AbstractSpicule deposition was studied by electron microscopy in fixed embryos and larvae of the haplosclerid sponge Reniera sp. and the hexactinellid Oopsacas minuta. Spicules form in centrally located vacuoles within cells and within syncytia, as in the adult sponges. In Reniera, scleroblasts differentiate from micromeres prior to gastrulation. At gastrulation the scleroblasts migrate to the periphery of the embryo and commence spicule deposition around a hexagonal axial filament. Sclerocytes have numerous pseudopodia and migrate to the posterior pole where they become aligned along the antero‐posterior axis in the free‐swimming larva. Reniera larvae possess some 40–50 oxeas, each 70–75 μm long and 1 μm wide. Mature Oopsacas larvae have up to 14 stauractin spicules, which are produced on a rectangular axial filament in tissues that lie under the smooth epithelium at the posterior pole of the larva. Young sclerocytes have many pseudopodia. The 4‐rayed spicules elongate along both the antero‐posterior and medial axes, until the longitudinal rays become anchored in a lipid‐filled cytoplasm at the anterior of the larva and the lateral rays intersect around the midline. The length of the transverse rays of the stauractins in free‐swimming larvae are 27–45 μm each, while the length of the two longitudinal rays is 40–80 μm. Although spicule deposition begins in cells with a similar morphology in both cellular and syncytial sponges, the elaboration and organization of the spicules differ markedly in cellular and syncytial sponges and appear to be an outcome of the very distinct cellular differentiation and larval morphogenesis that occur in each of these groups. Microsc. Res. Tech. 62:300–311, 2003. © 2003 Wiley‐Liss, Inc.