Articulation begins in all vertebrates at a very early embryonic stage, and this indicates the considerable phylogenetic age of the process. When the chordula (Figs. 83–86) 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 (Figs. 131–135).Transverse sections (Fig. 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 “protovertebræ.” (Figs. 153–155 _uw_). Fig.156. Embryo of the amphioxus, sixteen hours old, seen from the back. Fig. 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_ cœlom-pouches, _m_ their first segment, _n_ medullary tube, _i_ entoderm, _e_ ectoderm, _s_ first segment-fold. Among the mammals the embryos of the marsupials have three pairs of somites (Fig. 131) after sixty hours, and eight pairs after seventy-two hours (Fig. 135). They develop more slowly in the embryo of the rabbit; this has three somites on the eighth day (Fig. 132), and eight somites a day later (Fig. 134). In the incubated hen’s egg the first somites make their appearance thirty hours after incubation begins (Fig. 153). At the end of the second day the number has risen to sixteen or eighteen (Fig. 155). The articulation of the stem-zone, to which the somites owe their origin, thus proceeds briskly from front to rear, new transverse constrictions of the “protovertebral plates” forming continuously and successively. The first segment, which is almost half-way down in the embryonic shield of the amniote, is the foremost of all; from this first somite is formed the first cervical vertebra with its muscles and skeletal parts. It follows from this, firstly, that the multiplication of the primitive segments proceeds backwards from the front, with a constant lengthening of the hinder end of the body; and, secondly, that at the beginning of segmentation nearly the whole of the anterior half of the sole-shaped embryonic shield of the amniote belongs to the later head, while the whole of the rest of the body is formed from its hinder half. We are reminded that in the amphioxus (and in our hypothetic primitive vertebrate, Figs. 98–102) nearly the whole of the fore half corresponds to the head, and the hind half to the trunk.