Final answer:
The mesoderm is the middle embryonic germ layer that develops into muscles, bones, connective tissue, heart, blood vessels, and various organs like kidneys during embryonic development.
Step-by-step explanation:
The mesoderm, which is the middle layer of the three primary germ layers in an embryo, differentiates into various tissues and organs. During embryonic development, the mesoderm becomes muscles and bones, connective tissue, heart, blood vessels, and kidneys. It is responsible for forming all specialized muscle tissues, including cardiac and intestinal muscles, connective tissues such as the skeleton and blood cells, and most other visceral organs.