Bilaterally symmetrical arrangement of CISs in the VNE of Pseudopallene sp. (ES 4). Optical sections of tubulin- and PH3-labelled embryos. (A) 2D projection of a curved composite section. Vertical dashed lines indicate the VMR. White spots mark the basal nucleus-containing portions of flask-shaped cells in CISs. Arrowheads mark the apically converging cell extensions of flask-shaped cells in CISs. VNE of ovigeral and walking leg 1 segments, presumptive segment borders marked by horizontal dashed lines. Note differences in the distinctness of the tubulin-labelled spots of the CISs, the apically converging cell extensions of flask-shaped cells being in some cases placed less condensed. A largely bilaterally symmetrical pattern of CISs is recognizable. Note apical mitoses. Arrows indicate a conspicuous site of cell immigration lateral to each ovigeral hemi-neuromere, relating either to peripheral nervous system or gland development. (B) Transverse section through anterior portion of ovigeral neuromere. The two contra-lateral CISs of a bilaterally symmetrical pair do not always comprise a similar number of immigrating cells. (C) Transverse section through posterior portion of ovigeral neuromere. The transversally arranged CISs are closely spaced with only one or maximally two apical cells in between. (D,E) Transverse sections through anterior and posterior portion of walking leg neuromere 1, respectively. While the morphologically left CISs are well-defined, their contra-lateral counterparts are less distinct, being part of a wider region of cells with basally displaced nucleus (compare also to A). Presumably, the morphologically right hemi-neuromere would have predated the left one in the formation of the central invagination characteristic of ES 5.