Final answer:
The embryonic germ layer that forms structures such as the skeleton, muscles, and cardiovascular system, but not the epidermis, nervous system, or certain glandular epithelial linings, is known as the mesoderm.
Step-by-step explanation:
The germ layer that forms structures such as the skeleton, muscles, connective tissue, heart, blood vessels, and kidneys, but not the epidermis of the skin, nervous system, or epithelial linings and glands, is known as the mesoderm. This is the middle embryonic germ layer that differentiates into various organ systems during the developmental process. The ectoderm and endoderm are responsible for the formation of the epidermis and nervous system, and the epithelial linings of various internal organs, respectively.
During embryonic development, a phenomenon called gastrulation leads to the formation of these three distinct germ layers. The mesoderm emerges between the ectoderm, the outermost layer, which gives rise to the nervous system and the epidermal skin cells, and the endoderm, the innermost layer, which gives rise to the lining of the digestive and respiratory systems and many internal organs. Each germ layer is programmed to develop specific tissues and organs indicative of its positioning within the embryonic structure.
The embryonic germ layer in question is the mesoderm. It forms various systems but excludes the epidermis, nervous system, and certain glands and linings.