Embryonic Development: Origin of Heart, Muscles, Bones & Blood - Mesoderm Explained

The correct answer is mesoderm.

The heart, muscles, bones, and blood all develop primarily from the mesoderm. The mesoderm is one of the three germ layers that form during embryonic development. It is located between the ectoderm and endoderm. The mesoderm gives rise to a variety of tissues and organs, including the heart, muscles, bones, blood, kidneys, and reproductive organs.