Drosophila embryo stage embryonic stage 16 Catherine Leroy Stage 16 begins when the intersegmental grooves can be distinguished at mid-dorsal level, and ends when the dorsal ridge (frontal sac) has overgrown the tip of the cylpeolabrum, which is thereby enclosed in the atrium. During this stage the ventral cord retracts to about 60% egg length. Duration at 25 degrees C: approximately 180 minutes (780-960 minutes after egg laying). FBdv:00005334 Temporal ordering number - 380.