AbstractIn the mammalian embryo, the articular end of a long bone consists of a mass of fetal cartilage which undergoes transformation into two types of tissue as the animal ages: The hyaline cartilage surface of the adjacent joint; and the bone of the ossific nucleus of the epiphysis. The junction of these two areas, the calcified zone (or basal layer) of the articular cartilage, was studied in the distal femur of 110 albino rabbits of various ages by: tritiated thymidine autoradiography; routine histologic staining and mitotic counts; special stain for glycogen; and special stain for calcium (murexide).In the immature rabbit (under two months) the cartilage cells are arranged in columns and there is evidence of active proliferation. In its most basal portion adjacent to the vascular tufts of the epiphyseal nucleus, the cells are hypertrophic and contain glycogen. The intervening matrix bars are calcified. It is apparent that, at this age,endochondralossification, indistinguishable from that at the epiphyseal plate, is occurring.In the older rabbit (age four months) proliferation, columniation, and glycogen content are minimal, and the basal layer of cartilage is diffusely calcified. Proliferation is intense in and about the capillary loops and a thin layer of bone surrounds the capillary.In the adult (over six months) rabbit, calcification and ossification predominate. No proliferative activity is noted. Apparently growth has ceased completely or is so slow as to be unmeasurable by available techniques.