Chick embryo vertebral development has been studied during the period from 3 to 19 days of incubation. Whole mount preparations stained with alcian blue for cartilage and alizarin red for bone show the vertebral bodies as cartilaginous at 5 days, with dorsal spinous processes at 6 1/2 days, the onset of bone formation at the ventral and dorsal surfaces of the bodies at 13 days, ossification extending dorsally within the neural arches at 13 1/2 days, and bone development occurring throughout the vertebrae at 16 days. Descriptions at each time period center on the thoracic vertebrae with occasional inclusion of adjacent lower cervical and upper lumbar vertebrae. Histologic development is correlated with the Hamburger-Hamilton stages, which are based on external characteristics. The notochord and neural tube are well developed by 3 days and surrounded by sclerotome, myotome, and dermatome cells. Cartilage formation in the perinotochordal region of the vertebral body is seen at 5 days and precedes development of the neural arches and spinous processes. Separate centers of chondrocyte hypertrophy occur in the body (9 days), the lateral neural arches and the dorsal spinous process. Bone formation is under way at 13 days, beginning in the vertebral bodies. Intramembranous periosteal bone formation is seen adjacent to internal regions of chondrocyte hypertrophy. Vascular invasion of hypertrophic chondrocyte regions occurs, but the mechanism of endochondral ossification differs from that of mammals. The cartilage is resorbed by multinucleated chondroclasts and marrow round cells. Clumps of growth plate cartilage cells and matrix are surrounded occasionally by newly synthesized bone, but invasion of individual hypertrophic chondrocyte lacunae by vessels with bone deposition by accompanying osteoblasts on single trabeculae of cartilage does not occur. 3H-thymidine autoradiography shows high uptake at 3 and 5 days in the germinal neuroepithelial cells of the neural tube (spinal cord) and notochord. By 7 days, notochordal uptake is markedly diminished, and no uptake of isotope occurs from 8 days onward. Spinal cord uptake is highest in the first 8 days but persists in lessened amounts to 19 days. From 5 days onward, both undifferentiated mesenchymal cells and differentiated chondrocytes show positive 3H-thymidine uptake, but labeling is never seen in hypertrophic chondrocytes.