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.)
Among the mammals the embryos of the marsupials have three pairs of somites (Figure 1.131) after sixty hours, and eight pairs after seventy-two hours (Figure 1.135). They develop more slowly in the embryo of the rabbit; this has three somites on the eighth day (Figure 1.132), and eight somites a day later (Figure 1.134). In the incubated hen's egg the first somites make their appearance thirty hours after incubation begins (Figure 1.153). At the end of the second day the number has risen to sixteen or eighteen (Figure 1.155). 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, Figures 1.98 to 1.102) nearly the whole of the fore half corresponds to the head, and the hind half to the trunk.
The number of the metamera, and of the embryonic somites or primitive segments from which they develop, varies considerably in the vertebrates, according as the hind part of the body is short or is lengthened by a tail. In the developed man the trunk (including the rudimentary tail) consists of thirty-three metamera, the solid centre of which is formed by that number of vertebrae in the vertebral column (seven cervical, twelve dorsal, five lumbar, five sacral, and four caudal). To these we must add at least nine head-vertebrae, which originally (in all the craniota) constitute the skull. Thus the total number of the primitive segments of the human body is raised to at least forty-two; it would reach forty-five to forty-eight if (according to recent investigations) the number of the original segments of the skull is put at twelve to fifteen. In the tailless or anthropoid apes the number of metamera is much the same as in man, only differing by one or two; but it is much larger in the long-tailed apes and most of the other mammals. In long serpents and fishes it reaches several hundred (sometimes 400).
(FIGURES 1.158 TO 1.160. Embryo of the amphioxus, twenty four hours old, with eight somites. (From Hatschek.) Figures 1.158 and 1.159 lateral view (from left). Figure 1.160 seen from back. In Figure 1.158 only the outlines of the eight primitive segments are indicated, in Figure 1.159 their cavities and muscular walls. V fore end, H hind end, d gut, du under and dd upper wall of the gut, ne canalis neurentericus, nv ventral, nd dorsal wall of the neural tube, np neuroporus, dv fore pouch of the gut, ch chorda, mf mesodermic fold, pm polar cells of the mesoderm (ms), e ectoderm.)
In order to understand properly the real nature and origin of articulation in the human body and that of the higher vertebrates, it is necessary to compare it with that of the lower vertebrates, and bear in mind always the genetic connection of all the members of the stem. In this the simple development of the invaluable amphioxus once more furnishes the key to the complex and cenogenetically modified embryonic processes of the craniota. The articulation of the amphioxus begins at an early stage—earlier than in the craniotes. The two coelom-pouches have hardly grown out of the primitive gut (Figure 1.156 c) when the blind fore part of it (farthest away from the primitive mouth, u) begins to separate by a transverse fold (s): this is the first primitive segment. Immediately afterwards the hind part of the coelom-pouches begins to divide into a series of pieces by new transverse folds (Figure 1.157). The foremost of these primitive segments (us1) is the first and oldest; in Figures 1.124 and 1.157 there are already five formed. They separate so rapidly, one behind the other, that eight pairs are formed within twenty-four hours of the beginning of development, and seventeen pairs twenty-four hours later. The number increases as the embryo grows and extends backwards, and new cells are formed constantly (at the primitive mouth) from the two primitive mesodermic cells (Figures 1.159 to 1.160).
(FIGURES 1.161 AND 1.162. Transverse section of shark-embryos (through the region of the kidneys). (From Wijhe and Hertwig.) In Figure 1.162 the dorsal segment-cavities (h) are already separated from the body-cavity (lh), but they are connected a little earlier (Figure 1.161), nr neural tube, ch chorda, sch subchordal string, ao aorta, sk skeletal-plate, mp muscle-plate, cp cutis-plate, w connection of latter (growth-zone), vn primitive kidneys, ug prorenal duct, uk prorenal canals, us point where they are cut off, tr prorenal funnel, mk middle germ-layer (mk1 parietal, mk2 visceral), ik inner germ-layer (gut-gland layer).)
This typical articulation of the two coelom-sacs begins very early in the lancelet, before they are yet severed from the primitive gut, so that at first each segment-cavity (us) still communicates by a narrow opening with the gut, like an intestinal gland. But this opening soon closes by complete severance, proceeding regularly backwards. The closed segments then extend more, so that their upper half grows upwards like a fold between the ectoderm (ak) and neural tube (n), and the lower half between the ectoderm and alimentary canal (ch; Figure 1.82 d, left half of the figure). Afterwards the two halves completely separate, a lateral longitudinal fold cutting between them (mk, right half of Figure 1.82). The dorsal segments (sd) provide the muscles of the trunk the whole length of the body (1.159): this cavity afterwards disappears. On the other hand, the ventral parts give rise, from their uppermost section, to the pronephridia or primitive-kidney canals, and from the lower to the segmental rudiments of the sexual glands or gonads. The partitions of the muscular dorsal pieces (myotomes) remain, and determine the permanent articulation of the vertebrate organism. But the partitions of the large ventral pieces (gonotomes) become thinner, and afterwards disappear in part, so that their cavities run together to form the metacoel, or the simple permanent body-cavity.
The articulation proceeds in substantially the same way in the other vertebrates, the craniota, starting from the coelom-pouches. But whereas in the former case there is first a transverse division of the coelom-sacs (by vertical folds) and then the dorso-ventral division, the procedure is reversed in the craniota; in their case each of the long coelom-pouches first divides into a dorsal (primitive segment plates) and a ventral (lateral plates) section by a lateral longitudinal fold. Only the former are then broken up into primitive segments by the subsequent vertical folds; while the latter (segmented for a time in the amphioxus) remain undivided, and, by the divergence of their parietal and visceral plates, form a body-cavity that is unified from the first. In this case, again, it is clear that we must regard the features of the younger craniota as cenogenetically modified processes that can be traced palingenetically to the older acrania.