“GERM CELLS that eventually turn into sperm or eggs are identical in both sexes, and whether a sperm or an egg is produced depends only by how late meiosis occurs.” These are the cells that make the gametes, ova and sperm. The ova is the largest, and sperm, the smallest cells in the human body.
http://www.sciencedaily.com/releases/2006/04/060404090939.htm