The Evolution of Man, V.1. - Ernst Haeckel (learn to read activity book TXT) 📗

in size, in complexity of structure, and in the advanced character of its functions. From the point of view of both anatomy and physiology, the vertebrate stem outstrips all the other, or invertebrate, animals.

There is only one among the twelve stems of the animal kingdom that can in many respects be compared with the vertebrates, and reaches an equal, if not a greater, importance in many points. This is the stem of the articulates, composed of three classes: 1, the annelids (earth-worms, leeches, and cognate forms); 2, the crustacea (crabs, etc.); 3, the tracheata (spiders, insects, etc.). The stem of the articulates is superior not only to the vertebrates, but to all other animal-stems, in variety of forms, number of species, elaborateness of individuals, and general importance in the economy of nature.

When we have thus declared the vertebrates and the articulates to be the most important and most advanced of the twelve stems of the animal kingdom, the question arises whether this special position is accorded to them on the ground of a peculiarity of organisation that is common to the two. The answer is that this is really the case; it is their segmental or transverse articulation, which we may briefly call metamerism. In all the vertebrates and articulates the developed individual consists of a series of successive members (segments or metamera = "parts"); in the embryo these are called primitive segments or somites. In each of these segments we have a certain group of organs reproduced in the same arrangement, so that we may regard each segment as an individual unity, or a special "individual" subordinated to the entire personality.

The similarity of their segmentation, and the consequent physiological advance in the two stems of the vertebrates and articulates, has led to the assumption of a direct affinity between them, and an attempt to derive the former directly from the latter. The annelids were supposed to be the direct ancestors, not only of the crustacea and tracheata, but also of the vertebrates. We shall see later (

Chapter 2.

20) that this annelid theory of the vertebrates is entirely wrong, and ignores the most important differences in the organisation of the two stems. The internal articulation of the vertebrates is just as profoundly different from the external metamerism of the articulates as are their skeletal structure, nervous system, vascular system, and so on. The articulation has been developed in a totally different way in the two stems. The unarticulated chordula (Figures 1.83 to 1.86), which we have recognised as one of the chief palingenetic embryonic forms of the vertebrate group, and from which we have inferred the existence of a corresponding ancestral form for all the vertebrates and tunicates, is quite unthinkable as the stem-form of the articulates.

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 vertebrae (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 1.

11), 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.

(FIGURES 1.153 TO 1.155. Sole-shaped embryonic disk of the chick, in three successive stages of development, looked at from the dorsal surface, magnified about twenty times, somewhat diagrammatic. Figure 1.153 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). Figure 1.154 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. Figure 1.155 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.)

It is, therefore, wrong to describe the first rudimentary segments in the vertebrate embryo as primitive vertebrae or provertebrae; the fact that they have been so called for some time has led to much error and misunderstanding. Hence we shall give the name of "somites" or primitive segments to these so-called "primitive vertebrae." If the latter name is retained at all, it should only be used of the sclerotom--i.e., the small part of the somites from which the later vertebra does actually develop.

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.