Eggs develop from primordial germ cells that migrate into the developing gonad, becoming oogonia and then primary oocytes. Primary oocytes undergo meiotic division I and arrest at prophase I for days to years, during which they grow, synthesize a coat, and accumulate ribosomes, mRNAs, and proteins with help from surrounding accessory cells. Maturation involves completing meiotic division I, forming a small polar body and large secondary oocyte, which then arrests until fertilization stimulates it to complete meiosis and begin embryonic development.