Placement of the primary mesenchyme cells. (A) Nomarski videomicrograph showing a long, thin filopodium from a primary mesenchyme cell extending to the ectodermal wall of the gastrula, as well as a shorter filopodium extending inward from the ectoderm. The mesenchymal filopodia extend through the extracellular matrix and directly contact the cell membrane of the ectodermal cells. (B) The localization of the micromeres to form the calcium carbonate skeleton is determined by the ectodermal cells. The primary mesenchyme cells are stained green, while β-catenin is stained red. The primary mesenchyme cells appear to accumulate in those regions characterized by high β-catenin concentrations. (A from Miller et al. 1995; photographs courtesy of J. R. Miller and D. McClay.)