develops from stage 15 embryo late embryo stage 16 embryo fly_anatomy.ontology DEPRECATION WARNING. 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 clypeolabrum, which is thereby enclosed in the atrium. During this stage the ventral nerve cord retracts to about 60% egg length. Duration at 25 degrees C: approximately 180 minutes (780-960 minutes after egg laying). FBbt:00005334