David Page studies the human sex chromosomes and genes that play critical roles in the making of sperm and eggs. The propagation of our species, as in other animals, depends on the production of specialized reproductive cells, again in two forms, one generated by each sex. Males produce sperm, which apart from their long tails are tiny.
Females produce eggs, which are larger.
We refer to sperm, eggs, and their precursors as germ cells. In embryonic development in humans and many other animals, the germ cells are first discernible outside the portion of the embryo that will form the body. These primordial germ cells then invade the developing body and migrate to the gonads, which at this early stage are indistinguishable in males and females. Soon after, the gonads differentiate into ovaries in females or testes in males. The primordial germ cells, finding themselves in either an ovarian or a testicular environment, become committed to give rise to either eggs or sperm.