Actomyosin contractility and flow, as well as neighboring cell extensions, shape epithelial sheets during morphogenesis. (a) Actomyosin dynamics are not limited to these circum-apical bundles but are also found within the apical and basolateral cell cortex. (b) Classical 'purse-string' constriction draws circum-apical bundles of F-actin closed in a way analogous to the closure of a purse or noose. A constricting cell changes from cuboidal or columnar to adopt a wedge shape. Such a movement may concentrate proteins in the apical cortex or necessitate their removal by endocytosis. (c) Jacobson and colleagues hypothesized in their cortical tractor model that a flow of actomyosin over junctional adhesions may reshape neighboring cells, leading to cell wedging and folding. In the case of a single cell a cortical tractor (asterisk) could result in ingression; but when a field of cells (marked by asterisks) engages in tractor-tread like flows the entire sheet may fold.