substage of immediately preceded by developmental stage late embryonic stage embryonic stage 16 embryonic stage 17 Temporal ordering number - 390. FlyBase_development_CV Stage 17 begins when the dorsal ridge (frontal sac) has overgrown the tip of the clypeolabrum, 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). FBdv:00005335