[Experimental changes in the sites of presumptive articular areas]

Following proximo-distal and dorso-ventral reorientation of about the proximal (or distal) half of the chick embryo tibial rudiment, changes of the original morphogenetic fate of the latter occur. In the embryos operated upon between stage 24 and stage 26, from the bisected blastema two diminutive, tandem skeletal pieces each built of a diaphysis and two articular ends can develop. More restricted organogenetic changes take place when the tibial rudiment is divided during early phases of its chondrification (stage 26-27 1/2); in these cases, the presumptive diaphyseal (or metaphyseal) zone lying at one of the ends of both segments of the bisected tibia gives rise to an epiphysis sometimes reduced but often exhibiting an articular surface. The early rudiment of a long bone can, therefore, be viewed as an unitary morphogenetic field endowed with a remarkable plasticity. The site of the joints seems not to be definitely determined until the articular ends of the skeletal pieces undergo chondrification.