Book - Vertebrate Zoology (1928) 3
Vertebrate Zoology G. R. De Beer (1928)
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Chapter III Petromyzon, a Chordate with a Skull, Heart, and Kidney=
Externals
Petromyzon, commonly known as the lamprey, is an elongated animal not unlike a fish, but without paired fins or jaws. Some species live in fresh water, and others in the sea. Their length varies from a few inches to about four feet.
The slimy epidermis is about a dozen cells in thickness and contains glands. In the middle line the skin is produced into median fins, two on the back and one round the tail. These fins are stiffened only by rays of cartilage.
At the front there is a circular mouth surrounded by horny teeth. Behind the mouth on each side is a small deep-set eye, and then seven apertures in a row. These are the external openings of the gill-pouches. Dorsally, in the middle line near the front, there is a small hole which is the single median opening of the nasal sacs and the hypophysial cavity (see p. 401). The anus is in the mid- ventral line, not far in front of the ventral portion of the tail-fin.
Through the mouth there protrudes a rasping organ called the tongue, which like the sides of the mouth is covered with horny teeth. These teeth are little cones, formed from the ectoderm, and replaced from underneath when worn away. They must be carefully distinguished from the teeth of all higher forms, which are of a different nature.
The lamprey fastens itself by means of its circular and sucker-like mouth onto its food (mostly fish), and rasps at it with its tongue. This method of feeding is very specialised and almost degenerate ; and it has brought about several specialisations in the structure of the animal. The horny teeth are one of these.
Head. — The outstanding advance which the lamprey shows over the condition of Amphioxus is the possession of a definite head. A head is a specialisation of the anterior region of the body brought about in connexion with :
the development of paired sense-organs for perception
at a distance ; the correlated specialisation of the nerve-tube into a
brain.
To these is added in higher forms the specialisation of the most anterior gill-bars into jaws for the capture of food ; but this need not be considered here as the lamprey has no jaws. The organs of the head are protected by a special skeletal structure called the skull.
Sense-Organs
The nose consists of two sacs invaginated from the skin, and whose epithelium is specialised for the perception and detection of chemical substances. This epithelium sends nerve-fibres to the brain, forming the olfactory nerves. While the paired nature of the olfactory organ is easily seen on dissection, it is outwardly obscured by the great expansion and upgrowth on each side of the region corresponding to the upper lip. This modification causes both the nasal pits and the hypophysial sac to open to the outside by a single common median dorsal pore.
The main rudiments of the paired eyes in development grow out from the brain on each side, giving rise to the optic vesicles. The outer side of each vesicle is pushed in so as to convert it into a cup, and the lens (developed from the superficial skin) fits into the mouth of the cup, just beneath its rim. The inner layer of the cup contains the cells which are sensitive to light, and form the retina ; the outer layer forms a backing of pigment. Outside this again, two mesodermal layers are laid on. The innermost of these is the choroid which contains blood-vessels, the outer, which is also the outermost of the whole eyeball, is the hard and protective sclerotic. The sclerotic encloses the whole eyeball, but in front of the lens it is transparent, forming the cornea. The cornea is in contact with the epidermis, which is here also transparent, forming the conjunctiva. The eye may then be regarded as a closed hollow ball, with the enclosed chamber divided into two by the lens : viz., a posterior chamber between the retina and the lens, and an anterior chamber between the lens and the cornea. The posterior chamber contains a jelly-like substance called the vitreous humour, the anterior chamber contains the aqueous humour.
Fig. 14. — Diagram showing the method of origin of the eyes in chordate animals and the relations of the sensory cells.
A, before the formation of the nerve-cord the sensory cells (sc) are on the outer surface ; B, when the nerve-cord has been formed the sensory cells line its cavity (c) ; C, formation of the optic vesicles (ov) ; D, origin of the lens (/) from the epidermis, conversion of the optic vesicles into optic cups (oc) and formation of the pineal vesicle (pv) ; E, condition with completely formed eyes, ac, anterior chamber ; ch, choroid ; cm, ciliary muscle ; en, conjunctiva ; cr, cornea ; e, epidermis ; i, iris ; /, lens ; on, optic nerve ; p, pigment layer ; pc, posterior chamber ; pi, pineal lens ; pr, pineal retina (erect) ; r, retina (inverted) ; s, sclerotic.
There is an important point to notice in connexion with the retina. In Amphioxus the cells which are sensitive to light line the central canal of the nerve-tube ; in all higher forms, that portion of the wall of the nerve-tube in which these cells lie is bulged out sideways to form the eye. The sensitive cells are, however, still morphologically on the inner side of the wall of the brain (i.e. adjacent to the central cavity or the cavity of the optic vesicle). The nerve-fibres which convey the impulses away from the retina cannot go through this cavity, they must run in the wall of the optic vesicle and of the brain. In so doing the nerve-fibres must therefore pass between the sensitive cells and the lens. This means that the image of the seen object reaches these sensitive cells after passing through the nerve-fibres. A retina of this kind is called inverted, and is characteristic of the paired eyes of all chordates. The pineal eyes (described below in connexion with the brain) have an erect retina, for here the nerve-fibres leave the retina on the side away from the lens. With regard to the paired eyes, it is essential to realise that the cavity of the primitive optic vesicle is not the same as that of the definitive eyeball (or posterior chamber). In the process of conversion of the optic vesicle into the optic cup, the cavity of the vesicle has been obliterated. The eyes are not immovable, but can be turned in various directions. This is effected by the myotomes of the first three segments, which are modified into so-called eye-muscles. The description given above applies to the paired eyes of all chordates ; the eyes of the lamprey are, however, somewhat degenerate.
Petromyzon has so-called auditory organs or ears, but it must be remembered that these organs primitively do not serve for the purpose of hearing, but are organs of balance. They take the form of sacs on each side of the brain behind the eyes, giving off canals in the form of half- hoops, each end of which opens into the sac. These are the semicircular canals ; each one bears a swelling or ampulla containing a statolith or organ of balance (see p. 395). In all vertebrates above Petromyzon there are three such canals, in planes at right angles to one another, but Petromyzon has only two.
Brain
The anterior end of the nerve-tube is modified and enlarged in connexion with the paired sense-organs to form the brain. The brain can be divided into three main regions : fore-, mid-, and hindbrain. The forebrain bears the olfactory lobes in front, and on each side it connects with the eyes (which are really part of it) by the optic nerves. The roof bears the pineal and parapineal eyes. These lie in the middle line, the pineal above the parapineal which is degenerate. The pineal eye is a vesicle of which the dorsal wall forms a lens, and the ventral wall a retina backed with pigment. The nerve-fibres lead away from the underside of this retina, which is therefore not inverted but erect. Above the pineal eye, the skull is thin and the tissues are more or less transparent. Beneath the forebrain is a simple pituitary body (see p. 399), the pars intermedia of which is apposed to the feebly developed infundibulum of the brain. The pituitary body in Petromyzon has lost connexion with the hypophysial cavity ; the latter extends backwards beneath the brain forming the hypophysial sac, and connects with the exterior through the median dorsal pore.
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Fig. 15. — Petromyzon : view of median longitudinal section through the brain. c, cerebellum ; cp, choroid plexus ; hs, hypophysial sac ; n, notochord ; nc, nerve-cord ; ns, nostril ; oc, optic chiasma ; 00, olfactory organ ; pb, pituitary body ; pe, pineal eye ; ppe, parapineal eye.
The midbrain bears the optic lobes, and the roof .of the hindbrain is modified into a rudimentary cerebellum. It is important to notice that the roof of the brain in Petromyzon is very thin and membranous ; with the exception of the transverse commissures in the forebrain, the optic lobes, and the cerebellum, it contains scarcely any nerve-cells at all.
Although at first sight the brain differs considerably from the more posterior part of the nerve- tube or spinal cord, it is easy to see how it was derived from the latter.
In each segment on each side there is a ventral nerve supplying the segmented muscles formed from the myotomes of the vertebral plate (somatic muscles), and a dorsal nerve supplying the sense-organs. In the gill-region, the dorsal nerves also supply the muscles formed from the unsegmented lateral plate (visceral or splanchnic muscles). In the region of the trunk these segmental nerves are called spinal nerves, those which emerge from the brain are called cranial nerves.
Nerves. — The ventral nerves of the first three segments innervate the muscles which actuate the eyeball. They are respectively the oculomotor, trochlear, and abducens. The dorsal nerves corresponding to them are the profundus, trigeminal, and facial ; the auditory nerve is a branch of the facial. The ventral root of the 4th segment supplies the most anterior complete myotome, and its corresponding dorsal nerve, the glossopharyngeal, passes down in the arch behind the first gill-slit (and in front of the second). The next dorsal nerve, the vagus, is a composite one, formed by the aggregation of portions of several other posterior dorsal nerves. It sends a branch down behind each of the remaining gill-slits, as well as to the " lateral-line " organs (see p. 38), and to the heart and gut. The ventral roots corresponding to the vagus supply the 5th and following myotomes, and the muscles beneath the gills. The muscles of the tongue are supplied by the trigeminal nerve.
In Amphioxus, the dorsal or sensory nerves are formed of fibres produced inwards from the sensory cells themselves. In Petromyzon and all higher forms this method of formation applies only to the olfactory nerves. All the other sensory nerves are formed in a different way. There are special nerve- cells which send one fibre to the sensory cell and another into the central nervous system. These nerve-cells are not in the nerve-tube but just outside it. They lie on the track of the sensory dorsal nerves and form swellings or ganglia. In Petromyzon and all higher forms, on every dorsal nerve- root, whether cranial or spinal, there is a ganglion. The ventral nerves consist of nerve-fibres which are formed from nerve-cells which lie inside the nerve-tube ; they therefore do not have ganglia.
In Petromyzon there are two important primitive features to note in connexion with the nerves. One is that the dorsal and ventral roots of each segment do not join together, but remain separate. The other is that the nerves are simple and uncovered by insulating material, i.e. they are non-medullated.
The nerves which innervate striated muscles go straight from the central nervous system to the muscle. On the other hand, those nerves which supply the smooth muscle-fibres of the gut and of the arteries (and in higher forms certain other structures also) do not run direct from the central nervous system to the muscle. Instead, they run to other nerve-cells, and these run to the muscle. These latter nerve-cells form part of the autonomic nervous system (see Chapter XXXI). In Petromyzon this system is only feebly developed. It is represented by some groups of nerve-cells along the gut, supplied by the intestinal branch of the vagus (see p. 47), and also by some cells close to blood-vessels near the spinal cord.
Skull and Skeleton
The brain, paired sense-organs, and roots of the cranial nerves are protected by a case of cartilage forming the skull. This is characteristic of all forms above Amphioxus which are therefore referred to as Craniata. The brain is surrounded by the cranium proper ; the sense-organs are protected by capsules. The nasal capsules are fixed on to the front of the cranium by connective tissue ; the auditory capsules are firmly fused on to the sides of the cranium by cartilage. The spinal cord enters the skull at the hind end through the foramen magnum, but in Petromyzon the nerves of the fourth and following segments (glossopharyngeal and vagus) come out from the brain behind the hindmost limit of the skull. This shows that the process of cephalisation or specialisation of the skull has not extended very far. The walls of the skull are very incomplete, as is the roof. The notochord extends forwards as far as the forebrain. In the trunk-region, in each segment on each side of the notochord, is a pair of cartilaginous pegs, one behind the other. The anterior peg in each segment (interdorsal) is in front of the ventral nerve-root ; the hinder peg (basidorsal) is in front of the dorsal root. These pegs are the rudiments of the vertebral column, as yet very incomplete and not in any way constricting or interrupting the notochord.
Between every two gill-slits, behind the last and in front of the first, are cartilaginous rods, the branchial arches or gill- arches. Together they constitute the branchial basket. The most anterior branchial arches, together with cartilages belonging perhaps to vanished gill-clefts and others in connexion with the rasping tongue, form a skeletal framework attached to the skull and termed the splanchnocrarsium. The braincase and sense-capsules are called the neurocranium. In higher forms the term " skull " is usually applied to both these structures, but it should be realised that they are fundamentally distinct. Petromyzon has no biting jaws ; instead, its mouth is round, for which reason the group to which it belongs is known as the Cyclostomes. The splanchnocranium of the Cyclostome is unimportant from the present point of view, because it is so much specialised that it can throw little light on the skulls of higher forms.
The fins are supported by rays of cartilage.
Alimentary System
In order to understand the structure of the alimentary canal and associated organs more easily, it is necessary to leave the adult Petromyzon and to turn to its larval form, which is known as the Ammoccete. The mouth is situated in a buccal cavity separated from the pharynx by a velum. The side walls of the pharynx are pierced by seven pairs of gill-slits.
Along the floor of the pharynx runs a groove which is continuous anteriorly with a pair of peripharyngeal bands. These rise up on each side of the mouth, behind the velum. Posteriorly the groove runs into a ventral hollow downgrowth of the pharynx. The floor of this downgrowth is folded and contains four rows of glandular cells. It is obvious that this structure is practically identical with that of the endostyle of Amphioxus. Behind the pharynx, the gut leads straight back through an intestine to the anus.
During the metamorphosis from the Ammocoete larva to the adult, certain important changes take place. The buccal cavity develops into a sucker with horny teeth, and the rasping tongue is formed in the floor of the pharynx. The endostyle closes up and its glandular and ciliated cells disappear but transverse section through the endostyle of an Ammocoete larva.
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Fig. 16. — Petromyzon
gc, the four tracts of glandular cells ; fp, floor of the pharynx ; va, branch of the ventral aorta. (Compare with Amphioxus.) its duct gives rise to the thyroid gland. From its habit of adhering closely to its food with its sucking mouth, water cannot easily pass through the animal's mouth to its gills. The latter are modified into pouches which take water in through their external apertures and then expel it again. Inside, the pharynx becomes divided into two parts, one above the other. The upper portion becomes the definitive passage from the mouth to the intestine ; the lower becomes the branchial tube. The latter is blind behind, receives the inner openings of the seven pairs of gill-pouches, and opens in front into the buccal cavity guarded by the remnants of the velum.
These changes which take place during the life of Petromyzon show that it is a form descended from ancestors which practised the ciliary mode of feeding, since when, it developed a specialised and somewhat degenerate method of feeding of its own.
Behind the pharyngeal region the gut runs straight as the intestine to the anus : there is no indication of a curved and enlarged region known in all higher forms as the stomach. In the intestine the surface of absorption is slightly increased by a small inwardly projecting ridge which, as it winds helicoidally down the intestine, is known as a " spiral valve." The lining of the gut is ciliated.
Ventral outgrowths from the front of the intestine form the liver. It is more or less degenerate in the adult, and it is said that its communication with the intestine by means of the bile-duct becomes lost, so that its only communication is with the blood-vessels. The pancreas is very rudimentary, and represented only by scattered packets of cells along the intestine.
Vascular System
Running forwards beneath the intestine, and therefore in the splanchnopleur, is a subintestinal vessel. It runs to the liver where it breaks up into capillaries forming a hepatic portal system. From the liver, the vessel proceeds forwards as the hepatic vein, and soon swells out beneath the pharynx and becomes specialised to form a muscular pump : the heart.
The heart is composed of the following structures : a sinus venosus, into which the hepatic and other veins enter ; leading on to a thin- walled auricle and a thick- walled ventricle. The entry to and exit from the ventricle, which does the propelling of the blood, are guarded by valves so that blood cannot flow in the reversed direction. The length of the structures composing the heart is greater than that of the space (pericardium) in which they lie ; consequently the heart is slightly bent on itself into the form of an S.
From the ventricle the ventral aorta runs forward beneath the branchial duct, and gives off paired afferent branchial vessels to the gill-arches, where they break down into the capillaries of the gills. Paired efferent branchial vessels then gather up the oxygenated blood and lead it to the dorsal aorta, which runs back just beneath the notochord, and is continued forwards into the head as the internal carotid artery.
On each side of the dorsal aorta are paired anterior and posterior cardinal veins which lie of course in the body-wall or somatopleur. At the level of the sinus venosus, the cardinals of each side communicate with the heart by means of the ductus Cuvieri. But as the cardinals are in the body- wall and the sinus venosus is in the gut- wall, the ductus Cuvieri have to cross the ccelom. This they do by means of a bridge of ccelomic epithelium called the transverse septum. The coelom is thus divided into an anterior region surrounding the heart : the pericardium ; and a posterior perivisceral splanchnocoel. This division is incomplete in the Ammoccete larva, but complete in the adult. In the adult there are peculiar median ventral veins in connexion with the tongue, and the ductus Cuvieri on the left side disappears.
The blood is red owing to the presence of haemoglobin in corpuscles. There is no spleen.
Excretory System. — No nephridia are found in any Craniate. The excretory organ is derived from the segmented nephrotomes, between the myotomes and the lateral plate. Typically, each nephrotome contains a cavity, the nephrocoel, which opens into the splanchnocoel by ciliated funnels (ccelomostomes), one in each segment. The nephroccels swell out into little cavities known as Bowman's capsules, into each of which a glomerulus projects. Each glomerulus is formed from an arteriole from the dorsal aorta and a venule leading to the posterior cardinal vein of its side. Glomerulus and Bowman's capsule together form what is known as a Malpighian corpuscle. From each capsule, a tubule grows backwards and into the tubule of its next posterior neighbour. A collecting duct is thus formed on each side, and it grows back, meets its fellow in the middle line, and opens behind the anus on a small papilla. This is the typical structure of the vertebrate kidney, and it is to be noted that the tubules which form it are of mesodermal origin, coming from the ccelomic epithelium, and are sharply to be distinguished from nephridia, which are ectodermal in origin.
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Fig. 17. Method of formation of the kidneys in chordate animals. A, early stage showing the cavity of the splanchnoccel (sc) M"™^** with those of the myocoels (tnc) by means of the nephrosis (nc). 1, intestine B hypothetical archinephros. The nephrocoels now known as Bowman' s capsules (Be) preserve their connexions with the . jp anc fcnoccd by means of ciliated funnels (,/). From each capsule 1 tubu le Ogrow back and joins that formed from the next posterior c fP s ^%\ so f ^ ed th ^ archineohric duct (ad). With each Bowman's capsule is associated a SoSlu" torfomed from an arteriole from the dorsal aorta (da) and a
Originally these coelomostomes must have served to free the genital products (as indeed the spermatozoa are freed in all higher forms), and excretion also took place through them from the coelom to the exterior. Then the excretory products were brought to the tubules by blood-vessels and the coelom lost its excretory function. The tubules lose their connexion with the splanchnocoel. In Petromyzon the coelomostomes were never open. In other forms, the coelomostomes may persist. In Myxine, a close relative of Petromyzon, the Malpighian corpuscles retain their segmental arrangement, but in all other forms they become increased in number, and the segmental correspondence is lost.
The tubules arise in two sets. First an anterior lot, opening into the pericardium, form the pronephros, and their duct (the pronephric duct) grows right back to the papilla. The pronephros nearly disappears in the adult Petromyzon, and is replaced in function by an identical but more posterior set of tubules which form the mesonephros. The mesonephric tubules grow into the pronephric duct which they find readymade for them, and which becomes known as the mesonephric duct. The mesonephric tubules develop later than the pronephric tubules, but are of essentially the same nature. This is illustrated in Bdellostoma, another relative of Petromyzon, in which a continuous series of tubules arises, the more anterior of which become the pronephros, and the posterior venule to the posterior cardinal vein (pc). C, condition in most young chordates, in which an anterior set of pronephric tubules (pt) gives rise to a pronephric duct (pd) before the more posterior mesonephric tubules (dmt) are properly formed, pf, pronephric funnels ; mf, mesonephric funnels. D, condition in older chordates, and retained throughout life in Myxine. The pronephros has degenerated, and the mesonephric tubules (mi) have joined the pronephric duct which now bears the name mesonephric duct (md). The latter is no longer continuous with the anterior portion of the pronephric duct. The capsules of the mesonephros lose their connexion with the splanchnocoel, but they retain their simple segmental arrangement. E, condition in Petromyzon. The pronephros has degenerated, and the number of capsules in the mesonephros has been increased by the formation of secondary Bowman's capsules (sBc).
D the mesonephros. Such a primitive kidney approaches very closely to the hypothetical Archinephros, with its archinephric duct. It is important to note that in the Cyclostomes there is but one kidney-duct on each side. The kidneys and their ducts hang down in the ccelom as the so-called nephric fold.
Genital System
The gonads are situated in a ridge hanging down from the roof of the coelom. Originally paired, the gonad is single and median in Petromyzon. In both sexes the genital products are shed freely into the splanchnoccel. In front of the place where the two mesonephric ducts join, each duct has a small pore opening into the splanchnoccel, and it is through these pores that the genital products escape to the exterior ; they have no special ducts.
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FlG. i 8. — Petromyzon : view of a dissection from the left side of the anus (a) and urinogenital (ug) aperture. df, dorsal fin ; i, intestine ; kd, kidney duct ; n, notochord ; nc, nerve- cord ; pv, perivisceral coelomic cavity. The arrow is passed through the genital pore from the coelom.
Coelom and Mesoderm
The eventual division of the coelom by the transverse septum into pericardial and peri- visceral cavities has already been mentioned. The myoccels are completely obliterated, and the only other portions of coelomic cavity are the kidney- tubules and ducts. The first three somites are drawn off into the service of the eyeballs, and the fourth forms the first proper trunk-muscle. The series of somites is therefore complete, and no segment has lost its somite. The myotomes are W-shaped, which condition can easily be derived from the V-shaped myotomes of Amphioxus by the turning forwards of their upper and lower ends. They are not subdivided into dorsal and ventral portions as in higher forms.
The characters of Petromyzon and Cyclostomes can be analysed under three headings : those which show an advance from the condition of Amphioxus, those which are primitive when compared with higher forms, and those which are secondary and specialised.
Characters shown by Petromyzon, absent in Amphioxus and typical of Craniata :
Formation of a distinct head, brain, and skull ;
Formation of a distinct heart ;
Formation of pro- and mesonephric kidneys ;
Epidermis several cells in thickness ;
Dorsal nerves with ganglia ;
Rudimentary vertebral column ;
Myotomes W-shaped ;
Rudimentary sympathetic nervous system.
Characters shown by Petromyzon {and Cyclostomes) which are primitive when compared with higher forms :
Endostyle, ciliated groove and velum of the Ammocoete
larva ; Emergence of glossopharyngeal behind the cranium ; Dorsal and ventral nerves separate and unconnected in
each segment ; Fourth segment forming a complete myotome : no
myotomes lost ; Notochord unconstricted by vertebral column ; Kidney-tubules segmental in Myxine ; Myotomes not divided into dorsal and ventral portions ; Persistence (although slight) of the pronephros ; Absence of biting jaws ; Absence of paired fins ;
Absence of dermal skeleton (fin-rays or teeth) ; Absence of special stomach ; Absence of special genital ducts ; Absence of medullated nerves.
Specialised characters of Petromyzon :
Rasping tongue ; Sucking mouth ; Horny teeth ; Sac-like gill-pouches ; Separate branchial duct ; Large hypophysial sac ;
Single median dorsal pore for nasal organs and hypophysial sac.
Literature
Goodrich, E. S. Vertebrata Craniata, Cyclostomes and Fishes. Black, London, 1909.
Parker, T. J. A Course of Instruction in Zootomy (Vertebrata). Macmillan, London, 1884.
- 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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- 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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