In urodele embryos, primordial germ cells arise epigenetically from the region of the ventral mesoderm, as a result of inductive influences from the ventral endoderm. In contrast, primordial germ cells in anuran embryos originate from cells which are in the endoderm and which contain germ plasm. In urodeles, the mechanism which allows primordial germ cells to differentiate from mesoderm cells remains to be discovered. Similarly, in anurans, whether germ plasm is the determinant and if so, how and when it acts, has yet to be determined.