The video sequence below is a computer- generated morph of tracings from an original journal article by Keller and Danilchik (1987) in which the behavior of "Keller sandwiches" was extensively documented for the first time. For clarity, the animal cap and dorsal non-involuting marginal zone (DNIMZ) of the explant are colored blue (since they both give rise to ectoderm), and the dorsal involuting marginal zone (DIMZ) is colored red (its deep cells give rise to mesoderm. The deep cells of the DIMZ undergo massive convergence and extension, forming axial mesoderm, including notochord and somites, even within the sandwich. At the same time, the cells of the DNIMZ also undergo convergence and extension. The animal cap, in contrast, flattens and forms a hollow cavity (a blastocoel equivalent?).