Figure 1.

(A-E): Morphology of the secondary invagination sites. Confocal micrograph (A, inverted) of a flat preparation of an embryo stained with phalloidin-rhodamine and light micrographs (B-D) and electron micrograph (E) of transverse sections through prosomal hemi-neuromeres. The midline is to the right. (A) Final pattern of the primary invagination sites in the opisthosomal segments 1 and 2. The invagination sites are arranged in 7 rows. The black dots correspond to the constricted cell processes of the individual precursor groups that are attached to the apical surface (arrow). (B) Morphology of the secondary invagination sites. At 250 hours the secondary invaginating cell groups (asterisks) are still attached to the apical surface. The individual groups are isolated by brighter sheath cells (arrowhead). The primary precursor groups have dissociated (arrow) and form basal cell layers. The longitudinal connective (lc) is already visible at the basal side. (C) The secondary invagination sites (asterisks) loose contact to the apical surface, when the epidermis (arrow) overgrows the ventral neuromeres. (D) After invagination the secondary neural precursors (asterisks) remain attached to each other forming epithelial vesicles. The cell processes run parallel to each other and extend to a lumen (arrow). (E) The cell processes (o) of the invaginating cells of a group are opposed to each other and the lumen between the cell processes is filled with microvilli (arrow). Cell junctions connect the individual processes (arrowheads). lc, longitudinal connective; o2 to o3, opisthosomal hemi-segments 2 to 3.