Note
Two types of oogenesis have evolved in polychaetes: one (extraovarian) in which germ cells are ovulated into the coelomic cavity where they undergo vitellogenesis, and another (intraovarian) in which the germ cells are released from the ovary after or near the completion of vitellogenesis. There are many mechanisms of yolk synthesis and significant variation as to the sources of yolk precursors, their route of entry into developing oocytes, and the oocytic organelles involved in yolk synthesis. The evolution of the coelom and its role in nutrient storage and transfer, as well as the origin of germ cells, are intimately related to the evolution of vitellogenic mechanisms in polychaetes.