The moment of conception Normally, only one sperm can break through the outer membrane of the egg; once the sperm enters the egg, the sperm loses its tail. Implantation After six days, the cluster of cells develops a hollow cavity. Called a blastocyst, it attaches itself to the wall of the womb. Some cells grow into the womb''''s lining and the fetal life-support system (the placenta) begins to form. |