INTRODUCTIONThe basic Keller explant is a rectangle of dorsal mesendoderm and ectoderm from an early-gastrula-stage Xenopus laevis embryo. It is ~60° to 90° wide, extending from the bottle cells to the animal pole. This protocol describes how to dissect, assemble, and cultivate Keller explants. The purpose of Keller explants was initially to allow observation of gastrulation movements, particularly convergent extension, in culture. This is difficult to do when explants curl up, but in Keller sandwiches, the explants are cultured flat, either as a single sheet (open-face explant) or more frequently as two sheets sandwiched together with their inner surfaces apposed (closed sandwich). Explants are cultured beneath a coverslip fragment or a glass bridge resting on silicone vacuum grease until the desired stage, usually during or after neurulation. Instead of involuting beneath the ectoderm, mesoderm elongates in a plane with adjacent ectoderm. Explants are made at the onset of gastrulation before significant vertical juxtaposition of ectoderm and mesoderm has occurred.