Coordinates cell shape changes during formation of the ventral furrow and invagination of the posterior midgut primordium, by inducing apical constriction of cells in spatially and temporally defined manners. Could function as a secreted signal to initiate apical constriction by acting as a ligand for an unidentified G protein-coupled receptor, which in turn activates the G protein alpha subunit encoded by concertina, in neighboring cells. Such an intracellular pathway would ultimately induce contraction of the apical actin-myosin network. In the ventral furrow, fog appears to ensure that all the cells initiate constriction within several minutes of each other. In the posterior midgut invagination, fog appears to direct the ordered progression of constriction initiations out from a central region and also to delimit the peripheral extent of this spreading. Expressed in the invagination primordia in a pattern that precisely precedes the pattern of constrictions. Maternal transcripts are deposited into the egg and uniformly distributed throughout the cortex of cleavage stage and syncytial blastoderm embryos. Zygotic transcription is first found in the ventral furrow primordium during the beginning of cellularization, about 30 min before the start of constrictions also expressed about 30 min before the start of constrictions in the posterior midgut primordium. The ventral-most cells are last to express fog. Controlled by zygotic patterning genes. May be highly O-glycosylated in its Ser/Thr-rich C-terminal part.