Incorporation of 5-bromo-2'-deoxyuridine (BrdU) and subsequent immunocytochemical detection in embryos of the onychophoran Euperipatoides rowelli (Peripatopsidae). Confocal micrographs. Note the intense labelling in the ventral organs and anlagen of the hypocerebral organs, which are derivatives of the anterior-most pair of ventral organs. (A, B) Ventral and lateral views of a late stage 6 embryo after 12 hours incubation in BrdU. Arrows point to the ventral organs. (C) Ventral view of the head of a late stage 6 embryo after 10 hours incubation in BrdU. Double-labelling with the DNA-selective dye Hoechst (Bisbenzimide, blue) and anti-BrdU antibody (green). (D) Detail of two pairs of ventral organs from a stage 6 embryo after 12 hours incubation in BrdU. (E) Optical cross-section of an anlage of the hypocerebral organ after 12 hours incubation in BrdU showing a superficial layer of anti-BrdU labelled cells. (F) Detail of ventral organ nuclei after 3 hours incubation in BrdU (ventral view). Double-labelling with anti-BrdU (green) and anti-phospho-histone H3 antibodies (α-PH3, red). Arrowheads point to BrdU incorporation foci in each nucleus. Abbreviations: br, presumptive brain tissue; ho, anlagen of hypocerebral organs; mo, presumptive mouth opening; vo, ventral organs. Scale bars: A and B, 500 μm; C, 100 μm; D, 50 μm; E and F, 20 μm.