The morphologic features of bone bark, a structure surrounding the distal and proximal ends of long bones, were studied in the distal femur, proximal tibia, and proximal fibula of 77 spontaneously aborted human fetuses varying in gestational age from 10 to 20 weeks. Standard histologic techniques used in addition to in situ immunohistochemical staining allowed the examination of the structure of the bone bark and localization of Types 1, 2, and 3 collagens at different gestational ages. The bone bark was shaped like a cylindrical sheath of bone lamellae of varying thickness. The epiphyseal end of the bone bark, known as the groove of Ranvier, was covered outwardly by a fibrous layer and inwardly by the epiphyseal cartilage and contained mesenchymal cells, chondroblastic precursor cells, and densely packed cells differentiating into osteoblasts. Neither the cell density in the groove nor the thickness of the bone bark were identical circumferentially, indicating an unequal growth in width. In addition, the presence of periosteal apposition and endosteal resorption of the bone bark on one side and of endosteal bone deposition accompanied by periosteal resorption of the bone bark on the opposite side support the concept of a spatial drift of bones. These observations furnish histologic proof that groove and bone bark, although assuring an equal growth in length, contribute to an unequal and eccentric growth in width.