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An embryo develops only with an adequate supply of oxygen and metabolites. In its early stages these may be provided by diffusion. Because the rate of diffusion becomes limiting beyond a certain size, however, the circulatory system becomes functional early in development, often before other organs and systems are obvious. The heart develops from the middle embryonic tissue layer, the mesoderm, just below the anterior part of the gut. It begins as a tube that joins with blood vessels also forming in the mesoderm. Other mesodermal cells form a coat around the heart tube and become the muscular wall, or ...(100 of 13162 words)