The histogenesis and mechanism of joint clefting of the developing chick embryo up to the fifteenth day of incubation have been studied morphologically, ultrastructurally, and by histochemical methods. Cell degeneration was consistently noted 24 hours after differentiation of the joint tissue, and it is postulated that this early cell necrosis might account for the loosening of the medial part of articular mesenchyme (interzone) leading to differentiation of a three-layered embryonic joint. At the time of joint clefting degenerative cells were also seen in the peripheral parts of the developing articular cavity. In some cases clefting was immediately preceded and/or accompanied by the appearance of elongated, basophilic and electron-opaque cells closely arranged along the zone of tissue cleavage. These cells were thought to be implicated in some way in the clefting process and later to constitute a surface cell layer of articular cartilage. In addition to these observations clear morphological and histoautoradiographic evidence was found for the presence of an organic component, presumably mucopolysaccharide, in the primitive synovial fluid. Fluid secretion might also account for tissue cleavage at the sites of its accumulation. The data reported here suggest that joint cavity formation results from a combination of both intrinsic, genetically expressed and extrinsic mechanical factors acting synchronously.