There is also a complete agreement between the fore and hind limb in the stem or shaft. The first section of the stem is supported by a single strong bone--the humerus in the fore, the femur in the hind limb. The second section contains two bones: in front the radius (r) and ulna (u), behind the tibia and fibula. (Cf. the skeletons in Figures 2.260, 2.265, 2.270, 2.278 to 2.282, and 2.348.) The succeeding numerous small bones of the wrist (carpus) and ankle (tarsus) are also similarly arranged in the fore and hind extremities, and so are the five bones of the middle-hand (metacarpus) and middle-foot (metatarsus). Finally, it is the same with the toes themselves, which have a similar characteristic composition from a series of bony pieces before and behind. We find a complete parallel in all the parts of the fore leg and the hind leg.

When we thus learn from comparative anatomy that the skeleton of the human limbs is composed of just the same bones, put together in the same way, as the skeleton in the four higher cla.s.ses of Vertebrates, we may at once infer a common descent of them from a single stem-form.

This stem-form was the earliest amphibian that had five toes on each foot. It is particularly the outer parts of the limbs that have been modified by adaptation to different conditions. We need only recall the immense variations they offer within the mammal cla.s.s. We have the slender legs of the deer and the strong springing legs of the kangaroo, the climbing feet of the sloth and the digging feet of the mole, the fins of the whale and the wings of the bat. It will readily be granted that these organs of locomotion differ as much in regard to size, shape, and special function as can be conceived. Nevertheless, the bony skeleton is substantially the same in every case. In the different limbs we always find the same characteristic bones in essentially the same rigidly hereditary connection; this is as splendid a proof of the theory of evolution as comparative anatomy can discover in any organ of the body. It is true that the skeleton of the limbs of the various mammals undergoes many distortions and degenerations besides the special adaptations (Figure 2.342). Thus we find the first finger or the thumb atrophied in the fore-foot (or hand) of the dog (II). It has entirely disappeared in the pig (III) and tapir (V). In the ruminants (such as the ox, IV) the second and fifth toes are also atrophied, and only the third and fourth are well developed (VI, 3). Nevertheless, all these different fore-feet, as well as the hand of the ape (Figure 2.340) and of man (Figure 2.341), were originally developed from a common pentadactyle stem-form. This is proved by the rudiments of the degenerated toes, and by the similarity of the arrangement of the wrist-bones in all the pentanomes (Figure 2.342 a to p).

If we candidly compare the bony skeleton of the human arm and hand with that of the nearest anthropoid apes, we find an almost perfect ident.i.ty. This is especially true of the chimpanzee. In regard to the proportions of the various parts, the lowest living races of men (the Veddahs of Ceylon, Figure 2.344) are midway between the chimpanzee (Figure 2.343) and the European (Figure 2.345). More considerable are the differences in structure and the proportions of the various parts between the different genera of anthropoid apes (Figures 2.278 to 2.282); and still greater is the morphological distance between these and the lowest apes (the Cynopitheca). Here, again, impartial and thorough anatomic comparison confirms the accuracy of Huxley's pithecometra principle (Chapter 1.15).

The complete unity of structure which is thus revealed by the comparative anatomy of the limbs is fully confirmed by their embryology. However different the extremities of the four-footed Craniotes may be in their adult state, they all develop from the same rudimentary structure. In every case the first trace of the limb in the embryo is a very simple protuberance that grows out of the side of the hyposoma. These simple structures develop directly into fins in the fishes and Dipneusts by differentiation of their cells. In the higher cla.s.ses of Vertebrates each of the four takes the shape in its further growth of a leaf with a stalk, the inner half becoming narrower and thicker and the outer half broader and thinner. The inner half (the stalk of the leaf) then divides into two sections--the upper and lower parts of the limb. Afterwards four shallow indentations are formed at the free edge of the leaf, and gradually deepen; these are the intervals between the five toes (Figure 1.174). The toes soon make their appearance. But at first all five toes, both of fore and hind feet, are connected by a thin membrane like a swimming-web; they remind us of the original shaping of the foot as a paddling fin. The further development of the limbs from this rudimentary structure takes place in the same way in all the Vertebrates according to the laws of heredity.

The embryonic development of the muscles, or ACTIVE organs of locomotion, is not less interesting than that of the skeleton, or Pa.s.sIVE organs. But the comparative anatomy and ontogeny of the muscular system are much more difficult and inaccessible, and consequently have hitherto been less studied. We can therefore only draw some general phylogenetic conclusions therefrom.

It is incontestable that the musculature of the Vertebrates has been evolved from that of lower Invertebrates; and among these we have to consider especially the unarticulated Vermalia. They have a simple cutaneous muscular layer, developing from the mesoderm. This was afterwards replaced by a pair of internal lateral muscles, that developed from the middle wall of the coelom-pouches; we still find the first rudiments of the muscles arising from the muscle-plate of these in the embryos of all the Vertebrates (cf. Figures 1.124, 1.158 to 1.160, 2.222 to 2.224 mp). In the unarticulated stem-forms of the Chordonia, which we have called the Prochordonia, the two coelom-pouches, and therefore also the muscle-plates of their walls, were not yet segmented. A great advance was made in the articulation of them, as we have followed it step by step in the Amphioxus (Figures 1.124 and 1.158). This segmentation of the muscles was the momentous historical process with which vertebration, and the development of the vertebrate stem, began. The articulation of the skeleton came after this segmentation of the muscular system, and the two entered into very close correlation.

The episomites or dorsal coelom-pouches of the Acrania, Cyclostomes, and Selachii (Figure 1.161 h) first develop from their inner or median wall (from the cell-layer that lies directly on the skeletal plate [sk] and the medullary tube [nr]) a strong muscle-plate (mp). By dorsal growth (w) it also reaches the external wall of the coelom-pouches, and proceeds from the dorsal to the ventral wall. From these segmental muscle-plates, which are chiefly concerned in the segmentation of the Vertebrates, proceed the lateral muscles of the stem, as we find in the simplest form in the Amphioxus (Figure 2.210).

By the formation of a horizontal frontal septum they divide on each side into an upper and lower series of myotomes, dorsal and ventral lateral muscles. This is seen with typical regularity in the transverse section of the tail of a fish (Figure 2.346). From these earlier lateral muscles of the trunk develop the greater part of the subsequent muscles of the trunk, and also the much later "muscular buds" of the limbs.* (* The ontogeny of the muscles is mostly cenogenetic. The greater part of the muscles of the head (or the visceral muscles) belong originally to the hyposoma of the vertebrate organism, and develop from the wall of the hyposomites or ventral coelom-pouches. This also applies originally to the primary muscles of the limbs, as these too belong phylogenetically to the hyposoma. (Cf.

Chapter 1.14))

CHAPTER 2.27. THE EVOLUTION OF THE ALIMENTARY SYSTEM.

The chief of the vegetal organs of the human frame, to the evolution of which we now turn our attention, is the alimentary ca.n.a.l. The gut is the oldest of all the organs of the metazoic body, and it leads us back to the earliest age of the formation of organs--to the first section of the Laurentian period. As we have already seen, the result of the first division of labour among the h.o.m.ogeneous cells of the earliest multicellular animal body was the formation of an alimentary cavity. The first duty and first need of every organism is self-preservation. This is met by the functions of the nutrition and the covering of the body. When, therefore, in the primitive globular Blastaea the h.o.m.ogeneous cells began to effect a division of labour, they had first to meet this twofold need. One half were converted into alimentary cells and enclosed a digestive cavity, the gut. The other half became covering cells, and formed an envelope round the alimentary tube and the whole body. Thus arose the primary germinal layers--the inner, alimentary, or vegetal layer, and the outer, covering, or animal layer. (Cf. Chapter 2.19.)

When we try to construct an animal frame of the simplest conceivable type, that has some such primitive alimentary ca.n.a.l and the two primary layers const.i.tuting its wall, we inevitably come to the very remarkable embryonic form of the gastrula, which we have found with extraordinary persistence throughout the whole range of animals, with the exception of the unicellulars--in the Sponges, Cnidaria, Platodes, Vermalia, Molluscs, Articulates, Echinoderms, Tunicates, and Vertebrates. In all these stems the gastrula recurs in the same very simple form. It is certainly a remarkable fact that the gastrula is found in various animals as a larva-stage in their individual development, and that this gastrula, though much disguised by cenogenetic modifications, has everywhere essentially the same palingenetic structure (Figures 1.30 to 1.35). The elaborate alimentary ca.n.a.l of the higher animals develops ontogenetically from the same simple primitive gut of the gastrula.

This gastraea theory is now accepted by nearly all zoologists. It was first supported and partly modified by Professor Ray-Lankester; he proposed three years afterwards (in his essay on the development of the Molluscs, 1875) to give the name of archenteron to the primitive gut and blastoporus to the primitive mouth.

Before we follow the development of the human alimentary ca.n.a.l in detail, it is necessary to say a word about the general features of its composition in the fully-developed man. The mature alimentary ca.n.a.l in man is constructed in all its main features like that of all the higher mammals, and particularly resembles that of the Catarrhines, the narrow-nosed apes of the Old World. The entrance into it, the mouth, is armed with thirty-two teeth, fixed in rows in the upper and lower jaws. As we have seen, our dent.i.tion is exactly the same as that of the Catarrhines, and differs from that of all other animals (Chapter 2.23). Above the mouth-cavity is the double nasal cavity; they are separated by the palate-wall. But we saw that this separation is not there from the first, and that originally there is a common mouth-nasal cavity in the embryo; and this is only divided afterwards by the hard palate into two--the nasal cavity above and that of the mouth below (Figure 2.311).

At the back the cavity of the mouth is half closed by the vertical curtain that we call the soft palate, in the middle of which is the uvula. A glance into a mirror with the mouth wide open will show its shape. The uvula is interesting because, besides man, it is only found in the ape. At each side of the soft palate are the tonsils. Through the curved opening that we find underneath the soft palate we penetrate into the gullet or pharynx behind the mouth-cavity. Into this opens on either side a narrow ca.n.a.l (the Eustachian tube), through which there is direct communication with the tympanic cavity of the ear (Figure 2.320 e). The pharynx is continued in a long, narrow tube, the oesophagus (sr). By this the food pa.s.ses into the stomach when masticated and swallowed. Into the gullet also opens, right above, the trachea (lr), that leads to the lungs. The entrance to it is covered by the epiglottis, over which the food slides. The cartilaginous epiglottis is found only in the mammals, and has developed from the fourth branchial arch of the fishes and amphibia.

The lungs are found, in man and all the mammals, to the right and left in the pectoral cavity, with the heart between them. At the upper end of the trachea there is, under the epiglottis, a specially differentiated part, strengthened by a cartilaginous skeleton, the larynx. This important organ of human speech also develops from a part of the alimentary ca.n.a.l. In front of the larynx is the thyroid gland, which sometimes enlarges and forms goitre.

The oesophagus descends into the pectoral cavity along the vertebral column, behind the lungs and the heart, pierces the diaphragm, and enters the visceral cavity. The diaphragm is a membrano-muscular part.i.tion that completely separates the thoracic from the abdominal cavity in all the mammals (and these alone). This separation is not found in the beginning; there is at first a common breast-belly cavity, the coeloma or pleuro-peritoneal cavity. The diaphragm is formed later on as a muscular horizontal part.i.tion between the thoracic and abdominal cavities. It then completely separates the two cavities, and is only pierced by several organs that pa.s.s from the one to the other. One of the chief of these organs is the oesophagus.

After this has pa.s.sed through the diaphragm, it expands into the gastric sac in which digestion chiefly takes place. The stomach of the adult man (Figure 2.349) is a long, somewhat oblique sac, expanding on the left into a blind sac, the fundus of the stomach (b apostrophe), but narrowing on the right, and pa.s.sing at the pylorus (e) into the small intestine. At this point there is a valve, the pyloric valve (d), between the two sections of the ca.n.a.l; it opens only when the pulpy food pa.s.ses from the stomach into the intestine. In man and the higher Vertebrates the stomach itself is the chief organ of digestion, and is especially occupied with the solution of the food; this is not the case in many of the lower Vertebrates, which have no stomach, and discharge its function by a part of the gut farther on. The muscular wall of the stomach is comparatively thick; it has externally strong muscles that accomplish the digestive movements, and internally a large quant.i.ty of small glands, the peptic glands, which secrete the gastric juice.

(FIGURE 2.349. Human stomach and duodenum, longitudinal section. a cardiac (end of oesophagus), b fundus (blind sac of the left side), c pylorus-fold, d pylorus-valves, e pylorus-cavity, fgh duodenum, i entrance of the gall-duct and the pancreatic duct. (From Meyer.)

FIGURE 2.350. Median section of the head of a hare-embryo, one-fourth of an inch in length. (From Mihalcovics.) The deep mouth-cleft (hp) is separated by the membrane of the throat (rh) from the blind cavity of the head-gut (kd). hz heart, ch chorda, hp the point at which the hypophysis develops from the mouth-cleft, vh ventricle of the cerebrum, v3, third ventricle (intermediate brain), v4 fourth ventricle (hind brain), ck spinal ca.n.a.l.)

Next to the stomach comes the longest section of the alimentary ca.n.a.l, the middle gut or small intestine. Its chief function is to absorb the peptonised fluid ma.s.s of food, or the chyle, and it is subdivided into several sections, of which the first (next to the stomach) is called the duodenum (Figure 2.349 fgh). It is a short, horseshoe-shaped loop of the gut. The largest glands of the alimentary ca.n.a.l open into it--the liver, the chief digestive gland, that secretes the gall, and the pancreas, which secretes the pancreatic juice. The two glands pour their secretions, the bile and pancreatic juice, close together into the duodenum (i). The opening of the gall-duct is of particular phylogenetic importance, as it is the same in all the Vertebrates, and indicates the princ.i.p.al point of the hepatic or trunk-gut (Gegenbaur).

The liver, phylogenetically older than the stomach, is a large gland, rich in blood, in the adult man, immediately under the diaphragm on the left side, and separated by it from the lungs. The pancreas lies a little further back and more to the left. The remaining part of the small intestine is so long that it has to coil itself in many folds in order to find room in the narrow s.p.a.ce of the abdominal cavity. It is divided into the jejunum above and the ileum below. In the last section of it is the part of the small intestine at which in the embryo the yelk-sac opens into the gut. This long and thin intestine then pa.s.ses into the large intestine, from which it is cut off by a special valve. Immediately behind this "Bauhin-valve" the first part of the large intestine forms a wide, pouch-like structure, the caec.u.m.

The atrophied end of the caec.u.m is the famous rudimentary organ, the vermiform appendix. The large intestine (colon) consists of three parts--an ascending part on the right, a transverse middle part, and a descending part on the left. The latter finally pa.s.ses through an S-shaped bend into the last section of the alimentary ca.n.a.l, the r.e.c.t.u.m, which opens behind by the a.n.u.s. Both the large and small intestines are equipped with numbers of small glands, which secrete mucous and other fluids.

For the greater part of its length the alimentary ca.n.a.l is attached to the inner dorsal surface of the abdominal cavity, or to the lower surface of the vertebral column. The fixing is accomplished by means of the thin membranous plate that we call the mesentery.

Although the fully-formed alimentary ca.n.a.l is thus a very elaborate organ, and although in detail it has a quant.i.ty of complex structural features into which we cannot enter here, nevertheless the whole complicated structure has been historically evolved from the very simple form of the primitive gut that we find in our gastraead-ancestors, and that every gastrula brings before us to-day.

We have already pointed out (Chapter 1.9) how the epigastrula of the mammals (Figure 1.67) can be reduced to the original type of the bell-gastrula, which is now preserved by the amphioxus alone (Figure 1.35). Like the latter, the human gastrula and that of all other mammals must be regarded as the ontogenetic reproduction of the phylogenetic form that we call the Gastraea, in which the whole body is nothing but a double-walled gastric sac.

We already know from embryology the manner in which the gut develops in the embryo of man and the other mammals. From the gastrula is first formed the spherical embryonic vesicle filled with fluid (gastrocystis, Figure 1.106). In the dorsal wall of this the sole-shaped embryonic shield is developed, and on the under-side of this a shallow groove appears in the middle line, the first trace of the later, secondary alimentary tube. The gut-groove becomes deeper and deeper, and its edges bend towards each other, and finally form a tube.

As we have seen, this simple cylindrical gut-tube is at first completely closed before and behind in man and in the Vertebrates generally (Figure 1.148); the permanent openings of the alimentary ca.n.a.l, the mouth and a.n.u.s, are only formed later on, and from the outer skin. A mouth-pit appears in the skin in front (Figure 2.350 hp), and this grows towards the blind fore-end of the cavity of the head-gut (kd), and at length breaks into it. In the same way a shallow a.n.u.s-pit is formed in the skin behind, which grows deeper and deeper, advances towards the blind hinder end of the pelvic gut, and at last connects with it. There is at first, both before and behind, a thin part.i.tion between the external cutaneous pit and the blind end of the gut--the throat-membrane in front and the a.n.u.s-membrane behind; these disappear when the connection takes place.

Directly in front of the a.n.u.s-opening the allantois develops from the hind gut; this is the important embryonic structure that forms into the placenta in the Placentals (including man). In this more advanced form the human alimentary ca.n.a.l (and that of all the other mammals) is a slightly bent, cylindrical tube, with an opening at each end, and two appendages growing from its lower wall: the anterior one is the umbilical vesicle or yelk-sac, and the posterior the allantois or urinary sac (Figure 1.195).

The thin wall of this simple alimentary tube and its ventral appendages is found, on microscopic examination, to consist of two strata of cells. The inner stratum, lining the entire cavity, consists of larger and darker cells, and is the gut-gland layer. The outer stratum consists of smaller and lighter cells, and is the gut-fibre layer. The only exception is in the cavities of the mouth and a.n.u.s, because these originate from the skin. The inner coat of the mouth-cavity is not provided by the gut-gland layer, but by the skin-sense layer; and its muscular substratum is provided, not by the gut-fibre, but the skin-fibre, layer. It is the same with the wall of the small a.n.u.s-cavity.

If it is asked how these const.i.tuent layers of the primitive gut-wall are related to the various tissues and organs that we find afterwards in the fully-developed system, the answer is very simple. It can be put in a single sentence. The epithelium of the gut--that is to say, the internal soft stratum of cells that lines the cavity of the alimentary ca.n.a.l and all its appendages, and is immediately occupied with the processes of nutrition--is formed solely from the gut-gland layer; all other tissues and organs that belong to the alimentary ca.n.a.l and its appendages originate from the gut-fibre layer. From the latter is also developed the whole of the outer envelope of the gut and its appendages; the fibrous connective tissue and the smooth muscles that compose its muscular layer, the cartilages that support it (such as the cartilages of the larynx and the trachea), the blood-vessels and lymph-vessels that absorb the nutritive fluid from the intestines--in a word, all that there is in the alimentary system besides the epithelium of the gut. From the same layer we also get the whole of the mesentery, with all the organs embedded in it--the heart, the large blood-vessels of the body, etc.

(FIGURE 2.351. Scales or cutaneous teeth of a shark (Centrophorus calceus). A three-pointed tooth rises obliquely on each of the quadrangular bony plates that lie in the corium. (From Gegenbaur.))

Let us now leave this original structure of the mammal gut for a moment, in order to compare it with the alimentary ca.n.a.l of the lower Vertebrates, and of those Invertebrates that we have recognised as man's ancestors. We find, first of all, in the lowest Metazoa, the Gastraeads, that the gut remains permanently in the very simple form in which we find it transitorily in the palingenetic gastrula of the other animals; it is thus in the Gastremaria (Pemmatodiscus), the Physemaria (Prophysema), the simplest Sponges (Olynthus), the freshwater Polyps (Hydra), and the ascula-embryos of many other Coelenteria (Figures 2.233 to 2.238). Even in the simplest forms of the Platodes, the Rhabdocoela (Figure 2.240), the gut is still a simple straight tube, lined with the entoderm; but with the important difference that in this case its single opening, the primitive mouth (m), has formed a muscular gullet (sd) by inv.a.g.i.n.ation of the skin.

(FIGURE 2.352. Gut of a human embryo, one-sixth of an inch long, magnified fifteen times. (From His. Showing: Epiglottis, Tongue, Hypophysis, Hepatic duct, Tail, Allantoic duct, Tail-gut, Umbilical cord, Larynx, Rudimentary lungs, Stomach, Pancreas, Bladder, Wolffian duct, Rudimentary kidneys.))

We have the same simple form in the gut of the lowest Vermalia (Gastrotricha, Figure 2.242, Nematodes, Sagitta, etc.). But in these a second important opening of the gut has been formed at the opposite end to the mouth, the a.n.u.s (Figure 2.242 a).

We see a great advance in the structure of the vermalian gut in the remarkable Balanoglossus (Figure 2.245), the sole survivor of the Enteropneust cla.s.s. Here we have the first appearance of the division of the alimentary tube into two sections that characterises the Chordonia. The fore half, the head-gut (cephalogaster), becomes the organ of respiration (branchial gut, Figure 2.245 k); the hind half, the trunk-gut (truncogaster), alone acts as digestive organ (hepatic gut, d). The differentiation of these two parts of the gut in the Enteropneust is just the same as in all the Tunicates and Vertebrates.

It is particularly interesting and instructive in this connection to compare the Enteropneusts with the Ascidia and the Amphioxus (Figures 2.220 and 2.210)--the remarkable animals that form the connecting link between the Invertebrates and the Vertebrates. In both forms the gut is of substantially the same construction; the anterior section forms the respiratory branchial gut, the posterior the digestive hepatic gut. In both it develops palingenetically from the primitive gut of the gastrula, and in both the hinder end of the medullary tube covers the primitive mouth to such an extent that the remarkable medullary intestinal duct is formed, the pa.s.sing communication between the neural and intestinal tubes (ca.n.a.lis neurentericus, Figures 1.83 and 1.85 ne). In the vicinity of the closed primitive mouth, possibly in its place, the later a.n.u.s is developed. In the same way the mouth is a fresh formation in the Amphioxus and the Ascidia. It is the same with the human mouth and that of the Craniotes generally. The secondary formation of the mouth in the Chordonia is probably connected with the development of the gill-clefts which are formed in the gut-wall immediately behind the mouth. In this way the anterior section of the gut is converted into a respiratory organ. I have already pointed out that this modification is distinctive of the Vertebrates and Tunicates. The phylogenetic appearance of the gill-clefts indicates the commencement of a new epoch in the stem-history of the Vertebrates.

In the further ontogenetic development of the alimentary ca.n.a.l in the human embryo the appearance of the gill-clefts is the most important process. At a very early stage the gullet-wall joins with the external body-wall in the head of the human embryo, and this is followed by the formation of four clefts, which lead directly into the gullet from without, on the right and left sides of the neck, behind the mouth.

These are the gill or gullet clefts, and the part.i.tions that separate them are the gill or gullet-arches (Figure 1.171). These are most interesting embryonic structures. They show us that all the higher Vertebrates reproduce in their earlier stages, in harmony with the biogenetic law, the process that had so important a part in the rise of the whole Chordonia-stem. This process was the differentiation of the gut into two sections--an anterior respiratory section, the branchial gut, that was restricted to breathing, and a posterior digestive section, the hepatic gut. As we find this highly characteristic differentiation of the gut into two different sections in all the Vertebrates and all the Tunicates, we may conclude that it was also found in their common ancestors, the Prochordonia--especially as even the Enteropneusts have it. (Cf. Chapters 1.12, 1.14 and 2.20, and Figures 2.210, 2.220, 2.245.) It is entirely wanting in all the other Invertebrates.

(FIGURE 2.353. Gut of a dog-embryo (shown in Figure 1.202, from Bischoff), seen from the ventral side, a gill-arches (four pairs), b rudiments of pharynx and larynx, c lungs, d stomach, f liver, g walls of the open yelk-sac (into which the middle gut opens with a wide aperture), h r.e.c.t.u.m.

FIGURE 2.354. The same gut seen from the right. a lungs, b stomach, c liver, d yelk-sac, e r.e.c.t.u.m.)

There is at first only one pair of gill-clefts in the Amphioxus, as in the Ascidia and Enteropneusts; and the Copelata (Figure 2.225) have only one pair throughout life. But the number presently increases in the former. In the Craniotes, however, it decreases still further. The Cyclostomes have six to eight pairs (Figure 2.247); some of the Selachii six or seven pairs, most of the fishes only four or five pairs. In the embryo of man, and the higher Vertebrates generally, where they make an appearance at an early stage, only three or four pairs are developed. In the fishes they remain throughout life, and form an exit for the water taken in at the mouth (Figures 2.249 to 2.251). But they are partly lost in the amphibia, and entirely in the higher Vertebrates. In these nothing is left but a relic of the first gill-cleft. This is formed into a part of the organ of hearing; from it are developed the external meatus, the tympanic cavity, and the Eustachian tube. We have already considered these remarkable structures, and need only point here to the interesting fact that our middle and external ear is a modified inheritance from the fishes. The branchial arches also, which separate the clefts, develop into very different parts. In the fishes they remain gill-arches, supporting the respiratory gill-leaves. It is the same with the lowest amphibia, but in the higher amphibia they undergo various modifications; and in the three higher cla.s.ses of Vertebrates (including man) the hyoid bone and the ossicles of the ear develop from them. (Cf. Chapter 2.25.)

(FIGURE 2.355. Median section of the head of a Petromyzon-larva. (From Gegenbaur,) h hypobranchial groove (above it in the gullet we see the internal openings of the seven gill-clefts), v velum, o mouth, c heart, a auditory vesicle, n neural tube, ch chorda.)

From the first gill-arch, from the inner surface of which the muscular tongue proceeds, we get the first structure of the maxillary skeleton--the upper and lower jaws, which surround the mouth and support the teeth. These important parts are wholly wanting in the two lowest cla.s.ses of Vertebrates, the Acrania and Cyclostoma. They appear first in the earliest Selachii (Figures 2.248 to 2.251), and have been transmitted from this stem-group of the Gnathostomes to the higher Vertebrates. Hence the original formation of the skeleton of the mouth can be traced to these primitive fishes, from which we have inherited it. The teeth are developed from the skin that clothes the jaws. As the whole mouth cavity originates from the outer integument (Figure 2.350), the teeth also must come from it. As a fact, this is found to be the case on microscopic examination of the development and finer structure of the teeth. The scales of the fishes, especially of the shark type (Figure 2.351), are in the same position as their teeth in this respect (Figure 2.252). The osseous matter of the tooth (dentine) develops from the corium; its enamel covering is a secretion of the epidermis that covers the corium. It is the same with the cutaneous teeth or placoid scales of the Selachii. At first the whole of the mouth was armed with these cutaneous teeth in the Selachii and in the earliest amphibia. Afterwards the formation of them was restricted to the edges of the jaws.

Hence our human teeth are, in relation to their original source, modified fish-scales. For the same reason we must regard the salivary glands, which open into the mouth, as epidermic glands, as they are formed, not from the glandular layer of the gut like the rest of the alimentary glands, but from the epidermis, from the h.o.r.n.y plate of the outer germinal layer. Naturally, in harmony with this evolution of the mouth, the salivary glands belong genetically to one series with the sudoriferous, sebaceous, and mammary glands.

Thus the human alimentary ca.n.a.l is as simple as the primitive gut of the gastrula in its original structure. Later it resembles the gut of the earliest Vermalia (Gastrotricha). It then divides into two sections, a fore or branchial gut and a hind or hepatic gut, like the alimentary ca.n.a.l of the Balanoglossus, the Ascidia, and the Amphioxus.

The formation of the jaws and the branchial arches changes it into a real fish-gut (Selachii). But the branchial gut, the one reminiscence of our fish-ancestors, is afterwards atrophied as such. The parts of it that remain are converted into entirely different structures.

(FIGURE 2.356. Transverse section of the head of a Petromyzon-larva.

(From Gegenbaur.) Beneath the pharynx (d) we see the hypobranchial groove; above it the chorda and neural tube. A, B, C stages of constriction.)

But, although the anterior section of our alimentary ca.n.a.l thus entirely loses its original character of branchial gut, it retains the physiological character of respiratory gut. We are now astonished to find that the permanent respiratory organ of the higher Vertebrates, the air-breathing lung, is developed from this first part of the alimentary ca.n.a.l. Our lungs, trachea, and larynx are formed from the ventral wall of the branchial gut. The whole of the respiratory apparatus, which occupies the greater part of the pectoral cavity in the adult man, is at first merely a small pair of vesicles or sacs, which grow out of the floor of the head-gut immediately behind the gills (Figures 2.354 C, 1.147 l). These vesicles are found in all the Vertebrates except the two lowest cla.s.ses, the Acrania and Cyclostomes. In the lower Vertebrates they do not develop into lungs, but into a large air-filled bladder, which occupies a good deal of the body-cavity and has a quite different purport. It serves, not for breathing, but to effect swimming movements up and down, and so is a sort of hydrostatic apparatus--the floating bladder of the fishes (nectocystis, Chapter 2.21). However, the human lungs, and those of all air-breathing Vertebrates, develop from the same simple vesicular appendage of the head-gut that becomes the floating bladder in the fishes.

At first this bladder has no respiratory function, but merely acts as hydrostatic apparatus for the purpose of increasing or lessening the specific gravity of the body. The fishes, which have a fully-developed floating bladder, can press it together, and thus condense the air it contains. The air also escapes sometimes from the alimentary ca.n.a.l, through an air-duct that connects the floating bladder with the pharynx, and is ejected by the mouth. This lessens the size of the bladder, and so the fish becomes heavier and sinks. When it wishes to rise again, the bladder is expanded by relaxing the pressure. In many of the Crossopterygii the wall of the bladder is covered with bony plates, as in the Tria.s.sic Undina (Figure 2.254).

This hydrostatic apparatus begins in the Dipneusts to change into a respiratory organ; the blood-vessels in the wall of the bladder now no longer merely secrete air themselves, but also take in fresh air through the air-duct. This process reaches its full development in the Amphibia. In these the floating bladder has turned into lungs, and the air-pa.s.sage into a trachea. The lungs of the Amphibia have been transmitted to the three higher cla.s.ses of Vertebrates. In the lowest Amphibia the lungs on either side are still very simple transparent sacs with thin walls, as in the common water-salamander, the Triton.

It still entirely resembles the floating bladder of the fishes. It is true that the Amphibia have two lungs, right and left. But the floating bladder is also double in many of the fishes (such as the early Ganoids), and divides into right and left halves. On the other hand, the lung is single in Ceratodus (Figure 2.257).

(FIGURE 2.357. Thoracic and abdominal viscera of a human embryo of twelve weeks, natural size, (From Kolliker.) The head is omitted.

Ventral and pectoral walls are removed. The greater part of the body-cavity is taken up with the liver, from the middle part of which the caec.u.m and the vermiform appendix protrude. Above the diaphragm, in the middle, is the conical heart; to the right and left of it are the two small lungs.)