Book - Vertebrate Zoology (1928) 7

From Embryology

Chapter VII Triton : A Chordate With 5-Toed Limbs

Vertebrate Zoology G. R. De Beer (1928)

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Triton, the newt, is sharply distinguished from all the types so far described, because its limbs end in fingers and toes instead of being fins. The foot has five toes, but in the newt and allied animals, the number of fingers on the hand has been reduced from five to four. Triton and all higher vertebrates are typically land-animals, and are collectively called the Tetrapoda. Some of them, however, have reverted to the condition of living in water, in varying degrees. So the whales, seals, and hosts of extinct marine reptiles have come to live almost if not entirely in water, and the newt also spends more of its time in water and is more adapted to it than its ancestors were. This secondary return to aquatic conditions is, however, easily and fundamentally distinguished from the primitive aquatic habit of the fish. The possession of typically 5-fingered, " pentadactyl," limbs is a sure criterion of a terrestrial animal, or of one whose ancestors were terrestrial. As an example of the secondary readaptation to aquatic conditions may be mentioned the webs of skin which in some species of newts extend between the fingers, and are used for swimming.

The skin is soft and slimy owing to the presence of glands, and is used largely as a respiratory surface for the oxygenation of the blood. There are no scales or fin-rays of any sort. The tail carries a continuous median dorsal and ventral fin, and in the male animals of some species, there is also a fin along the back, which becomes enlarged at the breeding season.

The first part of the life is spent in the water in which the eggs are laid and hatched, and since the early stages are aquatic and the later ones terrestrial, these animals (newts, toads, and frogs) are called Amphibia. In the early larval condition there are external gills, and subsequently three pairs of gill- slits. These disappear when the animals metamorphose and come out on land.

The aquatic larvae have lateral-line organs, disposed in a similar manner to those of fish, only they are sunk in a groove instead of being in a canal. When the newt emerges from the water in the summer, these organs degenerate somewhat. They reappear when the newt returns to the water, as it does at the next breeding season, if not before.

The mouth is wide, and the external nostrils are just above it. The eyes are small. The alimentary, excretory, and genital apertures are situated in a cloaca just in front of the base of the tail.


The cartilaginous neurocranium is very similar to that of Ceratodus. The suspension of the jaws is autostylic, and besides the basal and otic processes, there is also an ascending process. These processes have precisely the same relations to the neighbouring blood-vessels and nerves as they have in Ceratodus. Only a little of this cartilaginous brain case and olfactory and auditory capsules is replaced by cartilage- bone. On each side, anteriorly, are the orbitosphenoids. Posteriorly are the prootics and exoccipitals, which form the condyles with which the skull articulates with the first vertebra.

The membrane-bones covering the skull dorsally are paired nasals, prefrontals, frontals, and parietals ; on each side of the latter are the squamosals which overlie the quadrates. On the under side are the paired prevomers, pterygoids, and the parasphenoid.

Paired maxillae are present, and the two premaxillae have fused together in front. The pterygoids (dermal bones) extend freely forwards from the quadrates. In the lower jaw, part of Meckel's cartilage ossifies as the articular, which is encased anteriorly between two membrane-bones ; the (lateral) dentary and the (medial) splenial. The surfaces by which the quadrate and the articular are in contact are cartilaginous. The ceratohyals and the ventral elements of the first two branchial arches form a framework often called the " hyoid," lying under the floor of the mouth, and of importance in breathing. When this floor is lowered, air is drawn in through the nostrils. These are then closed and the floor raised, with the result that the air has to pass down the throat and into the lungs.

Teeth are carried by the premaxilla, maxilla, prevomer, and dentary. They are similar to the teeth of fish already described, but now they are firmly fixed on to the bones which carry them.

The vagus nerves emerge just in front of the back of the skull, and the hypoglossal roots emerge behind it. Six segments are included in the skull.

Vertebral Column

The vertebrae are elongated cylinders, articulating with one another by zygapophyses and cartilaginous pads on their front and rear surfaces. The notochord is much constricted. Each vertebra bears a pair of neural arches above, and those of the tail also have haemal arches beneath. The trunk- vertebrae (except the first) bear lateral transverse processes, with which the ribs articulate. The latter are " true " or dorsal ribs. The first vertebra is modified in connexion with its articulation with the condyles of the skull. The ribs belonging to one vertebra are modified and attached to the ilia of the pelvic girdle, forming the sacrum.

Pectoral Limb and Girdle

The pectoral girdle is represented only by the primitive cartilaginous girdle of the endoskeleton, and indeed it remains largely unossified. The dermal or membrane-bones of the girdle of other forms have disappeared. There is a dorsal scapula, and a ventral portion in which these may be distinguished : an anterior cartilaginous precoracoid and a posterior partly ossified coracoid. A sternum is present as a median plate of cartilage, overlapping the coracoid.

The fore limb is divisible into three regions : upper arm, forearm, and hand. The skeleton of the limbs is derived from the radials of the fins of fish, and consequently its bones are cartilage-bones. The bone in the upper arm is the humerus, which fits into the glenoid cavity of the girdle, proximally.

Distally, it articulates with the radius and ulna of the forearm. The radius is the anterior of the two, i.e. preaxial. These two bones in turn articulate with the bones of the wrist or carpus, which is composed of seven small " carpal " bones. The radiale (scaphoid) and ulnare (cuneiform) articulate with the radius and ulna respectively. (Typically there are three bones in this proximal row, the additional one being the intermedium (lunar) which here is fused with the ulnare.) Next, there is a centrale, and the distal row of carpals is formed of four bones. The bones of the fingers are the metacarpals, and two or more phalanges, according to the finger. The first finger has been lost, so the others are numbered 2 to 5. The digital formula by which the number of phalanges is expressed, is : o, 2, 2, 3, 2.

Pelvic Limb and Girdle. — The pelvic girdle also consists of a dorsal and two ventral elements. Dorsally, the ilium leads from the acetabulum to the modified rib of the vertebra forming the sacrum. The posterior ventral element is the ischium : both ilium and ischium are bony. The anterior element is the pubis which remains cartilaginous, and bears another cartilaginous process akin to the sternum projecting forwards. The ischium and pubis meet their fellows of the opposite side in the middle line.

Like the fore limb, the hind limb can be divided into three parts : thigh, shank, and foot. The bone of the thigh is the femur, which articulates with the acetabulum of the girdle. Distally, it joins the tibia (preaxial) and fibula of the shank. The ankle or tarsus is very similar to the wrist. The three proximal tarsal bones are separate : tibiale, intermedium (together possibly equivalent to the astragalus of higher forms), and fibulare (calcaneum). The second row is represented by the centrale (navicular), and the distal row by four bones. The proximal bones of the toes are the metatarsals, bearing the phalanges. The digital formula is 2, 2, 3, 3, 2.

Alimentary System

The floor of the mouth carries a tongue, of which the hind edge is free.* Salivary glands are present, opening into the mouth. The glottis lies on the floor of the pharynx and leads to the windpipe. The stomach is typical and the intestine makes a few loops before leading to the rectum, which opens into the cloaca. Ventral to the rectum is a sac which is formed as an outgrowth from it, the bladder. This sac is of great importance in the evolution of the higher vertebrates, where it gives rise to the allantois. For this reason the bladder of Amphibia is called " allantoic," to distinguish it from the swellings of the urinary ducts or " bladders " of fish. The liver is divided into several lobes, a gall-bladder is present and the bile-duct which leads from it joins the duct from the pancreas to open into the intestine.

  • This tongue has nothing to do with the similarly named structure in Petromyzon.

Fig. 46. — Triton : dissection of female seen from the ventral side. b, bladder ; b d, bile-duct ; ca, carotid arch ; cl, cloaca ; da, dorsal aorta; gb, gall-bladder; h, heart; i, intestine ; k, mesonephric kidney; /, liver ; In, lung ; Md, Miillerian duct ; o, ovary ; p, pharynx ; pn, pancreas ; r, rectum ; s, stomach ; sa, systemic arch ; sp, spleen ; Wd, Wolffian duct.

Respiratory and Vascular Systems

The windpipe, or trachea, leads back ventral to the gut and divides into the two bronchi leading to the lungs. These differ from the lung of Ceratodus only in that they are paired, and that they maintain their primitive position ventral and lateral to the gut. In the adult the gill-slits have disappeared, and the afferent and efferent branchial arteries are directly continuous instead of being separated by the capillaries of the gills. These arteries are now called arterial arches, and they run round the pharynx from the ventral to the dorsal side as if they were still separated from one another by the gill-slits. The ventral aorta is shortened up so much that the arterial arches come off from the truncus arteriosus, close to the heart. The ist arterial arch runs in the mandibular arch, and the 2nd arterial arch likewise ascends the hyoid arch. Although present in the embryo, these arterial arches disappear. The 3rd arch persists as the carotid. At its base is the lingual artery which represents the anterior prolongation of the original ventral aorta. The carotid then passes through the carotid gland (see p. 401) and ascends the 3rd visceral (ist branchial) arch until it reaches a position dorsal to the pharynx. Here it turns forwards and enters the skull. It is the anterior prolongation of the original dorsal aorta.

The 4th arterial arch is known as the systemic. It goes up in what was the 4th visceral (2nd branchial) arch and turns backwards. Arrived here, dorsal to the pharynx, this vessel is exactly in the position of and corresponds to the lateral dorsal aorta of fish, only it has lost its connexion with its anterior prolongation which is now the internal carotid. This connexion, when present, is called the ductus caroticus.

The 5th arterial arch disappears in Triton, though it is present in the nearly related form Salamandra, where it leads from the truncus up the 5th visceral arch and joins the lateral dorsal aorta.

The 6th arterial arch persists as the pulmonary. It also runs up round the pharynx and joins the lateral dorsal aorta, and, just as in Ceratodus, it gives off an artery which runs backwards to the lungs, on each side. The lungs are symmetrical, and the pulmonary arteries do not twist round the gut as in Ceratodus. The connexion between the pulmonary artery and the lateral dorsal aorta is called the ductus arteriosus, or ductus Botalli.

The lateral dorsal aortae join one another in the middle line above the gut to form the dorsal aorta. On the way, several arteries are given off : the subclavians to the fore limbs, the cutaneous to the skin, and, near the point of junction, the cceliaco-mesenteric which runs ventrally in the mesentery suspending the gut, and supplies blood to the viscera. The dorsal aorta continues running back, supplying the kidneys on the way, and gives off the two iliac arteries which supply the hind limbs ; thence it runs on into the tail.

As in Ceratodus, the sinus venosus receives three large veins. These are the paired ductus Cuvieri or superior vena? cavae, and the single inferior vena cava. Each superior vena cava is made up of four principal veins; external jugular (from the ventral regions of the head and tongue) ; internal jugular, corresponding to the anterior cardinal (from the dorsal regions of the head, brain, and skull) ; subclavian, itself made up of the brachial from the fore limb and the cutaneous from the skin ; and the posterior cardinal (anterior portion).

The inferior vena cava receives the hepatic veins from the liver, and the renal veins from the kidneys. Blood returns from the tail by the caudal vein which divides into two ; each portion connects with two veins coming from the hind limb (femoral and sciatic) and runs as the renal portal vein to the kidney of its side. All the blood from the hind regions of the body is not bound to take this course, for instead of engaging in the renal portals it may enter the pelvic veins which run towards one another, join in the midventral line, and proceed forwards as the anterior abdominal. This vein connects with the hepatic portal vein, which collects up the blood from the intestine and takes it to the liver.

Fig. 47. — Salamandra : dissection of male seen from the ventral side.

The nerves are shown on the left and the urinogenital ducts on the right. 5a, fifth arterial arch ; aa, anterior abdominal vein ; ab, adrenal bodies ; bp, brachial plexus ; c, cloaca ; ca, carotid arch ; cd, collecting ducts from the excretory portion of the kidney ; da, coeliac artery ; cv, caudal vein ; d, dorsal aorta ; da, ductus arteriosus ; ej, external jugular vein ; /, femoral vein ; ij, internal jugular vein ; k, mesonephric kidney ; pa, pulmonary artery ; pc, posterior cardinal vein ; pv, pelvic vein ; r, rectum ; rp, renal portal vein ; sa, subclavian artery ; sv, subclavian vein ; sy, systemic arch ; syc, sympathetic nerve-chain ; syg, sympathetic ganglion and supra-renal body ; t, testis ; ta, truncus arteriosus ; vci, vena cava inferior ; vcs, vena cava superior ; vd> vas deferens ; ve, vasa efferentia.


From the lungs, the pulmonary veins return the oxygenated blood to the left auricle of the heart.

The heart consists of the two auricles and the single ventricle. It is to be noticed, however, that the septum separating the two auricles is perforated, allowing blood to pass from one side to the other. Leading from the ventricle is the conus arteriosus, which, as in Ceratodus, is provided with valves. The truncus represents the ventral aorta very much shortened up ; in its anterior region just before giving off the arterial arches, it is divided into two by a horizontal septum, forming a cavum pulmonale (leading to the pulmonary arch) and a cavum aorticum (leading to the systemic and carotid arches).

The circulatory system is on the whole very similar to that of Ceratodus. It is to be noted that the separation of the venous blood from the arterial is still far from complete. The blood is oxygenated in the lungs and in the skin. It returns from the lungs to the left auricle by the pulmonary veins, and from the skin to the right auricle by the cutaneous, subclavian, superior vena cava, and sinus venosus. The remainder of the blood entering the right auricle is venous.

Urino-genital System

The mesonephric kidneys project downwards from the roof of the ccelomic cavity, covered over by ccelomic epithelium ; they are therefore more easily visible than those of fish. The tubules of the kidneys are drained by the Wolffian ducts which lead into the bladder. In the male, the vasa efferentia from the testis lead through the anterior tubules of the kidney and so into the Wolffian ducts which become the vasa deferentia. The more posterior tubules of the kidney are solely excretory in function, and they do not connect with the Wolffian duct until the latter is close to the cloaca. This is a step in the direction of separating the genital from the excretory ducts, which would be effected if the ducts from the purely excretory part of the kidney were to move still farther down the Wolffian duct and eventually open directly into the cloaca.

In the female, the Wolffian ducts are solely excretory in function, and the Miillerian ducts or oviducts, which open into the coelomic cavity anteriorly, receive the eggs and convey them down to the cloaca into which they open separately.

The kidneys retain their open ciliated funnels, leading into the coelomic cavity (ccelomostomes).

Nervous System

The brain has large elongated cerebral hemispheres, in the roof of which nerve-cells appear. The floor and side of the hemispheres form the corpus striatum. The cavities of these hemispheres (ist and 2nd ventricles) communicate with that of the diencephalon (3rd ventricle) by the foramina of Monro. The pineal projects upwards from the roof, and the infundibulum down from the floor of the diencephalon ; and a choroid plexus projects into the 3rd ventricle. There is no saccus vasculosus. The roof of the midbrain forms the optic lobes which are joined in the middle line, and do not present a double appearance. The hind brain has a cerebellum and a choroid plexus projecting into the 4th ventricle.

The cranial nerves are similar to those of the dogfish, except for the fact that the disappearance of the lateral-line organs (or their very great reduction) entails the disappearance of those nerves which supply them, viz. superficial ophthalmic, buccal and mandibularis externus of the facial, and lateralis of the vagus. There is a further simplification owing to the closure of the gill-slits. The glossopharyngeal is distributed to the tongue and pharynx. The vagus supplies the muscles of the larynx, and also sends parasympathetic fibres to the heart, stomach, and intestine. The hypoglossal comes out behind the skull and is counted as the ist spinal nerve ; it runs to the muscles beneath the tongue which actuate the " hyoid " plate for the purpose of breathing. The spinal nerves to the limbs are grouped, forming the brachial and sciatic plexus for the fore and hind limb respectively.

As in the dogfish, there are sympathetic nerve-chains on each side of the dorsal aorta. They continue forwards accompanying the internal carotids into the head. They join the sympathetic ganglia to one another, each receiving in addition a ramus communicans from its corresponding spinal nerve.


The lateral-line sense-organs have already been mentioned. The ears are in a degenerate condition in Triton, for although they appreciate vibrations in air, i.e. sound, their structure is not typical of land- vertebrates, and will not be considered here. It may be mentioned, however, that the tympanic cavity which is characteristic of the ears of other Tetrapods and which is homologous with the spiracular slit of the dogfish, is not developed ; and that the ear-drum or tympanic membrane is also absent.

The lens of the eye is attached to a protractor lentis muscle, contraction of which increases the distance between the lens and the retina and accommodates the eye for near vision. There is a retractor bulbi muscle which pulls the eyeball in, and depresses the roof of the mouth, which action assists in the process of swallowing.

Mesoderm and Coelom

An important feature is that the wall separating the pericardium from the perivisceral ccelom is very thin and membranous, and unlike the stiff partition present in the dogfish and Gadus. The amphibian condition is already foreshadowed in Ceratodus, and it results in the fact that the heart and pericardium project back into the perivisceral cavity, ventral to the gut.

The first three somites give rise to the eye-muscles. The 4th disappears during development, and the 5th produces muscle-fibres which persist.

Ductless Glands . — The thyroid arises from a downgrowth from the floor of the pharynx and afterwards divides into two, right and left. The groups of vesicles of which it is composed are surrounded by connective tissue. Close to it are the parathyroids, on each side, and developed from the ventral region of the gill-slits which close up at metamorphosis. Their origin is therefore segmental, and it is worth noticing that parathyroids do not appear in vertebrates with persisting water- breathing gills.

The thymus glands arise from the dorsal sides of the gill- pouches, and are therefore also segmental in origin.

The adrenal bodies are in a very interesting condition. They consist of islands of tissue overlying the ventral surface of the kidneys and extending forward as isolated lumps at the side of the dorsal aorta. The bodies consist of two kinds of tissue : cortical, corresponding to the inter-renal of the dogfish, and medullary or chromaffme tissue, corresponding to the suprarenals of the dogfish, and like them derived from the sympathetic nervous system. The cortex (and interrenal) is formed from the ccelomic epithelium. In the region of the kidney, the adrenal bodies are composed of both cortical and medullary tissue, as in the higher vertebrates. The bodies in front of the kidneys, however, may consist entirely of medullary tissue, as in the supra-renals of the dogfish. These animals therefore provide a very interesting intermediate condition.

The pituitary consists of four parts. The pars nervosa is formed from the floor of the infundibulum, the remaining three (anterior, intermedia, and tuberalis) arise from the hypophysis.

Characters of Triton which show an advance over the conditions in Fish, and which are typical of Tetrapoda : Limbs ending in digits (fingers and toes) ; Formation of arterial arches, short-circuiting the gill-capillaries ; Interruption of dorsal aorta from internal carotid ; Pelvic girdle composed of three elements : one dorsal and two ventral ; Presence of an allantoic bladder ; Joining of cortical and medullary tissue to form the adrenal bodies ; Parathyroid glands ; Salivary glands.

Characters of Triton which are specialised when compared with higher forms : Reduction of bone in the skull and girdles ; Absence of membrane-bones in the pectoral girdle ; Incompleteness of the interauricular septum ; Degenerate condition of the ear.

Characters of Triton which are typical of Amphibia : Heart with two auricles and single undivided ventricle ; Skin naked, i.e. without horny scales ; Aquatic larval stage.

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Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology

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