Which of the following structures is NOT derived from the mesoderm?

The brain is the correct answer because it is derived from the ectoderm, which is the outermost layer of the three primary germ layers formed during embryonic development. The ectoderm gives rise to structures such as the nervous system and skin. In contrast, the heart, skeletal muscle, and kidneys are all derived from the mesoderm, which is the middle germ layer. The mesoderm is responsible for forming various structures, including muscles, bones, and most organs. Understanding the origins of these structures is crucial in developmental biology, as it helps explain how tissues and organs form in the body.