Mechanics of animal development.

Morphogenetic movements are active processes created by forces located within the moving cells themselves. As a rule, these forces are generated by cytoskeleton structures (contraction of actin microfilaments organized as subcortical bundles or actine gel) and by the membranes and vacuole mediated processes of osmotic water transport. The intercellular forces lead to contraction and thus give rise to long-range mechanical stresses, mostly tensile ones. Tensile stresses create the regularly space/time arranged fields which remain topologically invariable within certain developmental periods, which then change drastically. The model of epithelial morphogenesis is discussed which postulates the segregation of an initially homogeneous cell sheet to the proportional domains of polarized and tangentially stretched cells as a result of self-organization. Some other models which tend to explain the different kinds of fold formation are also suggested. One of the models implies a simple "curvature increasing rule" which derives a common trefoiled archetype as a fundamental trend of development of an epithelial rudiment.