The heterocellular female gonad of Rhabdocoela consists of germaria and vitellaria both enveloped by a tunica comprising of an outer extracellular lamina and an inner cellular sheath of accessory cells. In the germaria oocyte maturation occurs completely during the prophase of the first meiotic division. The main feature of oocyte differentiation is the development of RER profiles and Golgi complexes involved in the production of small inclusions that become localized in the cortical cytoplasm of mature oocytes. In Kalyptorhynchia, Typhloplanida and Dalyelliida the peripheral egg inclusions contain polyphenols and have been interpreted as residual eggshell granules. By contrast the oocytes of Temnocephalida possess peripheral inclusions containing glycoproteins and devoid of polyphenols and have been interpreted as cortical granules. In addition, the scutariellid Troglocaridicola sp. shows proteinaceous acorn-shaped granules which remain scattered in the ooplasm throughout oogenesis and have been interpreted as residual yolk.

The maturation process of vitellocytes shows similar ultrastructural features in all the taxa of rhabdocoels examined and is typical of secretory cells producing membrane-bound inclusions:  eggshell globules and yolk globules. Eggshell globules contain polyphenols and show a multigranular/mosaic like pattern. The exception is represented by the scutariellid Troglocaridicola sp. where the polyphenolic eggshell globules show a meandering/ring shaped pattern similar to that observed in Proseriata and Tricladida. Mature vitellocytes contain, in addition, glycogen and lipids as further reserve material. The phylogenetic significance of some ultrastructural features of oocytes and vitellocytes are discussed.