Mesoderm
One of the three primary germ cell layers -- the other two are the ectoderm and endoderm -- in the very early embryo.
The mesoderm is the middle layer.
It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the middle layer of the skin.
Some cells in mesodermal tissues retain the capacity to differentiate in diverse directions.
For example, some cells in the bone marrow (mesoderm) can become liver (endoderm)