In the present work, principles of formation of germ line cells are reviewed. Germ line cells separate themselves from the rest of the embryo at the early stages of embryogenesis. In certain animal groups, formation of precursors of germ cells occurs by induction by surrounding cells. However, for most animal taxons, formation of primordial germ cells (PGCs) is determined by inheritance of certain maternal determinants--the so-called germ plasm. It is formed by mitochondria, electron-dense granules with the complex structure, and maternal RNAs and proteins necessary for formation of germ line. In Xenopus, the source of material for germ plasm is a mitochondrial cloud, which also specifically binds and transports to the vegetal pole maternal RNAs important for PGC formation. Cis elements determining the transport of these RNAs are usually located in the 3' untranslated region of RNA, and their function is mediated by binding of trans acting protein factors. In addition to a specific localization of certain macromolecules in germ plasm, special status of germ line cells is provided by degradation of RNA and protein components of germ plasm in somatic cells, silencing of transcription in PGCs until advanced stages of embryogenesis, and specific regulation of RNA translation in somatic and germ cells. In this review, we also briefly discuss results obtained by authors regarding the properties of a novel component of Xenopus germ plasm, namely maternal RNA germes, and encoded protein.