Final answer:
In chordates, embryonic structures like the notochord and pharyngeal slits often serve temporary purposes and are transformed as development progresses; for example, the human embryonic tail is lost before birth. These features support the study of embryology and indicate a shared evolutionary ancestry.
Step-by-step explanation:
In many chordate species, the notochord and other embryonic structures such as pharyngeal slits are reduced or transformed during embryonic development. The notochord, a flexible, rod-like mesodermal structure, is present in all chordate embryos and provides skeletal support. However, in vertebrates, it is replaced by a vertebral column as development progresses. Similarly, pharyngeal slits, which in some species develop into gills, in others, including humans, develop into structures such as the middle ear.
The presence of these structures during embryonic development is evidence of a common ancestry among chordates, despite divergence into vastly different adult forms. This concept is a key aspect of embryology, the study of an organism's development from embryo to adult. Structures like the tail found in great ape embryos, including humans, which is lost before birth, and sessile adult tunicates that lose the notochord, highlight the varying functions and fates of these embryonic features across different chordate species.