While the notochord is forming (see neuroembryology), the mesoderm is growing on either side of the midline (as the primitive streak recedes)- forming 3 columnar structures:paraxial, intermediate and lateral plate mesoderm.
The paraxial mesoderm (like those that have formed the notochord), originally come from epiblast cells migrating through the primitive node/cranial portion of primitive streak. It forms a pair of cylindrical structures that are adjacent to the neural tube and notochord.
- At around day 24/25 the paraxial mesoderm begins to segment (beginning at the cranial end, working caudally) into somites. By day 32, around 35-37 somite pairs are present. Somites will go on to become the axial skeleton (sclerotome), striated muscle (myotome) and subcutaneous tissue and skin (dermatome). The arrangement and order of the somites will determine that of the spine, spinal nerves, trunk and chest (ribs etc).