Regional developmental capacities of the rat embryonic endoderm at the head-fold stage.

Three areas, composed of all three germ layers, were isolated from Fischer strain rat embryonic shields at the head-fold stage, and grafted separately under the kidney capsule of adult male rats of the same strain. The areas were from the neural plate, Hensen's node and the primitive streak. The resulting teratomas were examined histologically for the presence of derivatives of the primitive gut. The grafts differed strikingly in their capacity to develop into different segments of the gut. Endoderm underlying the neural plate developed into derivatives of the foregut, while endoderm underlying the primitive streak developed mainly into derivatives of the midand hindgut. It was concluded that, at the head-fold stage, the capacities to develop into different segments of the definitive gut are already roughly limited to particular areas of the endoderm.