The Evolution of Man, vol 1, Ernst Haeckel [books to read this summer TXT] 📗

the principles which I laid down in my Monograph on the Sponges, and developed in the Study of the Gastraea Theory. We have first to distinguish the unicellular animals (protozoa) from the multicellular tissue-forming (metazoa). Only the latter exhibit the important processes of segmentation and gastrulation; and they alone have a primitive gut, and form germinal layers and tissues.

The metazoa, the tissue-animals or gut-animals, then subdivide into two main sections, according as a body-cavity is or is not developed between the primary germinal layers. We may call these the coelenteria and coelomaria, the former are often also called zoophytes or coelenterata, and the latter bilaterals. This division is the more important as the coelenteria (without coelom) have no blood and blood-vessels, nor an anus. The coelomaria (with body-cavity) have generally an anus, and blood and blood-vessels. There are four stems belonging to the coelenteria: the gastraeads (“primitive-gut animals”), sponges, cnidaria, and platodes. Of the coelomaria we can distinguish six stems: the vermalia at the bottom represent the common stem-group (derived from the platodes) of these, the other five typical stems of the coelomaria—the molluscs, echinoderms, articulates, tunicates, and vertebrates—being evolved from them.

Man is, in his whole structure, a true vertebrate, and develops from an impregnated ovum in just the same characteristic way as the other vertebrates. There can no longer be the slightest doubt about this fundamental fact, nor of the fact that all the vertebrates form a natural phylogenetic unity, a single stem. The whole of the members of this stem, from the amphioxus and the cyclostoma to the apes and man, have the same characteristic disposition, connection, and development of the central organs, and arise in the same way from the common embryonic form of the chordula. Without going into the difficult question of the origin of this stem, we must emphasise the fact that the vertebrate stem has no direct affinity whatever to five of the other ten stems; these five isolated phyla are the sponges, cnidaria, molluscs, articulates, and echinoderms. On the other hand, there are important and, to an extent, close phylogenetic relations to the other five stems—the protozoa (through the amoebae), the gastraeads (through the blastula and gastrula), the platodes and vermalia (through the coelomula), and the tunicates (through the chordula).

How we are to explain these phylogenetic relations in the present state of our knowledge, and what place is assigned to the vertebrates in the animal ancestral tree, will be considered later (Chapter 2.20).

For the present our task is to make plainer the vertebrate character of man, and especially to point out the chief peculiarities of organisation by which the vertebrate stem is profoundly separated from the other eleven stems of the animal kingdom. Only after these comparative-anatomical considerations shall we be in a position to attack the difficult question of our embryology. The development of even the simplest and lowest vertebrate from the simple chordula (Figures 1.83 to 1.86) is so complicated and difficult to follow that it is necessary to understand the organic features of the fully-formed vertebrate in order to grasp the course of its embryonic evolution.

But it is equally necessary to confine our attention, in this general anatomic description of the vertebrate-body, to the essential facts, and pass by all the unessential. Hence, in giving now an ideal anatomic description of the chief features of the vertebrate and its internal organisation, I omit all the subordinate points, and restrict myself to the most important characteristics.

Much, of course, will seem to the reader to be essential that is only of subordinate and secondary interest, or even not essential at all, in the light of comparative anatomy and embryology. For instance, the skull and vertebral column and the extremities are non-essential in this sense. It is true that these parts are very important PHYSIOLOGICALLY; but for the MORPHOLOGICAL conception of the vertebrate they are not essential, because they are only found in the higher, not the lower, vertebrates. The lowest vertebrates have neither skull nor vertebrae, and no extremities or limbs. Even the human embryo passes through a stage in which it has no skull or vertebrae; the trunk is quite simple, and there is yet no trace of arms and legs. At this stage of development man, like every other higher vertebrate, is essentially similar to the simplest vertebrate form, which we now find in only one living specimen. This one lowest vertebrate that merits the closest study—undoubtedly the most interesting of all the vertebrates after man—is the famous lancelet or amphioxus, to which we have already often referred. As we are going to study it more closely later on (Chapters 2.16 and 2.17), I will only make one or two passing observations on it here.

The amphioxus lives buried in the sand of the sea, is about one or two inches in length, and has, when fully developed, the shape of a very simple, longish, lancet-like leaf; hence its name of the lancelet. The narrow body is compressed on both sides, almost equally pointed at the fore and hind ends, without any trace of external appendages or articulation of the body into head, neck, breast, abdomen, etc. Its whole shape is so simple that its first discoverer thought it was a naked snail. It was not until much later—half a century ago—that the tiny creature was studied more carefully, and was found to be a true vertebrate. More recent investigations have shown that it is of the greatest importance in connection with the comparative anatomy and ontogeny of the vertebrates, and therefore with human phylogeny. The amphioxus reveals the great secret of the origin of the vertebrates from the invertebrate vermalia, and in its development and structure connects directly with certain lower tunicates, the ascidia.

When we make a number of sections of the body of the amphioxus, firstly vertical longitudinal sections through the whole body from end to end, and secondly transverse sections from right to left, we get anatomic pictures of the utmost instructiveness (cf. Figures 1.98 to 1.102). In the main they correspond to the ideal which we form, with the aid of comparative anatomy and ontogeny, of the primitive type or build of the vertebrate—the long-extinct form to which the whole stem owes its origin. As we take the phylogenetic unity of the vertebrate stem to be beyond dispute, and assume a common origin from a primitive stem-form for all the vertebrates, from amphioxus to man, we are justified in forming a definite morphological idea of this primitive vertebrate (Prospondylus or Vertebraea). We need only imagine a few slight and unessential changes in the real sections of the amphioxus in order to have this ideal anatomic figure or diagram of the primitive vertebrate form, as we see in Figures 1.98 to 1.102. The amphioxus departs so little from this primitive form that we may, in a certain sense, describe it as a modified “primitive vertebrate.” (

The ideal figure of the vertebrate as given in Figures 1.98 to 1.102

is a hypothetical scheme or diagram, that has been chiefly constructed on the lines of the amphioxus, but with a certain attention to the comparative anatomy and ontogeny of the ascidia and appendicularia on the one hand, and of the cyclostoma and selachii on the other. This diagram has no pretension whatever to be an “exact picture,” but merely an attempt to reconstruct hypothetically the unknown and long extinct vertebrate stem-form, an ideal “archetype.”) The outer form of our hypothetical primitive vertebrate was at all events very simple, and probably more or less similar to that of the lancelet. The bilateral or bilateral-symmetrical body is stretched out lengthways and compressed at the sides (Figures 1.98 to 1.100), oval in section (Figures 1.101 and 1.102). There are no external articulation and no external appendages, in the shape of limbs, legs, or fins. On the other hand, the division of the body into two sections, head and trunk, was probably clearer in Prospondylus than it is in its little-changed ancestor, the amphioxus. In both animals the fore or head-half of the body contains different organs from the trunk, and different on the dorsal from on the ventral side. As this important division is found even in the sea-squirt, the remarkable invertebrate stem-relative of the vertebrates, we may assume that it was also found in the prochordonia, the common ancestors of both stems. It is also very pronounced in the young larvae of the cyclostoma; this fact is particularly interesting, as this palingenetic larva-form is in other respects also an important connecting-link between the higher vertebrates and the acrania.

(FIGURES 1.98 TO 1.102. The ideal primitive vertebrate (prospondylus).

Diagram. Figure 1.98 side-view (from the left). Figure 1.99 back-view.

Figure 1.100 front view. Figure 1.101 transverse section through the head (to the left through the gill-pouches, to the right through the gill-clefts). Figure 1.102 transverse section of the trunk (to the right a prorenal canal is affected). a aorta, af anus, au eye, b lateral furrow (primitive renal process), c coeloma (body-cavity), d small intestine, e parietal eye (epiphysis), f fin border of the skin, g auditory vesicle, gh brain, h heart, i muscular cavity (dorsal coelom-pouch), k gill-grut, ka gill-artery, kg gill-arch, ks gill-folds, l liver, ma stomach, md mouth, ms muscles, na nose (smell pit), n renal canals, u apertures of same, o outer skin, p gullet, r spinal marrow, a sexual glands (gonads), t corium, u kidney-openings (pores of the lateral furrow), v visceral vein (chief vein). x chorda, y hypophysis (urinary appendage), z gullet-groove or gill-groove (hypobranchial groove).)

The head of the acrania, or the anterior half of the body (both of the real amphioxus and the ideal prospondylus), contains the branchial (gill) gut and heart in the ventral section and the brain and sense-organs in the dorsal section. The trunk, or posterior half of the body, contains the hepatic (liver) gut and sexual-glands in the ventral part, and the spinal marrow and most of the muscles in the dorsal part.

In the longitudinal section of the ideal vertebrate (Figure 1.98) we have in the middle of the body a thin and flexible, but stiff, cylindrical rod, pointed at both ends (ch). It goes the whole length through the middle of the body, and forms, as the central skeletal axis, the original structure of the later vertebral column. This is the axial rod, or chorda dorsalis, also called chorda vertebralis, vertebral cord, axial cord, dorsal cord, notochorda, or, briefly, chorda. This solid, but flexible and elastic, axial rod consists of a cartilaginous mass of cells, and forms the inner axial skeleton or central frame of the body; it is only found in vertebrates and tunicates, not in any other animals. As the first structure of the spinal column it has the same radical significance in all vertebrates, from the amphioxus to man. But it is only in the amphioxus and the cyclostoma that the axial rod retains its simplest form throughout life. In man and all the higher vertebrates it is found only in the earlier embryonic period, and is afterwards replaced by the articulated vertebral column.

The axial rod or chorda is the real solid chief axis of the vertebrate body, and at the same time corresponds to the ideal long-axis, and serves to direct us with some confidence in the orientation of the principal organs. We therefore take the vertebrate-body in its original, natural disposition, in which the long-axis lies horizontally, the dorsal side upward and the ventral side downward (Figure 1.98). When we make a vertical section through the whole length of this long axis, the body divides into two equal and symmetrical halves, right and left. In each half we have ORIGINALLY

the same organs in the same disposition and connection; only their disposal in relation to the vertical plane of section, or median plane, is exactly reversed: the left half is the reflection of the right. We call the two halves antimera (opposed-parts). In