Stage 4 (Appearance of the ‘Scheitelplatten’, (a-c)) and stage 5 (Head-V, (d-i)): (a, b) Sytox; (c, i) Schematic drawing; (d-h) Hoechst. (a) Stage 4 (appearance of the ‘Scheitelplatten’); a paired ‘Scheitelplatten’ anlage appears anterior to the gastrulation zone. Each ‘Scheitelplatte’ consists of a cluster of about ten to twelve cells; this number increases during further development. The bar marks the distance between the pair of ‘Scheitelplatten’. The arrow marks a field of densely packed cells, which is the later mouth field. (b) Detail of one kidney-shaped ‘Scheitelplatte’ which becomes more crescent-shaped during further development. (c) Schematic drawing of an early stage 4embryo. (d) Stage 5 (Head-V); in the head region a V-shaped area becomes apparent. Its posterior tip extends to the prospective posterior border of the mandibular segment. The cells show loosely scattered nuclei resulting in a lower fluorescent nuclei staining compared to the adjacent area. The bar marks the decreased distance between the ‘Scheitelplatten’. The arrows mark the regions of the emerging naupliar appendages which are the first and second antennae and the mandible. The arrowhead marks the area of the later stomodeum. (e) Lateral view of the same embryo. (f) Detail of the ‘Scheitelplatte’, which shows a crescent-shape, enclosing clusters of small cells with bright nuclear staining (arrow). (g) At the posterior end the area of the prospective proctodeum is evident due to a cluster of internal cells and an external roundish ‘flap’-like incision (arrow). (h) The ‘Scheitelplatten’ move even closer together during further development. (i) Schematic drawing of the Head-V stage. sp = ‘Scheitelplatte’.