Drosophila embryo stage embryonic stage 17 Temporal ordering number - 390. FBdv:00005335 Stage 17 begins when the dorsal ridge (frontal sac) has overgrown the tip of the cylpeolabrum, which is thereby enclosed in the atrium. It lasts until hatching of the embryo (approximately 24 hours after egg laying), during which time much terminal differentiation occurs, the tracheal tree fills with air, so becoming completely visible, and the ventral cord continues to retract. Duration at 25 degrees C: approximately 8 hours (16-24 hours after egg laying). Catherine Leroy