This allows the sperm to penetrate through the egg wall and release its genetic material into the egg, after which no other sperm are allowed inside of the egg. Genetic material (the chromosomes containing DNA ) from both parents mix within the egg to form the earliest preimplantation embryo (2 pronucleus or 2PN) stage, which is considered day 1. The embryo then begins to develop as it passes through the Fallopian tube to the uterus (see diagram at our Embryo Development page showing the embryo moving into the uterus from the site of fertilization.
