Stereo pair showing Lytechinus gastrula during the dispersal of the secondary mesenchyme cells. Convergent extension has occured, and a few mitotic spindles (amall arrows) can be seen. (PMC, primary mesenchyme cell; SMC, secondary mesenchyme cell.) If you cross your eyes, a 3 dimensional “central image” will form. (From Martins et al. 1998; photographs courtesy of the authors.)