In the accompanying figure (fig. 4) the notochord is seen in section fully formed and lying between the entoderm and the neural canal. Its first formation is at an earlier period than this, before the neural groove has closed into a canal, and it appears at first as an upward groove from the most dorsal part of the ento derm in what will later on be the cervical region of the embryo. The groove, by the union of its edges, becomes a tube, but the cavity of this is soon obliterated by the growth of its cells, so that a solid elastic rod is formed which grows forward as far as the pituitary region of the skull and backward to where the end of the coccyx will be.
While the development of the notochord is going on the meso derm on each side of it is dividing itself into a series of masses called mesodermic somites (see fig. 4, PS) or protovertebrae. This process begins in the cervical region and proceeds forward and backward until thirty-eight pairs have been formed for the neck and trunk and probably four extra ones for the occipital region of the skull. Each of these somites consists of three parts : that nearest the surface ectoderm is the cutaneous lamella (fig. 4, CL). Deep to this and separated in the earlier-formed somites by a space is the muscle layer (fig. 4, ML) while deepest of all and nearest the nerve cord and notochord is the scleratogenous layer (fig. 4, SL). It is this layer which gradually meets its fellow of the opposite side and encloses the nerve cord and the notochord in continuous tubes of mesodermal tissue, thus forming the membra nous vertebral column, which is perforated for the exit of the spinal nerves, but the intervals between the successive mesodermic somites are still marked by the tissue being rather denser there.
The next stage is conversion into cartilage of each segment of the membranous vertebral column surrounding the notochord. In this way the bodies of the cartilaginous vertebrae are formed and each of these is segmental, that is, it corresponds to a muscle segment and a spinal nerve. The cartilaginous neural arch, however, which surrounds the nerve cord is intersegmental and is formed in the denser fibrous tissue which separates each somite from the next. This also applies to the cartilaginous ribs which appear in the fibrous intervals (myocommata) between the muscle plates (myo tomes), and so it is easy to realize that each typical rib must articulate with the bodies of two adjacent vertebrae, but with the neural arch, through its tranverse process, of only one.
The intersegmental tissue between the bodies of the vertebrae becomes the intervertebral discs and in the centre of these a pulpy mass is found which contains some remnants of the notochord. Elsewhere this structure is pressed out of existence and there is no further use for it when the cartilaginous vertebrae are once formed. One other series of structures must be mentioned though they do not play any great part in human development. In the intersegmental tissue ventral to each of the intervertebral discs a transverse rod of cells, known as a hypochordal bar, is formed which connects the heads of two opposite ribs. In man the greater number of these disappear, but in the case of the atlas the rod chondrifies to form the anterior (ventral) arch which is therefore intersegmental, while the segmental body of the atlas, through which the notochord is passing, joins the axis to form the odontoid process. These hypochordal bars are the last remnant in man of the haemal arch of the vertebrae of fishes (see subsection on comparative anatomy). In the cervical region the ribs form the ventral boundary of the foramen for the vertebral artery. They are so short that they become fused with the centrum and trans verse process, leaving the vertebrarterial canal between. Some times in the seventh cervical vertebra the rib element is much longer and remains as a separate cervical rib with definite joints.