Behavioural transformation in chick yolk-sac cells.

During the first 3 to 4 days of development in the chick, the extraembryonic part, the yolksac, expands to encompass the yolk mass. The yolk-sac, initially a two-layered epithelial sheet, is pulled out by the action of a specialized band of cells at its periphery. These attach to the overlying vitelline membrane and move out over it. However, unincubated blastoderms are not attached to the vitelline membrane, and each comprises a loose assemblage of rounded cells, not an epithelial sheet. The transformation of the unincubated blastoderm into an actively expanding epithelial sheet has been followed in culture, using hanging-drop cultures of fragments to observe largescale behaviour, and disaggregated blastoderms to observe individual cell behaviour. The timing of events in culture accords well with related events in ovo, and the possibility that /// vitro changes are merely a response to culture conditions has been largely excluded. 0-6 h in ovo. No cells attached to the vitelline membrane. All cells loosely attached to one another. 0-6 h in vitro. Cells do not attach to the glass. The cells of disaggregated blastoderms are rounded and stationary. 6-10 h in ovo. Cells at the blastoderm periphery attach to the vitelline membrane inner surface, but expansion does not start. The cells remain loosely attached to one another. 6-10 h in vitro. Cells begin to stick to the glass surface. Cells from disaggregated cultures move around as individuals. They remain rounded with long, narrow filopodia. If two cells collide, the adhesion tends to be brief. 10+ h in ovo. The blastoderm starts to expand rapidly as a cohesive epithelial sheet, pulled by its specialized 'edge' cells. All yolk-sac cells become tightly attached to one another. 10+ h in vitro. Blastoderm fragments start to spread rapidly as flattened epithelial sheets. There is no sign of specialized 'edge' cells. Cells at the periphery of any fragment take on the role of the edge. Cells from disaggregated cultures flatten out on the glass with wide lamellae all round. When two cells collide, they now tend to stick permanently together. The role of these changes in the mechanics of blastoderm expansion is briefly discussed.