Development of major neuropils in the onychophoran brain. Heads of embryos at progressively older developmental stages in dorsal view. Anti-acetylated α-tubulin immunolabelling. Confocal maximum projections (A, B) and depth-coded projections (C, D). Anterior is up. (A) Central neuropil (cn) arises from a single transverse commissure in the antennal segment in an early Euperipatoides rowelli embryo. Scale bar: 200 μm. (B-D) Further stages of brain development in embryos of Epiperipatus isthmicola. Scale bars: 100 μm. A second transverse neuropil (arrowhead in B) and a third neuropil (arrowheads in C and D) arise anterior to the original commissure. Abbreviations: an, antennal nerve; at, antenna; cn, developing central brain neuropil; jw, jaw anlage; np, future nerve cord neuropil; ph, pharynx; sp, slime papilla.