During which developmental stage is the notochord present in members of the phylum Chordata?

The notochord is a flexible, rod-like structure that is a defining characteristic of members of the phylum Chordata. It is present during some stage of development in all chordates, which is why the selected answer is correct.

In many chordates, such as amphibians and fish, the notochord provides structural support during the embryonic stage and can persist into adulthood, although in some vertebrates like mammals, it is largely replaced by the vertebral column (spine) during development. In other cases, such as in adults of certain species, remnants of the notochord can still be found. Thus, the presence of a notochord at various stages throughout a chordate's lifecycle is a fundamental aspect of their development, supporting the classification of these organisms within the phylum Chordata.

This understanding highlights that the notochord is not exclusively confined to a single phase of development, reinforcing its significance in the evolutionary biology of these animals.