Fig. 4. (A-H) Confocal micrographs of the prosomal hemisegment corresponding to leg 2 (fourth prosomal segment) of embryos stained with phalloidin-rhodamine. Anterior is towards the top, the medial furrow towards the left. The white lines indicate the segmental borders. (A) No invagination sites can be detected up to 120 hours after egg laying. The limb buds are already visible. (B) At 130 hours, the first five to eight invagination sites arise in the anterior most lateral region of the prosomal and the first opisthosomal hemisegments (arrow). (C) At 150 hours, nine to 12 new invagination sites have formed posteriorly and medially to the anterior region, where the first invagination sites occurred (arrows). (D) At 175 hours, the next five to eight invagination sites are visible laterally and medially in the posterior region of the hemisegment (arrows). (E) At 190 hours, seven to ten invagination sites have been added between row three and four and at the posterior end of the hemisegment. (F) At 220 hours, the number of invagination sites decreases. (G) Confocal micrograph of a transverse section through the neuroectoderm of the fourth prosomal segment at 240 hours. Apical is at the bottom. At this time a growing neuropil can be detected basally (arrow). (H) Horizontal optical section through the neuropil of the fourth prosomal segment at 240 hours. The arrowhead points to the neuropil. The outgrowing axons of the invaginated neuroectodermal cells join the developing neuropil (arrow). In C-F, the legs were removed or put to the side. l2, walking leg 2 (corresponding to the fourth prosomal segment). Scale bars : 50 µm in A-F; 20 µm in G; 50 µm in H.