The organization and the development of primary bone were studied in a number of newborn or foetal mammals. Specimens were derived from long bones, calvaria and mandibles. The surface of the mineral component was exposed by treatment with an organic solvent (hot 1,2 ethane diamine or cold sodium hypochlorite solution) and in the case of some internal surfaces (endosteal surfaces of long bone shafts, endocranial aspects of calvaria and the lining of the dental groove or crypts of mandibles) some success was achieved in dissecting the soft tissues from the bone matrix surface. These surface were studied by scanning electron microscopy. Ground sections of similar material were studied by microradiography.

Primary membrane bone was found to be most different from adult lamellar bone on rapidly growing (periosteal) surfaces and in the following respects. 1) Mineralization of both the ground substance and collagen occured close to the mineralizing front, so that a higher level was reached rapidly and without a “maturation” phase. 2) The mineral clusters at the mineralizing front did not reflect the collagen fibre distribution. 3) The collagen fibre bundles at the matrix surface, at least at internal surfaces, were irregular in size and random in orientation.

Mineralizing fronts intermediate in character between those of rapidly forming, periosteal primary bone and adult lamellar bone were found on the uncommon “internal” forming surfaces and in the walls of vascular channels.

Most internal surfaces were resorbing. There was a wide range in the size of the resorption lacunae — the many which were only as large as the osteocyte lacunae exposed by the resorption process may have been caused by uninucleated cells.