Notochord

The notochord, a flexible rod in the back, was observed in many fishlike vertebrates and was used to understand evolutionary relationships among species, playing a significant role in the study of comparative embryology.
 Karl Ernst von Baer Estonia 1828 Notochord

The notochord, a flexible rod in the back, was observed in many fishlike vertebrates and was used to understand evolutionary relationships among species, playing a significant role in the study of comparative embryology.