A useful approach to the investigation of embryonic morphogenesis is the study of the factors that control cell movement in cell aggregates in organ culture. Previous studies, in which aggregates of embryonic chick heart ventricle tissue were paired in organ culture, supported the hypothesis that the associative behaviour is dominated by the mesenchymal cell (at the stages used the ventricle is composed of approximately 25% mesenchyme (Mes) and 75% myocyte tissue (My)) by virtue of this cell's ability to establish a pericellular matrix rich in fibronectin. In aggregate pairs, the aggregate types that develop a fibronectin-rich matrix rapidly are spread over by the aggregate types that are less able to deposit fibronectin in the matrix. In sorting conditions, Mes sorts to the surface of My. This is explained as a consequence of a requirement that Mes have access to a component in the serum fraction of the culture medium for deposition of fibronectin in the matrix. It is proposed that the factor penetrates to a shallow depth in aggregates, limiting the establishment of a fibronectin-rich matrix to superficially located Mes. As fibronectin appears in the matrix, Mes becomes more cohesive than My, allowing it to exclude myocytes and establish itself as a pure tissue that increases in volume as mesenchyme cells migrating within the interior contact the surface zone, becoming immobilized and also activated to secrete fibronectin. The analysis presented includes an experimental investigation of the different elements of this hypothesis and also explores some of the predictions of the hypothesis.