Germ cells are the only type of cell that can transmit all genomic information to the next generation. Such cells have unique characteristics, being not only germline-competent but also serving as stem cells. In Caenorhabditis elegans and Drosophila, germ cell lineages are predetermined. In contrast, in mammals, primordial germ cells, which are precursors of germ cells, are induced from multipotentepiblasts via extra-embryonic signaling. In avian species, however, the means by which germ cells are defined remain to be elucidated. Manipulation of germ cells is valuable in studies on germline development and in production of valuable cell lines.