In the sponge Petrobiona massiliana, a Calcarea related to "pharetronid" fossils, the oocyte and the embryo both receive an unusual amount of maternal nurse cells. Yolk granules are large and display a lamellar structure throughout the entire growth period, which allows them to be used as markers of the oocyte reserves. The cruciform cells (" cellules en croix") of the embryo appear to degenerate at an early stage. These features are compared to those found in other Calcarea.