The goal of this proposal is to integrate the biomechanics of morphogenesis across a number of size-scales from subcellular generation of forces to the macroscopic forces and bulk tissue properties that guide convergence and extension in the developing frog embryo. We propose to address a series of questions that arise from a biomechanical analysis of mediolateral cell intercalation and its effects on tissue deformation and establish a toolkit for future biomechanical analyses of morphogenesis. What are the forces generated by polarized protrusions and how are they converted into mediolateral cell intercalation? This question concerns the cellular origin of force during gastrulation. To resolve these forces during gastrulation we will adapt "force-traction" microscopy techniques for use with frog embryo explants and combine images of traction-forces with high resolution imaging of cell protrusions during mediolateral cell intercalation in these explants. In order to understand how local force generation effects tissue movement we need to understand the mechanical context, i.e. the viscoelastic properties of tissues in the developing embryo and how they relate to tissue architecture. To identify key features of the 3D tissue architecture that are responsible for tissue material properties we propose to measure bulk mechanical properties of the dorsal mesoderm and assess with histology the features of the tissue architecture that are relevant. Lastly, we want to understand how the tissue architecture modulates conversion of protrusive activity into the bulk forces of tissue extension. In order to test hypotheses on the interactions between cell behaviors and tissue extension we propose to measure the force of extension of intact explants after treatments that alter protrusive activity, local force generation, or tissue material properties. The experimental framework established in this work will complement ongoing studies of the molecular regulators of convergence and extension by providing "nuts-and-bolts" models of morphogenesis with testable hypotheses. Our biomechanical approach will also address basic questions from the field of tissue engineering on the source of mechanical properties in embryonic tissues and presents a relevant case study of how embryos build a complex tissue.