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Calvariae of fetal mice were investigated with electron microscopy in order to demonstrate initial stages of immature, membrane bone calcification. As a consequence, nucleation sites for initial calcification of hydroxyapatite crystals in woven bone were found in extrusions from osteoblasts within the osteoid. Growth of the crystals epitaxially from these initial calcification loci into the surrounding collagen was spherulitic, forming discrete spheres referred to as "bone nodules." These nodules, when coalesced, formed seams of bone. Decalcified, the organic substructure of the bone nodules was revealed. It contained a centrally disposed initial calcification locus, a peripheral zone of decomplexing collagen and an intermediate zone of altered collagen. Fully formed collagen fibrils remained interposed between most of the coalesced nodules. Polysaccharides were localized within the initial calcification locus and in the peripheral zone of the bone nodules where collagen was shown to be decomplexing.