Ectoderm , the most exterior germ layer, forms skin, brain, the nervous system, and other external tissues. Mesoderm , the the middle germ layer, forms muscle, the skeletal system, and the circulatory system. Although the details of gastrulation differ between various groups of animals, the cellular mechanisms involved in gastrulation are common to all animals. Gastrulation involves changes in cell motility, cell shape, and cell adhesion. Below are schematic diagrams of the major types of cell movements that occur during gastrulation.
Primary invagination is thought to result from changes in the shape of cells in the vegetal plate.
This high magnification view shows a filopodium extended by a secondary mesenchyme cell. These images show the rearrangement of a labelled clone of cells during archenteron elongation. This movie was constructed from a series of cross-sectional images taken by confocal microscopy during Xenopus gastrulation. The animal pole is up, and dorsal is to the right. The posterior end of the embryo is pointed at you.