The primary germ layers form quite early in embryogeny. They appear, for example, during the 16th day of human embryonic development. The outermost layer is called the ectoderm; the innermost, the endoderm. Between these two layers lies the mesoderm. At first, the three layers are more or less uniform. Soon, however, they begin to take on different characteristics. In each layer of the embryo, unique tissues and organs begin to take shape.
Every adult organ is identified with one of the germ layers. However, it should be understood that no structure in the adult body is the exclusive product of a single germ layer. Actually, only the physiologically unique portion of a particular organ can be traced to just one of the layers. Thus, though the intestines are said to originate from the endoderm, this is true only of their secreting and absorbing inner lining. The muscles, connective tissue, blood vessels, and nerves that make up the bulk of the intestines originate from the mesoderm or the ectoderm
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