Chordates are the animals that have notochord, a skeletal rod present at some stage in life cycle. In lower vertebrates, notochord persists throughout life while in higher vertebrates it is replaced by vertebral column in adults. Nonchordates never develop notochord, not even in embryonic stage.

Introduction:
The notochord is a flexible rod-like structure that is present in the embryonic stage of all chordates. It serves as an important developmental structure and is eventually replaced by the vertebral column in most vertebrates. The presence of a notochord is a defining characteristic of chordates.

Explanation:
Chordates are a diverse group of animals that includes fish, amphibians, reptiles, birds, mammals, and some invertebrate species. The notochord is present in the embryonic stage of all chordates, regardless of whether they are vertebrates or non-vertebrates.

The Presence of Notochord:
The notochord is located dorsally to the digestive tract and ventrally to the neural tube in the developing embryo. It provides support and rigidity to the body, acting as an axis around which other structures develop. The notochord also plays a crucial role in the early development of the nervous system.

Role of Notochord:
The notochord acts as a signaling center and produces important signaling molecules that influence the development of surrounding tissues and organs. It also serves as a template for the formation of the vertebral column in vertebrates. As development progresses, the notochord is gradually replaced by the vertebrae, which provide more efficient support and protection for the spinal cord.

Examples:
1. In fish, the notochord is present throughout life, providing structural support to the body.
2. In amphibians, reptiles, birds, and mammals, the notochord is present only during the embryonic stage. It is eventually replaced by the vertebral column.

Conclusion:
In conclusion, the notochord is present in the embryonic stage of all chordates, including both vertebrates and non-vertebrates. It is an important developmental structure that provides support and influences the development of surrounding tissues and organs. However, in most vertebrates, the notochord is eventually replaced by the vertebral column.