The correct answer is (a) Mesodermal and dorsal to nerve cord.

The notochord is a defining characteristic of chordates, which include vertebrates (animals with a backbone) and some invertebrate groups. It is an important developmental structure that plays a crucial role in the formation of the vertebrate body plan.

Formation of the Notochord:
The notochord is formed during embryonic development from the mesoderm, one of the three primary germ layers. The mesoderm is located between the ectoderm (outer layer) and endoderm (inner layer) in the developing embryo.

The notochord develops as a rod-like structure that extends along the dorsal (back) side of the embryo. It forms just below the ectoderm and runs parallel to the central nervous system. The central nervous system consists of the neural tube, which later develops into the brain and spinal cord.

Functions of the Notochord:
The notochord serves several important functions during embryonic development and in the adult vertebrate:

1. Structural Support: The notochord provides a rigid and yet flexible structure that serves as a temporary skeletal axis for the developing embryo. It helps in maintaining the shape and integrity of the developing body.

2. Induction of Neural Tube: The notochord plays a crucial role in the induction of the neural tube, which later develops into the brain and spinal cord. It secretes signaling molecules that influence the development of the neural plate, leading to the formation of the neural tube.

3. Formation of Vertebral Column: In vertebrates, the notochord is eventually replaced by the vertebral column (backbone). The notochord acts as a template for the formation of the vertebral bodies, which surround and protect the spinal cord.

Dorsal Position:
The notochord is located dorsally to the nerve cord, which is the precursor to the central nervous system. It runs parallel to the nerve cord and provides structural support to the developing embryo.

In conclusion, the notochord is a mesodermal structure that develops dorsal to the nerve cord during embryonic development in chordates. It plays a crucial role in the formation of the vertebrate body plan and acts as a temporary skeletal axis.