Fertilization is the union of gametes to initiate development of a new individual. The female gamete is formed during oogenesis. The process begins when, in the early embryo, primordial germ cells arise and subsequently colonize the genital ridges. They differentiate into oogonia, start meiosis, and become primary oocytes. The cell cycle of the primary oocytes then becomes arrested in mid-meiosis for an extended period of time. Prior to ovulation the oocytes undergo a growth phase and their sizes increase significantly. A hormonal cue then triggers oocyte maturation that involves the resumption of meiosis, the completion of the first meiotic division, and, as a result, the reduction in the diploid chromosome number. The cell cycle then stops again; in vertebrates this arrest occurs at the metaphase stage of the second meiotic division. Meiosis resumes at fertilization, when the sperm activates the egg, i.e., it causes a series of changes that are required for the initiation of embryo development. This is achieved by triggering an elevation in the egg's intracellular free calcium concentration. In response, the fertilized egg completes meiosis and enters the first embryonic cell cycle.