1. Primordial germ cells (PGCs) give rise to gametes through a two-step process - PGCs are first determined in a specific embryonic location, then migrate to the gonads. 2. PGC determination occurs either autonomously via cytoplasmic determinants ("germ plasm") or conditionally through signals from surrounding cells. In mammals it is conditional. 3. In many animals like nematodes and frogs, germ plasm localized in the early embryo autonomously specifies PGCs, which then migrate to develop into eggs and sperm in the gonads.