The synovial joints constitute the most common variety of joints of the body. During the development of synovial joints the interzonal mesenchyme between the developing bones becomes cavitated by the appearance of clefts in the central region of the mesenchyme during the third month of development. The clefts gradually coalesce to form a joint cavity. The peripheral mesenchyme differentiates into two layers: (1) the thick outer layer differentiates into dense fibrous tissue and forms the joint capsule and ligaments; (2) the thin inner layer of mesenchyme differentiates into
synovial membrane which lines the inner surface of the capsule and covers the articular cartilages (which coat the articular surface of component bones). j Later, as a result of joint movements,the synovial membrane disappears from the surface of the articular cartilages.