A form of coordinated cell movement occurring in a vertebrate embryo during gastrulation—an early developmental stage during which rudimentary tissues form and body axes are defined. It involves the spreading of a sheet of cells from the animal pole of the blastula to envelop the vegetal pole. Cells in a single layer can extend by becoming flatter to increase their surface area, whereas multiple layers extend by rearranging into fewer layers covering a larger area, a process called intercalation.