The present studies indicate that in the growth of cartilage canals the cartilage is removed by chondroclasts which stem from the primitive connective tissue cells of the perichondrium. Besides fusing to form multinucleated chondroclasts, these cells also provide the osteoblasts which establish the secondary centre of ossification. The growing tips of the blood vessels within the canals are also fashioned from these primitive connective tissue cells as they invade the microhaemorrhages at the ends of the canals. This is an identical procedure to that found in the vascular invasion of the primary growth plate. The cartilage canals are clearly useful sites in which to study the development of osteoclasts and chondroclasts from more primitive (osteoprogenitor) cells.