaction to occur, allowing the acrosomal process to form and release proteolytic enzymes. (4) The sperm adheres to the vitelline envelope and lyses a hole in it. (5) The sperm adheres to the egg plasma membrane and fuses with it. The sperm pronucleus can now enter the egg cytoplasm. (B) Mammalian fertilization is internal. (1) The contents of the female reproductive tract capacitate, attract, and activate the sperm. (2) The acrosome-intact sperm binds to the zona pellucida, which constitutes a thicker envelope than that of sea urchins. (3) The acrosomal reaction occurs on the zona pellucida. (4) The sperm digests a hole in the zona pellucida. (5) The sperm adheres to the egg, and their plasma membranes fuse.