Which germ layer contributes to the formation of the circulatory system?

The mesoderm is the germ layer responsible for the development of the circulatory system. During embryonic development, the mesoderm gives rise to several key structures, including the heart, blood vessels, and blood cells. These components are critical for the proper functioning of the circulatory system, which is essential for transporting nutrients, gases, and waste products throughout the body.

The ectoderm primarily develops into structures associated with the nervous system and the outer layer of skin, while the endoderm forms the internal linings of organs such as the gastrointestinal tract and lungs. Therefore, they do not play a direct role in forming the circulatory system. The option stating "all of the above" would imply that the ectoderm and endoderm contribute to the circulatory system, which is not accurate, as their developmental pathways focus on other organ systems. Thus, the correct answer emphasizes the specific contribution of the mesoderm to the circulatory system's development.