The primordial germ cells (PGCs) originate in the epiblast, which can be identified by the alkaline phosphatase reactivity and by staining for octamer-binding transcription factor 3/4. These cells migrate to the genital ridge, after which they are referred to as gonocytes. They differentiate either to pre-spermatogonia or oocytes. Embryonic stem cells (ESCs) are derived from the inner cell mass, whereas embryonic germ cells (EGCs) can be isolated from PGCs until day 12.5 of development. The ESCs show a biparental pattern of genomic imprinting, whereas in EGCs this is erased. ESCs and EGCs can give rise to pluripotent teratomas, of which the embryonal carcinoma cells are the stem cells. Teratomas can also be formed directly from PGCs in vivo. During spermatogenesis, the paternal pattern of genomic imprinting is established, whereas the maternal pattern is formed during oogenesis. The timing of meiotic I arrest is different between male and female germ cells.