All articulated animals came originally from unarticulated ones. This phylogenetic principle is as firmly established as the ontogenetic fact that every articulated animal-form develops from an unarticulated embryo. But the organisation of the embryo is totally different in the two stems. The chordula-embryo of all the vertebrates is characterised by the dorsal medullary tube, the neurenteric canal, which passes at the primitive mouth into the alimentary canal, and the axial chorda between the two. None of the articulates, either annelids or arthropods (crustacea and tracheata), show any trace of this type of organisation. Moreover, the development of the chief systems of organs proceeds in the opposite way in the two stems. Hence the segmentation must have arisen independently in each. This is not at all surprising; we find analogous cases in the stalk-articulation of the higher plants and in several groups of other animal stems.
The characteristic internal articulation of the vertebrates and its importance in the organisation of the stem are best seen in the study of the skeleton. Its chief and central part, the cartilaginous or bony vertebral column, affords an obvious instance of vertebrate metamerism; it consists of a series of cartilaginous or bony pieces, which have long been known as _vertebræ_ (or _spondyli_). Each vertebra is directly connected with a special section of the muscular system, the nervous system, the vascular system, etc. Thus most of the “animal organs” take part in this vertebration. But we saw, when we were considering our own vertebrate character (in Chapter XI), that the same internal articulation is also found in the lowest primitive vertebrates, the acrania, although here the whole skeleton consists merely of the simple chorda, and is not at all articulated. Hence the articulation does not proceed primarily from the skeleton, but from the muscular system, and is clearly determined by the more advanced swimming-movements of the primitive chordonia-ancestors.
Figs. 153-155. Sole-shaped embryonic disk of the chick, in three successive stages of development, looked at from the dorsal surface, magnified, somewhat diagrammatic. Figs. 153–155—Sole-shaped embryonic disk of the chick, in three successive stages of development, looked at from the dorsal surface, magnified, somewhat diagrammatic. Fig. 153 with six pairs of somites. Brain a simple vesicle (_hb_). Medullary furrow still wide open from _x_; greatly widened at _z. mp_ medullary plates, _sp_ lateral plates, _y_ limit of gullet-cavity (_sh_) and fore-gut (_vd_). Fig. 154 with ten pairs of somites. Brain divided into three vesicles: _v_ fore-brain, _m_ middle-brain, _h_ hind-brain, _c_ heart, _dv_ vitelline-veins. Medullary furrow still wide open behind (_z_). _mp_ medullary plates. Fig. 155 with sixteen pairs of somites. Brain divided into five vesicles: _v_ fore-brain, _z_ intermediate-brain, _m_ middle-brain, _h_ hind-brain, _n_ after-brain, _a_ optic vesicles, _g_ auditory vesicles, _c_ heart, _ dv_ vitelline veins, _mp_ medullary plate, _uw_ primitive vertebra.