During Xenopus gastrulation, the mesoderm involutes at the blastopore lip and moves on the inner surface of the BCR toward the animal pole of the embryo. Active cell migration is involved in this mesoderm translocation. In vitro, mesoderm cells migrate non-persistently and intermittently by extending and retracting multiple lamellipodia, which pull the cell body in their direction. Lamellipodia formation is induced by FN. FN fibrils are present on the BCR as part of the in vivo substrate of mesoderm migration. Mesoderm cells can attach to the BCR independently of FN, but interaction with FN is required for lamellipodia extension and cell migration on the BCR. In contrast to preinvolution mesoderm, involuted migrating mesoderm always stays on the surface of the BCR cell layer: migrating mesoderm cells do not mix with BCR cells, and a stable interface between tissues is maintained. A corresponding change in cell sorting behavior occurs during mesoderm involution. In Xenopus, the mesoderm moves as a multilayered coherent cell mass held together by cadherin-mediated cell adhesion. Aggregate formation changes mesoderm cell behavior, rendering it more continuous, persistent and directional, i.e. more efficient. The mesoderm possesses an intrinsic tissue polarity which biases the direction of its movement. In addition, the fibrillar FN matrix of the BCR contains guidance cues which also direct the mesoderm toward the animal pole. Haptotaxis is most likely not involved in this substrate-dependent guidance of the mesoderm, but intact FN fibrils seem to be required. A polarity of the BCR cell layer which underlies this anisotropy of the BCR matrix develops under the influence of the marginal zone in the late blastula. Although in other amphibian species, gastrulation depends critically on mesoderm cell migration, in Xenopus, convergent extension of the axial mesoderm seems to provide the main driving force for gastrulation.