The PriSCs of Caenorhabditis elegans . Schematic of C. elegans early development. The zygote (P0) gives rise to the somatic AB cell and the series of germline blastomeres (P1, P2, P3 and P4), which retain the P granules (green dots). Each of them has both somatic and germ potential and divides to give rise to one somatic cell and another germline cell, except P4, which only has germ potential and can, therefore, be called PGC. P4 arises from the division of the presumptive primordial germ cell (P3, pPGC), and gives rise to the PGCs Z2 and Z3, which will remain silent during development and later proliferate to give rise to the GCs. Therefore, P1 to P3 are the rudimentary PriSCs of C. elegans. The sizes and shapes of cells are purely schematic and do not represent their actual shapes and relative sizes. PGCs, primordial germ cells; PriSCs, primordial stem cells.