Primordial germ cells (PGCs) are the precursors of sperm and eggs1. In most animals, segregation of the germ line from the somatic lineages is one of the earliest events in development2; in avian embryos, PGCs are first identified in an extra-embryonic region, the germinal crescent, after approximately 18 h of incu- bation. After 50–55 h of development, PGCs migrate to the gonad and subsequently produce functional sperm and oocytes3,4. So far, cultures of PGCs that remain restricted to the germ line have not been reported in any species5,6. Here we show that chicken PGCs can be isolated, cultured and genetically modified while main- taining their commitment to the germ line. Furthermore, we show that chicken PGCs can be induced in vitro to differentiate into embryonic germ cells that contribute to somatic tissues.