develops from late embryo stage 16 embryo stage 17 embryo 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 nerve cord continues to retract. Duration at 25 degrees C: approximately 8 hours (960-1440 minutes after egg laying). FBbt:00005335 fly_anatomy.ontology DEPRECATION WARNING.