While the solid, massive bodies of the vertebræ represent the real central axis of the skeleton, the dorsal arches serve to protect the central marrow they enclose. But similar arches develop on the ventral side for the protection of the viscera in the breast and belly. These lower or ventral vertebral arches, proceeding from the ventral side of the vertebral bodies, form, in many of the lower Vertebrates, a canal in which the large blood-vessels are enclosed on the lower surface of the vertebral column (aorta and caudal vein). In the higher Vertebrates the majority of these vertebral arches are lost or become rudimentary. But at the thoracic section of the column they develop into independent strong osseous arches, the ribs (costæ). In reality the ribs are merely large and independent lower vertebral arches, which have lost their original connection with the vertebral bodies.
Fig. 331—Intervertebral disk of a new-born infant, transverse section. a rest of the chorda. (From Kölliker.)
If we turn from this anatomic survey of the composition of the column to the question of its development, I may refer the reader to earlier pages with regard to the first and most important points (pp. 145–148). It will be remembered that in the human embryo and that of the other vertebrates we find at first, instead of the segmented column, only a simple unarticulated cartilaginous rod. This solid but flexible and elastic rod is the axial rod (or the chorda dorsalis). In the lowest Vertebrate, the Amphioxus, it retains this simple form throughout life, and permanently represents the whole internal skeleton (Fig. 210 i). In the Tunicates, also, the nearest Invertebrate relatives of the Vertebrates, we meet the same chorda—transitorily in the passing larva tail of the Ascidia, permanently in the Copelata (Fig. 225 c). Undoubtedly both the Tunicates and Acrania have inherited the chorda from a common unsegmented stem-form; and these ancient, long-extinct ancestors of all the chordonia are our hypothetical Prochordonia.
Long before there is any trace of the skull, limbs, etc., in the embryo of man or any of the higher Vertebrates—at the early stage in which the whole body is merely a sole-shaped embryonic shield—there appears in the middle line of the shield, directly under the medullary furrow, the simple chorda. (Cf. Figs. 131–135 ch). It follows the long axis of the body in the shape of a cylindrical axial rod of elastic but firm composition, equally pointed at both ends. In every case the chorda originates from the dorsal wall of the primitive gut; the cells that compose it (Fig. 328 b) belong to the entoderm (Figs. 216–221). At an early stage the chorda develops a transparent structureless sheath, which is secreted from its cells (Fig. 328 a). This chordalemma is often called the “inner chorda-sheath,” and must not be confused with the real external sheath, the mesoblastic perichorda.
Fig. 332—Human skull.
But this unsegmented primary axial skeleton is soon replaced by the segmented secondary axial skeleton, which we know as the vertebral column. The provertebral plates (Fig. 124 s) differentiate from the innermost, median part of the visceral layer of the cœlom-pouches at each side of the chorda. As they grow round the chorda and enclose it they form the skeleton plate or skeletogenetic layer—that is to say, the skeleton-forming stratum of cells, which provides the mobile foundation of the permanent vertebral column and skull (scleroblast). In the head-half of the embryo the skeletal plate remains a continuous, simple, undivided layer of tissue, and presently enlarges into a thin-walled capsule enclosing the brain, the primordial skull. In the trunk-half the provertebral plate divides into a number of homogeneous, cubical, successive pieces; these are the several primitive vertebræ. They are not numerous at first, but soon increase as the embryo grows longer (Figs. 153–155).