A group of cells in an embryo is known as the germ layer.
These layers associate with each other as the development of the embryo takes place and will finally give rise to all the organs and tissues of an organism.
It is any of the three primary cell layers produced in the initial stages of embryonic development, comprising the ectoderm (outer layer), the endoderm (inner layer), and the mesoderm (middle layer).
These layers are produced during the process of gastrulation.
Mesoderm, which lies between the ectoderm and endoderm, produces the tissues of the body like the heart, the dermis of the skin, the urogenital system, the muscle system, the bone marrow, and the bones.
Thus, it is the mesoderm that develops into bones.