The germline cycle in freshwater planarians. Freshwater planarians possess a population of stem cells, the so called neoblasts, which represents the PriSCs in these organisms (PriSC). Neoblasts are able to give rise to the GCs (gc) and to somatic cells (sc). GCs give rise to oocytes (oc) and sperm (s), which jointly give rise to the zygote (z). The zygote gives rise to both somatic cells and the PriSCs. The planarian PriSCs have unlimited self-renewal (sr) and both germ potential (gp) and somatic potential (sp). Green dots represent the presence of nuage granules and germ plasm components. The dotted blue line represents the position of the germ-to-soma boundary, as classically understood. The dotted red line represents the proposed position of the germ-to-soma boundary as postulated in the Primordial Stem Cells hypothesis, which coincides with the Weismann barrier (solid black line) in freshwater planarians. PriSCs, primordial stem cells.