Practice question

Gastrulation in Drosophila begins after formation of cellular blastoderm, with ventral furrow as first morphogenetic movement. Ventral midline cells express twist and snail downstream of high Dorsal concentration, activating Fog ligand via concertina G protein and recruiting Rho1 guanine exchange factor. Apical constriction driven by Myosin II contractility causes ventral epithelial sheet to invaginate, forming transient tube that internalizes presumptive mesoderm. This tube collapses and mesodermal cells spread dorsally to underlie ectoderm. Cephalic furrow and dorsal folds contribute mechanically, but mesoderm internalization specifically occurs through ventral furrow.