Articulation begins in all vertebrates at a very early embryonic stage, and this indicates the considerable phylogenetic age of the process. When the chordula (Figures 1.83 to 1.86) has completed its characteristic composition, often even a little earlier, we find in the amniotes, in the middle of the sole-shaped embryonic shield, several pairs of dark square spots, symmetrically distributed on both sides of the chorda (Figures 1.131 to 1.135). Transverse sections (Figure 1.93 uw) show that they belong to the stem-zone (episoma) of the mesoderm, and are separated from the parietal zone (hyposoma) by the lateral folds; in section they are still quadrangular, almost square, so that they look something like dice. These pairs of "cubes" of the mesoderm are the first traces of the primitive segments or somites, the so-called "protovertebrae." (Figures 1.153 to 1.155 uw). (FIGURE 1.156. Embryo of the amphioxus, sixteen hours old, seen from the back. (From Hatschek.) d primitive gut, u primitive mouth, p polar cells of the mesoderm, c coelom-pouches, m their first segment, n medullary tube, i entoderm, e ectoderm, s first segment-fold. FIGURE 1.157. Embryo of the amphioxus, twenty hours old, with five somites. (Right view; for left view see Figure 1.124.) (From Hatschek.) V fore end, H hind end. ak, mk, ik outer, middle, and inner germinal layers; dh alimentary canal, n neural tube, cn canalis neurentericus, ush coelom-pouches (or primitive-segment cavities), us1 first (and foremost) primitive segment.)