[Morphogenetic factors and mechanisms in the articulation of limbs]

After excision or reorientation of a part of the tibia rudiment in the chick embryo, an articular head or a heterotopic joint may develop by regulation without direct or indirect participation of the interzonal mesenchyme. The undifferentiated cells which take part in the regulation arise from the prospective perichondrium: their proliferation, organization and incorporation are controlled by influences exerted by the adjacent parts of the skeletogenous blastema. Other experiments have been performed on the tibia rudiment in the attempt to modify the pressures exerted along the zeugopod by its growing skeleton. When the pressures are reduced, a joint can form between the distal end of the tibia and the proximal tarsal, which undergo fusion in normal development. As an effect of increased longitudinal pressures, fusion of the proximal tarsal and the proximal epiphysis of the 180 degrees inverted tibia may occur in spite of the interposition of a part of the interzonal mesenchyme of the knee. Our results stress the primary role seemingly exerted by the growing articular ends of the skeletal pieces during joint formation; under the conditions of our experiments, joint development does not seem causally related to any special morphogenetic property of the interzonal mesenchyme. In some joints, menisci, ligaments, fatty pads differentiate from parts of the interzonal mesenchyme which are apparently endowed with a special and early determination; it is still unknown, however, whether the interzonal mesenchyme also plays a more general role in all the joints.