Conversion of primitive lumen into central canal.

<p>(A–L), Contraction of opposed apical surfaces reflects recession of the primitive lumen into a central canal between 24 and 72 hpf. Immunofluorescent staining by a combination of anti-GFP with anti-β-catenin antibody (A–D), anti-ZO-1 antibody (E–H), and phalloidin that detects F-actin (I–L). Increase in cell number and build-up of axonal tracts (*), plastic transverse sections of the spinal cord (M–P). (Q–T), Schematics corresponding to (M–P) showing cell nuclei.