Figure 4.

(A-j): Invagination of secondary neural precursors and formation of epithelial vesicles. (A-F) Confocal micrographs of flat preparations of embryos stained with phalloidin-rhodamine. (B-G) Flat preparations of the fourth prosomal hemi-segments. (A) At 220 hours about 25 secondary invagination sites form (arrow). There is no clear dividing line between the formation of secondary invagination sites (arrow) and invagination of primary neural precursors. Some primary invagination sites are still visible (arrowhead) The bars indicate the segment borders. (B) Apical optical section of the pattern of secondary invagination sites (arrow) at 240 hours of development. (C) Epidermal cells overgrow the ventral neuromeres between 250 and 300 hours (arrowheads) The arrow points to a secondary invagination group. (D) After invagination the individual cells of a groups remain attached to each other forming epithelial vesicles (arrow). (E) At 300 hours the anterior-posterior extension of the individual hemi-segments has been reduced leading to a rearrangement in the positions of the invaginated cell groups (arrow). (F) After 320 hours 8 of the 25 invaginated cell groups are no longer visible indicating that the cells have detached from each other. The arrow points to an invaginated cell group. (G) 10 cell groups are still visible at hatching (arrow). (H) Overview of the arrangement of epithelial vesicles (arrow) of the four prosomal hemi-segments corresponding to the four walking legs. The anterior-posterior reduction in size is clearly visible (compare to A). The bars indicate the segment borders. (I) Flat preparation of the prosoma at hatching. Epithelial vesicles are still visible (arrow). The bars indicate the segment borders, the arrowhead points to the midline. (J) Flat preparation of the brain at 350 hours. The arrow points to epithelial vesicles. ch, chelicera; l1 to l4, prosomal neuromeres corresponding to walking leg 1 to 4; leg 1, walking leg 1.p, pedipalp; ped, pedipalpal neuromere.