To investigate the contribution of the epiblast cell behavior to the primitive streak formation, we examined the motility of a single epiblast cell from pre-streak stage embryo in vitro. On the substratum that was evenly coated with laminin gel, epiblast cells attached well to the gel and one or a few very long and broad cellular processes protruded from their spherical cell bodies; however, they hardly locomoted on it. Unexpectedly, after overnight culture, half of the single cells dissolved the laminin gel beneath them to make well-like holes, and invaded in the holes. On the substratum lined parallel with the fibrous laminin gels supplemented with fibronectin, they locomoted actively in accordance with the alignment. That is, they were subjected to contact guidance. In locomotion they looked like snails, extending one or a few long and broad processes in a forward direction from the spherical cell bodies. However, on the substratum lined with laminin or fibronectin only, they did not locomote actively. Individual chick pre-streak epiblast cells had already been committed to invade, and their migratory nature existed in each cell, even though they were isolated from the epithelial sheet. The implication of these findings on the cellular basis of primitive streak formation will be discussed.