Measurements of several indices of shape, contact, position and arrangement of deep cells in the late blastula and gastrula were made from scanning electron micrographs of carefully staged, fractured embryos in order to describe the cellular processes which account for the increased area of the deep region of the gastrula during extension of the dorsal marginal zone and epiboly of the animal region. At the onset of gastrulation, the deep cells of the dorsal marginal zone become elongated, extend protrusions between one another along radii of the embryo and interdigitate to form fewer layers of cells of greater area in a process of radial interdigitation. When interdigitation, is complete, the deep region consists of one layer of columnar cells which then flatten and spread and thus account for additional increase in area of the deep region. During epiboly of the animal region, interdigitation occurs and the number of cell layers decreases without the changes in cell shape seen in the dorsal marginal zone. These differences may be related to the anisotropy of expansion (extension and convergence) in the dorsal marginal done as opposed to uniform spreading in the animals region, or they may reflect an active cell motility in the dorsal marginal zone as opposed to a passive behavior in the animal region. A cellular and mechanical model is presented in which active (autonomous) spreading is brought about by active, force-producing interdigitation and subsequent flattening of deep cells. A model of passive spreading (stretching) is also presented. These observations suggest experiments that would determine the relationship of cell behavior to the mechanics of gastrulation.