Summary of the formation of dorsal tissue during embryonic development in Glomeris marginata. Embryos are stained with the nuclear fluorescent dye DAPI. The developmental stages are as indicated; note that these stages are before dorsal closure, the lateral tissue on both sides of the germ band is the future dorsal tissue (details see text). (A) Trunk region of a stage 3 embryo (staging see [4,6]). At this stage the head segments (only gnathal segments visible here) and the first four metameric units of the ventral trunk have formed. Additional trunk metameric units will be added from the posterior growth zone (gz). At this stage the de novo formation and outgrowth of the dorsal tissue (lateral plates) commences (filled circle and asterisks). The asterisks mark the dorsal tissue aligned with the trunk segments. The filled circle marks the common dorsal tissue of the gnathal segments. (B) Stage 4 embryo. The outgrowth of the lateral plates proceeds (asterisks). Note that the growth zone now gives rise to ventral as well as dorsal tissue (arrowhead). (C) Stage 4.1 embryo. The fifth ventral trunk metamere has formed from the growth zone. (D) Stage 5 embryo. The embryo starts to bend in to form the characteristic shape of a "pill". The sixth ventral trunk metamere has formed. Note that the fifth and sixth ventral metameres share one common dorsal tissue that has been added from the growth zone. (E) Stage 6.1 embryo. The embryo has bent in completely. Roman numerals denote the future tergites. (F) Stage 7 embryo. The bipartite body plan of the millipede composed of head and trunk becomes clear. The outgrowth of the dorsal plates has almost finished and the dorsal tissues meet at the back of the embryo to complete dorsal closure. Note that the constrictions between the tergites are clearly visible at this late embryonic stage. Anterior is always to the left. A–D are ventral views, E and F are lateral views. Abbreviations: lb: labrum; an: antennal segment; pmd: pre-mandibular segment; md: mandibular segment; mx: maxillary segment; p: post-maxillary segment; gz: growth zone; 1–6: ventral leg bearing metameric unit 1–6; I–VI: tergites I–VI.