Definition: "Transient structure that lies immediately ventral to the notochord in the embryo, likely required for the formation of the dorsal aorta. A slight depression marks the future hypochord at NF stage 20 and its segregation from the dorsal endoderm begins at NF stage 24. It progresses from anterior to posterior and is complete by NF stage 28. At NF stage 32 it is present as a dorso-laterally flattened ribbon of yolk-laden cells. It degenerates by apoptosis and by NF stage 43 is no longer detectable as a distinct structure in the trunk region of the tadpole. The most posterior hypochord persists and begins to ossify, fusing with the coccyx to form the urostyle in the pelvic girdle around NF stage 63."