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The extracellular osteoid matrix of the spongiosa of the metaphysis at the distal end of the femur in newborn infants can be divided into three zones.The first is uncalcified osteoid composed of a network of collagen fibrils set in ground or cement substance. This has been seen in some degree between the osteoblasts and calcified osteoid in all sites in the material examined. The width of this uncalcified zone varies but is usually too narrow to be resolved by the light microscope.The matrix becomes calcified and so forms the second zone. The dividing line between the calcified and uncalcified osteoid matrix is fairly clear-cut. The crystals are deposited with their long axes parallel with the long axes of the fibrils and probably lie mostly outside the fibrils and are not concentrated at 630-Angstrom intervals along the fibrils. At this stage the fibrils still tend to form an intertwining network.In the third zone, fibrils are more generally grouped in a parallel fashion, and their periodic band areas are in phase. The crystals lie with their long axes parallel with the axes of the fibrils and are concentrated in rows at about 630 Angstrom-unit intervals along the fibrils. The rows of crystals are in register with the periodic bands of the fibrils.