How does the archenteron elongate, given that both autonomous extension and secondary mesenchyme cell dependent extension occur? By sectioning through the archenteron, Ettensohn (1985) and Hardin and Cheng (1986) showed that sea urchin gastrulae elongate their archenterons via convergent extension. A more direct demonstration of this was provided by Hardin (1989), who used fluorescent clones of cells in chimeric embryos to show that the epithelial cells of the archenteron rearrange as it elongates. Note that on the left, the clone has smooth boundaries; by the end of gastrulation, the clone has intercalated with neighboring unlabeled cells to generate a jagged boundary.
