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FIGURE 1
Schematic depicting the two major mechanisms of germline differentiation, preformation and induction. In preformative species, as shown on left, maternally derived germline determinants, also known as germ plasm ribonucleoparticles (GP-RNPs), are already present in the zygote and subsequently localize in a subset of cells to confer germline fate. In inductive species, as shown on right, no maternally derived germline determinants are present in the zygote, and primordial germ cells gain their identity later in embryonic development through cell–cell signaling. The lower quotes represent opposing 19th century-ideologies, from which the terms preformation and epigenesis (induction) were historically associated.