Intraembryonic Coelom Development
In the development of the human embryo the intraembryonic coelom (or somatic coelom) is a portion of the conceptus that forms in the mesoderm. During the second week of development, the lateral mesoderm splits into a dorsal somatic mesoderm (somatopleure) and a ventral splanchnic mesoderm (splanchnopleure). By the third week of development, this process has given rise to a cavity between the somatopleure and splanchnopleure, which is referred to as the intraembryonic coelom. This space later gives rise to both the thoracic and abdominal cavities.
In the developing vertebrate embryo, somites are masses of mesoderm which can be found distributed along the two sides of the neural tube. They will eventually become dermis (dermatome), skeletal muscle (myotome), and vertebrae (sclerotome).
The mesoderm found lateral to the neural tube is called the paraxial mesoderm. It is separate from the chordamesoderm underneath the neural tube. The mesoderm ultimately becomes the notochord. In chick embryos, the paraxial mesoderm is initially called the "segmental plate. " It is referred to as the "unsegmented mesoderm" in other vertebrates. As the primitive streak regresses and neural folds gather preceding the formation of the neural tube, the paraxial mesoderm divides into blocks called somites. Somites play a critical role in early development participating in the specification of the migration paths of neural crest cells and spinal nerve axons. Later in development, somites separate into four compartments: the sclerotome, which forms the vertebrae and the rib cartilage; the myotome, which forms the musculature of the back, the ribs and the limbs; the dermatome, which forms the skin on the back; and the syndetome, which forms the tendons and some blood vessels.