1. Embryos of Rana temporaria have been dissected and shape alterations of different parts of the embryo, taking place within 1 h of separation, have been studied. Two categories of deformation have been revealed. 2. The first category comprises those deformations which take place immediately after separation. They are insensitive to cooling, cyanide and Cytochalasin B treatment. These deformations, which consist of a shortening of initially elongated cells, are considered to be the passive relaxations of previously established elastic tensile stresses. 3. Deformations of the second category proceed more slowly. They are inhibited by cooling, cyanide and Cytochalasin B treatment, are accompanied by elongation and migration of cells and occasionally lead to rather complex morphodifferentiations of isolated fragments. These processes are considered to be the result of the active work of intracellular contractile systems, either pre-existing or induced de novo. 4. By analysing the arrangement of the passive deformations we have constructed maps of mechanical stresses in embryos from late blastula up to the early tail-bud stage. At several embryonic stages drastic transformations of the stress pattern occur, these transformations being separated by periods during which the pattern of stress distribution remains topologically constant. 5. A correlation between the arrangement of stress lines and the presumptive morphological pattern of the embryo is pointed out. 6. Some possible relations between tensile tissue stresses and active mechanochemical processes are discussed.