Final answer:
The early central nervous system is formed by the neural tube, which differentiates into the brain and spinal cord as the embryo develops.
Step-by-step explanation:
The structure that serves as the early central nervous system is the neural tube. This simple, hollow tube forms early in embryonic development and extends from the anterior to the posterior end of the embryo. Around 25 days after conception, the anterior end of the neural tube starts to develop into the brain while the posterior portion gives rise to the spinal cord. As the neural tube develops, it differentiates into three primary vesicles, which correspond to the forebrain, midbrain, and hindbrain in the adult brain. These three vesicles further differentiate into five secondary vesicles, leading to the formation of the cerebrum from the telencephalon and the thalamus and hypothalamus from the diencephalon.
The rest of the neural tube, which does not differentiate into the brain, continues to develop into the spinal cord. This retains the hollow structure of the neural tube with the central canal running through it. The central canal of the spinal cord and the ventricles within the brain, which contain cerebrospinal fluid, are remnants of this early neural tube structure throughout the nervous system's development.