Drosophila embryo stage embryonic stage 12 Catherine Leroy FBdv:00005327 Temporal ordering number - 320. Germ band retraction. Stage 12 begins when germ-band retraction starts and ends when this process is complete so that the prospective anal plate occupies the posterior pole. During this stage the posterior and anterior midgut primordia meet and fuse and the tracheal pits fuse to form the tracheal tree. Duration at 25 degrees C: approximately 120 minutes (440-560 minutes after egg laying).