1. In vertebrates like amphibians, reptiles, birds, and mammals, germ cells known as primordial germ cells (PGCs) migrate from their origin site to the developing gonads, or genital ridges. 2. In amphibians, PGCs form in the animal pole of the blastula and migrate through the gut epithelium to reach the developing gonads. 3. In mammals, PGCs form in the yolk sac near the hindgut and migrate through the gut and up the dorsal mesentery into the developing gonads in the genital ridges. Monoclonal antibodies are used to identify migrating PGCs in mice embryos.