Asymmetrical divisions of NSCs during advanced neurogenesis of Pseudopallene sp. (ES 6 – ES 10). F-actin- (A–C’) and tubulin- and PH3-labelled embryos (D–K) with nuclear counterstain. Optical sections showing Imaris volume, except (A). Oblique slicers have been aligned to show the asymmetry of forming sister cells as clearly as possible. Dashed vertical lines indicate VMR. White spots mark prospective epidermal cells surrounding the central invaginations. Asterisks mark NSCs that are not in division. White arrows mark larger sister cells of a NSC division (presumably self-renewing NSCs), arrowheads mark smaller sister cells of the same division. Note the asymmetrical positioning and different appearance of the newly forming nuclei. (A) Ventral view of entire embryo in late ES 7. Note F-actin-positive and largely nuclei-free paired invaginations in walking leg neuromeres 1 and 2 (black-framed arrows). In the proboscis and chelifore anlagen, intensely labelled structures represent differentiating muscles and parts of the spinning and chela glands. (B) Magnification of central invagination highlighted in (A). Note absence of grid-like array of F-actin-positive spots within the invagination, indicating a lack of defined CISs at this stage. Four NSCs in division are apically visible, two of them in telophase (details in C,C’). (D–H) Horizontal/slightly oblique sections showing asymmetrical NSC divisions in hemi-ganglion anlagen of walking leg neuromeres. Question marks in (D) indicate possible sister cells of a previous NSC division. Note slight cell distortions in late embryonic stages (G,H) due to compression by the covering walking leg anlagen. (I–K) Transverse sections showing asymmetrical NSC divisions in hemi-ganglion anlagen of walking leg neuromeres. Stars mark smaller immigrating cells. Note radial arrangement of the forming nuclei at the rim of the invagination in ES 8 (K). It remains unresolved whether the marked cells to the left (question marks) represent sister cells.