Mesoderm gives rise to all the structures except

The correct option/answer is: nervous system

During embryonic development in triploblastic animals, the gastrula stage forms three primary germ layers: the ectoderm (outer layer), the mesoderm (middle layer), and the endoderm (inner layer). Each of these layers is responsible for giving rise to specific tissues, organs, and systems in the mature organism.

Let us analyze the origin of the structures listed in the options:
1. Gonads: The reproductive organs (testes and ovaries) originate from the mesoderm.
2. Circulatory system: The heart, blood vessels, and blood cells develop from the mesoderm.
3. Muscular system: Skeletal, smooth, and cardiac muscles are derived from the mesoderm.
4. Nervous system: The nervous system, including the brain, spinal cord, and peripheral nerves, develops from the ectoderm (specifically through the process of neurulation).

Since the nervous system is derived from the ectoderm rather than the mesoderm, it is the exception. Therefore, the mesoderm gives rise to all the listed structures except the nervous system.