Fertilization is initiated by the following steps. (1) Sperm chemotaxis. (2) Binding of spermatozoa to the VC. (3) Interaction of self/nonself-recognition proteins that induce the signal transduction cascade leading to the SI response. (4) Penetration of nonself-recognized sperm through the VC. (5) Fusion of the egg and sperm plasma membranes resulting in fertilization and subsequent activation of zygote development.