The formation of the mesoderm in early post-implantation mouse embryos is described and analysed. The outgrowth of the mesoderm was found to depend on the changes in the shape of the embryonic ectoderm, which lead to a relative displacement of the primitive streak in the caudal direction. The primitive streak deposits its cells laterally in the case of the lateral mesoderm, and medially in the case of the headprocess. In doing so, the primitive streak leaves a trail of mesoderm cells. This means that mesoderm cells do not migrate actively from the caudally located primitive streak towards more frontal positions in the embryo. This is confirmed by the results of scanning electron microscopy, which revealed that mesoderm cells show no polarity at all in the caudofrontal direction. In may therefore be concluded that these cells probably do not migrate. Studies on the cell-cycle parameters of the embryonic ectoderm, showed that mesoderm cells-to-be are probably recruited not only from the proliferation zone, but also from the lateral ectoderm. It is postulated that the lateral ectoderm gives rise, via the largest part of the primitive streak, to most of the mesoderm cells, whereas the proliferation zone gives rise to the head-process mesoderm, via the anterior part of the primitive streak.