(FIGURE 1.92. Transverse section of the chordula-embryo of a bird (from a hen's egg at the close of the first day of incubation). (From Kolliker,) h horn-plate (ectoderm), m medullary plate, Rf dorsal folds of same, Pv medullary furrow, ch chorda, uwp median (inner) part of the middle layer (median wall of the coelom-pouches), sp lateral (outer) part of same, or lateral plates, uwh structure of the body-cavity, dd gut-gland-layer.)

Hertwig even succeeded in showing, in the coelomula-embryo of the water salamander (Triton), between the first structures of the two middle layers, the relic of the body-cavity, which is represented in the diagrammatic transitional form (Figures 1.87 and 1.88). In sections both through the primitive mouth itself (Figure 1.89) and in front of it (Figure 1.90) the two middle layers (pb and vb) diverge from each other, and disclose the two body-cavities as narrow clefts.

At the primitive-mouth itself (Figure 1.90 u) we can penetrate into them from without. It is only here at the border of the primitive mouth that we can show the direct transition of the two middle layers into the two limiting layers or primary germinal layers.

The structure of the chorda also shows the same features in these coelomula-embryos of the amphibia (Figure 1.91) as in the amphioxus (Figures 1.79 to 1.82). It arises from the entodermic cell-streak, which forms the middle dorsal-line of the primitive gut, and occupies the s.p.a.ce between the flat coelom-pouches (Figure 1.91 A). While the nervous centre is formed here in the middle line of the back and separated from the ectoderm as "medullary tube," there takes place at the same time, directly underneath, the severance of the chorda from the entoderm (Figure 1.91 A, B, C). Under the chorda is formed (out of the ventral entodermic half of the gastrula) the permanent gut or visceral cavity (enteron) (Figure 1.91 B, dh). This is done by the coalescence, under the chorda in the median line, of the two dorsal side-borders of the gut-gland-layer (ik), which were previously separated by the chorda-plate (Figure 1.91 A, ch); these now alone form the clothing of the visceral cavity (dh) (enteroderm, Figure 1.91 C). All these important modifications take place at first in the fore or head-part of the embryo, and spread backwards from there; here at the hinder end, the region of the primitive mouth, the important border of the mouth (or properistoma) remains for a long time the source of development or the zone of fresh construction, in the further building-up of the organism. One has only to compare carefully the ill.u.s.trations given (Figures 1.85 to 1.91) to see that, as a fact, the cenogenetic coelomation of the amphibia can be deduced directly from the palingenetic form of the acrania (Figures 1.79 to 1.84).

(FIGURE 1.93. Transverse section of the vertebrate-embryo of a bird (from a hen's egg on the second day of incubation). (From Kolliker.) h horn-plate, mr medullary tube, ch chorda, uw primitive segments, uwh primitive-segment cavity (median relic of the coelom), sp lateral coelom-cleft, hpl skin-fibre-layer, df gut-fibre-layer, ung primitive-kidney pa.s.sage, ao primitive aorta, dd gut-gland-layer.)

The same principle holds good for the amniotes, the reptiles, birds, and mammals, although in this case the processes of coelomation are more modified and more difficult to identify on account of the colossal acc.u.mulation of food-yelk and the corresponding notable flattening of the germinal disk. However, as the whole group of the amniotes has been developed at a comparatively late date from the cla.s.s of the amphibia, their coelomation must also be directly traceable to that of the latter. This is really possible as a matter of fact; even the older ill.u.s.trations showed an essential ident.i.ty of features. Thus forty years ago Kolliker gave, in the first edition of his Human Embryology (1861), some sections of the chicken-embryo, the features of which could at once be reduced to those already described and explained in the sense of Hertwig's coelom-theory. A section through the embryo in the hatched hen's egg towards the close of the first day of incubation shows in the middle of the dorsal surface a broad ectodermic medullary groove (Figure 1.92 Rf), and underneath the middle of the chorda (ch) and at each side of it a couple of broad mesodermic layers (sp). These enclose a narrow s.p.a.ce or cleft (uwh), which is nothing else than the structure of the body-cavity. The two layers that enclose it--the upper parietal layer (hpl) and the lower visceral layer (df)--are pressed together from without, but clearly distinguishable. This is even clearer a little later, when the medullary furrow is closed into the nerve-tube (Figure 1.93 mr).

Special importance attaches to the fact that here again the four secondary germinal layers are already sharply distinct, and easily separated from each other. There is only one very restricted area in which they are connected, and actually pa.s.s into each other; this is the region of the primitive mouth, which is contracted in the amniotes into a dorsal longitudinal cleft, the primitive groove. Its two lateral lip-borders form the primitive streak, which has long been recognised as the most important embryonic source and starting-point of further processes. Sections through this primitive streak (Figures 1.94 and 1.95) show that the two primary germinal layers grow at an early stage (in the discoid gastrula of the chick, a few hours after incubation) into the primitive streak (x), and that the two middle layers extend outward from this thickened axial plate (y) to the right and left between the former. The plates of the coelom-layers, the parietal skin-fibre-layer (m) and the visceral gut-fibre-layer (f), are seen to be still pressed close together, and only diverge later to form the body-cavity. Between the inner borders of the two flat coelom-pouches lies the chorda (Figure 1.95 x), which here again develops from the middle line of the dorsal wall of the primitive gut.

(FIGURES 1.94 AND 1.95. Transverse section of the primitive-streak (primitive mouth) of the chick. Figure 1.94 a few hours after the commencement of incubation, Figure 1.95 a little later. (From Waldeyer.) h horn-plate, n nerve-plate, m skin-fibre-layer, f gut-fibre-layer, d gut-gland-layer, y primitive streak or axial plate, in which all four germinal layers meet, x structure of the chorda, u region of the later primitive kidneys.)

Coelomation takes place in the vertebrates in just the same way as in the birds and reptiles. This was to be expected, as the characteristic gastrulation of the mammal has descended from that of the reptiles. In both cases a discoid gastrula with primitive streak arises from the segmented ovum, a two-layered germinal disk with long and small hinder primitive mouth. Here again the two primary germinal layers are only directly connected (Figure 1.96 pr) along the primitive streak (at the folding-point of the blastula), and from this spot (the border of the primitive mouth) the middle germinal layers (mk) grow out to right and left between the preceding. In the fine ill.u.s.tration of the coelomula of the rabbit which Van Beneden has given us (Figure 1.96) one can clearly see that each of the four secondary germinal layers consists of a single stratum of cells.

Finally, we must point out, as a fact of the utmost importance for our anthropogeny and of great general interest, that the four-layered coelomula of man has just the same construction as that of the rabbit (Figure 1.96). A vertical section that Count Spee made through the primitive mouth or streak of a very young human germinal disk (Figure 1.97) clearly shows that here again the four secondary germ-layers are inseparably connected only at the primitive streak, and that here also the two flattened coelom-pouches (mk) extend outwards to right and left from the primitive mouth between the outer and inner germinal layers. In this case, too, the middle germinal layer consists from the first of two separate strata of cells, the parietal (mp) and visceral (mv) mesoblasts.

(FIGURE 1.96. Transverse section of the primitive groove (or primitive mouth) of a rabbit. (From Van Beneden.) pr primitive mouth, ul lips of same (primitive lips), ak and ik outer and inner germinal layers, mk middle germinal layer, mp parietal layer, mv visceral layer of the mesoderm.

FIGURE 1.97. Transverse section of the primitive mouth (or groove) of a human embryo (at the coelomula stage). (From Count Spee.) pr primitive mouth, ul lips of same (primitive folds), ak and ik outer and inner germinal layers, mk middle layer, mp parietal layer, mv visceral layer of the mesoblasts.)

These concordant results of the best recent investigations (which have been confirmed by the observations of a number of scientists I have not enumerated) prove the unity of the vertebrate-stem in point of coelomation, no less than of gastrulation. In both respects the invaluable amphioxus--the sole survivor of the acrania--is found to be the original model that has preserved for us in palingenetic form by a tenacious heredity these most important embryonic processes. From this primary model of construction we can cenogenetically deduce all the embryonic forms of the other vertebrates, the craniota, by secondary modifications. My thesis of the universal formation of the gastrula by folding of the blastula has now been clearly proved for all the vertebrates; so also has been Hertwig's thesis of the origin of the middle germinal layers by the folding of a couple of coelom-pouches which appear at the border of the primitive mouth. Just as the gastraea-theory explains the origin and ident.i.ty of the two primary layers, so the coelom-theory explains those of the four secondary layers. The point of origin is always the properistoma, the border of the original primitive mouth of the gastrula, at which the two primary layers pa.s.s directly into each other.

Moreover, the coelomula is important as the immediate source of the chordula, the embryonic reproduction of the ancient, typical, unarticulated, worm-like form, which has an axial chorda between the dorsal nerve-tube and the ventral gut-tube. This instructive chordula (Figures 1.83 to 1.86) provides a valuable support of our phylogeny; it indicates the important moment in our stem-history at which the stem of the chordonia (tunicates and vertebrates) parted for ever from the divergent stems of the other metazoa (articulates, echinoderms, and molluscs).

I may express here my opinion, in the form of a chordaea-theory, that the characteristic chordula-larva of the chordonia has in reality this great significance--it is the typical reproduction (preserved by heredity) of the ancient common stem-form of all the vertebrates and tunicates, the long-extinct Chordaea. We will return in Chapter 2.20 to these worm-like ancestors, which stand out as luminous points in the obscure stem-history of the invertebrate ancestors of our race.

CHAPTER 1.11. THE VERTEBRATE CHARACTER OF MAN.

We have now secured a number of firm standing-places in the labyrinthian course of our individual development by our study of the important embryonic forms which we have called the cytula, morula, blastula, gastrula, coelomula, and chordula. But we have still in front of us the difficult task of deriving the complicated frame of the human body, with all its different parts, organs, members, etc., from the simple form of the chordula. We have previously considered the origin of this four-layered embryonic form from the two-layered gastrula. The two primary germinal layers, which form the entire body of the gastrula, and the two middle layers of the coelomula that develop between them, are the four simple cell-strata, or epithelia, which alone go to the formation of the complex body of man and the higher animals. It is so difficult to understand this construction that we will first seek a companion who may help us out of many difficulties.

This helpful a.s.sociate is the science of comparative anatomy. Its task is, by comparing the fully-developed bodily forms in the various groups of animals, to learn the general laws of organisation according to which the body is constructed; at the same time, it has to determine the affinities of the various groups by critical appreciation of the degrees of difference between them. Formerly, this work was conceived in a teleological sense, and it was sought to find traces of the plan of the Creator in the actual purposive organisation of animals. But comparative anatomy has gone much deeper since the establishment of the theory of descent; its philosophic aim now is to explain the variety of organic forms by adaptation, and their similarity by heredity. At the same time, it has to recognise in the shades of difference in form the degree of blood-relationship, and make an effort to construct the ancestral tree of the animal world. In this way, comparative anatomy enters into the closest relations with comparative embryology on the one hand, and with the science of cla.s.sification on the other.

Now, when we ask what position man occupies among the other organisms according to the latest teaching of comparative anatomy and cla.s.sification, and how man's place in the zoological system is determined by comparison of the mature bodily forms, we get a very definite and significant reply; and this reply gives us extremely important conclusions that enable us to understand the embryonic development and its evolutionary purport. Since Cuvier and Baer, since the immense progress that was effected in the early decades of the nineteenth century by these two great zoologists, the opinion has generally prevailed that the whole animal kingdom may be distributed in a small number of great divisions or types. They are called types because a certain typical or characteristic structure is constantly preserved within each of these large sections. Since we applied the theory of descent to this doctrine of types, we have learned that this common type is an outcome of heredity; all the animals of one type are blood-relatives, or members of one stem, and can be traced to a common ancestral form. Cuvier and Baer set up four of these types: the vertebrates, articulates, molluscs, and radiates. The first three of these are still retained, and may be conceived as natural phylogenetic unities, as stems or phyla in the sense of the theory of descent. It is quite otherwise with the fourth type--the radiata. These animals, little known as yet at the beginning of the nineteenth century, were made to form a sort of lumber-room, into which were cast all the lower animals that did not belong to the other three types. As we obtained a closer acquaintance with them in the course of the last sixty years, it was found that we must distinguish among them from four to eight different types. In this way the total number of animal stems or phyla has been raised to eight or twelve (cf. Chapter 2.20).

These twelve stems of the animal kingdom are, however, by no means co-ordinate and independent types, but have definite relations, partly of subordination, to each other, and a very different phylogenetic meaning. Hence they must not be arranged simply in a row one after the other, as was generally done until thirty years ago, and is still done in some manuals. We must distribute them in three subordinate princ.i.p.al groups of very different value, and arrange the various stems phylogenetically on 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 sub-divide 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 a.n.u.s. The coelomaria (with body-cavity) have generally an a.n.u.s, 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 a.s.signed 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 pa.s.s 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 pa.s.ses 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 pa.s.sing 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 a.s.sume 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 a.s.sume that it was also found in the prochordonia, the common ancestors of both stems. It is also very p.r.o.nounced 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 pro-renal ca.n.a.l is affected). a aorta, af a.n.u.s, 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 ca.n.a.ls, u apertures of same, o outer skin, p gullet, r spinal marrow, a s.e.xual 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 s.e.xual-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 ma.s.s 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 princ.i.p.al 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 the vertical plane of section that divides the two halves the sagittal ("arrow") axis, or "dorsoventral axis," goes from the back to the belly, corresponding to the sagittal seam of the skull. But when we make a horizontal longitudinal section through the chorda, the whole body divides into a dorsal and a ventral half. The line of section that pa.s.ses through the body from right to left is the transverse, frontal, or lateral axis.

The two halves of the vertebrate body that are separated by this horizontal transverse axis and by the chorda have quite different characters. The dorsal half is mainly the animal part of the body, and contains the greater part of what are called the animal organs, the nervous system, muscular system, osseous system, etc.--the instruments of movement and sensation. The ventral half is essentially the vegetative half of the body, and contains the greater part of the vertebrate's vegetal organs, the visceral and vascular systems, s.e.xual system, etc.--the instruments of nutrition and reproduction. Hence in the construction of the dorsal half it is chiefly the outer, and in the construction of the ventral half chiefly the inner, germinal layer that is engaged. Each of the two halves develops in the shape of a tube, and encloses a cavity in which another tube is found. The dorsal half contains the narrow spinal-column cavity or vertebral ca.n.a.l ABOVE the chorda, in which lies the tube-shaped central nervous system, the medullary tube. The ventral half contains the much more s.p.a.cious visceral cavity or body-cavity UNDERNEATH the chorda, in which we find the alimentary ca.n.a.l and all its appendages.

The medullary tube, as the central nervous system or psychic organ of the vertebrate is called in its first stage, consists, in man and all the higher vertebrates, of two different parts: the large brain, contained in the skull, and the long spinal cord which stretches from there over the whole dorsal part of the trunk. Even in the primitive vertebrate this composition is plainly indicated. The fore half of the body, which corresponds to the head, encloses a k.n.o.b-shaped vesicle, the brain (gh); this is prolonged backwards into the thin cylindrical tube of the spinal marrow (r). Hence we find here this very important psychic organ, which accomplishes sensation, will, and thought, in the vertebrates, in its simplest form. The thick wall of the nerve-tube, which runs through the long axis of the body immediately over the axial rod, encloses a narrow central ca.n.a.l filled with fluid (Figures 1.98 to 1.102 r). We still find the medullary tube in this very simple form for a time in the embryo of all the vertebrates, and it retains this form in the amphioxus throughout life; only in the latter case the cylindrical medullary tube barely indicates the separation of brain and spinal cord. The lancelet's medullary tube runs nearly the whole length of the body, above the chorda, in the shape of a long thin tube of almost equal diameter throughout, and there is only a slight swelling of it right at the front to represent the rudiment of a cerebral lobe. It is probable that this peculiarity of the amphioxus is connected with the partial atrophy of its head, as the ascidian larvae on the one hand and the young cyclostoma on the other clearly show a division of the vesicular brain, or head marrow, from the thinner, tubular spinal marrow.

Probably we must trace to the same phylogenetic cause the defective nature of the sense organs of the amphioxus, which we will describe later (Chapter 2.16). Prospondylus, on the other hand, probably had three pairs of sense-organs, though of a simple character, a pair of, or a single olfactory depression, right in front (Figures 1.98 and 1.99, na), a pair of eyes (au) in the lateral walls of the brain, and a pair of simple auscultory vesicles (g) behind. There was also, perhaps, a single parietal or "pineal" eye at the top of the skull (epiphysis, e).

In the vertical median plane (or middle plane, dividing the bilateral body into right and left halves) we have in the acrania, underneath the chorda, the mesentery and visceral tube, and above it the medullary tube; and above the latter a membranous part.i.tion of the two halves of the body. With this part.i.tion is connected the ma.s.s of connective tissue which acts as a sheath both for the medullary tube and the underlying chorda, and is, therefore, called the chord-sheath (perichorda); it originates from the dorsal and median part of the coelom-pouches, which we shall call the skeleton plate or "sclerotom"

in the craniote embryo. In the latter the chief part of the skeleton--the vertebral column and skull--develops from this chord-sheath; in the acrania it retains its simple form as a soft connective matter, from which are formed the membranous part.i.tions between the various muscular plates or myotomes (Figures 1.98 and 1.99 ms).

To the right and left of the cord-sheath, at each side of the medullary tube and the underlying axial rod, we find in all the vertebrates the large ma.s.ses of muscle that const.i.tute the musculature of the trunk and effect its movements. Although these are very elaborately differentiated and connected in the developed vertebrate (corresponding to the various parts of the bony skeleton), in our ideal primitive vertebrate we can distinguish only two pairs of these princ.i.p.al muscles, which run the whole length of the body parallel to the chorda. These are the upper (dorsal) and lower (ventral) lateral muscles of the trunk. The upper (dorsal) muscles, or the original dorsal muscles (Figure 1.102 ms), form the thick ma.s.s of flesh on the back. The lower (ventral) muscles, or the original muscles of the belly, form the fleshy wall of the abdomen. Both sets are segmented, and consist of a double row of muscular plates (Figures 1.98 and 1.99 ms); the number of these myotomes determines the number of joints in the trunk, or metamera. The myotomes are also developed from the thick wall of the coelom-pouches (Figure 1.102 i).

Outside this muscular tube we have the external envelope of the vertebrate body, which is known as the corium or cutis. This strong and thick envelope consists, in its deeper strata, chiefly of fat and loose connective tissue, and in its upper layers of cutaneous muscles and firmer connective tissue. It covers the whole surface of the fleshy body, and is of considerable thickness in all the craniota. But in the acrania the corium is merely a thin plate of connective tissue, an insignificant "corium-plate" (lamella corii, Figures 1.98 to 1.102 t).

Immediately above the corium is the outer skin (epidermis, o), the general covering of the whole outer surface. In the higher vertebrates the hairs, nails, feathers, claws, scales, etc., grow out of this epidermis. It consists, with all its appendages and products, of simple cells, and has no blood-vessels. Its cells are connected with the terminations of the sensory nerves. Originally, the outer skin is a perfectly simple covering of the outer surface of the body, composed only of h.o.m.ogeneous cells--a permanent horn-plate. In this simplest form, as a one-layered epithelium, we find it, at first, in all the vertebrates, and throughout life in the acrania. It afterwards grows thicker in the higher vertebrates, and divides into two strata--an outer, firmer corneous (horn) layer and an inner, softer mucus-layer; also a number of external and internal appendages grow out of it: outwardly, the hairs, nails, claws, etc., and inwardly, the sweat-glands, fat-glands, etc.

It is probable that in our primitive vertebrate the skin was raised in the middle line of the body in the shape of a vertical fin border (f).

A similar fringe, going round the greater part of the body, is found to-day in the amphioxus and the cyclostoma; we also find one in the tail of fish-larvae and tadpoles.

Now that we have considered the external parts of the vertebrate and the animal organs, which mainly lie in the dorsal half, above the chorda, we turn to the vegetal organs, which lie for the most part in the ventral half, below the axial rod. Here we find a large body-cavity or visceral cavity in all the craniota. The s.p.a.cious cavity that encloses the greater part of the viscera corresponds to only a part of the original coeloma, which we considered in Chapter 1.10; hence it nay be called the metacoeloma. As a rule, it is still briefly called the coeloma; formerly it was known in anatomy as the pleuroperitoneal cavity. In man and the other mammals (but only in these) this coeloma divides, when fully developed, into two different cavities, which are separated by a transverse part.i.tion--the muscular diaphragm. The fore or pectoral cavity (pleura-cavity) contains the oesophagus (gullet), heart, and lungs; the hind or peritoneal or abdominal cavity contains the stomach, small and large intestines, liver, pancreas, kidneys, etc. But in the vertebrate embryo, before the diaphragm is developed, the two cavities form a single continuous body-cavity, and we find it thus in all the lower vertebrates throughout life. This body-cavity is clothed with a delicate layer of cells, the coelom-epithelium. In the acrania the coelom is segmented both dorsally and ventrally, as their muscular pouches and primitive genital organs plainly show (Figure 1.102).

The chief of the viscera in the body-cavity is the alimentary ca.n.a.l, the organ that represents the whole body in the gastrula. In all the vertebrates it is a long tube, enclosed in the body-cavity and more or less differentiated in length, and has two apertures--a mouth for taking in food (Figures 1.98 and 1.100 md) and an a.n.u.s for the ejection of unusable matter or excrements (af). With the alimentary ca.n.a.l a number of glands are connected which are of great importance for the vertebrate body, and which all grow out of the ca.n.a.l. Glands of this kind are the salivary glands, the lungs, the liver, and many smaller glands. Nearly all these glands are wanting in the acrania; probably there were merely a couple of simple hepatic tubes (Figures 1.98 and 1.100 l) in the vertebrate stem-form. The wall of the alimentary ca.n.a.l and all its appendages consists of two different layers; the inner, cellular clothing is the gut-gland-layer, and the outer, fibrous envelope consists of the gut-fibre-layer; it is mainly composed of muscular fibres which accomplish the digestive movements of the ca.n.a.l, and of connective-tissue fibres that form a firm envelope. We have a continuation of it in the mesentery, a thin, bandage-like layer, by means of which the alimentary ca.n.a.l is fastened to the ventral side of the chorda, originally the dorsal part.i.tion of the two coelom-pouches. The alimentary ca.n.a.l is variously modified in the vertebrates both as a whole and in its several sections, though the original structure is always the same, and is very simple. As a rule, it is longer (often several times longer) than the body, and therefore folded and winding within the body-cavity, especially at the lower end. In man and the higher vertebrates it is divided into several sections, often separated by valves--the mouth, pharynx, oesophagus, stomach, small and large intestine, and r.e.c.t.u.m. All these parts develop from a very simple structure, which originally (throughout life in the amphioxus) runs from end to end under the chorda in the shape of a straight cylindrical ca.n.a.l.

As the alimentary ca.n.a.l may be regarded morphologically as the oldest and most important organ in the body, it is interesting to understand its essential features in the vertebrate more fully, and distinguish them from unessential features. In this connection we must particularly note that the alimentary ca.n.a.l of every vertebrate shows a very characteristic division into two sections--a fore and a hind chamber. The fore chamber is the head-gut or branchial gut (Figures 1.98 to 1.100 p, k), and is chiefly occupied with respiration. The hind section is the trunk-gut or hepatic gut, which accomplishes digestion (ma, d). In all vertebrates there are formed, at an early stage, to the right and left in the fore-part of the head-gut, certain special clefts that have an intimate connection with the original respiratory apparatus of the vertebrate--the branchial (gill) clefts (ks). All the lower vertebrates, the lancelets, lampreys, and fishes, are constantly taking in water at the mouth, and letting it out again by the lateral clefts of the gullet. This water serves for breathing.

The oxygen contained in it is inspired by the blood-ca.n.a.ls, which spread out on the parts between the gill-clefts, the gill-arches (kg).

These very characteristic branchial clefts and arches are found in the embryo of man and all the higher vertebrates at an early stage of development, just as we find them throughout life in the lower vertebrates. However, these clefts and arches never act as respiratory organs in the mammals, birds, and reptiles, but gradually develop into quite different parts. Still, the fact that they are found at first in the same form as in the fishes is one of the most interesting proofs of the descent of these three higher cla.s.ses from the fishes.

Not less interesting and important is an organ that develops from the ventral wall in all vertebrates--the gill-groove or hypobranchial groove. In the acrania and the ascidiae it consists throughout life of a glandular ciliated groove, which runs down from the mouth in the ventral middle line of the gill-gut, and takes small particles of food to the stomach (Figure 1.101 z). But in the craniota the thyroid gland (thyreoidea) is developed from it, the gland that lies in front of the larynx, and which, when pathologically enlarged, forms goitre (struma).

From the head-gut we get not only the gills, the organs of water-breathing in the lower vertebrates, but also the lungs, the organs of atmospheric breathing in the five higher cla.s.ses. In these cases a vesicular fold appears in the gullet of the embryo at an early stage, and gradually takes the shape of two s.p.a.cious sacs, which are afterwards filled with air. These sacs are the two air-breathing lungs, which take the place of the water-breathing gills. But the vesicular inv.a.g.i.n.ation, from which the lungs arise, is merely the familiar air-filled vesicle, which we call the floating-bladder of the fish, and which alters its specific weight, acting as hydrostatic organ or floating apparatus. This structure is not found in the lowest vertebrate cla.s.ses--the acrania and cyclostoma. We shall see more of it in Volume 2.

The second chief section of the vertebrate-gut, the trunk or liver-gut, which accomplishes digestion, is of very simple construction in the acrania. It consists of two different chambers.

The first chamber, immediately behind the gill-gut, is the expanded stomach (ma); the second, narrower and longer chamber, is the straight small intestine (d): it issues behind on the ventral side by the a.n.u.s (af). Near the limit of the two chambers in the visceral cavity we find the liver, in the shape of a simple tube or blind sac (l); in the amphioxus it is single; in the prospondylus it was probably double (Figures 1.98 and 1.100 l).