In polychaete annelids, two types of vitellogenesis have been described: a heterosynthetic mechanism (in a number of different of worms) and an autosynthetic process (other includingNereis). Recent biochemical results suggest that the heterosynthetic mechanism is more general than previously thought. InNereis, the vitellogenin (the precursor) is synthesized in coelomocytes and after transfer through coelomic fluid incorporated into oocytes. In germinal cells, a conversion process, involving proteolytic cleavages of vitellogenin, produces mature vitellins which are accumulated in yolk granules.