This is a photograph of an individual deep cell from a killifish (Fundulus heteroclitus) embryo. These cells migrate extensively throughout the yolk sac, and details of the locomotory machinery can be observed. Note the clear cytoplasm near the leading edge (at top of image), the rounded rear edge, and two trailing retraction fibers. These cells can be induced to migrate directionally, by making a small wound in the overlying epithelium.
For more information, see Fink, R.D. and J.P.Trinkaus. 1988. Fundulus deep cells: Directional migration in response to epithelial wounding. Developmental Biology 129:179-190.