Our experimental results, as well as those of others, lead us to suggest the following steps in the dorsalization and axialization of the Xenopus egg and embryo: (1) the sperm aster determines the direction of rotation of the cortex relative to the deeper cytoplasm (endoplasm); (2) the rotation of the cortex activates latent dorsalizing-axializing agents in the vegetal hemisphere. The extent of rotation determines the amount of activation. The direction of rotation determines the location of the activated agents. (3) The activated agents determine the level of mesoderm-inducing activity of the vegetal cells cleaved from that cytoplasmic region. (4) The level of inducing activity determines at least the time at which marginal zone cells will begin gastrulation movements. (5) The time of its initiation of gastrulation may determine how anterior and dorsal a particular marginal zone cell can become.