Establishment and organization of germ layers in the gastrulating mouse embryo.

By following the distribution of wheat germ agglutinin-gold-labelled cells in primitive streak stage embryos, we obtained direct evidence for a continuous recruitment of the embryonic ectoderm cells to the definitive endoderm and to the embryonic and extraembryonic mesoderm during gastrulation. The majority of the definitive endodermal cells ingressed through the anterior end of the primitive streak and were incorporated initially into the midline endoderm at the archenteron, but a small population of endodermal cells may be recruited by direct delamination from the embryonic ectoderm. The pre-existing visceral embryonic endoderm was progressively replaced, but not totally, by the newly recruited population which colonized the embryonic foregut and the notochord. The developmental fate of the recruited endoderm and that of cells in the embryonic ectoderm and the mesoderm of late primitive streak stage embryos indicate that concomitant with the establishment of the germ layers, an orderly allocation of prospective fetal tissues to specific parts of the body occurs simultaneously in all three germ layers.