SUMMARY

The orientation of developing myoblasts extending a bipolar axis in the presence of explanted myoblasts or whole notochord has been studied in vitro. Myoblasts tended to elongate perpendicular to the lines of diffusion of substances from these tissues. A slow-release source of agar impregnated with medium conditioned by segmented somitic myoblast or notochord also caused myoblasts to elongate perpendicular to the lines of diffusion from the source. Medium conditioned by neural tube cells or unsegmented mesoderm cells did not influence the orientation of myoblasts. It is concluded that somites and notochord release diffusible substances in vitro which are capable of directing the orientation of developing myoblasts. In vivo, a somite-derived material could play a role in determining the direction of myoblast elongation in the presomitic mesoderm. An interaction between somite and notochord-derived secretions could influence the rotation of presomitic myoblasts to form a segmented somite