In a series of 26 human fetuses, the author performed histochemical studies on the development of the knee joint and the proximal tibio-fibular joint. The following histochemical reactions were used (1) Metachromatic staining with 0.1% azure A in 30% ethanol. (2) Staining with 0.1% Alcian blue in 3% acetic acid followed by ordinary PAS staining. (3) Periodic acid-Schiff reagent (PAS). (4) Carbol fuchsin-picro-ponceau. According to the results, the patella appears to be derived from the blastema like the femur, tibia, and fibula. Moreover, the menisci arise from the blastema as semilunar structures. They contain cartilaginous matrix from the 23 mm stage and collagen fibers from the 30 mm stage. The cruciate ligaments also appear to be blastemal derivatives. The joint cavities arise as 3 separate cavities: 1 femoro-patellar and 2 fe]moro-tibial, at the 35-41 mm stage, the joint cavities starting centrally in the 3 joints. During the subsequent stages, up to 46 mm, the 3 cavities become confluent. Prior to cavity formation, large quantities of chondroitin sulphate A and C are formed in the 3 layers of the inter-zone and in the loose infrapatellar tissue. At 46 mm, a central joint cavity may be seen in the proximal tibio-fibular joint.