(A–E, A'-E') fog mutant SGs of late stages, from fully internalized (A, A'), partially internalized (B, B') to completely externalized glands, either folded (C, C') or elongated (D, D', E, E'). The externalized SG that migrates along the embryo surface (dashed bracket in E) formed a longer tube than the internalized one (solid bracket in E). All SGs have proper apicobasal polarity. (F, G) Whereas the surface epithelium is in the same plane as the SG in WT (F), a deep ingression between the SG and the neighboring surface epithelium is often observed in fog mutants (G), with some SG cells perpendicular to the rest of the SG on the apical side of the epithelial sheet (arrowheads). Z sections along the lines (F’, G’). Two SG cells are outlined with white dotted lines for each sample; one of them is perpendicularly positioned at the edge in the fog mutant (arrowhead). ML, midline. Yellow dashed lines, the boundary of the neighboring surface epithelia. (H) Strong myosin signals are often observed along the edge of the fog mutant SG (arrows). Arrowheads, nuclei of the slipped-out cells. (I, J) fog mutant SGs with slipped-out cells (arrowheads) in the anterior region of the placode form an externalized tube as cells invaginate (I). fog mutant SGs with slipped-out cells in the posterior region of the placode form an externalized tube as cells invaginate (J). Z sections along the lines (I’, J’). White lines, the boundary of SGs. Green lines, embryo surface. Arrows, the direction of invagination. Asterisks, non-specific signals. (K) SG-specific expression of Fog in fog mutant embryos did not rescue the externalized SG phenotype. Numbers inside bars indicate the number of SGs counted. P value was calculated using the Chi-square test. (L) A model for SG invagination. Fkh-dependent high-level Crb expression in the SG versus low levels of Crb in the surrounding ectodermal tissues regulates formation of the circumferential myosin cable that surrounds the entire SG to provide a compressing force during invagination. Regulation of the SG expression of fog by Fkh controls apicomedial localization of Rok and formation of apicomedial myosin, which drives clustered apical constriction to ensure the proper tissue geometry. Fkh-dependent uncharacterized target genes are proposed to drive the cell rearrangement that also contributes to SG internalization.