AN ELEMENTARY COURSE OF PRACTICAL ZOOLOGY BY THE LATE T. JEFFERY PARKER, D.Sc, F.R.S. PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO DUNEDIN, NEW ZEALAND AND W. N. PARKER, PH.D. PROFESSOR OF ZOOLOGY AT THE UNIVERSITY COLLEGE OF SOUTH WALES AND MONMOUTHSHIRE, IN THE UNIVERSITY OF WALES FIFTH EDITION With One Hundred and Sixty-eight Illustrations MACMILLAN AND CO., LIMITED ST. MARTIN'S STREET, LONDON 1922 \ CHARACTERS OF THE CLASS PISCES — -THE DOGFISH .... 430 PRACTICAL DIRECTIONS 472 CHAPTER XI CHARACTERS OF THE CLASS MAMMALIA — -THE RABBIT . . 482 PRACTICAL DIRECTIONS . . 542 CHAPTER XII THE MINUTE STRUCTURE OF CELLS — -CELL-DIVISION — -STRUC- TURE OF THE OVUM -GAMETOGENESIS — -(SPERMATO- GENESIS AND OOGENESIS) — -MATURATION AND FERTILISA- TION OF THE OVUM -DIFFERENT TYPES OF OVA AND OF SEGMENTATION -EFFECT OF FOOD-YOLK ON DEVELOP- MENT 558 PRACTICAL DIRECTIONS 575 CHAPTER XIII DEVELOPMENT OF THE LANCELET I EARLY DEVELOPMENT OF OTHER TYPES, INCLUDING THE CHICK AND RABBIT — • FORMATION OF THE CHIEF ORGANS OF VERTEBRATES, AND OF THE AMNION, ALLANTOIS, AND PLACENTA . . . 577 PRACTICAL DIRECTIONS 615 INDEX 619 AN ELEMENTARY COURSE OF PRACTICAL ZOOLOGY PART I CHAPTER I SCOPE OF THE SCIENCE OF BIOLOGY — THE FROG : PRE- LIMINARY SKETCH OF ITS STRUCTURE, LIFE-HISTORY, AND VITAL FUNCTIONS — HINTS ON DISSECTION AND DRAWING Biology, Zoology, and Botany; — There is a good deal of misconception as to the scope of the science of Biology. One often meets with students who think that while the study of animals as a whole is Zoology and the study of plants as a whole Botany, Biology is the study of a limited number of animals and plants, treated as if they had no connection with anything else, — even with one another. This is quite wrong. Biology is the master-science which deals with all living things, whether animals or plants, under whatever aspect they may be studied. Physiology, treated for practical purposes as a separate subject, is a branch of biology ; so is anatomy, to which the medical student devotes so much time ; so are botany PRACT. ZOOL. j£ 2 SCOPE OF THE SCIENCE OF BIOLOGY CHAP. and zoology, in the ordinary sense of the words, i.e., the study of the structure, the mutual relations, and the arrangement or classification of plants and animals. But biology may also be pursued, and very profitably pursued toe, quite independently of teachers, class- rooms, and examinations. The country boy who knows the song of every bird, its nesting place, the number of its eggs, the nature of its food, the lurking place of the trout in the stream or the frogs in the marsh ; who has watched the ants with their burden of grain, or the bees with their loads of honey or pollen ; has began the study of biology in one of its most important branches. The intelligent gardener who observes the habits of plants, their individual tastes as to the soil, moisture, sunshine and the like, is also something of a biologist without knowing it. So also is the collector of eggs, shells, or insects, provided he honestly tries to learn all he can about the objects he collects, and does not consider them merely as a hoard or as objects for barter. In- deed, all that is often spoken of as natural history, so far as it deals with living things — plants and animals — and not with lifeless natural objects, such as rocks and minerals, is included under the head of biology. What then is the connection between biology in this wide sense and the kind of thing you are expected to learn in a limited number of lessons ? Simply this : — In the class-room nature cannot be studied under her broader aspects : indeed, much out-door natural history cannot be taught at all, but must be picked up by those who have a love of the subject, a keen eye and patience. But there is one thing we can do within the narrow limits of the class-room : we can confine ourselves to some department of biology small enough to be manage- able : we can take, for instance, one or more familiar animals and plants, and, by studying them in some i THE STUDY OF ZOOLOGY 3 detail, get some kind of conception of animals and plants as a whole. This book deals with the zoological side of biology only ; and what we have now to do is, in fact, what you have often done in the study of English : you take a single verse of a poem at a time, analyse it, parse it, criticise its construction, try to get at its1 exact mean- ing. If you have any real love of literature this detailed study of the part will not blind you to the beauty of the whole. And so if you have any real love of nature, the somewhat dry and detailed study we have now to enter upon should serve to awaken your interests in the broader aspects of biology by showing you, in a few instances, what wonderful and complex things animals are. One word of warning before we begin work. You must at the outset disabuse your mind of the fatal error that zoology or any other branch of natural science can be learnt from books alone. In the study of languages the subject-matter is furnished by the words, phrases, and sentences of the language ; in mathematics, by the figures or other symbols. All these are found in books, and, as languages and mathematics are commonly the chief subjects studied at school, they tend to produce the habit of looking upon books as authorities to which a final appeal may be made in disputed questions. But in natural science the subject-matter is furnished by the facts and phenomena of nature ; and the chief educa- tional benefit of the study of science is that it sends the student direct to nature, and teaches him that a state- ment is to be tested, not by an appeal to the authority of a teacher or of a book, but by careful and repeated observation and experiment. The object of this book, therefore, is not only to give you some idea of what animals are, but also to induce you to verify the statements contained in it for your- B 2 4 THE FROG CHAP. self. The description of each animal you should follow with the animal before you ; and if you find the account in the book does not agree with what you see, you must conclude, not that there is something wrong with your subject, but either that the description is imperfect or erroneous, or that your observation is at fault and that the matter must be looked into again. In a word, zoology must be learnt by the personal examination of animals : a text-book is merely a guide-post, and all doubtful points must be decided by an appeal to the facts of nature. It matters very little what animal we choose as a starting-point — a rabbit, a sparrow, or an earthworm — one will serve almost as well as another to bring out the essential nature of an animal, how it grows, how it is nourished, how it multiplies. On the whole, one of the best subjects to begin with is a frog : partly because it is easily obtained, partly because its examination presents no difficulties which an intelligent student may not be expected to surmount by due exercise of patience. Let us therefore begin our studies by catching a frog and placing it in a convenient position for examination, as, for instance, under an inverted glass bell-jar or even a large tumbler. External Characters. — Notice, first of all, the short, broad trunk, passing insensibly in front into the flattened head — there -being no trace of a neck — and ending behind without the least vestige of a tail : these constitute the axial parts of the animal. In the ordinary squatting position the back has a bend near the middle, producing a peculiar humped appearance. The head ends in front in a nearly semicircular snout, round the whole edge of which extends the huge slit-like mouth. On the top of fore-end of the snout are the two small nostrils, one on I EXTERNAL CHARACTERS 5 each side of the middle line ; and some distance behind them, the large, bright, prominent eyes, in which we can distinguish, as in our own eyes, a coloured ring or iris, surrounding a roundish black space or pupil. The eyelids, however, are rather different from our own : the upper is fairly well developed, but the lower is a mere fold of skin, incapable itself of covering the eye, but produced into a thin transparent skin, the nictitating membrane, which can be drawn upwards over the eye. The entire absence of eyebrows and eyelashes is a point worthy of notice. Extending backwards from the eye is a large dark patch, in the middle part of which is a circular area of tightly stretched skin, reminding one of the parchment of a tambourine ; this is the drum-membrane or tympanic membrane, a part of the ear. Here again we see a striking difference from our own organs : in ourselves the drum- membrane, instead of being flush with the surface of the head, is placed at the inner end of a deep passage or tunnel, the entrance to which is guarded by the large external ear. Of the latter there is no frace in the frog. Attached to the trunk are two pairs of offshoots or appendages, the arms and legs, or fore- and hind-limbs, in which the resemblance to our own limbs will be at once obvious. The arms are very short : each consists of an upper-arm, a fore-arm, and a hand, the latter provided with four fingers, which are slender and tapering and have no nails. The legs, on the other hand, are very long : each consists of a stout thigh, a long shank, with a well-marked " calf," and a very curious foot. The ankle-region is long — almost like a second shank — and has no heel : it is followed by five toes, the first or inner- most short, the second of moderate length, the third longer, the fourth longer still, and the fifth of about the 6 THE FROG CHAP. same length as- the third. All the toes are joined to- gether by thin transparent webs, and, like the fingers, have no nails. The name digit is conveniently applied both to fingers and toes. Between the bases of the thighs, at the hinder end of the trunk, is a small aperture, the vent or anus. In the squatting posture the body is raised upon the arms, which are kept slightly bent at the elbows, with the fingers spread out and directed forwards. In this position the innermost of the four fingers corresponds with our own index-finger, the frog having no thumb. The hind-limb, under similar circumstances, is bent into a sort of Z, the knee being directed forwards and the ankle-joint backwards. The toes are turned forwards, and the inner one, which is the -smallest of all, corre- sponds with our own great toe. Owing to the bent position of the limbs, we cannot very well, as in our own arms and legs, speak of- their upper and lower ends. It is therefore customary to call the end of a limb, or of any division of a limb, which is nearest to the trunk, the proximal end, that which is furthest away the distal end. Thus the proximal end of the fore-arm is the elbow region, the distal end of a digit is its tip. The whole body, including head, trunk, and limbs, is covered with a soft, slimy skin, of a brown colour, irregularly spotted with brown or black on the upper or dorsal surface, and whitish on the under or ventral surface. The colouring is, however, not constant ; in a frog kept in the dark the black spots increase to such an extent that the whole animal becomes almost black, while if kept in full daylight a corresponding brightening of the tints takes place. Moreover, the spots and patches of brighter colour are very variable : if you examine a dozen specimens you will see at once that no two are i MOVEMENTS AND GROWTH 7 alike in this respect. The large dark patch situated behind the eye and containing the tympanic membrane, is, however, always present, and is one of the chief distinguishing marks of the common British frog as compared with other kinds, such as the " edible frog " of the Continent. Sexual Characters. — As in so many of the more familiar animals there are two sexes of frogs, easily distinguished from one another. If you examine several of them you will find that a certain number have on the palm of the hand, towards the inner side, a large swelling, rather like the ball of our own thumb, but much more prominent and of a black colour. Frogs having this structure are males ; it is not present in the females. Actions performed by the Living Frog. — Kept under suitable conditions a frog very soon shows evidences of life. If touched or otherwise alarmed it attempts to escape by making a series of vigorous leaps — suddenly extending the hind legs and jumping to a considerable height. Thrown into water it swims by powerful strokes of the hind-limbs. It has thus, like so many living things with which we are familiar, the power of voluntary movement. If kept under observation for a sufficient time — weeks or months — it will be found that frogs grow until they reach a certain limit of size. Growth, in the case of the frog, is an increase in size and weight affecting all parts of the body, so that the proportions remain practically unaltered, and no new parts are added. Careful observation shows that the throat is constantly rising and falling, and the nostrils opening and shutting. These movements, like the expansion and contraction of the human chest, are respiratory or breathing movements, and serve to pump air into and out of the lungs. It requires frequent watching and sharp observation to see a frog feed. It lives upon insects, worms, slugs, and 8 THE FROG CHAP. the like. Opening its mouth it suddenly darts out a tolerably long, nearly colourless, and very sticky tongue ; if the prey is a small insect, such as a fly, it adheres to the end, and the tongue is quickly drawn back into the mouth, the whole operation being performed with almost incon- ceivable rapidity. Like other animals the frog discharges waste matters from its body. Its droppings or faces, discharged from the vent, are black and semi-solid. From the same aperture, it expels periodically a quantity of fluid, the urine, which is perfectly clear and colourless, and contains little besides water. Sometimes a frog will escape from confinement, leaving its damp box or vivarium for the warm, dry atmosphere of an ordinary room. When this happens the animal is usually found next morning dead and shrunken, and with its naturally moist skin dry and hard. From this it may be inferred that there is a constant evaporation of -water from the skin, which, under ordinary circumstances, is checked by a damp atmosphere or by occasional immer- sion in water. Hibernation. — In winter frogs bury themselves in damp places, and become sluggish, manifestations of life be- coming hardly apparent until the following spring, when they emerge from their holes. In this way they escape the dangers of frost which would otherwise be fatal to them. This suspension of activity during winter is known as hibernation, or the winter-sleep. Reproduction and Development. — If you examine a number of frogs towards the end of winter — about February — you will find that the full-grown females are distinguished from the males, not only by the absence of the pad on the hand, but by the swollen condition of the trunk, due to the interior being distended with eggs. After a time the eggs are laid, being passed out of the i EGGS AND TADPOLES 9 vent by hundreds ; each is a little globular body about -j^-th inch in diameter, half black and half white, and surrounded by a sphere of clear jelly, by means of which the eggs adhere together in large irregular masses, the well-known " frog-spawn." As the eggs are laid, the male passes out of his body, also by the vent, a milky spermatic substance or milt, which gets access to the eggs and impregnates or fertilises them. Without impregna- tion they are incapable of developing. Neither male nor female takes the slightest care of the eggs when once they are deposited and fertilised. They are simply left in the water unprotected in any way ; and, naturally enough, the mortality among them during the course of development is very great, the majority being eaten or otherwise destroyed, and only a very small per- centage coming to maturity. The first noticeable change in the spawn is that the sphere of jelly surrounding each egg swells up so as to acquire several times the diameter of the enclosed egg. The egg itself, or embryo, as it must now be called, gradually becomes entirely black, then elongates, and takes on the form of a little creature (Fig. i, i) with a large head, a short tail, and no limbs ; which after wrig- gling about for a time, escapes from the jelly and fixes itself, by means of a sucker on the underside of its head, to a water-weed. Great numbers of these tadpoles, as the free-living immature young or larvce of the frog are called, may be seen attached in this way. At first they are sluggish and do not feed, but, before long, they begin to swim actively by lashing movements of their tails, and to browse on the weeds. They are thus in the main vegetable-feeders, not carnivorous, like the adult frog. On each side of the head appear three little branched tufts or gills, which serve as respiratory organs (2, 2a), the tadpole, like a fish, breathing air which is dissolved 10 THE FROG CHAP. in the water. After a while the gills begin to shrivel up (3, 4), and the tadpole then comes periodically to the surface to breathe, lungs having in the meantime made FIG. i. — Stages in the life-history of a Frog, from the newly-hatched tadpoles (i) to the young frog (8). Natural size. — 20, is a magnified view of 2. (After Mivart.) their appearance. A pair of little hind-limbs appears at the root of the tail, and a pair of fore-limbs behind the head (5, 6). As these increase in size the tail slowly dwindles, the head and trunk assume the characteristic frog-form, and the animal now comes on land and hops i SUMMARY OF CHAPTER 11 about as a small tailed frog (7). As growth goes on, the tail further diminishes and finally disappears alto- gether, the transformation or metamorphosis being thus completed (8). Death and Decomposition; — Frogs may live for many years, but, sooner or later, either in the ordinary course of nature or by accident, they die. The heart stops beat- ing, the flesh undergoes what is called " death-stiffening," becoming hard and rigid, and all vital manifestations cease. Before long the process of decomposition ensues, the flesh, viscera, etc., soften and emit a bad smell, and in course of time rot away completely, leaving only the bones. Summary of Chapter. — The very brief and cursory study we have made so far shows us (i) that a frog has certain definite parts arranged in a particular way; (2) that it performs characteristic movements, some of them, such as leaping and swimming, voluntary ; others, such as breathing, involuntary ; (3) that it takes in solid food, consisting mainly of vegetable matter in the tadpole, of living animals in the adult ; (4) that it gives off waste matters ; (5) that it reproduces its kind by laying eggs, which develop only if impregnated ; (6) that it undergoes transformation or metamorphosis, the egg giving rise to a larva, the tadpole, which, after living for a time the life of a fish, gradually changes into a frog- 12 PRACTICAL WORK CHAP. HINTS ON DISSECTION AND DRAWING Instruments and other Requisites for Dissection.1 — In order to carry out the dissection of the frog and other animals successfully it is necessary to be provided with proper tools. The most important are — 1. Three or four sharp dissecting knives, or scalpels, of different sizes. 2. A large and a small pair of straight dissecting- forceps ; the small pair should have a peg on one leg fitting into a hole on the other, to prevent the points crossing; the points should be roughened. 3. A large and a small, fine-pointed pair of dissecting scissors ; the small pair for the more delicate work, and the large pair for coarser work and for cutting through bones. For the latter purpose a pair of bone-forceps is useful, but is not necessary in the case of such a small animal as the frog. 4. A seeker, i.e., a blunt needle mounted in a handle. 5. Three or four probes ; a seeker or knitting needle, or a thin slip of whalebone will answer for some purposes, but the most generally useful form of probe is made by sticking the end of a hog's bristle into melted sealing wax, and immediately withdrawing it so as to affix a little knob or guard. 6. An anatomical blowpipe, or, failing this, a piece of glass-tubing, 6 or 8 inches long, with one end drawn out in the flame until it is not more than -f^th to ^\jth of an inch in diameter. 7. An ordinary " medicine-dropper," or " feeder " of a self-feeding pen (see Fig. 25), made of a piece of glass-tubing about three inches long, drawn out in the flame at one end, and thickened at the other so as to form a collar, over which an india-rubber cap — an ordinary non-perforated teat — is fixed. This is useful for washing fine dissections, as well as for injecting. 8. A dissecting-dish. Get a common pie-dish, about 6 or 8 inches long, with rather low sides. Cut out a piece of cork-carpet or thick linoleum the size of the bottom of the dish, and a piece of sheet-lead of the same size, and fasten the two together by three or four ties of copper wire or 1 A suitable box of dissecting instruments can be bought from most scientific instrument makers for about 135.— £i. (For further apparatus required in connection with injection and micro- scopical work, see pp. 99, 119, and 135.) i HINTS ON DISSECTION 13 strong thread. Place this in the dish with the lead (which is simply to keep the cork-board from floating in .water) downwards. Or, place a few strips of sheet lead in the bottom of the dish, and then pour in some melted paraffin- wax into which a little lamp-black has been stirred, so as to make a layer half an inch or more in thickness. For larger animals than the frog, in addition to a larger dish, a dissecting -board will also be required. Get a piece of soft deal or pine about 18 inches x n inches and § inch thick, and nail round its edge a strip of wood about J inch X i inch, so as to form a projecting rim. 9. A magnifying glass. Any good pocket-lens or a com- mon watchmaker's glass will answer the purpose. As it is often desirable to have both hands free while using the lens, a stand of some kind is useful. One of the simplest is made by fixing a piece of J-inch brass-tubing, about 6 inches long, into a heavy block of wood, about 3 inches in diameter, and coiling roujid it, in a close spiral, one end of a piece of thick wire, which can be raised and lowered on the tube ; leave 6 or 7 inches of the wire standing out at right angles, and bend the free end into a loop to carry the lens. Or, get a piece of narrow clock-spring, about 13 inches long, and rivet one end to the outside of the rim of a watchmaker's glass, and the other to a small piece of zinc or brass ; on passing the spring round the head, the lens is kept in place at the eye without exertion. 10. Medium and small-sized pins. Large blanket pins are useful for fixing down larger animals. 11. A small sponge and a duster. 12. One or more wide-mouthed bottles or jars, containing a preservative in which to place your subjects after each day's work. The most convenient preservative for the purpose in most cases is the fluid sold as formaline,1 which can be diluted as it is wanted. For preserving your dissections from day to day, a i per cent, solution of formaline is strong enough in many cases — i.e., i cubic centimetre of formaline to 99 c.c. of water, or three-quarters of a dram of formaline to half a pint of water. For permanent preservation, a stronger solution — 2 to 5 per cent., according to circum- stances— should be used, or methylated spirit. If formaline is not available, use strong methylated spirit (i.e., about 90 per cent.) diluted with one-third of its bulk of water. 13. A plentiful supply of clean water. 14. An ounce or two of chloroform. A 40 per cent, solution of the gas formic aldehyde. 14 PRACTICAL WORK CHAP. Rules to be Observed in Dissection. — Many of the parts and organs of animals are bound together by means of a substance known as " connective-tissue," and the main object of dissection is to tear away and remove this substance so as to separate the parts from one another. The subject should be firmly fixed down in the dissecting- dish or on the dissecting-board by means of pins, inserted obliquely, so that they do not interfere with the dissection. The dissecting-dish must always be used for finer dissections, which should be done under water ; only just enough water being put into the dish to cover the dissection, which should be washed under the tap from time to time. When dissecting a part keep it on the stretch, and avoid fingering it or damaging it with the forceps. Never remove anything until you know what you are removing. Dissect along and not across such structures as blood-vessels and nerves. See that your instruments are kept clean and sharp, and never use the smaller scissors and scalpels for coarse work. Drawing. — You should make a point of drawing as many of your preparations, as well as of the living animals, as possible : an accurate sketch, taken from nature, no matter how rough, is of more value in teaching observation and in impressing the facts on your memory than the examination and copying of more perfect drawings, made by others. Anyone can soon learn to make sketches of this kind, even without having any previous knowledge of drawing. Each sketch should be made to scale, and small objects should be enlarged several times ; it is much easier to insert details in a large drawing than in a small one. Mark the scale against each drawing — e.g., x 2, x J. Using a rule and compasses, first sketch in an outline of the principal parts with a hard pencil ; if your object is bilaterally symmetrical, draw a faint line down the middle of the paper and then sketch in one side first. When you have sketched in all the outlines correctly, go over them again with a softer pencil, so as to make them clear and distinct. Do not attempt any shading unless you have some knowledge of drawing. Then tint the various parts in different colours, using very light tints except for such structures as vessels and nerves. You should keep to the same colours for the corresponding organs or tissues in all the animals you examine : thus you i HINTS ON DRAWING 15 might in all cases colour the alimentary canal yellow, the arteries red, the veins blue, glands brown, cartilage green, and so on. Make your drawings on one side of the page only ; the opposite side can then be used for explanations of the figures. Never insert on your original sketches anything you have not actually seen ; you can copy as many other figures as you like from various sources, but these should be kept apart from your own original drawings. Directions for the examination of the external characters of the adult frog, as described in this chapter, are given at the end of Chapter II., p. 31 ; and of the eggs and tadpoles in Chapter XII., p. 212. CHAPTER II THE FROG (continued) : GENERAL INTERNAL STRUCTURE You have now seen that a frog can perform a number of very complicated actions ; and, if you have any curiosity in these matters, you will probably want to know something of the mechanism by which these actions are brought about. Now, the best way to understand the construction of a machine, such as a clock or a steam-engine, is to begin by taking it to pieces ; and, in the same way, you can find out the parts of which the living machine we call a frog is made, and the way they are related to one another, only by taking it to pieces, or dissecting it. First notice, in addition to the external characters described in the last chapter, that the various parts of the body are strengthened or stiffened, as in ourselves, by a number of bones, which together form the greater part of the skeleton. It is quite easy to ascertain by feel- ing that the head contains a hard skull ; the lower jaw, a lower- jaw-bone or mandible ; that running through the back is a jointed backbone or vertebral column; that the region of the chest is protected by a breastbone or sternum ; and that each division of the limbs has its own bone or bones. The Mouth-Cavity. — There are also several points to CHAP, ii THE FROG: MOUTH AND PHARYNX 17 be observed in the interior of the mouth. All round the edge of the upper jaw is a row of small conical teeth (Fig. 7). There are no teeth in the lower jaw ; but on the roof of the mouth, a short distance behind the snout, are two little patches of teeth, called the vomerine teeth (vo. t), and nearer the jaw are two apertures, called the internal nostrils (p. no) : a guarded bristle passed into one of the external nostrils and pushed gently backwards and down- wards will be found to enter the mouth by the corre- sponding internal nostril. Behind the internal nostrils are two large hemispherical projections, due to the roof of the mouth being bulged out by the huge eyes, as can be readily made out by pushing the eyes from outside. On the floor of the mouth is the large, flat tongue (tng), remarkable for the fact that it is attached at its front end, its hinder end being free and double-pointed. When the frog uses it to catch insects it is suddenly thrown for- wards, almost like a released spring. Just behind the back- wardly-turned tips of the tongue is an oval elevation, having on its surface a longitudinal slit, called the glottis (gl), which leads, as we shall see afterwards, into the lungs. The back of the mouth narrows considerably, and the soft skin or mucous membrane lining it is here thrown into folds. A probe gently pushed backwards passes, as we shall see, into the stomach. The narrowed region of the mouth is the throat, or pharynx. On its upper wall, near the angles of the mouth, are two pits : a guarded bristle passed into one of these will be found to come into contact with the corresponding tympanic membrane, which will be pierced if sufficient force is used. The pits are known as the Eustachian recesses or tubes (em. t). Dissection of the Frog : Skin and Muscles. — If a slit is made in the skin of the belly, and a probe pushed in PRACT. ZOOL. r» 18 THE FROG CHAP. under it, it will be seen that the skin, instead of being firmly attached to the underlying flesh, as in a rabbit or a sheep, is for the most part quite loose, a spacious cavity lying between it and the flesh. Not, however, a single continuous cavity for the whole body : the probe, gently pushed in various directions, is stopped, in front, at about the level of the arms ; behind, at the junction of the thighs with the trunk ; and at each side, along an oblique line joining the armpit with the thigh. Moreover, by opening the skin of the back, throat, and limbs, and inserting the probe as before, similar cavities will be found in these regions, all separated from one another by partitions, along which the skin is firmly united to the underlying flesh. It will be noticed also that the probe, when withdrawn from any of these cavities, is wet. The cavities contain a watery fluid, called lymph, and are hence known as sub-cutaneous lymph-sinuses (Fig. 7, d. ly. s, v. ly. s). When the skin is removed it will be seen that under the skin and separated from it by the lymph-sinuses is a nearly colourless, semi-transparent, fibrous substance, the flesh. At first this appears to be continuous over the whole body, but, by careful dissection with a sharp scalpel, a very delicate, transparent membrane, called the fascia, can be separated from the flesh, which is then seen to consist of a number of separate bands (Fig. 2, my. hy, pet, ret. abd ; see also Fig. 16), covered as aforesaid by the fascia, and separated from one another by a kind of packing substance, also very delicate and transparent and known as connective-tissue. These bands or sheets are the muscles, and the whole of the flesh is made up of distinct muscles, readily separated from one another when once the requisite anatomical skill is attained. Here and there — for instance on the top of the head and the front of the shanks — there are no muscles, BODY-WALL 19 and the bones are covered only by skin and connective- tissue. Passing along the middle line of the belly is a dark longitudinal streak (Fig. 2, abd. v) : this is a blood-vessel, the abdominal vein. On each side of the body another sJt pel cc.st FIG. 2. — A^Frog with the skin (sk) of the ventral surface cut through and turned back right and left, so as to expose the muscles. Of these the mylohyoid (my. hy), pectoralis (pet), external oblique (ext. obi), and rectus abdominis (ret. abd) are lettered. On the right side (left in the figure), the posterior portion of the pectoral muscle is cut away, its two ends (pet', pet") only being left. The cartilaginous extremity of the breast-bone (xiphisternum, x. st) is shown, as well as the abdominal (abd. v), musculo-cutaneous (m. c. v), and brachial (scl. v) veins, and the cutaneous artery (c. a). (Natural size.) vein (m. c. v) is seen forming a loop, one limb of which is on the turned-back flap of skin, while the other passes between the muscles not far from the armpit : this vessel is the musculo-cutaneous vein. Both these veins, and many others which will be seen in the course of the C 2 20 THE FROG CHAP. dissection, are thin- walled tubes full of blood, as will be proved if you should happen to cut one of them, when the blood will escape in considerable quantity. Between the right and the left fore-limbs the ventral region of the trunk is protected by certain bones which form part of the shoulder-girdle : projecting backwards from this in the middle line is a flat, heart-shaped plate of a softer, gristle-like substance, known as cartilage (compare Fig. 12). Immediately between the thighs a cartilage called the pubis, part of the hip-girdle (Fig. 14), can be felt. Between the shoulder- and hip-girdles the ventral body-wall is soft, being formed only of muscle and connective-tissue. The Abdomen and its Contents. — By cutting through the muscles of the belly or abdomen, a large cavity, the body-cavity or ccelome, is exposed, in which are contained numerous structures presently to be described. In order, however, to open the whole of the cavity the ventral part of the shoulder-girdle must be removed. In the middle line, between the fore-limbs, and there- fore covered in the entire animal by the shoulder-girdle, is a pink conical body (Figs. 3 and 4, v) connected in front with a thin- walled bag (r. au, I. au), of a purplish colour. The whole thing is the heart : the pink posterior portion is called the ventricle : the purple anterior part consists of two chambers, the uuricles. The heart is enclosed in a transparent, membranous bag, the pericardium (pcd), which contains a lymphatic fluid. Just behind or posterior to the heart are two large masses (Ir) which have a dark reddish-brown colour ; these are the right and left lobes of the liver. They extend forwards, one on either side of the heart : between them is a globular bag of a greenish colour (Fig. 3, gl. bl), the gall-bladder. In front of the liver and left and right TT DISSECTION OF A MALE FROG 21 FIG. 3. — Dissection of a Male Frog. The enteric canal and liver are displaced to the animal's left, and part of the liver is cut away. Some of the muscles are cut away and certain nerves and blood-vessels traced into the head and limbs (xii). abd. v. abdominal vein ; br. brachial artery, vein, and nerve ; cod. mes. splanchnic or coeliaco-mesenteric artery ; cp. ad. right fat-body ; dm. duodenum ; ext. ju. external jugular vein ; fm. femoral vein ; g/. bl. gall-bladder ; hp. pt. hepatic portal vein ; hp. v. hepatic vein ; Ir. liver ; m. c. v. musculo-cutaneous vein ; my. hy. mylohyoid muscle ; pcd. pericardium ; p. c. hy. anterior horn of hyoid ; pt. cv. postcaval vein ; put. a. pulmonary artery ; pul. v. pulmonary vein ; pv. pelvic vein ; r. au. right auricle ; ret. rectum ; r. kd. right kidney ; r. Ing. right lung ; r. pr. cv. right precaval vein ; rn. pt. right renal portal vein ; r. spy. right spermary ; s. int. ileum ; spl. spleen ; st. stomach ; s. v. sinus venosus ; tr. a. conus arteriosus ; u. bl. urinary bladder ; ur. ureter ; v. ventricle ; vs. sm. seminal vesicle. 22 THE FROG CHAP. of the heart are two thin-walled transparent sacs (r. Ing, 1. Ing) with a honeycombed surface, the lungs. Their appearance varies very much according to their state of distension. When full of air they are about two inches in length in a full-sized frog, and protrude freely as soon as the abdomen is opened : when empty they hardly show unless the liver is turned aside. Emerging from beneath the left lobe of the liver (beneath in the present position of the animal, actually above) is a wide whitish tube (Fig. 3, st), which almost immediately turns to the right (the frog's right, not yours), so as to form a U-shaped bend (st, dm). This is the stomach, which is connected with the pharynx by a short tube called the gullet or cesophagus (compare Fig. 7, gul, st), and which varies considerably in size according to whether it is empty or distended with food. The stomach becomes continuous with a narrower tube, the first part of which (dm) passes forwards parallel - with the stomach, thus forming the narrow limb of the U, while the rest of it (s. int) is thrown into a rather com- plex coil. This tube is the small intestine ; the part in immediate connection with the stomach (dm) is distin- guished as the duodenum and the coiled part as the ileum. Between the stomach and duodenum, in the bend of the U, is a small yellowish-white body of irregular form, the pancreas (Figs. 7, pn, and 18, P). The stomach and intestine are kept in place and suspended to the dorsal wall of the body-cavity by a delicate membrane, the mesentery (Fig. 5, mes)t which is folded in correspondence with the various coils. As we shall see, the mesentery is really a portion of a thin, moist membrane, the peritoneum, with which the body- cavity is lined. The small intestine becomes continuous, posteriorly, with a much wider tube (Figs. 3 and 7, ret), lying against ii ABDOMINAL VISCERA 23 the dorsal wall of the abdomen, and called the large intestine or rectum. It is continued into a short tube, the cloaca (cl), which passes backwards, between the back- bone above and the pubis below, to open externally by the vent. Thus the mouth-cavity, pharynx, gullet, stomach, small intestine, rectum, and cloaca form a continuous tube, opening externally at each end, by mouth and vent respectively, and, for the greater part of its extent, contained within the body-cavity. The whole tube is known as the enteric or alimentary canal. Attached to the mesentery, close to the anterior end of the rectum, is a rounded body of a deep-red colour, the spleen (Figs. 3 and 7, spl). Quite at the posterior end of the abdominal cavity a very thin-walled and transparent sac (u. bl) will be seen, connected with the ventral side of the cloaca, and varying much in size according to its state of distension. This is the urinary bladder, which communicates by an aperture (Fig. 7, bl} with the cloaca, and when distended will be seen to be bilobed and of considerable size. If your specimen should be an adult female, and the time of year approaching the breeding season, you will already have observed, as the most prominent organs in the body, two large, lobed structures of a dark colour, protruding one on either side, and partly obscuring the view of the other organs. Each (Fig. 4, /. ovy) contains a great number of small globular bodies, half black and half white, and is suspended to the roof of the body- cavity by a sheet of peritoneum. These bodies are the ovaries, or organs for the manufacture of the eggs ; the rounded bodies of which they are largely composed are the eggs themselves. To each ovary is attached a yellow structure, produced into a number of streamer- like lobes (cp. ad) ; this is the fat-body, which serves as a storehouse of reserve nutriment. FIG. 4. — Dissection of a young Female Frog. The gullet (gul) and rectum (ret) have been cut through, and the enteric canal removed between these two points. The liver is removed, with the exception of a small portion (Ir) surrounding the postcaval vein (pf.. cv). The ventricle of the heart (v) i§ turned forwards, and CHAP, ii ABDOMINAL VISCERA 25 the abdominal vein (abd.v) is severed and turned backwards. The right ovary and fat-body are removed, and the right oviduct (r. ovd) is slightly displaced outwards, (x ii.) abd. v. abdominal vein ; cocl. mes. splanchnic or cceliaco-mesenteric artery ; cp. ad. corpus adiposum, or fat-body ; d. ao. dorsal aorta ; gul. gullet ; hu. cut end of humerus or upper-arm bone ; /. au. left auricle ; /. Ing. left lung ; 1. ovd. left oviduct ; I. ovd'. its opening into the body-cavity ; /. ovd". its pos- terior dilatation ; /. ovy. left ovary ; Ir. portion of liver ; pt. cv. postcaval vein ; pt. c-S . its anterior portion passing between the liver and the heart ; r. au. right auricle ; ret. rectum ; r. kd. right kidney ; r. Ing. right lung ; rn. pt. renal portal vein ; r. ovd. right oviduct ; r. ovd'. its opening into the body-cavity ; r. ovd". its posterior dilatation ; syst. tr. systemic arterial trunks at their point of union ; u. bl. urinary bladder ; ur. ureter ; v. ventricle. By lifting up either of the ovaries there is seen beneath it — in the natural position of the parts, above or dorsal to it — a greatly convoluted colourless tube (/. ovd, r. ovd) of about the same diameter as the intestine. This is the oviduct, through which the eggs pass from the ovary to the cloaca. . If the specimen is allowed to remain long in water the oviducts will be found to swell and finally to become disintegrated ; this is due to the fact that in them is formed the jelly in which the laid eggs are enclosed, and which, as already mentioned (p. 9), swells in water. In the male there is seen, on turning the intestines aside, a pair of yellow ovoidal bodies (Fig. 3, r. spy) about half an inch long, attached by peritoneum to the dorsal wall of the body-cavity. These are the spermaries or testes ; they manufacture the spermatic substance or milt by which the eggs are impregnated. To the anterior end of each is attached & fat-body (cp. ad], like that of the female. In young specimens of both sexes the reproductive organs — spermaries, ovaries, and ovi- ducts— are very small. When the intestine is turned aside there will also be seen, in both sexes, a pair of flattened, irregularly-oval bodies (Figs. 3 and 4, r. kd} lying in the posterior part of the abdominal cavity just above or dorsal to the ovaries or spermaries. These are the kidneys. With the outer edge of each is connected a tube, the ureter 26 THE FROG CHAP. (ur), by which the urine, formed in the kidneys, is carried to the cloaca (Fig. 7). It has been pointed out that the abdomen is lined by peritoneum, and that the various organs are sus- pended by folds of the same membrane, called, in the case of the enteric canal, the mesentery. The relations of this membrane are best seen in a diagrammatic transverse section of the body (Fig. 5), though many int 3 CU. FIG. 5. — Diagrammatic transverse section through the trunk of a Frog, to show the relations of the peritoneum, (x 2^.) abd. v. abdominal vein ; d. ao. dorsal aorta ; il. ilium ; int. intestine ; kd. kidney ; m. muscles of back ; m'. muscles of abdomen ; rnes. mesentery ; p. per. parietal layer of peritoneum ; p. per', the same, turning down to cover the kidney ; pi. cv. postcaval vein ; sk. skin ; s. cu. ly. s. sub-cutaneous lymph-sinuses ; spy. spermary ; s. v. ly. s. sub-vertebral lymph-sinus ; u. st. prostyle (part of the vertebral column) ; v. per. visceral layer of peritoneum, investing intestine ; v. per', the same, investing spermary. points can be perfectly well made out from the actual specimen. The body-cavity is lined by what is called the parietal layer of the peritoneum (p. per), which ad- heres closely to the body-wall except in the middle dorsal region, where it becomes closely applied to the ventral surface of the kidneys and reproductive organs. Leaving these, the peritoneum of the right side ii PERITONEUM— NEURAL CAVITY 27 approaches that of the left, and the two, coming into contact, form a double vertical sheet, the mesentery (mes), which extends ventrally towards the enteric canal. On reaching the latter, the two layers diverge again and surround the canal, forming the visceral layer of the peritoneum (v. per). The liver, oviducts, etc., are sus- pended and covered in the same way. Thus the lining of the body-cavity, the investment of the various organs contained in it, and the folds by which they are sus- pended, are all parts of one continuous membrane. The space left between the two diverging layers of peritoneum, in the mid-dorsal region, contains lymph, and is known as the sub-vertebral lymph sinus (s. v. ly. s). We have already noticed the abdominal and musculo- cutaneous veins. Other veins of greater or less size will be seen everywhere, passing, for instance, to the head and limbs (Fig. 3), and in the mesentery. Running parallel with many of the veins are smaller vessels, many of which have pigment in their walls, and which are of distinctly stouter texture. These are the arteries. They contain little blood in the dead animal, and, owing to the stoutness and elasticity of their walls, do not collapse when empty. Hence they are quite easy to see in a frog from which all the blood has been drained, while the thin-walled veins are almost invisible under like cir- cumstances. Finally, there will be seen in many parts of the body, often lying parallel to an artery and a vein, white cords, the nerves. The Neural Cavity and its Contents. — By turning the frog with its back upwards and cutting through the muscles of the back and the arches of the vertebrae as well as, in front, the roof of the skull (see Fig. 6), you will see that the backbone contains a distinct cavity, the neural canal, in which lies a white rod, made of the same soft kind of substance as the nerves, and called the spinal 28 THE FROG CHAP. cord (sp. cd), which ends behind in a thread-like prolong- ation (/. t), some distance in front of the thighs. It will also be found that the neural canal is continued, with a slightly increased diameter, into the skull, and that the spinal cord becomes continuous with the brain (br), a v.i ttst FIG. 6. — Dissection of a Frog to which the entire neural canal (n. c) has been opened from above, and the brain (br) and the spinal cord (sp. cd) laid bare. The brain consists of olfactory lobes (olf. I), cerebral hemispheres (crb. h), diencephalon (dien), optic lobes (opt. I), cerebellum (cblm), and medulla oblongata (med. obi), which will be referred to in Chapter X. The spinal cord ends in a delicate pro- longation, the filum terminale (/. t). The nasal bones (no), eyes (e), auditory region of the skull (au), transverse processes of the nine vertebras (v. i — v. g), urostyle (u. st) and ilia (il) are indicated in outline, and serve as landmarks - (x If.) (After Howes, slightly altered.) complex organ formed of several parts, which will be referred to hereafter. General Structure of the Limbs. — A transverse section cut across one of the legs, at about the middle of the thigh, will show in the middle of the cut surface the SUMMARY OF CHAPTER 29 thigh-bone, around it the flesh or muscle, and around this again the skin. Similar cuts through various parts of both fore- and hind-limbs show that these appendages of the body are solid, containing no cavities, except the sub-cutaneous lymph-sinuses previously observed. prt s.int FIG. 7. — Dissection of a Male Frog from the left side. The left fore- and hind-limbs and the left sides of the head and trunk have been cut away, the enteric canal and liver are displaced downwards, and the mouth, pharynx, and cloaca laid open, (x i|.) an. anus ; b. d. bile-duct ; b. hy. body of hyoid ; bl. urinary bladder ; bl'. its opening into the cloaca ; c. art. conus arteriosus ; cblm. cerebellum ; cl. cloaca ; en. j, centrum of third vertebra ; cp. ad. fat body ; crb. h. cerebral hemisphere ; d. /y. s. dorsal lymph-sinus ; du. duodenum ; ep. cor. epicoracoid ; (us. t. Eustachian recess ; FR. PA. fronto-parietal ; gl. glottis ; gul. gullet ; IL. ilium ; IS. ischium; kd. kidney; /. au. left auricle; /. Ing. left lung; Ir. liver; M.MCK. mento-meckelian bone ; n. a. /, arch of first vertebra ; olf.l. olfactory lobe ; opt. 1. optic lobe ; 0. ST. omosternum ; pcd. pericardium ; PMX. pre- maxilla : pn. pancreas ; p. na. internal nostril ; pu. pubis ; ret. rectum ; r. Ing. right lung; s. int. ileum ; s/>. cd. spinal cord ; SPH.ETH. sphenethmoid ; spl. spleen ; st. stomach ; s. v. sinus venosus ; tng. tongue ; /s. spermary ; ur. ureter; ur'. its aperture into the cloaca ; UST. urostyle ; v. ventricle ; v. ly. s. ventral lymph-sinus; vo. t. vomerine teeth; vs. sem. seminal vesicle. (From Parker and Haswell's Zoology.) Summary. — We thus get a notion of the general plan of construction of a frog as follows. It consists of a central or axial portion, the head and trunk, and of two pairs of lateral offshoots or appendages, the fore- and hind-limbs. 30 THE FROG CHAP. The trunk is hollowed out into two cavities : the ab- dominal or body-cavity (ccelome) below, and the neural canal above ; of these the neural cavity alone is continued into the head. The abdominal cavity contains the greater part of the enteric canal, the liver, gall-bladder, pancreas, spleen, lungs, heart, kidneys, urinary bladder, and reproductive organs. The neural canal contains the brain and spinal cord. The anterior end of the enteric canal is continued forwards into the head, forming the mouth-cavity, and opens externally by the aperture of the mouth ; its posterior end opens externally by the vent. The enteric canal passes through the containing body-cavity, having no communication with it. The lungs open into the pharynx, and thus communicate with the exterior not only by the mouth but also by the nostrils. The kidneys, bladder, and oviducts communi- cate with the cloaca, and thus with the exterior through the vent. Neither the neural nor the abdominal cavity has any communication with the exterior. The walls of the head and trunk consist largely of muscles and bones covered with skin. The limbs are solid outgrowths of the trunk, formed mainly of muscle, with bony supports and a covering of skin. Organs. — Notice that the body consists of various definite structures, or organs as they are technically termed, which have various purposes or functions to perform. The enteric canal, together with the liver and pancreas, are organs of digestion ; the lungs and skin, organs of respiration or breathing ; the heart and blood- vessels organs of circulation, serving as they do to propel and conduct the blood through the body ; the kidneys, aided by the skin, organs of excretion, for getting rid of waste matters ; the ovaries and spermaries, organs of reproduction ; the muscles, organs of movement ; the brain and spinal cord, together with the nerves, organs ii PRACTICAL DIRECTIONS 31 of control, serving to direct or control the actions of the body : the skin, nose, eye, and ear, sensory organs, by which communications are kept up with the external world. Tissues. — Notice also that the various organs of the body are built up of different materials, or tissues as they are called. We have already distinguished muscle, bone, cartilage, connective-tissue and nervous tissue. Other tissues we shall meet with in the course of a more care- ful examination. PRACTICAL DIRECTIONS To Kill a Fr&g for Dissection. — Place a frog on a plate, and cover it with a tumbler, or put it into a stoppered bottle. Soak a little bit of cotton-wool or sponge in chloroform, and push it under the edge of the tumbler, or drop it into the bottle. In a few minutes the vapour will make the animal quite insensible, and a somewhat longer exposure will kill it painlessly. External Characters. — Observe the voluntary and the involuntary respiratory movements of the living animal, and compare with a dead frog when making out the external characters (pp. 4-8) and the position of the various parts of the skeleton (p. 16). Sketch the entire animal from the side or from above. The Cavity of the Mouth.— Gently open the mouth of a dead frog as wide as possible, and make out the points described on p. 17. Sketch. The Body-wall. — Lay the frog on its back in the dissect- ing-dish, and fix it firmly by sticking pins through the skin of the arms and legs. With the forceps, held in the left hand, pinch up the skin of the abdomen near the middle line between the thighs, and make a nick in it with the point of the scissors. Then, holding the edge of the hole thus made with the forceps, pass in a probe and push it forwards as far as it will go without opposition. Note : — The sub-cutaneous lymph-sinuses, and the underlying muscle. 32 THE FROG CHAP. With the scissors extend the incision made in the skin of the belly forwards, in a straight line, to the lower jaw. Holding up the edge of the skin with the forceps, cut through, with a scalpel, the partitions between adjacent lymph- sinuses, so as to separate the whole of the skin of the ventral surface from the muscle ; and, having done so, pin back the flaps, right and left (see Fig. 2). Similar cuts should be made in the skin of the limbs and back. Observe — The fascia, the muscles of the body-wall,v the abdominal and musculo-cutaneous veins, and the position of the hyoid (p. 40), sternum (p. 47), shoulder-girdle, and pubic region of the hip-girdle (pp. 46 and 50). The Abdomen and its Contents. — Pinch up the muscles on one side of the abdominal vein with the forceps, and make an incision in them by a single snip of the scissors. Then, holding the edge of the wound with the forceps, extend the cut forwards to the shoulder-girdle and back- wards to the pubis. Keep the cut parallel to the abdominal vein, and be careful not to wound the latter. You will find that the incision thus made opens a large body -cavity or coelome, in which a number of structures, the abdominal viscera, are contained. Note that the body-wall consists of three layers : (i) skin, (2) muscles, with their fasc-ia, and (3) peritoneum. So far, however, the cavity is not thoroughly opened. Lift up the side of the abdominal wall to which the abdominal vein is attached, and very carefully separate the vein by tearing through, with a needle or the point of a scalpel, the connective-tissue by which it is attached to the inner face of the muscles : or, in order to prevent the possibility of injuring the vein, cut through the muscles of the body-wall longitudinally on the other side of the abdominal vein, so as to leave a narrow strip of muscle attached to it. Then make two cross-cuts, starting from the anterior end of the longitudinal incision, and extending outwards towards the fore-limbs : take care not to injure the musculo-cutaneous veins, and pin back the two flaps into which the soft ab- dominal wall is now divided (Figs. 3 and 4). Next dissect away the muscles covering the shoulder-girdle, so as to expose the bones : identify the bones called coracoid and clavicle (compare a skeleton and Fig. 12). With the strong scissors cut through these bones on either side as near as possible to the shoulder-joint : then lift up the sternum and middle portions of the shoulder-girdle, and carefully dissect them away from the underlying parts. ii PRACTICAL DIRECTIONS 33 Having thus exposed the whole of the abdominal cavity, pour just enough water into the dissecting-dish to cover the animal, first washing away any blood which may have es- caped from cut vessels. If your specimen is female, dissolve a little common salt in the proportion of i per cent, in the water (i gramme to 100 c.c.), or mix it with about one-third of its bulk of methylated spirit, in order to prevent the excessive swelling of the oviducts. (If, however, you wish to make out the blood-vessels in this specimen without injecting them, it is as well to defer putting water into the dish until a later stage of dissection.) Note— 1. The peritoneum — parietal and visceral layers. 2. The pericardium, containing the heart. If not already opened, the pericardium should be slit through, so that the auricles and ventricle can be plainly seen. If the frog has been killed quite recently, you will be able to observe the pulsation of the*heart. 3. The right and left lobes of the liver, and the gall- bladder. 4. The two lungs : if contracted, inflate with a blowpipe through the glottis. 5. The enteric or alimentary canal, consisting of gullet or oesophagus, stomach, small intestine (duodenum and ileum), and large intestine or rectum communicating with the cloaca, which will be seen at a later stage, and which opens to the exterior by the vent. 6. The mesentery. 7. The pancreas. 8. The spleen. 9. The urinary bladder. If collapsed, insert a blowpipe into the vent and inflate. (You will very likely find some small parasitic flat-worms, called Polystomum integerrinum, in the bladder ; each worm has a ring of suckers round the hinder end.) 10. In the male the spermaries and fat-bodies, and in the female the ovaries, fat-bodies and oviducts. 11. The kidneys and ureters. 12. The mode of suspension of all these organs (p. 26), and the position of the sub-vertebral lymph-sinus. In order to understand clearly the relations of these parts, a thick transverse section should be made through another frog in PRACT. ZOOL. -n 34 THE FROG CHAP, n the region of the kidneys and examined under water (com- pare Fig. 5). Sketch the contents of the abdomen in situ. The Neural Cavity and its Contents. — Now turn the frog with its back upwards, and pin it firmly to the bottom of the dissecting-dish or to the dissecting-board as before. Pinch up the skin, make a longitudinal cut through it from the snout to within a short distance of the vent, and turn the flaps right and left. The muscles of the back will be exposed, and, in front, the roof of the skull, which lies just beneath the skin. Carefully dissect away the muscles along the middle line of the back until the vertebral column is seen. Compare a prepared skeleton and Fig. 8, and make out the arches of the vertebra?. Between the first of these and the back of the skull is a slight space : insert one blade of the strong scissors into this, directing the point backwards, and cut through the arch of the first vertebra, first on one, then on the other side, and finally detach and remove the little piece of bone. The neural canal will then be exposed, in which lies the spinal cord, ending behind in a thread-like prolongation (compare Fig. 6) : work backwards, cutting away the arches of the remaining vertebrae. Next, using the scissors in the same manner, cut away, bit by bit, the roof of the skull : two large bones — ihefronto- parietals (see Figs. 8 and 9), forming a considerable part of the roof, can be more easily removed by raising them up with the edge of a scalpel. Note the cavity of the skull and its contained brain. General Structure of the Limbs. — With a strong knife, cut across one of the legs at about the middle of the thigh. Compare p. 28, and notice the thigh-bone, muscles, and skin. Sketch. Now preserve your specimen in formaline (about 2 per cent.) or spirit (70 per cent.). CHAPTER III THE FROG (continued) : THE SKELETON IF you have followed the description given in the pre- ceding chapter with a frog before you, testing every statement as you proceeded by reference to the specimen, you will now have a very fair notion of the general build of the animal. The next thing to do is to study its various parts in somewhat greater detail. As the bones and cartilages form the framework on which all the other parts are supported, it is convenient to begin with them. You may study them on a pre- pared skeleton, but a far better plan is to make a skele- ton for yourself as directed on p. 53. Parts of the Skeleton. — The skeleton consists of the following regions : — 1. The skull (Figs. 8 and 9) : a complex mass of mingled bone and cartilage, enclosing the brain and the organs of smell and hearing, and supporting the upper jaw. Connected loosely with the skull, but really form- ing part of it, are the lower jaw and a cartilage in the floor of the mouth known as the hyoid. 2. The vertebral column or backbone, consisting of nine movably united pieces, the vertebra (Fig. 8 v.i — V.Q), and of a long bony rod, the urostyle (UST), representing a number of fused vertebrae belonging to the tail-region of the tadpole. 36 THE FROG CHAP, m 3. The shoulder-girdle or pectoral arch (Figs. 8 and 12), an inverted arch of bone and cartilage nearly encircling the anterior part of the trunk and giving attachment to— 4. The bones of the fore-limbs. 5. The hip-girdle or pelvic arch (Figs. 8 and 14), an apparatus shaped somewhat like a bird's " merry- thought " : it is attached in front to the ninth vertebra and behind gives attachment to — 6. The bones of the hind-limbs. The Vertebral Column. — The essential structure of a vertebra may be best studied by examining any of the nine from the second to the seventh : the first, eighth, and ninth present certain peculiarities, and so may be left till last. The whole vertebra (Fig. 8, B) has something the form of a signet-ring with its sides produced into two outstanding projections. The part comparable to the stone of the ring is ventral in position, and is called the body or centrum (en), the form of which is proccelous, i.e., its anterior face is concave, its posterior face convex, and both faces are covered with a thin layer of cartilage. The part corresponding with the circle of the ring is the neural arch (pd, Im) : it arches over the spinal cord and is produced in the middle line above into a blunt projec- tion, the neural spine (n. sp) . From the arch is given off, on either side, the large outstanding projection already referred to, the transverse process (tr. pr), which is tipped with cartilage in the second, third, and fourth vertebrae. The neural arch gives off from its anterior face, just above the origin of the transverse processes, a pair of small shelf -like projections, the articular processes or zygapophyses (a. zyg). Each has its upper surface flat and smooth, and covered with a thin layer of cartilage. A similar pair of processes spring from the posterior UST FIG. 8. — A, skeleton of Frog from the dorsal aspect ; B, anterior face of the 'fourth vertebra, (x if.) In A the left half of the shoulder-girdle and. the left fore- and hind-limbs are removed, as also are the investing bones of the left side of the skull. Cartilaginous parts are distinguished by dotting. The names of replacing bones are in thick capital letters, those of investing bones in italic capitals, other references in small italics. i. c. hy. anterior horn of hyoid ; actb. acetabulum ; AST. astragalus ; a. zyg. anterior articular processes, or zygapophysis ; b. hy. basi-hyal ; C. calcar ; GAL. calcaneum ; en. centrum ; EX. 00. exoccipital : FE. femur ; fon, fon'. fontanelles ; FR. PA. fron to-parietal ; HU. humerus ; IL. ilium; Im. roof of neural arch ; MX. maxilla ; n. sp. neural spine ; olf. cp. olfactory capsule ; ot. pr. otic process ; p. c. hy. posterior horn of hyoid ; pd. base of neural arch ; PMX. premaxilla ; PR. OT. pro-otic ; QU.JU. quadrato-jugal ; RA. UL. radio-ulna ; SP. ETH. sphenethmoid ; SQ. squamosal ; S. SCP. supra-scapula ; SMS. suspensorium ; TI. FI. tibio-fibula ; tr. pr. transverse process ; UST. uro- style ; V. 1, cervical vertebra ; V. 9, sacral vertebra ; VO. vomer ; / — V digits. (From Parker and Haswell's Zoology, after Howes, slightly altered.) 38 THE FROG CHAP. face of the arch, but have the smooth, cartilage-covered surface or facet looking downwards. When two vertebrae are placed in position, the convex posterior face of the foremost centrum fits into the con- cave anterior face of its successor, like a cup and ball, and at the same time the posterior articular processes of the first fit over the anterior articular processes of the second. All the touching surface is, as we have seen, capped with cartilage, and the vertebrae can be moved upon one another, either up and down or from side to side. The centra and articular processes are the only parts of the vertebrae which are in contact when the bones are in their natural positions. Large gaps or notches are left between the dorsal portions of the arches (see Fig. 8, A) to allow of the requisite amount of up and down movement, and there are similar gaps between the sides of the arches, bounded by the articular processes above and the centra below. These are called the .inter- vertebral foramina : through them the nerves pass from the spinal cord. The only differences of importance between the vertebrae now under consideration is in the form arid direction of their transverse processes, which are specially large in the third, short and devoid of cartilaginous tips in the fifth, sixth, and seventh. The first vertebra has no transverse processes, and on either side of the very small " body " its anterior face bears, instead of the ordinary articular processes, a pair of obliquely placed, oval, slightly concave surfaces or facets, covered with cartilage, and serving for the articulation of the condyles of the skull, presently to be described. The transverse processes of the ninth or sacral vertebra are very long and strong, directed obliquely backwards, and tipped with cartilage : to them the arms of the hip girdle are articulated. in AXIAL SKELETON 39 The eighth vertebra differs from its predecessors in having its body concave behind as well as in front. Corresponding with this, the ninth (v. 9) has its centrum convex in front, while behind it presents two little rounded elevations placed side by side. It will be seen that the vertebrae are all corresponding structures, following one another in a regular series from before backwards. A correspondence of this kind, in which there is a repetition of similar parts along the body, is termed a serial homology, and thus not only the vertebrae as a whole, but also their various parts are serially homologous, each to each, the correspondence being disturbed only by the first vertebra, in which the transverse processes are absent and the anterior face is modified for articulation with the skull. The urostyle (UST) is a long bone with a gradually diminishing ridge along its dorsal surface (see p. 35). Its anterior face has somewhat the appearance of a small vertebra with no transverse processes, . and has a double concavity for articulation with the double convexity on the ninth vertebra. Near its anterior end there is on each side a small aperture, representing an inter- vertebral foramen, for the last spinal nerve. The skull is a very complex structure, consisting partly of bone, partly of cartilage. It is divided into the following regions : — 1. The brain-case or cranium, a sort of oblong box containing the brain (Figs. 8 and 9) : it forms the middle portion of the skull and is a direct forward continuation of the vertebral column. 2. The auditory capsules (aud. cp), a pair of outstand- ing masses arising, right and left, from the posterior end of the brain-case. They lodge the organs of hearing. 3. The olfactory capsules (olf. cp), smaller masses pro- ceeding from the anterior part of the brain-case and 40 THE FROG CHAP. united with one another in the middle line. They lodge the organs of smell. 4. The suspensoria (sus), a pair of projections spring- ing from the outer and upper portions of the auditory capsules, and directed downwards, outwards and back- wards. To them the ends of the lower jaw are attached. 5. The upper jaw, a half-circle of bone and cartilage, united in front to the olfactory capsules and behind to the auditory capsules and suspensoria. On either side of the skull, between the cranium and upper jaw, is a large space, the orbit, in which the eye is contained. 6. The lower jaw, a roughly semicircular bar of bone and cartilage, articulated at its ends with the sus- pensoria. 7. The hyoid (b. hy), a shield-shaped cartilage connected by delicate curved rods (a. c. hy} with the auditory capsules. On the posterior surface of the brain-case is a large hole, the foramen magnum (Fig. 9, c, for. mag), on either side of the lower edge of which is a large oval elevation covered with cartilage, the occipital condyle (oc. en). The foramen magnum leads into the cavity in which the brain is contained. If the first vertebra is placed in its natural position with regard to the skull it will be seen that the foramen magnum corresponds with the neural canal of the vertebra, and that the condyles fit into its articular surfaces. Thus the skull readily moves up and down upon the vertebra, the condyles acting as rockers ; a space between the neural arch and the dorsal edge of the foramen magnum covered by membrane in the fresh state allows of the requisite amount of play. The discrimination of the separate bones and detailed structure of the skull is rather dinicult, and may very well be omitted by the beginner at the present stage. The occipital condyles are borne on a pair of irregular Ill SKULL 41 bones (EX.OC) which bound the sides of the foramen magnum, nearly uniting above and below it, and extending over a considerable portion of the posterior surface of the auditory capsule. These bones are the exoccipitals. In £1 "sBS|lgs- *' g *>" ^fclS^ o'S!0 5, S|-slt^82l- I SJljwvtf |l.«U front of each exoccipital is another irregular bone (PR.OT) forming the front part of the auditory capsule, and called the pro-otic. Each pro-otic is separated from the corre- sponding exoccipital, in young frogs, by a band of cartilage, but in old specimens the two bones are more or less completely united. 42 THE FROG CHAP. In the disarticulated skull it can be made out that the exoccipital and pro-otic of either side enclose a cavity ; in this the organ of hearing is contained. The exoccipital is perforated, just in front of the condyle, by a double aperture \Nv. 9, i o), through which two nerves, the glossopharyngeal and the vagus, pass on their way from the brain (see Chapter X.) . The pro-otic is similarly perforated or notched for the trigeminal and facial nerves (Nv. 5, 7). The dorsal surface of the brain-case is covered by two longish, flat bones (FR.PA). In the young condition each of these consists of two distinct bones, the front one the frontal, the hinder the parietal. As the young frog grows the frontal and parietal of either side become com- pletely fused, forming a single fr onto -parietal. On the upper surface of each olfactory capsule is a roughly triangular bone, the nasal (NA ) , in front of which is the corresponding nostril. The ventral surface of the brain- case is covered by a single bone (PA.SPH) having the shape of a T. The stem extends forwards in the middle line as far as the olfactory capsules, while the arms stretch outwards beneath the auditory capsules. This very characteristic bone is the parasphenoid. On the under surface of the olfactory capsules, corresponding to the nasals above, are a. pair of irregular bones, the vomers (VO). Their outer edges are notched and help to bound the internal nostrils : their posterior ends bear the vomerine teeth. The anterior end of the brain-case is surrounded by a bone (SP.ETH) which extends forwards into the region of the olfactory capsules, and is partly covered by the fronto- parietals and nasals above and by the parasphenoid below. This is the girdle-bone or sphenethmoid. In the disarti- culated skull it is seen to have a very peculiar shape. Its posterior half encloses a single cavity in which the fore-end of the brain (Fig. 6, olf. I) is lodged. Its anterior half encloses two cavities, right and left, separated from one another by a vertical partition, and serving to lodge the posterior ends of the olfactory sacs or organs of smell. Each of these cavities communicates with the single posterior cavity by a small hole through which the nerve of smell passes. Between the girdle-bone in front and the pro-otic behind, the side-walls of the skull are formed of cartilage perforated by a rounded aperture, the optic foramen (Nv. 2), for the nerve of sight. Forming the outer part of the suspensorium is a hammer- shaped bone, the squamosal (SQ) ; its head is applied to the auditory capsule and projects forwards into the orbit. in SKULL 43 The upper jaw is formed of three bones on either side. In front is the premaxilla (P. MX), a short bone, sending off an upward process towards the nostril. Next follows the maxilla (MX), a long, curved bone, forming the greater part of the upper jaw, and joined at its posterior end to a small, slender bone, the quadrato-jugal (QU.JU), which is firmly connected with the lower end of the suspensorium. Both premaxilla and maxilla are produced below into a prominent edge from which spring a number of small conical teeth, arranged in a single row. Besides these three bones there are two others which seem, as it were, to brace the upper jaw to the brain -case and suspensorium. The palatine (PA L) is a narrow, rod- like bone, placed transversely behind the olfactory capsule. The pterygoid (PTG) is a large three-rayed bone ; one ray is directed forward and connected with the outer end of the palatine and with the inner face of the maxilla ; another passes backwards and inwards and is connected with the auditory capsule ; the third extends backwards and out- wards and forms the inner and ventral portion of the sus- pensorium. The main mass or core of the suspensorium, between the squamosal outside and the pterygoid within, is a rod of cartilage (sus), which is continued forwards by a bar (pal. qu) supporting the pterygoid and palatine. There is an important distinction to be drawn between the bones of the skull which can be made out only by the exercise of a good deal of care and patience. By softening the connective-tissue which binds the bones together, it is possible to remove the majority of them without injuring the underlying cartilage (compare the right and left sides of the skull in Figs. 8 and 9, A and C), provided, of course, that the operation is skilfully performed : these bones are the nasals, vomers, fronto-parietals, parasphenoid, premaxilloe, maxillae, quadrato-jugals, palatines, pterygoids, and squa- mosals. A sort of foundation or groundwork (left side of figure in Figs. 8 and 9, A ; right side in Fig. 9, C) is then left behind, consisting mainly of cartilage, but containing the exoccipitals, pro-otics, and girdle-bone. These five bones cannot be removed without pulling the cartilaginous ground- work or chondrocranium to pieces. We thus get a distinc tion between replacing bones (" cartilage bones ") which are actually continuous with the cartilage and form part of the chondrocranium, and investing bones (" membrane bones ") which lie outside the chondrpcranium, united to it only by connective-tissue. The chondrocranium has a cartilaginous roof, underlying the fronto-parietals ; it is pierced by one large (Fig. 8, fon) 44 THE FROG CHAP. and two small (fonf) spaces, called fontanelles, covered by membrane. It has also a cartilaginous floor (Fig. 9, A) underlaid by the parasphenoid. The olfactory capsules (olf. cp) also have a cartilaginous roof and floor of irregular form, with the posterior end of which is united the cartilaginous palato -quadrate bar (pal. qu), with which the palatine and pterygoid bones are connected. Posteriorly this bar is continuous with the cartilaginous groundwork (quadrate) or core of the suspensorium (sus), which unites above with the auditory capsule by two processes (Fig. 9, C, ot. pr, ped) and below furnishes an articular surface for the lower jaw. Notice that in describing the vertebral column no dis- tinction was drawn between replacing and investing bones. As a matter of fact the vertebrae and the urostyle are all replacing bones ; each consists, in the tadpole, of cartilage which subsequently undergoes ossification, i.e., is replaced by bone in which a deposition of lime salts takes place. The lower jaw (Fig. 9, B) consists of two halves, or rami, united with one another in front by ligament. At its posterior end each half bears on its upper surface a I shallow pit, by which it articulates with the- suspen- sorium, and a little in advance of this pit is an elevation of the dorsal edge of the jaw, called the coronary process. Each half of the lower jaw consists of a cartilaginous core, the mandibular or Meckel's cartilage, which furnishes the articular surface referred to, and in front is ossified as a small replacing bone, the mento-meckelian (M.MCK). Outside the cartilage are two investing bones. One, the angulo-splenial, extends along the inner surface and lower edge of the jaw and forms the coronary process, while the dentary (DNT) forms the outer surface of the anterior half of the jaw. The hyoid is a thin, shield-shaped plate of cartilage (Figs. 8 and 9, b. hy) produced, both in front and behind, into a pair of processes or horns, as well as into less important offshoots. The anterior horns (Fig. 9, a.c.hy) are long, delicate, cartilaginous rods which curve back- wards and then upwards, finally joining with the audi- tory capsules. The posterior horns (p.c.hy) are short, Ill TYMPANIC CAVITY 45 bony rods which pass backwards, diverging as they go, one on either side of the glottis. Two apparently insignificant structures connected with the skull must be described because of their con- nection with the organ of hearing. Behind the suspen- sorium is a recess, roofed over by the squamosal, and, in the entire frog, converted by muscle and other tissues Trternb.Lab ctntymp lymp.cciv mp.TneTnb o.st FIG. 10. — Transverse section (diagrammatic) through the head of a Frog at the level of the tympanic cavity. The various parts of the skull shown in section are black, the muscles, &c., grey, and the skin and mucous membrane white. ( X 5.) an. tytnp. tympanic ring ; t. hy. body of hyoid ; buc. cav. cavity of pharynx ; ch. plx. choroid plexus ; col. columella ; eus. t. Eustachian tube ; fen. ov. fenestraovalis ; med. obi. medulla oblongata ; memb. lab. membranous labyrinth ; mnd. mandible ; Nv. VIII. auditory nerve ; o. st. omosternum ; pig. pterygoid ; qu. jn. quadrato-jugal ; stp. stapes ; tymp. cav. tympanic cavity ; tymp. memb. tympanic membrane. (From Parker and Haswell's Zoology.) into a chamber, the tympanic cavity (Fig. 10, tymp. cav), bounded externally by the tympanic membrane, and communicating with the mouth by the Eustachian tube. Supporting the tympanic membrane, as the frame of a tambourine supports the parchment, is a cartilaginous ring, the tympanic ring (an. tymp, shown in section). Stretching across the tympanic cavity from the outer wall of the auditory capsule to the tympanic 46 THE FROG CHAP. membrane is a small, hammer-shaped rod, the columella (Figs. 9 and 10, col), having a bony handle and a cartila- ginous head, the latter firmly fixed to the inner face of the tympanic membrane. The inner end of the handle is tipped with cartilage, and is attached to a small cartilaginous nodule, the stapes (st), which is inserted into an aperture in the auditory capsule known as the fenestra ovalis (fen. ov). With care the columella, in a wet skull, may easily be removed with small forceps, and examined under a magnifying glass. The shoulder-girdle has the form of an inverted FIG. ii. — Diagrammatic transverse section through the shoulder-girdle of a Frog, (x 2.) cor. coracoid ; ep. cor. epicoracoid ; gl. glenoid cavity ; hu. humerus ; scp. scapula ; s. scp. supra-scapula ; v. j, third vertebra. arch encircling the anterior region of the trunk, and having its dorsal ends turned inwards so as partly to cover the second to the fourth vertebrae (Figs. 8 and n). The dorsal region, on either side, is formed by a broad plate, the supra-scapula (s. scp), or upper blade-bone. It is mostly formed of bone, but its free edge consists of cartilage which, when dried, is seen to be impregnated with a granular deposit of lime-salts. This rough, brittle tissue is called calcified cartilage, and is distin- guishable from true bone, which has usually a smooth, enamelled surface. in SHOULDER-GIRDLE 47 Connected with the ventral end of the supra-scapula and passing vertically downwards is a flat bone, broadened at each end, the scapula or blade-bone (Fig. n, scp: Fig. 12, sc). From its lower end two bones (Fig. 12, cl, co : Fig. n, cor) pass directly inwards, parallel with one another, to end in a plate of cartilage which meets with its fellow of the opposite side in the middle line of the chest. The more anterior of these (cl) sc FIG. 12. — The shoulder- girdle of the Frog from the ventral aspect, the left scapula being straightened out. The cartilage is dotted, (x 3.) cl. clavicle ; co. coracoid ; g. glenoid cavity ; sc. scapula ; s. sc. supra-scapula ; si. sternum; st', omosternum; st". x phisternum. (From Howes's Atlas of Practical Elementary Zootomy.)* is a narrow bone called the clavicle, or collar-bone, the posterior one is broader and is known as the coracoid (co) . Between the scapula on the one hand and the clavicle and coracoid on the other, there is a cartilaginous interval, the posterior edge of which is scooped out into a depression, the j*lenoid cavity (Fig. 12, g ; Fig. n, gl) for the articulation of the upper-arm bone. Connected with the median ventral portion of the shoulder-girdle is the sternum, or breast-bone, which 48 THE FROG CHAP. consists of two separate parts, one extending forwards, the other backwards, in the middle line, and each formed of a flattened bony rod (st,r st"}, tipped with a flat plate of cartilage. The anterior bony rod with its terminal cartilage is called the omosternum (st'), the posterior bony rod the sternum (st), and the bilobed cartilage at its end the xiphisternum (st"}. The cartilages uniting the inner or ventral ends of the clavicles and coracoids are distinguished as the epicoracoids (Fig. n, ep. cor.). All the bones of the shoulder-girdle and sternum are replacing bones except the clavicle. This can be removed, and is seen partly to surround a bar of cartilage, the pro- coracoid, which stretches between the scapula and the epicoracoid and is ordinarily completely concealed by the clavicle. The Fore-limb. — The upper arm is supported by a single bone, the humerus (Fig. 8, HU), the first example we have had of what is conveniently called . a long bone. It consists of a roughly cylindrical shaft, formed of dense bone, and of two extremities — the proximal of partially calcified cartilage, the distal of spongy or cancellated bone. The proximal extremity or head is convex, and fits into the glenoid cavity of the shoulder girdle (Fig. n) ; the distal extremity or condyle is almost globular, and is articulated with the bone of the fore- arm. In a longitudinal section of the humerus which has not been allowed to dry you will see that the shaft (Fig. 13, A, sh) is not a solid rod, but a tube, containing a cavity, the marrow-cavity. In this way the weight of the bone is diminished without its strength being impaired. The marrow-cavity contains a substance called bone- marrow, composed chiefly of connective-tissue and fat, with blood-vessels. The proximal end of the hollow shaft is, as it were, plugged by the cartilaginous ex- tremity. Ill FORE -LIMB The fore-arm is also supported by a single bone, the radio-ulna (Fig. 8, RA.UL ; Fig. 13, B). Its proximal end is concave and articulates with the almost globular condyle of the humerus : the outer or posterior edge of the concavity is produced into a short process, the ^sk ct * ut FIG. 13. — Longitudinal sections of the principal long bones of the Frog, (x 2\.) A, humerus ; B, radio-ulna ; C, femur ; D, tibio-fibula ; en. condyle ; /. foramen for artery ; yt. fibula ; hd. head ; m. marrow ; oL olecranon process ; p. bony partition ; ra. radius ; sh. shaft ; ti. tibia ; ul. ulna. olecranon or elbow. The distal end is incompletely divided into two articular surfaces, and between these is a groove passing for some distance towards the proximal end of the bone. A section shows that at this end there are two distinct marrow-cavities, indicating that the bone is really double. That this is the case is PR ACT. ZOOL. -rp 50 THE FROG CHAP. proved by the examination of a very young frog, in which the single fore-arm bone is represented by two distinct cartilages, the radius on the inner or thumb side, and the ulna on the outer or little-finger side. The olecranon is a process of the ulna. The skeleton of the hand is divisible into three re- gions : the carpus or wrist, the metacarpus or mid-hand, and the phalanges or finger-bones. The carpus consists of six small irregular bones, arranged in two rows (Fig. 8). The proximal row articulates with the radio- ulna, while to the distal row are attached the metacarpals, which together constitute the metacarpus. Four of these are long, rod-like bones and support the bases of the four fingers or digits : to them are attached the phalanges, of which the first or innermost digit (II) has two, the next two, and the remaining two digits three apiece. A very small metacarpal, with a- single phalanx (I), occurs on the radial side and is concealed by the skin in the entire frog : it corresponds with our own thumb, so that the apparent first digit of the frog is really the second or index finger. The Hip-girdle. — This, as we have seen, has some- what the form of a bird's merrythought. It consists of two long arms (Fig. 8, IL ; Fig. 14, //), which are articulated with the transverse processes of the ninth vertebra, and sweeping backwards, unite in a disc- shaped mass, having on either side of it a deep, hemi- spherical cavity, the acetabulum (Fig. 8, actb ; Fig. 14, G), for the articulation of the thigh-bone. Two sutures, or lines of separation, nearly at right angles to one another, divide the disc-shaped portion into three parts. One of these, dorsal and anterior in position, is continued into one of the arms of the hip- girdle and forms half of the acetabulum ; this is the ilium (Fig. 14, //, P). The second, posterior in position, in HIND-LIMB 51 is the ischium (Is) ; like the ilium it is made of true bone. The third, or pubis (Kn), is ventral, and is formed of calcified cartilage. Originally each of these elements is paired, i.e., there is an ilium, an ischium, and a pubis on either side, the three together forming the innominate ; but in the adult the right and left ischia and pubes become united in the median plane, the ilia only remaining free. The Hind-limb.— The thigh, like the upper arm, is supported by a single long bone, the femur (Fig. 8, FE ; Fig. 13, C), harving a gently curved shaft and extremities of calcified cartilage. Its rounded proximal extremity, or head, fits into the acetabulum : its distal end articu- lates with the single bone of the shank, the tibio-fibula (TI. FI). This, the longest bone in the body, also has a shaft and extremities, and is further distinguished by grooves running from each end towards the middle of the shaft. Sections show that the grooved portions of the bone contain a double marrow-cavity (Fig. 13, D), and in the young animal there are found two shank-bones which afterwards unite, the tibia on the inner side, the fibula on the outer side. The foot, like the hand, is divisible into three regions : the tarsus or ankle, the metatarsus or mid-foot, and the phalanges or toe-bones. The tarsus, like the carpus, consists of two rows, but with only two bones in each; Those of the proximal row (astragalus and calcaneum) are greatly elongated (AST., CAL.), and furnish an additional segment to the limb, thus increasing the E 2 FIG. 14.— The hip-girdle of a Frog seen from the right side, (x 2.) G. acetabulum ; Kn. pubis ; //., P. ilium ; Is. ischium. (From Wiedersheim's Com- parative Anatomy.) 52 THE FROG CHAP. frog's leaping powers : those of the distal row are very small. The metatarsals are five in number : those of the first and second digits (I, II) bear two phalanges each, those of the third and fifth, three each, and that of the fourth, four. Attached to the inner side of the tarsus is a little claw-like structure (C) composed of two or three bones and called the calcar or spur. . Notice the striking correspondence in structure be- tween the fore- and hind-limbs, a correspondence which extends also, though less obviously, to the limb-girdles. The humerus corresponds or is serially homologous (p. 39) with the femur, the radius with the tibia, the ulna with the fibula, the carpals with the tarsals, the meta- carpals with the metatarsals, and the phalanges of the fingers with those of the toes. Then in the limb-girdles the glenoid cavity corresponds with the acetabulum, the scapula and supra-scapula (being above the articular cavity) with the ilium, the precoracoid and clavicle (being ventral and anterior in position) with the pubis, and the coracoid with the ischium. Thus not only are the limbs and limb-girdles serially homologous structures, but their several parts are also serially homologous, each to each. Nature of Bone.— It is a mistake to suppose that bones are made exclusively of hard mineral matter, like rocks or stones. If one of the long bones, for example, is put into weak acid, bubbles of gas will rise from the bone, showing that the phosphate and carbonate of lime, of which it is partly composed, is being decomposed with the liberation of carbonic acid gas. When the liberation of bubbles is over, the bone will be found to be unaltered in form, but to be quite flexible instead of hard and rigid. It can be bent in any direction, and a bone of sufficient length, such as a sheep's rib, can be in PRACTICAL DIRECTIONS 53 tied in a knot. This shows that the bone contains a large amount of organic or animal matter. On the other hand, a bone may be completely calcined by heat- ing to redness in a closed vessel, when its animal matter is completely consumed and its mineral matter left. Under these circumstances it becomes very brittle, falling to pieces at a touch, and its appearance is far more altered than by the removal of the mineral matter. PRACTICAL DIRECTIONS Preparation of the Skeleton. — Kill a frog with chloroform (p. 31), open the abdomen as directed on p. 32, but without cutting the shoulder-girdle, and remove the contained organs. Then, the frog being firmly pinned down, remove the skin and gradually cut away the flesh from the bones. In the case of the long bones of the limbs, it is best to cut through the muscles near one end of the bone and then gradually to strip them back towards the other end until the bone is exposed. The process is facilitated by dipping the frog occasionally into boiling water (maceration in cold water requires a considerable time) : this softens the connective-tissue by which the bones and muscles are bound together, and thus allows them to be more readily separated. While at work keep Fig. 8 before you, and be particularly careful not to injure those parts of the skeleton which are made of cartilage (dotted in the figure), and are therefore easily cut : the most important of these parts are the hyoid or tongue-cartilage lying in the floor of the mouth, the omosternum, the xiphisternum, and the supra-scapula (Fig. 12). Great care will also be required in cleaning the bones of the hands and feet, since the fine cords or tendons which pass to them from the muscles are very strong, and if pulled upon with much force are sure to bring away the small toe-bones with them : they should be separated as far as possible and then cut off, close to the bones, with scissors. Keep all the parts of the skeleton together, avoiding separation of the various bones, until the general charac- teristics of the entire skeleton have been made out : the only part which cannot be kept in connection with the rest is the shoulder-girdle, together with the fore-limbs. Examination of the Skeleton. — You should have two skeletons to examine — one dried, after it has been thoroughly 54 THE FROG CHAP.III cleaned, and one which has been kept from the first in spirit or formaline : the latter is the more instructive (see also the paraffin method, p. 473). An additional skull should be carefully cleaned, and then boiled until the numerous bones become separated from one another or disarticulated. After observing the form and relations of different parts of the skeleton as described on pp. 35 and 36 (Fig. 8, A), they may be separated from one another for more detailed examination. The individual vertebrae should be strung on a piece of wire or string so as to prevent them from being lost or misplaced. With the specimen before you, work through the characters of the axial skeleton (pp. 36-46) : if you omit the details given in small type at the present stage, do not forget to examine them subsequently. Make sketches of — a. any one of the vertebrae from the ist to the yth, from the side and from the front or back ; b. the ist vertebra ; c. the urostyle ; d. the skull from above and from below ; and e. the hyoid. It requires considerable skill to make a satisfactory preparation of the chondrocranium, and it is advisable to examine that of a Dogfish first ; but if you wish to attempt it, procure a large skull which has not been dried, and boil it in water. Carefully separate, by means of a scalpel, most of the investing bones (p. 43) ; the palatines, pterygoids and quadratojugals, and the dentaries and angulosplenials cannot well be disarticulated without destroying the soft cartilaginous parts beneath them. Make out — i. The brain-case and its fontanelle sand nerve- apertures. 2. The olfactory capsules. "3. The auditory cap- sules. 4. The palatoquadrate bar (to which the palatine, pterygoid, and quadratojugal bones have been left attached). 5. The mandibula,' or Meckel's cartilage (to which the angulosplenial and dentary have been left attached) . 6. The replacing-bones (exoccipitals, pro-otics, sphenethmoid, and mento-meckelians). 7. The columella, stapes, and fenestra ovalis. Sketch from above and from below. Now proceed to examine the appendicular skeleton (pp. 46-52), and sketch the shoulder-girdle and fore-limb, and the hip-girdle and hind-limb. Split some of the longer limb-bones longitudinally with a knife, and note the marrow-cavity in the shaft (Fig. 13). Place another of the long bones in 10 per cent, hydrochloric acid for an hour or two ; wash thoroughly in water and examine. CHAPTER IV THE FROG (continued) : THE JOINTS AND MUSCLES IN the previous chapter the bones — more than 150 in number — which together constitute the greater part of the skeleton of a frog have been considered as so many separate parts, fitting into or against one another in certain ways. We must now see how they are joined together in the entire animal so as to afford the requisite support, and, at the same time, to allow of free move- ment. The Hip-joint. — Let us begin by a study of the hip- joint (Fig. 15). The acetabulum (actb), as you have already seen (p. 50), is a hemispherical depression on the outer surface of the hip-girdle. It is formed of cartilage, continued into a projecting rim round the edge of the cavity. The head of the femur (hd) is also formed of cartilage, and fits accurately but rather loosely in the acetabulum. The acetabulum is lined, and the head of the femur is covered, by a thin layer of connective-tissue, the perichondrium (p. chd), which, in both cases, is continued on to the adjacent bone, where it receives the name of periosteum (p. ost). Attached all round the rim of the acetabulum is a strong sheet of connective-tissue called the capsular liga- 56 THE FROG CHAP. ment (cps. Ig), forming a short, fibrous tube. The other end of this tube is fixed to the femur, just below the head, the ligament being continuous, in each case, with the perichondrium. There is thus a space between the head of the thigh bone and the acetabulum, closed all round by the capsular ligament. This space is filled with a delicate, fibrous, closed bag, the synovial capsule (sy. cps), one side of which fits closely into the acetabulum, while the other as closely invests the head of the femur. p. chef FIG. 15. — Horizontal section of the Frog's hip-joint, (x 5.) actb. acetabulum ; cps. Ig. capsular ligament ; hd. head of femur ; il. ilium ; med. marrow ; p. chd. perichondrium ; p. ost. periosteum ; pu. pubis ; sh. shaft of femur ; sy. cps. synovial capsule. The capsule is filled with a watery fluid, the synovia, and thus forms a buffer or water-cushion between the adjacent parts of the skeleton. Thus the synovial capsule keeps the two parts slightly apart and prevents friction, while the capsular ligament keeps them together and prevents dislocation. It is obvious that, in such a joint as this, movement is possible in all directions. The femur can be inclined either upwards, downwards, or sideways, and is capable of a certain amount of rotation. The joint is, in fact, a cup-and-ball joint, and is capable of movement in any iv JOINTS 57 plane. A similar but less perfect cup-and-ball joint is that of the shoulder, in which the cup is furnished by the glenoid cavity, the ball by the head of the humerus. Other Joints. — The elbow- and knee-joints are con- structed on the same general plan, but, owing partly to the form of the adjacent surfaces, partly to the mode of attachment of the ligaments, they are capable of movement in one plane only, i.e., up and down, but not from side to side. They are therefore distinguished as hinge- joints. The vertebrae are connected with one another in a similar way. Between the convex hinder face of one centrum and the concave front face of its successor is a synovial capsule, and the two centra are bound together by ligament, a shallow cup-and-ball joint, with a very limited range of movement, being produced. There are also synovial capsules between the articular processes, which, being in contact with one another by flat surfaces and working mainly from side to side, form gliding- joints. Strong ligaments connect the neural arches with one another and join the first vertebra to the skull. In all cases where free movement is necessary the joints are formed in the same way ; the bones are bound together by ligaments, and a synovial capsule is inter- posed between their adjacent cartilage-covered surfaces. Where little or no movement is required, as between the bones of the shoulder- and hip-girdles, the union is effected by cartilage or ligament only, and there is no synovial capsule. Such joints are therefore distinguished as immovable or imperfect joints. The Muscles. — We see then that the bones of the skeleton are attached to or articulated with one another by means of ligaments, so arranged, in most cases, as to allow of more or less free movement between the bones. add.lonfjf FIG. 16. — The muscles of a Frog from the ventral aspect. On the left side (right of figure) many of the superficial muscles have been cut and reflected to show the deep layer. (Natural size.) add. brev. adductor brevis ; add. long, adductor longus ; add. mag. adductor mag- nus ; del. deltoid \ext. cr. extensor cruris ; ext. trs. extensor tarsi ; FE. femur; 58 CHAP, iv MUSCLES 59 gn. hy. genio-hyoid ; gstr. gastrocnemius ; hy. gl. hyoglossus ; ins. ten. tendinous inscription ; /. alb. linea alba ; my. hy. mylo-hyoid ; obi. int. obliquus internus obi. ext. obliquus externus ; o.st. omosternum ; p. c. hy. posterior horn of hyoid pet. pectoralis ; pctn. pectineus ; per. peronaeus ; ret . abd. rectus abdominis reel. int. rnaj. rectus internus major ; rect. int. min. rectus internus minor sar. sartorius ; sb. nit. submentalis ; sent. ten. semitendinosus ; tib. ant. tibialis anticus ; tib. post, tibialis posticus ; TI. FI. tibio-fibula ; vast. int. vastus internus ; x. st. xiphisternum. (From Parker and Haswell's Zoology.) We must now try to find out how the movements are effected in the living frog. It was pointed out in the second chapter that the flesh is made up of distinct bands or sheets, the muscles, some of which came under your notice in your first dissection. It is quite easy to convince yourself that the whole of the flesh has this character by skinning a frog and carefully removing the fascia (p. 18) which covers the muscles and the more delicate web of con- nective-tissue which forms a sort of packing substance between them. After noticing some of the muscles shown in Fig. 16, especially those of the leg, give your attention to the muscle marked gstr, a prominent spindle- shaped mass of flesh forming the calf of the leg, and known as the gastrocnemius (gstr). The spindle-shaped, fleshy mass or belly of this muscle is continued at either end into a band of strong, tough connective-tissue, the tendon (Fig. 17). The tendon at the proximal end is flat, and is attached to the distal end of the femur and to the proximal end of the tibio-fibula, in each case becoming continuous with the periosteum of the bone. The tendon at the distal end has the form of a stout cord, and is distinguished as the tendo A chillis ; it corresponds with the strong tendon just above the heel of the human foot. At its distal end it is continued into a broad sheet of connective-tissue, the plantar fascia, which spreads over the whole sole or plantar surface of the foot. If the foot is bent upon the shank as in the ordinary sitting position of the frog, and the gastrocnemius pulled 60 THE FROG CHAP. upwards or towards the thigh, the foot will instantly be bent backwards, so as to come into a straight line with the shank, the action being one of those performed by the living frog when leaping. It will be seen that the proximal tendon is attached to a relatively fixed point : it is distinguished as the tendon of origin, or the muscle is said to arise from the femur and tibio-fibula. The distal tendon is attached to a relatively movable part, the foot, and is called the tendon of insertion, the muscle being said to be inserted into the plantar fascia. Muscular Contraction. — Obviously, however, there is nothing to pull upon the muscle from outside in the living frog. We must, therefore, try to form some idea as to how the action of bending the foot, roughly imitated in the dead subject, is performed during life. If the gastro- cnemius be exposed in a recently killed frog, fhe foot bent up as before, and a smart pinch be given to the belly of the gastrocnemius, the foot will be bent back, although no pull has been exerted on the muscle. The same thing will happen if you drop on the gastrocnemius a single drop of weak acid or of a strong solution of common salt, or if you touch it with a hot wire, or if you apply to it the electrodes from an induction coil so as to pass an electric current through it. Careful observation shows that what happens under either of these circumstances is that the belly of the muscle decreases in length and at the same time increases in breadth, so as to become shorter and thicker (Fig. 17) . The result of this must necessarily be to cause its two ends to approach one another. As the tendon of origin is attached to the femur, which we suppose to be fixed, it is unable to move, and the insertion is therefore drawn upwards, bringing with it the movably articulated foot. In fact exactly the same thing takes place as IV MUSCULAR CONTRACTION 61 when we raise our own fore-arm. This action is per- formed by means of the biceps muscle which arises from the scapula and is inserted into the fore-arm. When the latter is raised we feel a lump rise on the front of the upper arm due to the thickening of the biceps. FIG. 17. — Diagram of apparatus for demonstrating the contraction of the gastro- cnemius muscle. A, upright, bearing two adjustable horizontal arms. To the upper of these (B) is fixed by a clamp the femur (fe), having the gastrocnemius (gstr) in connection with it. To the lower arm (C) is fixed a light lever (L) movable in a vertical plane, and having the tendon of insertion of the muscle attached to it by a thread. The dotted lines show the form of the gastrocnemius and the position of the lever during contraction of the muscle, sc. nv. the sciatic nerve. This shortening and thickening of the muscle is termed a contraction. Do not fail to notice that this word is used in a special sense. When we say that a red-hot bar of iron contracts on cooling, we mean that it becomes smaller in all dimensions — undergoes an actual decrease in bulk. But in muscular contraction there is no alteration in bulk : the decrease in length is 62 THE FROG CHAP. balanced by an increase in thickness, as when a stretched piece of india-rubber is relaxed. The external influence by which a contraction is in- duced is called a stimulus. As we have seen, a stimulus may be produced by actual contact of some external object (mechanical stimulus), or by chemical action (chemical stimulus), or by heat (thermal stimulus), or by an electrical current (electrical stimulus). Relation of Muscle and Nerve. — Evidently, however, we have by no means got to the bottom of the matter. In the living frog movements are always going on, and all are due to the contraction of muscles, and yet no stimuli of the kind enumerated are applied to any of them. As the muscles retain the power of contraction for some little time after the death of the animal, it is easy to make such experiments as that described in the next paragraph. Running longitudinally between the muscles on the dorsal side of the thigh is a shining, white cord, the sciatic nerve (Fig. 17, sc.nv), accompanied by an artery : it gives off branches to the muscles and skin, and, amongst others, one to the gastrocnemius. If, when quite fresh, this nerve be carefully separated as it traverses the thigh and pinched with the forceps, the gastrocnemius will contract just as if the stimulus had been applied to it directly, and the same will happen if a chemical, thermal, or electrical stimulus be applied. • Thus a stimulus applied to the nerve of a muscle has the same effect as if applied to the muscle directly : it gives rise to a nervous impulse, which, travelling along the nerve, induces contraction of the muscle. Once more, however, external stimuli are not applied to the frog's nerves during life, and it is obvious that we must carry our inquiry a little further. The sciatic nerve if traced upwards will be found to pass into the iv MUSCULAR SYSTEM 63 trunk (Fig. 51, sci), and finally to join the spinal cord, which, as we have seen, is in connection with the brain. In the living frog nervous impulses originate in the brain, without the direct intervention of an external stimulus, and are conducted along the cord and nerves to the muscles. But further consideration of this sub- ject must be deferred until we have made a special study of the nervous system. The Muscular System in General.— All over the body the muscles, though varying greatly in form — some being elongated and band-like (Fig. 16, sar), others spindle-shaped (gastr), others in the form of broad, flat sheets (my. hy, *3 T3 M •S£|JP .83° Mllil •t! 2 ° T3 rt O "3 « '^ -g 0 ft-n.S-d'S^^-S S3 S « S « £.2*5 o?3 g cfi rt- C >t'S rt tt 92 THE FROG CHAP. or spermary ; Cp. Hd. of head ; Cp. H. L. of hind-limb ; Cp. Kd. of kidney ; Cp. Lng. of lung ; Cp. Lvr. of liver ; Cp. Pn. of pancreas ; Cp. Sk. of skin ; Cp. Spl. of spleen; cu. a. cutaneous artery ; cu. v. cutaneous vein; Cu. Gl. cutaneous gland ; d. ao. dorsal aorta ; Ent. C. enteric canal ; Ep. Ent. epithelium or enteric canal ; Ep Lng. of lung ; Ep. Sk. of skin ; Ep. Ur. T. of urinary tubule ; glm. glomerulus ; il. a. iliac artery ; int. a. artery to stomach and intestine ; int. v. vein from stomach and intestine ; ju. v. jugular vein; hp. a. hepatic artery (to liver) ; hp.pt. v. hepatic portal vein; hp.v. hepatic vein ; /. au. left auricle ; Lng. lung ; Lvr. C. liver-cells ; ly. cp. lymph capillaries; ly. v. lymphatic vessels ; Mlp. Cp. Malpighian capsule ; wstf.nephro- stome ; p. ly. ht. posterior lymph-heart ; Pn. C. cells of pancreas ; Pn. D. pan- creatic duct; pr. cv. v. precaval vein ; pt. cv. v. postcaval vein ; pul. a. pul- monary artery ; pul. cu. tr. pulmo-cutaneous trunk ; pul. v. pulmonary vein ; r. au. right auricle ; rn. pt. v. renal portal vein ; scl. a. subclavian artery ; s. c. ly. s.sub-cutaneous lymph-sinus ; scl. v. subclavian vein ; sp. a. splanchnic artery; spl. a. splenic artery ; spl. v. splenic vein ; s. v. sinus venosus ; syst. tr. systemic trunk ; U. Bl. urinary bladder ; Ur. ureter ; v. valve in vein ; vs. a. vesical artery (to bladder) ; vs. v. vesical vein; vl, ventricle. contained blood, which, coming by the precavals and postcaval, is non-aerated, is acted upon in all directions and might therefore be forced either into the three great veins (pr. cv. v, pt. cv. v) or into the right auricle (r. au). But the veins are full of blood steadily flowing towards the heart, and any regurgitation is further prevented by their valves : the right auricle, on the other hand, has finished its contraction and is now relaxing ; it is there- fore empty. Thus, on the principle of least resistance, the contraction of the sinus fills the right auricle with blood from the great veins, and the sinus itself is re- filled from the same source as soon as it begins to relax. Immediately after the sinus has ceased to contract the two auricles contract together : the right, as we have seen, has just been filled from the sinus, the left (/. au) is full of aerated blood brought to it by the pulmonary vein (pul. v). The presence of the sinu-auricular valves prevents the blood in the right auricle from being forced back into the sinus : 'that in the left auricle is prevented from being forced back into the pulmonary veins by the steady onward flow in the latter. On the other hand the ventricle is beginning to relax and is empty. Con- sequently the auriculo-ventricular valves are forced back into the ventricle (vl) and the blood from both auricles flows into and fills that chamber, the right half of which vi CIRCULATION 93 becomes filled with non-aerated, the left with aerated blood, the two taking an appreciable time to mingle, owing to the spongy character of the ventricular wall. The instant it is thus filled, the contraction of the ventricle begins. As it does so . the blood, getting behind the auriculo-ventricular valves, forces them together, and thus prevents any backward flow into the auricle. At the same time the semilunar valves at the entrance of the conus (c. art) are pushed aside and the blood flows into that chamber. Since the conus opens from the right side of the ventricle, the blood first enter- ing it will be non-aerated ; there will then follow a certain amount of mixed blood ; and finally, as the ventricle reaches the limit of its contraction, the aerated blood from its left side will be forced into the conus (compare Fig. 22). Last of all the chambers of the heart, the conus begins its contraction. The semilunar valves are immediately filled with blood, and, closing together, stop all backward flow into the ventricle. Two alter- native courses are now open to the blood : it can pass either directly from the conus into the pulmo-cutaneous trunk (pul. cu. tr], or make its way into the bulbus aortae (b. ao). As a matter of fact it takes the former course owing to the circumstance that there is little resistance in the limited blood-system of the lungs, while that in the systemic and carotid trunks is very great. Hence the blood just received into the conus from the ventricle, which, as we have seen, is non-aerated, goes immediately to the lungs and skin to be aerated. Before long — in a fraction of a second — the flow of blood into them increases the pressure in the pulmonary vessels, and at the same time the blood is continually flowing onwards — i.e., away from the heart — in the systemic and carotid trunks. Consequently the pressure 94 • THE FROG CHAP. in these vessels rapidly diminishes, and the blood soon forces aside the valves between the conus and the bulbus and fills the latter. Here again the question of pressure comes in. It is easier for the blood to make its way into the wide systemic trunks (syst. tr) uniting immediately into the long dorsal aorta (d. ao) than into the comparatively narrow carotid trunks (car. tr}, obstructed by the carotid labyrinths. Hence, the non- aerated blood having been mostly driven into the pulmo- cutaneous trunk, the mixed blood, from the middle part of the ventricle, goes into the systemic trunk, and thence to the various arteries supplying the limbs (scl. a, il. a) and the viscera (sp. a, etc.). Finally, when the pressure is sufficiently raised in the systemic trunks the remaining blood, which, coming from the left side of the heart, is aerated, is pumped into the carotid trunks (car. tr), and thence to the head. Thus, owing to the arrangement of the valves, and to the varying pressures in different parts of the vascular system, the non-aerated blood returned from the various parts of the body to the heart is mostly sent to the lungs and skin to be aerated. Mixed blood is sent to the trunk, limbs, and viscera, while for the head with its contained brain — the directing and controlling organ of the whole animal — a special supply of pure, aerated blood is reserved. We see then that the course of the circulation may be proved, as a simple matter of induction, from the structure of the heart and its valves, the direct observa- tion of its beat, and the manner in which the flow from cut vessels takes place. It was by observation and experiments of this kind that the circulation of the blood in the higher animals was demonstrated by William Harvey in the seventeenth century. But the final and most conclusive proof of the circulation — from vi CAPILLARIES 95 directly observing the flow — became possible only after the invention of the microscope. This instrument, by furnishing a sufficiently high magnifying power, allows us to see for ourselves the actual movement of the blood in an animal or organ of sufficient transparency ; and at the same time clears up the question, previously insolvable, of how the blood, having reached a given part or organ by the arteries, finds its way into the veins to begin its return journey. The Circulation in the Frog's Web. — There are three parts in the frog transparent enough to allow of the blood-flow being seen in them — the web of the foot, the tongue, and the mesentery. Of these the web is the most convenient, and can be examined under the microscope without any injury to the animal. The Capillaries. — If you have the makings of a naturalist, you will acknowledge the sight to be one of the most wonderful you ever saw. In the thickness of the web is an irregular network of minute blood-vessels, called capillaries (Fig. 24), and through them the blood is seen to flow with great rapidity, its course being made especially evident by the minute particles or corpuscles it contains, the structure of which we shall study later on. You will also notice much larger vessels, the smallest arteries and veins. The arteries (a) are distinguished by the fact that the blood in them flows in the direction from the leg towards the margin of the web, while in the veins (v) it takes the opposite direction. You must remember, however, that under the microscope everything is reversed ; right appears left and left right, and a current actually flowing towards the observer appears to go in the opposite direction. By careful examination you will see that both arteries and veins are in connection, by minute branches, with 96 THE FROG CHAP. the capillary network, and will be able to trace the blood from an artery, through the capillaries, into a vein. The same thing can be seen in other transparent FIG. 24. — Blood-vessels of the web of a Frog's foot seen under a low magnifying power. a. small arteries ; v. small veins. The minute tubes joining the arteries to the veins are the capillaries. The arrows show the direction of the circulation. In the small portion marked off, the pigment cells of the skin, which occur throughout the web, are also represented. (From Huxley's Physiology.) organs ; and by injecting the vascular system with a fluid injection-mass, such as gelatine suitably coloured, it can be proved that all parts of the body are permeated with a capillary network into which the blood is passed vi LYMPHATIC SYSTEM 97 by the arteries, and from which it is received into the veins. Thus by means of the microscope we are able to take the final step in demonstrating the circulation. The fact that the blood can flow in one direction only is proved by the disposition of the valves of the heart and of the veins, but the passage of the blood from the smallest arteries to the smallest vein by a connecting system of minute tubes or capillaries can be proved only by the employment of considerable magnifying powers. We see that the vascular system of the frog is a closed system of vessels : the blood is everywhere confined within definite tubes through which it flows in a definite direction, never escaping, as in some of the lower animals, into large irregular spaces among the tissues. The Lymphatic System. — Included in the vascular system are certain cavities and vessels containing lymph, and together constituting the lymphatic system. We have already noticed the subcutaneous lymph sinuses (p. 18, and Fig. 23, s.c.ly.s) and the sub-vertebral lymph-sinus (p. 27, Fig. 5, s.v.ly.s). There are also found in nearly all parts of the body delicate, thin- walled, branching tubes, the lymphatic vessels (Fig. 23, ly. v). Unlike the blood-vessels, the lymphatics are all of one kind, there being no distinction into anything of the nature of arteries and veins. They arise in lymph-capillaries (ly. cp), which are, as it were, interwoven with the blood- capillaries, but have no connection with them. By the lymph-capillaries the fluid which has exuded from the blood in its passage through the tissues is taken up and passed into the lymphatic vessels or sinuses, and these in their turn finally communicate with certain trans- parent muscular organs called lymph-hearts. Of these there are two pairs. The anterior lymph-hearts (a. ly. ht) lie, one on either side, beneath the supra-scapula TRACT. ZOOL. 98 THE FROG CHAP. and just behind the transverse process of the third vertebra: the posterior pair (p. ly. ht) are situated one on each side of the posterior end of the urostyle. These organs pulsate regularly, like miniature hearts, and pump the lymph into the veins, the anterior pair com- municating with the subclavian, the posterior with the renal portal vein. The lymphatics of the enteric canal have an important function to perform in that they absorb the fatty portions of the food (p. 75). The fluid they contain has v, milky appearance, owing to the presence of minute suspended fat- globules, and for this reason they receive the name of lacteals. The ccelome is really a great lymph-sinus (Fig. 23, ccel). It communicates with the veins of the kidneys through certain microscopic apertures called nephro- stomes (nst). The spleen (p. 23, and Fig. 3, spl) has important relations with the blood- and lymph- vessels, and probably acts as a blood-filter, removing particles in the blood which are no longer wanted. PRACTICAL DIRECTIONS The Vascular System. a. Let some blood from a frog — or better from the veins of some larger, freshly-killed, warm-blooded animal, such as a rat or a rabbit — flow directly into a white cup or porce- lain capsule. Note that it soon coagulates, and shortly afterwards separates into clot and serum. Notice also the difference in colour between the blood freshly drawn from a vein (see p. 85), and that soon assumed by exposed portions of the clot. b. Pin a freshly-killed frog to the dissecting -board, dorsal side upwards, and cut through the skin of the back along the urostyle. The posterior lymph-hearts (p. 98, Fig. 23) will then be seen. To make out the anterior lymph-hearts, care- fully separate the supra-scapulae from the vertebral column. Some of the chief lymph-sinuses have already been seen : special methods are required to trace the lymph-vessels. vi INJECTION OF BLOOD-VESSELS 99 c. Now turn the frog the other way upwards, pin it down in the dissecting-dish, and open the body-cavity as before (p. 32), taking great care not to cut the abdominal, musculo- cutaneous, and other veins. Slit open the pericardium and remove as much of it as possible, so as to expose the entire heart. The structure of the heart and the course of many of the blood-vessels can also be made out in the specimen from which you have already removed the alimentary canal. In the following dissections, use a dissecting lens whenever necessary. I. In the heart (Figs. 3, 4, 7, 20 and 21), notice again the ventricle, and the right and left auricles (appearing single in the entire heart), and make out also the conus arteriosus, dividing into two distally, and the sinus venosus (dorsal). If the heart is still beating, notice the order of contraction of its different chambers (p. go). Injection of the Arteries. — The tracing of the arteries is greatly facilitated by filling them with some coloured sub- stance. The operation requires, therefore, a coloured fluid or injection-mass capable of traversing the arteries, and some contrivance by which it can be injected into them. The most convenient injection-mass is made as follows : — 1. Grind up in a mortar 4 grammes of " French blue " (to be had at the oilman's) with 4 cubic centimetres of glycerine and the same quantity of methylated spirit. 2. Grind up 50 grammes of common laundry-starch, with 50 cubic centimetres of water and 25 of methylated spirit, and add to the mixture the colour prepared as above. Mix thoroughly and strain through muslin. This injection-mass will keep for an indefinite period in a stoppered bottle, requiring only to be stirred up when used. If it is considered too troublesome to make, a simpler but less satisfactory mass may be made by simply stirring up some French blue in water in the proportion of a teaspoonful to a tumblerful. For injecting the mass into the blood-vessels, the most satisfactory instrument is a brass injecting syringe, holding about one ounce, provided with nozzles of various sizes. This is, however, expensive, and an ordinary glass syringe, to be had of any druggist, will answer the purpose very fairly if provided with a proper nozzle or cannula. This latter is made by drawing out one end of a piece of glass tubing about two inches long until it is fine enough to pass into the conus arteriosus ; a piece of india-rubber tubing is then used to connect it with the fixed nozzle of the syringe. H 2 100 THE FROG CHAP. c.art A still simpler injecting apparatus is furnished by a common " medicine dropper " (see p. 12, and Fig. 25). By alterna- tive squeezing and releasing the cap, fluid is drawn into or expelled from the tube. Having provided these requisites, proceed as follows. Open the abdomen of a freshly- killed frog in the usual way, taking great care not to injure the blood-vessels. Remove the middle portion of the shoulder-girdle, so as to expose the heart, lay open the pericardium, and with the scissors make a snip into the ventricle, allowing the blood to escape freely. Pass a piece of fine thread, about six inches long, round the heart, at about the junction of the auricles and ventricle, and give it a single loose tie, as shown in Fig. 25. When the bleeding has ceased, fill the medicine dropper, or syringe, with injection-mass and pass the riarrow end of the former, or the nozzle of the latter, through the cut end of the ventricle into the conus — take care not to push it into one of the auricles instead — and tighten the thread so as to keep it in place. Then squeeze the cap of the medicine dropper, or push in the piston of the syringe, and if the operation is successful, you will see the blue injection pass from the conus into the arterial trunks, and thence into the various arteries of the body. The contrast between the arteries, filled with the blue mass, and the veins, filled with blood, is then very striking, particularly in the mesentery. When the arteries are well filled, withdraw the nozzle from the heart and instantly draw the thread tight and knot it so as to prevent escape of the injection. Then place the whole frog in spirit (methylated spirit 3 parts, water i part) for a few hours, after which time the injection-mass will be found to have set hard FIG. 25. — Sketch showing the method of injecting the Frog's arteries. a. glass "medicine dropper" with india-rubber cap (b) ; its pointed end (dotted) is passed through the cut end of the ventricle (v) into the conus (c. art) ; au. auricular division of the heart ; t. thread. vi PRACTICAL DIRECTIONS 101 enough to allow of the arteries "being 'conveniently traced. Injection of the Veins. — The veins are much more difficult to inject than the arteries, but if you wish to make a double injection on the same specimen, colour the injection-mass with vermilion or carmine in the case of the arteries instead of with French blue, using the latter for the veins. The operation is best performed by inserting the nozzle into an incision in the abdominal vein (by directing the nozzle forwards, the portal vein will be injected : by directing it backwards the pelvic and renal portal veins), and also into one of the subclavians. But for a really satisfactory prepara- tion, it is best to inject from the heart through the conus, as directed above, with a weak, warm solution of gelatine (in the proportion of one part of gelatine to two parts of water), coloured with precipitated carmine. In this case the injection-mass, containing only microscopic particles, passes from the arteries through the capillaries into the veins, keeping throughout to the course taken by the blood during life, and therefore unimpeded by the valves of the veins. A syringe must be used, since the medicine dropper will not give sufficient pressure, and the animal should be placed in warm water during the process. II. Now pin down under wrater and make out the course of the chief veins (p. 82, Fig. 21) : — 1. The two precavals, and the external jugular, internal jugular, subclavian, and musculo-cutaneous. 2. The postcaval, to see which turn the viscera on one side (Figs. 3 and 4). Note the renal, spermatic or ovarian, and hepatic veins. 3. The hepatic portal vein and its factors. 4. The abdominal vein and pelvic veins. 5. The veins from the hind -legs can be more easily seen at a later stage, after the alimentary canal is removed, and so their examination is best left until certain of the arteries have been traced (or use the specimen you have dissected previously for this purpose). Remove the skin from the thigh, place the frog on its side, and make out the femoral, pelvic (already seen), renal portal, and sciatic veins, as well as a large vein from the muscles of the back. 6. The two pulmonary veins. Make a sketch of the heart and as many of the veins as you have followed out up to this point, inserting the others after removing the alimentary canal (see p. 102). 102 THE FROG CHAP. . Hi. The chief arteries may now be followed out (p. 80, Fig. 20) -.--• Note the carotid, the systemic, and the pulmo-cutaneous trunk, arising from the conus arteriosus, and then trace each of these out as follows : — 1. The carotid trunk gives off a f lingual artery and is con- tinued into the head as the carotid artery, having at its origin the carotid labyrinth. 2. The systemic trunk unites with its fellow to form the dorsal aorta, first giving off vertebral, subclavian, and cesophageal arteries. From the point of union of the two systemic trunks arises the splanchnic or cceliaco-mesenteric artery. After following this out to its distribution, remove the alimentary canal as directed on p. 76, when the following branches of the dorsal aorta will be more plainly seen : — the venal, spermatic or ovarian, and iliac arteries. 3. The pulmo- cutaneous trunk divides into a pulmonary artery, passing along the outer side of the corresponding lung, and a cutaneous artery. Sketch the heart and chief arteries, and then make out and sketch the renal portal system (p. 101), if you have not already done so. IV.1 Cut out the heart of a frog preserved in formaline, taking great care not to injure it. Fasten it down in a dissecting-dish, with the ventral surface upwards, by sticking very small pins through the arteries and veins — not through the" heart itself. Pinch up the ventricle with fine forceps, and with small scissors gradually snip away its ventral wall, noting that it is a hollow structure with thick, spongy walls and a small cavity, which will probably be full of clotted blood. Wash this out and then proceed to open the auricles in a similar way, and to wash out the blood they contain. Observe the right and left auricles, separated by a partition. Slit open the conus arteriosus, and continue the cut forwards to the origin of the main arteries. Examine with a lens and make out (p. 87, Fig. 22) : — 1. The auricula-ventricular aperture and its valves. 2. The longitudinal valve and the small semilunar valves in the conus arteriosus. 3. The origins of the carotid and systemic trunks from the 1 On account of its small size, the examination of the structure of the frog's heart is somewhat difficult, and the student is advised to dissect first the heart of a larger animal, such as a dogfish (pp. 449 and 477). vi PRACTICAL DIRECTIONS 103 bulbus aovtce, and the small^ aperture leading into the pulmo- cutaneous trunks. 4. The sinu-auricular aperture and its valves. 5. The aperture in the left auricle leading into the pul- monary veins. Sketch. Turn over the heart, so that its dorsal surface is upwards, and cut away enough of the dorsal wall of the sinus venosus to show the sinu-auricular aperture from the other side. V. Get a piece of thin board — e.g., the side of a cigar-box — about six inches long by three wide. Towards one end make a round hole about half an inch in diameter, and opposite this, on either side, make a notch, or rather slit, with a penknife. This is called a " frog -board." Next get as light coloured a frog as possible. The web may be examined in the living animal without hurting it, or the animal may be first chloroformed as directed on p. 31, but removed from the influence of the anrrsthetic as soon as it is insensible, so that there may be no suspicion of the frog feeling any inconvenience from this harmless experiment. Lay the frog on the frog-board with a piece of wet rag wrapped loosely round the body, and take one or two turns around both frog and board with a piece of tape — you must avoid tying it tightly or the circulation will be impeded. Stretch out one leg, and selecting the most transparent web, tie a piece of thick soft silk round each of the two toes by which it is bounded. Adjust the leg so that the web comes just over the hole in the frog-board, and bring the two pieces of silk through the slits, regulating them until the web is evenly stretched out over the hole. Lastly, place the frog- board on the stage of the microscope,1 with the hole over the aperture in the stage, and either fix it with the clips or rest the opposite end on some support : adjust the mirror so as to illuminate the web from beneath, and examine it with the low power. Note the network of capillaries and the circula- tion of the blood through the arteries, capillaries and veins (Fig. 24). 1 A brief description of the compound microscope will be given at the end of the next chapter (p. 119). CHAPTER VII THE FROG (continued) : THE MICROSCOPICAL EXAMINATION OF THE SIMPLE TISSUES BEFORE carrying our inquiries any further into the anatomy and physiology of the frog it will be necessary to devote some consideration to its microscopic structure or histology, since there are many matters in connection with the various organs which can be further elucidated only by the examination of the minute structure of the organs as revealed by the microscope (see p. 119). Let us, first of all, examine a drop of the frog's blood under the low power of the microscope. It will at once be seen that the blood is not a simple homogeneous fluid, but that it contains a large number of minute solid bodies floating in it. These are called by the general name of blood-corpuscles : the fluid part of the blood in which they float is called the plasma. At first, owing to currents in the fluid, the corpuscles will be found to move to and fro, but after a time they come to rest. Under the high power you will notice that the corpuscles are of two kinds. The greater number of them are regularly oval in form (Fig. 26, C), and of a yellow colour. If the drop of blood is thick enough in one part for the corpuscles to lie over one another, so that the light passes through two or three layers of them to reach the CHAP. VII BLOOD-CORPUSCLES 105 eye, they will appear red : they are hence called red corpuscles. Frequently they are seen turned on edge (D) , and their appearance in this position shows them to be flat, oval discs with a swelling in the centre. They are about TVtn °f a millimetre (about y^oo^h inch) in long diameter. Among the red corpuscles are found, in much smaller numbers, bodies not more than half the long diameter of the red corpuscles in size, quite colourless, distinctly granular — so as to have the appearance of ground glass — and with a slightly irregular outline (Fig. 26, A). These are the colourless corpuscles or leucocytes. They 7D FIG. 26. — Blood-corpuscles of the Frog. ( X 525.) A, colourless corpuscle ; B, the same in process of division ; C. red corpuscle, surface view ; D, the same, edge view. nu. nucleus. (From Parker's Biology.) are not flat, like the red corpuscles, but have the form of irregular lumps. The plasma, like the leucocytes, is quite colourless, so that the colour of the blood is seen to be due entirely to the large number of red corpuscles it contains. If the drop of blood has been prepared and examined under the high power with sufficient rapidity, a remark- able phenomenon can be made out with regard to the colourless corpuscles. This can be most easily demon- strated by making a series of outline sketches of the same leucocyte at intervals of a minute or two. You will then notice that the sketches all differ from one another : in one there will perhaps be a little projection going off to the right ; in the next this will have disappeared and a similar projection will have appeared on the left, and so 106 THE FROG CHAP. on. As a matter of fact, as long as the blood is quite fresh, the leucocytes are in constant movement, sending out and withdrawing little processes of their substance called pseudopods or " false feet," by means of which they can crawl slowly along like independent living things. These very peculiar and characteristic move- ments are called amoeboid movements. Occasionally a leucocyte may be seen to elongate itself and divide into two (Fig. 26, B) : this is a case of what is called simple fission. The red corpuscles neither move nor divide. If a drop of some dye or staining fluid (p. 121) be run in under the cover-glass, the corpuscles will be seen to become rather faint in outline, transparent, and lightly tinted ; but the most obvious effect is that in the middle of each is seen a rounded or oval granular body (nu) deeply stained by the dye, so as to make a very well- defined coloured area in the interior of the corpuscle. This body is called the nucleus : it is present both in the red and the colourless corpuscles. By adding to a fresh drop of blood, in the same manner, a drop of weak acetic acid, the nucleus again becomes distinct, while the body of the corpuscle is rendered very transparent and almost invisible : indeed it finally disappears altogether. It is thus proved that the corpuscles, both red and colourless, consist of a substance which is known as protoplasm, but slightly affected by dyes, and soluble in weak acids ; and enclosed in this is a nucleus, stained by dyes, and unaffected by weak acids. Both nucleus and protoplasm consist mainly of proteids (p. 72), together with water and a small pro- portion of mineral matters. When distilled water is added to a drop of blood on the slide, the corpuscles are seen to swell up and become partly dissolved : the red colouring matter of the red corpuscles is dissolved out, the plasma becoming tinged vii CHARACTERS OF BLOOD 107 with yellow. Thus the colouring matter is evidently a distinct substance from the protoplasm, and is called hczmoglobin. It is characterised, among other things, by a strong attraction for oxygen : in combination with that gas it assumes a bright scarlet colour ; when deprived of oxygen, it becomes purple. This affinity for oxygen accounts for the change undergone by the blood when exposed to the air, as described on p. 79. Coagulated blood, as seen under the microscope, is characterised by the plasma being traversed by ex- tremely delicate threads, forming a sort of network in which the corpuscles are entangled. These threads are formed of a substance called fibrin, which is separated from the plasma during coagulation, the remaining or fluid portion of the plasma constituting the serum. We may therefore express the coagulation of the blood in a diagrammatic form as follows : — Fresh Blood. Coagulated Blood. ( Serum Plasma j Fibrin 1 n . Corpuscles }^ Having observed the microscopic characters of a drop of blood, let us examine once more the circulation in the web, this time under the high power (Fig. 27). The red corpuscles (F) can be seen streaming through the vessels, those in the capillaries in single file, those in the small arteries and veins, two or more abreast : as they pass through narrow capillaries or round corners, they become bent or squeezed (G, H). The leucocytes (/) travel more slowly and often stick to the sides of the vessels. Columnar Epithelium. — By carefully teasing out a small piece of the inner surface of the mucous membrane 108 THE FROG CHAP. FIG. 27. — The circulation in the Frog's web, under a high power. A , wall of capillaries ; B, tissue of the web in which the capillaries lie ; C, epiderm- cells; D, their nuclei ; E, pigment-cells ; F, red corpuscles ; G, H,red corpuscles being squeezed through a narrow capillary ; K, capillary seen through the epiderm , /, colourless corpuscles. (From Huxley's Physiology.) VII EPITHELIUM 109 FIG. 28. — Columnar epithelial cells from the Frog's intestine. (X 500.) m. droplet of mucus exuding from cell : nu. nucleus. of the intestine into the smallest possible particles, it will be found that the process has detached numerous, minute, conical bodies, about polygonal in transverse sec- tion, and having one end flat and the other more pointed (Fig. 28). These bodies are called epithelial cells : in the natural position they lie closely cemented to one another, like the blocks of a wood-pavemept, their flattened ends facing the cavity of the intestine, while their narrower ends abut against the connective-tissue layer (p. 70). Thus the epithelial cells together form an epithelium or epithelial layer of the mucous membrane directly bounding the cavity of the enteric canal. Each cell consists of protoplasm and contains a rounded, granular nucleus (nu), which is made very conspicuous by staining, and in which are one or more small bodies or nucleoli. Cer- tain of these cells have a space towards their free ends containing slime or mucus, and thus have the form of little cups or goblets : they are known as goblet-cells (see right hand cell in Fig. 28). Ciliated Epithelium. — By the same method the mucous membrane of the mouth is also seen to be lined by an epithelium, but the cells comprising it (Fig. 29) are broader in proportion to their length, and' each is produced on its free surface into a number of delicate, transparent threads of protoplasm called cilia, which in the living condition are in constant movement, lashing backwards and forwards like minute FIG. 20. — Ciliated epithelial cells from the mu- cous membrane of the frog's mouth, (x 500.) (From Parker's Biology, after Howes.) 110 THE FROG CHAP. whip-lashes, or, more accurately, like the blades of grass in a field when acted upon by a strong wind. If you happen to get under the microscope a good-sized bit of mucous membrane with the cells in position, you will see that the cilia produce a strong current by which small particles are swept along, while detached cells swim about, like little independent animals, by the action of their own cilia. These ciliated epithelial cells, like the ordinary columnar cells of the intestine, are made of protoplasm, and each contains a nucleus with one or two nucleoli clearly brought into view by staining. The action of the cilia can be demonstrated, on a large scale, by placing a freshly-killed frog on its back, turning back or cutting away the lower jaw, and placing a very small cube of cork on the roof of the mouth near to the projection due to the eyes. The cork will be slowly swept back towards the throat. Squamous or Pavement Epithelium. — By scraping the outer surface of a piece of skin with a sharp knife, and examining the scrapings in a drop of water, after staining them, the super- ficial layer of the skin will be found to be made up of flattened, roughly hexa- gonal plates (Fig. 30 and FIG. 30.— Squamous epithelial cells Fig. 27, C, D) Set closely from the Frog's skin, (x 300.) ,, -i.-i .-• , ••• MM. nuclei. together, like the tiles of a mosaic pavement. Each plate has a nucleus, and, from its flattened form, is dis- tinguished as a squamous or scale-like epithelial cell. Meaning of the word " Cell." — We see thus that the body of the frog is partly made up of distinct elements, which, under a considerable diversity of forms, exhibit the same essential characters. Each structure consists VII UNSTRIPED MUSCLE 111 of a mass of living protoplasm, which appears almost clear or more or less granular, containing in its interior a rounded body, the nucleus, specially distinguished by the affinity of parts of its substance for colouring matters. To a body having this essential structure, whatever its form, the name cell is applied. Unstriped Muscle. — Examination of a teased pre- paration of the muscular coat of the intestine, stomach, or urinary bladder will show that it is composed of delicate fibres (Fig. 31) tapering at both ends, and each with a nucleus in the middle. These are called smooth or unstriped muscular fibres : they are obviously cells which have undergone a great elongation in length. During the peristaltic move- ments of the intestine (p. 75) each fibre alternately contracts and relaxes, becoming shorter and thicker during the former process, like the large muscles of the body (p. 60). The movements in this case, how- ever, are not under the control of the will, and unstriped muscular tissue is therefore often spoken of as involuntary muscle. Contractility of Protoplasm. —We have now studied three different kinds of movement in cells : — muscular move- ment in the unstriped muscle-fibres, ciliary movement in the ciliated epithelial cells, and amoeboid movement in the FIG. 31. — Unstriped muscular fibres from the Frog's intes- tine, (x 500.) To the right are shown fibres from the longitudinal and circular layers (see Chap. VIII) cross- ing one another ; to the left isolated fibres. (After Howes.) 112 THE FROG CHAP. colourless blood-corpuscles. Muscular movement is due to the fibre undergoing a sudden shortening in a particular direction and a consequent approximation of its two ends. Ciliary movement is due to the alternate bending and straightening of the cilia ; and the bending of a cilium in a particular direction is caused by the protoplasm of which it is composed shortening or con- tracting on the side towards which it bends. Amoeboid movement is the protrusion and withdrawal of irregular processes of the cell : this results from the protoplasm undergoing a contraction or squeezing in a given direc- tion, as a consequence of which one part of its substance is drawn in and another pushed out. Hence all three kinds of movement are movements of contraction ; and contractility, or the power of contraction, may be con- sidered as a general property of protoplasm. Striped Muscle. — If a small piece of any of the body- muscles is carefully teased out with the grain, i.e., in the direction of the length of the fibres, so as to break away the connective-tissue binding them together, the fibres, which are much larger than those of smooth muscle, will readily separate from one another, and they will be seen to be long and cylindrical. Under the microscope each fibre shows a delicate transverse striation (Fig. 32), being made up of alternate bright (b) and dim (d) bands — or more accurately discs, the fibre being cylindrical — set at right angles to its length. Hence the ordinary body-muscles or voluntary muscles are composed of striped muscular fibres.1 In addition to the transverse striation a fainter longitudinal striaticn is more or less distinctly visible. Each fibre is covered by a delicate membrane (s), i 1 The muscles of the heart, although not under the control of the will, are transversely striated ; but their structure differs ' from that of ordinary striped voluntary muscle. STRIPED MUSCLE 113 called sarcolemma, beneath which nuclei (n) occur at intervals. It will be seen that striped muscle, unlike the tissues previously considered, does not appear to be composed of cells, although the occurrence of nuclei seems to indicate their presence. In the embryo, however, the muscle is formed of ordinary nucleated B // FIG. 32.— A, part of a fresh muscular fibre of a Frog. B, the same after treatment i. u • u T distilled water followed by methyl-green. ( x about 150.) b. bright bands ; d. dim bands ; n. nuclei ; s, s'. sarcolemma, rendered visible as a minute blister (s') by absorption of water and by the rupture of the muscle-fibre at s. (A, from Huxley's Physiology.) cells, which, as growth goes on, increase in length while their nuclei multiply by fission, each enormously elongated cell thus containing a considerable number. Connective-tissue. — We will next examine a piece of the delicate web of connective-tissue which binds the muscles together. Under the high power, connective-tissue is seen to be PRACT. ZOOL. 114 THE FROG CHAP. composed of a sort of irregular network of delicate bundles of wavy fibres called white connective-tissue fibres (Fig. 33, w), which cross one another in all directions. Amongst them are found single fibres sweeping across the field in bold curves and called elastic fibres (e) : it is FIG. 33. — Connective-tissue from between muscles of the Frog's legs. ( x about 200.) c. cells ; e. elastic fibres ; w. white fibres : — all of which are imbedded in a delicate matrix not specially indicated in the figure. owing to their elasticity that the tissue cannot be spread out when wet. Scattered among the fibres are numer- ous nucleated cells (c) of very varied and often irregular form : these are the connective-tissue cells. The fibres, as well as the cells, are imbedded in a soft, homogeneous, ; ground substance or matrix. vii CARTILAGE 115 Thus connective-tissue consists partly of cells, but between these, and forming the main substance of the tissue, is a matrix or intercellular substance, enclosing fibres. In the embryo the tissue consists of closely packed cells, but, as development proceeds, these separate from one another, and the ground-substance is formed between them. Cartilage. — The ordinary clear or hyaline variety of this tissue is conveniently studied by examining a piece of the thin edge of the omo- or xiphi-sternum, or by fl A IW G ry. o c' FIG. 34. — Section of cartilage, from the head of the Frog's femur, (x 200.) c. cells ; c'. cells undergoing fission ; c. s. empty cell-space ; m. matrix. taking a thin section with a razor of the head of the humerus or femur. Cartilage consists of a tough, elastic, transparent, homogeneous matrix (Fig. 34, m) containing numerous cavities or cell-spaces (c. s), in each of which is a nucleated cell (c). The cell-spaces, or lacuna, are in many cases arranged in groups of two or four, sometimes close together, sometimes with a narrow space of matrix or intercellular substance between them. This is due to Ithe fact that cartilage grows by the cells undergoing i 2 116 THE FROG CHAP. binary fission, so that two cells are formed in one cell- space : the two then gradually separate from one another and intercellular substance is formed between them. In the embryo, this tissue consists entirely of closely packed cells which gradually separate and form a structureless matrix which is firm and elastic, and which in some parts (pp. 46 and 48) may become calcified. Bone. — As we have already seen (p; 52) bone is formed of two constituents, a basis of animal matter, in which lm FIG. 35. — Transverse section of dry femur of Frog, (x about 125.) c. canaliculi ; Ic. lacunae ; lm. lamellae ; m. marrow-cavity. mineral matter — calcium phosphate and carbonate — is deposited. In microscopic examination we may there- fore investigate either the mineral matter by examining dried bone, or the animal matter by examining decalci- fied bone. A thin section of a dried long bone, such as the femur, \ shows that it is formed of very numerous thin layers or lamella (Fig. 35, lm), surrounding and concentric with the marrow-cavity. The lamellae contain numerous VII BONE 117 cavities, the lacuna (Ic), with delicate, branching tubes, the canaliculi (c), radiating from them in all directions. Both lacunae and canaliculi commonly appear black, owing to their being filled either with air or with bone- dust produced in grinding the section. In a section of decalcified bone (Fig. 36) the marrow is seen to be surrounded by lamellae of a delicate fibrous substance, arranged in two layers, an outer (b), having the periosteum (p) closely investing it, and an inner (b) in contact with the marrow. In the fibrous substance of FIG. 36. — A, transverse section of decalcified Frog's femur under a low power. (x 30.) B, portion of the same under a high power, (x about 150.) b. outer, and b', inner layer of bone ; b. c. bone-cells ; m. marrow ; o. layer of osteoblasts in connection with periosteum ; o', layer of osteoblasts in connection with marrow ; p. periosteum. (After Howes.) the lamellae are cell-spaces, corresponding with the lacunae of the dried bone, and each containing a bone-cell (b. c}, which sends off delicate branched processes of its protoplasm into the canaliculi. Thus the bone, like connective-tissue and cartilage, consists of cells with an intercellular substance : the latter is in the form of concentric layers and is impregnated with lime-salts. The long bones of the frog grow in two directions. Between the periosteum and the bone is a layer of cells, the osteoblasts (o), by which new lamellae of bone are 118 THE FROG CHAP. formed on the outside of those already existing : thus the outer layer of bone (b) grows from within outwards. Between the marrow and the inner surface of the bone is another layer of osteoblasts (o') which forms new lamellae on the inner side of the existing bone, so that the inner layer (bf) grows from without inwards. Summary. — The various simple tissues studied in the present chapter consist either entirely of cells, or of cells separated by an intercellular substance. Formed en- tirely of cells are the various kinds of epithelium — columnar, ciliated, and squamous, and unstriped muscle. In striped muscle the cells have elongated into fibres and their nuclei have multiplied. Of the supporting tissues, consisting characteristically of cells with inter- cellular substance, connective-tissue has the matrix soft and homogeneous, with fibres imbedded in it ; in hyaline cartilage it is structureless and tough, though 'elastic ; and in bone laminated and calcified. In the blood, the plasma may be looked upon as a kind of liquid inter- cellular substance. Cells, wherever they occur, have the same essential structure, being formed of protoplasm with a nucleus. In nearly all cases they increase by binary fission, first the nucleus and then the protoplasm dividing into two. The distribution of the various tissues throughout the body is worth noting. Epithelium always bounds a free surface — e.g., that covering the outer surface of the body or lining the inner surface of the enteric canal. Striped muscle forms the " flesh/' unstriped muscle, e.g., the outer layer of the enteric canal (p. 70). Bone and carti- lage form the framework of the body, while connective- > tissue is the packing between the other tissues. VII THE MICROSCOPE 119 PRACTICAL DIRECTIONS General Structure of the Compound Microscope. — The compound microscope, with which you must now become acquainted, consists of a strong stand (Fig. 37, a) from which rises a vertical pillar (b) , To the latter are attached — a horizontal plate or stage (c}} perforated in the centre with an aperture (d), the size of which can be varied by means of a diaphragm ; an adjustable mirror (e), placed be- low the stage : and a vertical tube (/) attached above the stage by a horizontal arm. Two com- binations of lenses are used ; an objective or object-glass (h), con- sisting of a metal tube with two or more lenses fixed into it, which screws into the lower end of the tube : and an ocular or eye- piece (?'), consisting of a metal cylinder with a lens at each end which slides into the upper end of the tube. It is this arrange- ment of lenses which forms the essential feature of the com- pound microscope : the object, placed on the stage, is magnified by the objective, and the mag- nified image, thrown into the interior of the tube, is further enlarged by the ocular. The object is brought into focus — i.e., placed at such a distance from the objective that a per- fectly clear and well-defined image is obtained — in one of two ways. The tube can be raised or lowered either by sliding it up and down in an outer tube or collar (g), or, in the more expensive instruments, by a rack and pinion : this movement forms the coarse adjustment. In addition, all good micro- FIG. 37. — Diagram of compound microscope. a. stand ; b. pillar ; b' movable portion of pillar, raised and lowered by fine adjustment ; c. stage ; d. aperture in stage ; e. mirror ; /. tube ; /' milled ring for raising and lowering tube; g. collar; h. objective; «. ocular ; k. screw of fine adjustment. 120 THE FROG CHAP. scopes have a fine adjustment, usually consisting of a spring concealed in the pillar, and acting upon the horizontal arm which carries the tube : it is worked by a screw (k), and by means of it the tube can be adjusted to within 10^00^ °f an inch. When the object is transparent — as in most cases with which we shall have to deal — it is placed over the hole in the stage on a glass slide, and illuminated from below by adjust- ing the mirror until a beam of light from the window or lamp is reflected vertically upwards : a small hole in the dia- phragm should be used with the high power. The object is thus said to be viewed by transmitted light. In the case of opaque substances the mirror is not used, and the object is illuminated by the light falling upon it directly : it is then said to be viewed by reflected light. A student's microscope should have two objectives, one — the low power — magnifying about 80, the other — the high power — about 300 to 400 diameters. One eye-piece is sufficient, and a sliding coarse adjustment is nearly as convenient as a rack and pinion, besides being cheaper and less likely to get out of order. The mistake often made in choosing a microscope is to get one of elaborate construction, the money going largely in brass -work. The proper thing is to get the simplest form of stand consistent with perfect rigidity, fitted with the best possible fine adjustment and lenses : to save on either of the latter is false economy. Requisites for Microscopic Work. — In addition to the microscope, the following will be required before starting work : — 1. A few slides or slips of glass, 3 inches long by i inch wide, which can be obtained from an optician. They must be thoroughly cleaned before being used. 2. A supply (about J oz.) of cover-glasses, small pieces of very thin glass, to be had at the optician's. The most convenient size is j inch square. They are best cleaned by being soaked for a few minutes in strong nitric acid, and then thoroughly washed under the tap, after which they should be dried by being placed flat on a clean surface and rubbed with a handkerchief : if held between the finger and thumb they are very liable to be broken. 3. One or two thin glass rods, about 6 inches long and fth inch in diameter ; and one or two dipping tubes, or pieces of glass tubing about 6 inches long and ^th inch in diameter. The ends both of rods and tubes should be smoothed off in the flame of a blow-pipe. vii PRACTICAL DIRECTIONS 121 4. Three or four dissecting needles, made by sticking a fine sewing needle into the end of a wooden penholder, allowing the point to project about half an inch. 5. A few reagent-bottles for holding the various fluids used for applying what are called micro-chemical tests to the tissues. Special bottles can be bought for the purpose, but sufficiently convenient ones can be made from ordinary one-ounce phials, fitted with sound corks. Bore a hole lengthwise through the cork, and insert into the hole a piece of narrow glass rod, pointed at the end, just long enough to reach nearly to the bottom of the bottle when the cork is inserted. This arrangement allows of the ready application of a single drop of fluid to the object under examination. 6. The following micro-chemical reagents : a. Salt solution. Dissolve 0-75 gramme of sodium chloride in 100 c.c. of distilled water, so as to make a f per cent, solution. b. Acetic acid, i per cent. One c.c. of strong acetic acid to 99 c.c. of distilled water. c. Distilled water. d. Solutions of one or two aniline dyes. For fresh tissues, dissolve enough mefhyl-green in distilled water to make a deep bluish-green solution, and add a trace of acetic acid. For preserved (or fresh) tissues, make a saturated solution of magenta or safranin in strong alcohol, and dilute with an equal bulk of water. e. Glycerine, 50 per cent. Equal parts of pure glycerine and distilled water. Microscopical Examination of the Simple Tissues. For the following work, a freshly-killed frog must be used. (If you wish to measure each kind of tissue-element, you must learn to use a micrometer, which consists of a circular piece of glass, marked at regular intervals with lines or squares, the distance between which can be calculated by comparing them with a scale engraved on a slide known as a stage-micrometer.) i. The Blood. — Have ready a clean dry slide and cover- glass. In a freshly-killed frog open a vein or make an incision in the heart, and with a clean glass rod remove a drop of blood to the middle of a slide. Take hold of the edge of the cover-glass with small forceps, and supporting it with a mounted needle, gently lower it on to the drop of blood until the latter is spread out into an even, transparent, yellowish film. This operation of covering the drop of blood requires a little practice : if not done quickly (unless a drop of salt solution is added), there is danger of the blood 122 THE FROG CHAP. coagulating before it is covered, in which case it will not spread out into a transparent layer : if the cover is lowered too suddenly, bubbles of air are commonly included. Examine your preparation, first of all, under the low power, and learn to recognise the appearance of air -bubbles. Note the colourless plasma and the numerous minute blood- cells or corpuscles. Now replace the low by the high power. Bear in mind that the higher the power, the shorter the focal distance. With the low power you will probably find that the object is in focus when about half an inch from the bottom lens of the objective. The high power, on the other hand, has to be brought to within about ^th of an inch of the cover- glass, which is therefore liable" to be broken and the lens to be injured by careless focussing. The safest plan is to screw down the tube, keeping your eye at the level of the stage, until the objective almost touches the cover-glass : then, looking through the microscope, very slowly raise the tube by means of the coarse adjustment, until the object comes into view. Note (Fig. 26) : — a. The numerous flat, oval, red corpuscles, each with a central swelling in which the nucleus is contained. -Sketch. b. The colourless corpuscles or leucocytes, much less numer- ous, having a granular appearance, and an irregular or rounded outline. Focus a colourless corpuscle under the high power, and note its amceboid movements : sketch its outline rapidly but accurately, and after a minute or two make another sketch, and then another, until some half-dozen outline drawings of the corpuscle have been obtained : then compare your sketches. Now place on the slide, against one edge of the cover -glass, a drop of methyl-green, and against the opposite edge a small strip of blotting paper. If the blood is sufficiently fresh — and if it has coagulated you must get another drop — the blotting paper will slowly absorb the blood on one side, and the methyl green will be drawn in and will gradually mingle with the blood. When this has taken place, put a drop of salt solution in the place of the methyl-green, and allow it to be drawn across so as to remove the superfluous stain : then remove the blotting paper, and examine the blood once more under the high power . Notice the nucleus present in each kind of corpuscle, and the surrounding protoplasm. Sketch. To a fresh drop of blood add, in the same manner, a drop of I per cent, acetic acid. Note that the body of each cor- vii PRACTICAL DIRECTIONS 123 puscle becomes transparent, while the nucleus is rendered distinct. To another drop of blood add distilled water. The cor- puscles become swollen up and partly dissolved, and the colouring matter (hemoglobin) of the red corpuscles is dissolved out into the plasma. Examine some coagulated blood under the microscope, and note the threads of fibrin in which the red corpuscles are entangled, and the serum. Examine once more the circulation in the web (p. 103), using the high power, and follow the course of both red and colourless corpuscles through the vessels and capillaries. By focussing to the surface of the web the flattened epithelial cells of the epiderm or outer skin can be seen, and at a deeper level the black pigment-cells (see p. 128, and compare Figs. 24 and 27^). 2. Columnar Epithelium. — Take a small piece of frog's intestine, and place it for 24 hours in a mixture called Ran- vier's alcohol, consisting of one part of methylated spirit and two parts of water. With fine scissors snip off a very small piece — not larger than a pin's head — from the inner surface of the mucous membrane, place it on a slide in a drop of water, and with two dissecting needles tease it out by tearing it into the smallest possible particles. The operation is best done under a lens. Then put on a cover-glass, and examine first with the low and then with the high power. (Remember that a cover-glass must always be used wrth the high power.) Note the minute, more or less conical cells of columnar epithelium (Fig. 28), each containing a nucleus. Observe the goblet-cells amongst the ordinary columnar cells. Stain with magenta, which is more effective than methyl-green in speci- mens previously treated with alcohol, add a trace of acetic acid, wash with water (salt solution need only be employed in the case of fresh tissues), and add glycerine. Sketch. 3. Ciliated Epithelium. — Snip off a very small bit of mucous membrane from the roof of the mouth of a recently- killed frog, and tease it out in salt solution (Fig. 29). Note the form of the cells and their nuclei : they are rela- tively shorter than the columnar cells just examined, and each bears a number of delicate vibratile cilia at its free end. Observe the movements of the cilia. Sketch. Treat with methyl-green, magenta, or acetic acid, when the nucleus will become more apparent. Perform experiment described on p. no to show the action of the cilia as a whole in the entire animal. 124 THE FROG CHAP. 4. Pavement or Squamous Epithelium. — Take a bit of frog's skin which has been kept for a day or two in Ranvier's alcohol, scrape the outer surface with a sharp knife, tease out and examine the edges of the scrapings in a drop of water, afterwards staining with magenta (Fig. 30). Note the flattened cells fitting together like tiles in a pave- ment, each one with its nucleus. Sketch. 5. Unstriped Muscle. — Snip off a small piece from an inflated urinary bladder of a frog which has been preserved in formaline, wash with water, and mount. Or, snip off a very small piece — not bigger than a pin's head — from the muscular coat of the intestine or stomach (or from a urinary bladder) which has been in Ranvier's alcohol for at least twenty-four hours ; then tease out in a drop of water very thoroughly. Note the elongated unstriped muscular fibres tapering at both ends, each containing a nucleus (Fig. 31). Stain with magenta. Sketch. 6. Striped Muscle. — Snip off a small piece — about Jth inch long — from any of the body-muscles of a freshly-killed frog, put it on a slide in a drop of salt solution, and tease it out with the grain, i.e., in the direction of the length of the fibres. The fibres will readily separate from one another : the teasing process must be stopped as soon as they are apart, and care must be taken not to tear or crush the individual fibres, which are large enough to be readily dis- tinguishable with a magnify ing-glass. Observe under the low power of the microscope the long cylindrical fibres (Fig. 32, A), bound together by connective- tissue, and showing a distinct transverse striation and a less distinct longitudinal striation. Examine a single fibre under the high power (Fig. 32, B), and make out the sarcolemma and the numerous nuclei, which will be rendered more distinct by the addition of acetic acid or by staining. Sketch. 7. Connective-tissue. — Carefully separate two of the muscles of the leg in a fresh frog, and note the delicate web of connective-tissue between them : or, note the fine strands of connective-tissue between the skin and the muscles of the body- wall. With fine forceps lift up a small shred of this, snip it off with scissors, and place if on a dry slide. Then, with two needles, spread it out on a thin, even layer, breath- ing on it occasionally to prevent drying. Lastly, place a drop of salt solution on a cover-glass and quickly lower it on the preparation. The reason for this procedure is that if connective-tissue is placed in fluid, it contracts into a little lump, which is too opaque for examination and cannot be readily spread out. vii PRACTICAL DIRECTIONS 125 Examine first with the low and then with the high power, and note the bundles of white fibres, and the elastic fibres (Fig. 33). Sketch. Add acetic acid : the white fibres will swell up, the elastic fibres are more readily distinguished, and the connective- tissue cells seen : the latter and the delicate ground -substance will be rendered more distinct by staining. (For comparison with this areolar tissue, mount in salt solution a piece of one of the fine tendons of the toes : treat as before, and examine.) Sketch. 8. Cartilage. — Snip off the thin edge of the omo- or xiphi- sternum or supra-scapula and examine it as before in a drop of salt-solution. Or, cut a thin section of the head of the humerus or femur with a razor. Note the transparent, homogeneous matrix, containing numerous lacuna, in each of which is a nucleated cell : observe here and there the groups of cells formed by binary fission (Fig. 34). Stain, and sketch. 9. Bone. — For the examination of dried bone, cut a very thin slice of one of the long bones with a fret-saw : fasten it to a slide with Canada balsam (p. 136), and when the balsam has dried quite hard, rub down the section on a hone until it is thin enough to be quite transparent. Or, a transverse section of a bone from a larger animal may be prepared in the same way or bought from a dealer in microscopic objects. a. Examine first a transverse section of dry frog's bone (e.g. femur or humerus), and note the marrow-cavity, the lamella, and the lacuna and canaliculi ; the two last will probably appear black (p. 117 and Fig. 35). A section of human bone, such as is usually supplied ready prepared, or of the bone of some other larger animal, shows a more complicated structure : instead of a single system of la- mellae, the bone consists of a number of such systems, each surrounding a central canal, in which blood-vessels and nerves run, and which corresponds to the marrow-cavity in the simpler frog's bone described above. Sketch. b. Compare with a section of decalcified frog's bone,1 and notice — the fibrous lamella arranged in two layers, the outer of which is closely invested by the periosteum ; the lacuna, containing bone-cells ; and the outer and inner layers of osteoblasts (Fig. 36). Sketch. (For the histology of nervous tissue, see Chapter X.) 1 The method of preparing sections of this and other tissues will be described at the end of the next chapter (p. 136). CHAPTER VIII THE FROG (continued) : THE MICROSCOPIC EXAMINATION OF THE COMPOUND TISSUES — GLANDS — SECRETION AND ABSORPTION WITH the exception of the tissues of the nervous system, which will be described in Chapter X, we have now studied the principal simple tissues by the method of dissociation, i.e., by separating their constituent parts. We have now to consider the way in which these tissues are combined in the various organs, and for this purpose must adopt some method of examination by which they are seen in their natural relations. The method adopted for this purpose is that of section- cutting. You know how, by cutting sections, in various directions, of a bit of twig, the arrangement and natural relations of its various parts — wood, bark, and pith — can be ascertained. The same thing applies to the organs of the frog and other animals, but, owing to their soft and non-resistant texture, it is impossible to cut them into sections thin enough for microscopic examination with- out special preparation. The methods employed are by no means easy for the beginner, especially without verbal instruction and the resources of a biological labora- tory ; but in the event of your wishing to make the preparations described and figured in this chapter for 126 CHAP, vm SKIN 127 yourself, the directions given on pp. 136 — 139 are simple enough to be carried out with very limited appliances. The Skin. — A vertical section of the skin, i.e., one FIG. 38. — Vertical section of the Frog's skin, highly magnified, (x about 200.) D. derm, formed of hf, hf,' hf." horizontal, and v. f. vertical fibres of connective, tissue, and containing b. v. blood-vessels, and pg. pigment cells. E. epiderm, with m. /. active or Malpighian layer, and h. /. horny layer of epithelial cells ; c. gl. cutaneous gland in section ; c. gl' '. in surface view ; d. duct. (After Howes.) taken at right angles to its surface, will be seen to have the following structure. The skin is clearly divisible into two layers, an outer, 128 THE FROG CHAP. the epiderm (Fig. 38, E) and an inner, the derm (D). The epiderm is built up of several layers of epithelial cells. These differ greatly in form according to their position, those in the lower or internal layer (m. I) being columnar, while those in the upper or external layer (h. I) are squamous, and have their protoplasm converted into horny matter so as to furnish a comparatively hard and insensitive covering to the body. The horny layer is cast off periodically in shreds, and to make up for this, the cells of the inner, or deep, layer multiply by binary fission, the increase in their number necessarily resulting in a pushing upwards of the super- jacent layers. There is thus a constant travelling of cells from the inner to the outer surface of the epiderm : as they pass towards the outer surface they become more and more flattened, and at last squamous and horny. The whole process takes place in such a way that the multiplication of the columnar cells in the lower layer is just sufficient to make good the loss of the squamous cells in the superficial layer. The derm (D) is formed of connective-tissue, the fibres of which are mostly horizontal (h.f, h.f, h.f"), or parallel to the surface of the skin, but at intervals are found bands of vertical fibres (v. /). The derm also differs from the epiderm in having an abundant blood- supply (6. v), capillaries ramifying through it in all directions. It also contains nerves, the ultimate fibres of which have been traced into the deeper layers of the epiderm. Imbedded in the derm, especially in its external portion, are irregular cells (pg), the proto- plasm of which contains pigment, often appearing intensely black. It is to these pigment-cells that the coloured patches in the frog's skin are due (Figs. 24 and 27). In this as well as in the other sections described in vra CUTANEOUS GLANDS 129 the present chapter, the structure of the nuclei of the various cells can be more easily made out than in the fresh preparations you have already examined. Each nucleus will be seen to be enclosed by a definite nuclear membrane, and to contain in its interior a number of minute bodies, which take up the stain more deeply than the rest of the nucleus. One or more of these bodies may correspond to the nucleoli already seen (p. 109), but many of them are of a different nature and can often be seen to form a network : the material of which these are composed is known as chromatin, and is sur- rounded by a semi-fluid substance forming the ground- work of the nucleus. Cutaneous Glands — Secretion. — In the superficial part of the derm are seen numerous rounded spaces (Fig. 38, c. gl, c. gl'), each of which can be proved, by taking sections in various directions, to be a nearly globular cavity, from which a narrow canal (d), like the neck of a flask, passes through the epiderm to open on the external surface. Both the body and the neck of the flask are lined with epithelium, the cells lining the body being nearly cubical, those of the neck squamous. These structures are called cutaneous glands : they per- form the function of manufacturing the slimy fluid which, as we have seen, is constantly exuding on the surface of the skin. The epithelial cells of the gland have the power of forming minute droplets of the fluid out of the materials supplied to them by the blood : the droplets escape from the cells and accumulate in the interior of the gland, whence the fluid is finally dis- charged by the duct. The cells lining the duct are continuous on the one ihand with those of the gland, and on the other with those of the epiderm. The whole gland with its duct PRACT. ZOOL. J£ 130 THE FROG CHAP, is to be looked upon as a depression of the skin, lined by pushed-in epiderm cells. Epithelial cells having the power of manufacturing and discharging a specific substance are called gland-cells, and the process of manufacture is known as secretion. We have already met with isolated gland-cells in the case of the goblet-cells of the intestine (p. 109), which secrete mucus ; but commonly, as in the present in- am FIG. 39. — A, transverse section of Frog's ileum ( X 25) ; B, small portion of the same highly magnified, (x about 125.) b. v. blood-vessel ; c. m. circular layer of muscle-fibres ; ep. epithelium ; I. m. longitudinal layer of muscle-fibres; m. m. muscularis mucosae (seep. 133 ); pr. peritoneum ; m. muscular layer ; s. w. submucosa. (After Howes.) stance, gland-cells are aggregated into a definite organ called a gland. The Intestine. — A transverse section of the intestine shows also a very definite and characteristic combination of simple tissues. The mucous membrane, like the skin, is composed of two layers, an epithelial layer (Fig. 39, ep), corresponding to the epiderm, and a connective-tissue layer (s. m), corresponding to the derm, its deeper part being called the submucosa (see vm GASTRIC GLANDS 131 p. 70). The epithelium consists of a single layer of cells only (B, ep), all columnar, and with their long axes at right angles to the elevations into which, as we have seen (p. 71), the mucous membrane is thrown. Amongst the ordinary epithelial cells numerous mucus-secreting goblet-cells will be recognised. The submucosa, like the derm, contains blood-vessels, lymphatics, and nerves. The muscular layer (A, m) is also divisible into two : an outer layer of longitudinal fibres (B, /. m), running parallel with the long axis of the tube, and an inner, much thicker layer of circular fibres (c. m) which encircle it, and consequently lie at right angles to the longi- tudinal fibres. Thus in a transverse section, such as Fig. 39, the fibres of the circular layer are cut longi- tudinally, those of the longitudinal layer transversely, while the opposite would be the case in a longitudinal section. The peritoneum (pr) which, as we have seen (p. 27), forms an outer covering to the intestine, is formed of a thin inner layer of connective-tissue and an outer of squamous epithelium. The Stomach. — Transverse sections of the stomach (Fig. 40) show it to differ from the intestine not only in the much greater thickness of all its layers, but also in the fact that the epithelium, instead of simply forming an even layer over the ridges of mucous membrane, is sunk into the mucous membrane in the form of simple or branched tubes, the gastric glands (g. gl). These differ from the cutaneous glands in being not flask-shaped but test-tube-shaped, each being a long, narrow tube, with an extremely small cavity (B and C, c) . They are lined by a single layer of gland-cells, and open by minute apertures (m) on the surface of the mucous membrane. The cells of the gastric glands have the power of K 2 132 THE FROG CHAP. forming, out of the materials supplied to them by the blood, the gastric juice, by which, as we have learnt (p. 74), proteids are digested. Thus, while the raw material supplied to both cutaneous and gastric glands is the same, the manufactured article is entirely different FIG. 40. — A, part of a transverse section of the Frog's stomach ( x about 50) ; B, one of the gastric glands in longitudinal section ( x about 300) ; C, transverse section of a gastric gland. 6. v. blood-vessel ; c. cavity of gastric gland ; c. m. circular muscles ; c. m. m. circular layer of muscularis mucosae ; ep. epithelium ; g. gl. gastric glands ; /. m. longitudinal muscles ; /. m. m. longitudinal layer of muscularis mucosa? ; m. mouth of gastric gland ; nu. nucleus ; pr. peritoneum ; s. m. mucous membrane : the submucosa is external to the muscularis mucosae. in the two cases. Each kind of gland-cell has the faculty of picking and choosing, the material supplied being worked up in the one case into the cutaneous secretion, in the other into gastric juice. The mucous membrane of the stomach is traversed vm LIVER 133 by a narrow band of unstriped muscle, the muscularis mucosce, formed, like the main muscular layer, of an outer layer of longitudinal (/. m. m) and an inner of circular (c. m. m) fibres.1 The Pancreas. — Sections of this gland (Fig. 41) show it to be made up of microscopic masses or lobuhs (I), each of which consists of a cluster of gland-cells en- closing a very narrow central space. The cavities of adjacent lobules communicate with one another and B FIG. 41. — A, small portion of a section of the Frog's pancreas (x about 300) ; B, diagram showing the connection between the lobules and ducts. c. connective-tissue covering the lobules ; d. duct ; /. lobules ; MM. nuclei. open into tubes or ducts (d) lined with epithelium, which join with one another and finally discharge into the bile-duct as it traverses the pancreas (p. 70). The pancreas is distinguished as a racemose or grape-bunch gland : the duct is comparable to the branched stalk and the lobules to the grapes. The Liver. — Sections of the liver show it to be made up of innumerable large, polyhedral cells (Fig. 42, hp, c), arranged in columns, and bounding extremely fine 1 A very thin muscularis mucosae is also present in the intes- tine ; (compare Fig. 39, B). 134 THE FROG CHAP. channels or bile-passages (c. b, b. p). These are found to open into one another, and finally to discharge into definite tubes (B, d), lined with epithelium. These, in their turn, unite into larger and larger tubes, which hp FIG. 42. — A, portion of a section of the Frog's liver ( x 350) ; B, small portion of the same, showing the origin of a bile-duct. bl. blood-capillaries ; b.p., ,c. b., c. b', c. b", bile-passages ; c. b'", the same in trans- verse section ; c., hp. liver-cells ; d. smallest bile-duct ; nc, nu. nuclei. (A, after Howes ; B, after Hoffmann.) form the hepatic ducts and ultimately open into the common bile-duct (p. 68). The liver-cells are glandular and secrete the bile, which, as it is formed, exudes into the bile-passages and viii NUTRITION 135 passes into the hepatic ducts, thence making its way either directly into the intestine or into the gall-bladder (p. 70) . The whole liver, which is the largest gland in the body, is traversed by a complex network of capillaries (bl), arising partly from the hepatic artery, partly from the hepatic portal vein ; and from the blood thus supplied, the liver-cells obtain the materials neces- sary to enable them to discharge their function of secreting the bile. The liver-cells have, however, other functions, one of which is to manufacture a substance called glycogen or animal-starch. This is stored up in the cells in the form of minute insoluble granules, which, being after- wards transformed into soluble sugar, pass into the blood and so to the tissues. Connection of the foregoing facts with the Physiology of Nutrition. — You will now be able to understand more clearly the various processes connected with the nutrition of the frog, hitherto studied without the aid of histology. When the food enters the enteric canal the various gland-cells are stimulated into activity, and the gastric juice, bile, and pancreatic juice are poured out and mingled with the food, which is digested in the manner already described. The soluble products of digestion — peptones, sugar, salts, fatty acids, and glycerine — diffuse through the epithelium of the enteric canal into the blood- capillaries of the underlying mucous membrane, and the blood, now loaded with nutriment, is carried by the portal vein to the liver and thence by the hepatic and postcaval veins to the heart (see Fig. 23). At the same time the fats make their way into the lymph-capillaries and are finally pumped, by the lymph-hearts, into the veins. Thus the products of digestion all find their way ultimately into the blood, and are distributed, through the circulatory mechanism, to all parts of the body. 136 THE FROG CHAP. PRACTICAL DIRECTIONS Materials required for the Preparation and Sectioning of Animal Tissues. — In addition to the requisites mentioned in Chapters I and VII, the following will be required : — a. Corrosive sublimate : a saturated solution in water. Care should be taken in using this solution, as it is a very virulent poison. b. Absolute alcohol. c. Turpentine or xylol. d. Paraffin : " hard " and " soft." Get an ounce or two of each. e. A solution of Canada balsam in chloroform, xylol, or turpentine. This should be kept in a small glass bottle with a ground glass cap — not a stopper or cork. /. A solution of alcoholic borax carmine. This (like many other useful staining-solutions, such as Ehrlich's hcematoxy- lin) may be bought ready prepared : or it may be made as follows : — Grind up in a mortar 2 grammes of carmine and 4 grammes of borax, and dissolve in 100 c.c. of distilled water : to this solution add an equal volume of 70 per cent, alcohol : allow to stand for a day or two and filter. g. A water-bath, in which melted paraffin may be kept at a constant temperature. For a makeshift you can use a saucepan with a flat piece of tin over it : fill the saucepan about half full of water, and heat it over a spirit-lamp or a small oil-lamp or gas-burner, regulating the distance of the flame so as to keep the temperature of the water at about 55"C. (i3i°F.). h. Two or three watch-glasses or other small shallow vessels for containing melted paraffin. i. A sharp, flat-ground razor. j. A section-lifter, made by beating out flat about half an inch of the end of a piece of stout copper wire, about 6 in. long, and bending the flattened portion at an obtuse angle with the rest. Preparation of Tissues for Section- Cutting. a. Fixing, hardening, and decalcifying. Sections may be cut from specimens which have been care- fully preserved in alcohol : — first in 70 per cent., and after a day or two transferred to 90 per cent. But certain other reagents are more effective for the purpose of fixing the vm SECTION-CUTTING 137 tissues — i.e., quickly killing and coagulating the protoplasm of the cells with a minimum of shrinkage, — and of these the one most generally useful is a solution of corrosive sublimate (see a, above), in which, from a freshly-killed frog, place small pieces of the various organs and tissues to be examined — e.g., skin, intestine, stomach, liver, pancreas, kidney, ovary, spermary, and spinal cord, as well as the inner half of the eye-ball. The intestine and stomach should be first washed out in salt solution, and then cut into pieces about J inch long ; the liver should be cut into pieces not more than J inch cube. After about half-an-hour to two hours, according to the size of the piece, place in water under a tap, and wash thoroughly for a quarter of an hour or more, until the corrosive sublimate is removed. After washing, transfer to 50 per cent, alcohol for a few hours, and then to 70 per cent, for twenty-four hours, after which the pieces may be stained at once (see below), or transferred to strong methylated spirit (90-93 per cent.), in which they may be kept until wanted. This completes the process of hardening : it is done gradually, by alcohols of increasing strength, in order to avoid shrinkage. In order to decalcify such tissues as bone, from which the lime-salts must be extracted before cutting into sections, place a small piece for a few days in 70 per cent, alcohol to which 2 per cent, of strong nitric acid has been added : then wash thoroughly, transfer to alcohol, and stain. b. Staining. — Place the organs, cut into convenient sizes for imbedding — i.e., not more than J inch long, and, in the case of such organs as the liver, J inch in thickness — into borax-carmine or haematoxylin for one or two days or even more. They will become stained throughout, and the difficulty of staining the sections after cutting will thus be avoided. After staining, place them in weak alcohol (50-70 per cent.), slightly acidulated with hydrochloric acid : if a watch-glass or some such vessel is used, it is sufficient to dip the end of a glass rod into acid and stir it round in the alcohol. The effect of the acid is to remove much of the colour from the protoplasm of the cells, leaving the nuclei brightly tinted. After half-an-hour or less in the acid alcohol, place the tissues once more in strong methylated spirit. c. Dehydrating. — Transfer from the strong methylated spirit to absolute alcohol, which must be kept in a stoppered or tightly corked bottle, as it will otherwise deteriorate by absorption of water from the air. It has the effect of with- drawing the last traces of water from the tissues, an abso- 138 THE FROG CHAP. lutely necessary step in order that they may be permeated with paraffin. d. Imbedding. — Transfer the objects from absolute alcohol to turpentine or xylol. Either of these fluids acts as an intermediary between alcohol and paraffin, being freely miscible with both : it gradually replaces the alcohol in the tissues, rendering them transparent. If they are not transparent in the course of an hour or so, the process of dehydration has not been complete, and they must be returned to absolute alcohol. In the meantime, melt some paraffin over the water-bath, using various mixtures of hard and soft according to the season : in a cold room in winter soft paraffin will be hard enough ; in the height of summer hard paraffin alone will be suitable. The tem- perature of the water-bath must never be allowed to rise more than a degree or two above the melting- point of the paraffin. Transfer the objects from turpentine or xylol to melted paraffin and keep them in it for some hours — the time varies according to the size of the piece — FIG. 43-A, imbedding box made by wrap- until theY are thoroughly ping paper round a cork ; B, cork after permeated . removal of the paper, showing the jf wiqh -j-n rirf- cPr paraffin pared down to a convenient -1 you wlsn to ( size for sectioning, a, object to be tions by hand, get Some cut- ordinary medium - sized bottle-corks : around each wind a piece of paper, allowing it to project about J an inch beyond one end of the cork, and fixing it with a pin, as in Fig. 43, A. Into the little cylindrical vessel or imbedding box thus formed pour some melted paraffin, and immediately transfer to it, by means of a warmed section- lifter or forceps, one of the prepared pieces, adjusting its position by means of a heated needle. When the paraffin is quite cold remove the paper, and you will have fixed to the cork a solid block of paraffin containing the object to be cut. e. Section-cutting. — Pare away the block of paraffin until the object comes into view : then trim the block, as in- Fig. 43, B, until its upper surface, with the object in the vni PRACTICAL DIRECTIONS 139 middle, is not more than J inch square. Hold the cork firmly in the left hand, with the wrist resting on the table, and with a razor cut the thinnest possible slices of the paraffin block, including the imbedded object. The razor must be held firmly grasped at the junction of blade and handle, and kept with the surface of the blade parallel with that of the block : use almost the whole length of the edge for each section. With a little practice you will be able to cut sections so thin as to be quite transparent under the high power.1 /. Clearing and mounting. — Place the section on a slide and warm it gently on the water-bath until the paraffin melts, and then add a large drop of turpentine or xylol in order to dissolve the paraffin. Then draw off the turpentine with blotting-paper and replace it by a fresh drop, repeating the process until all the paraffin is dissolved : put on a cover-glass and Examine. If you wish to be sure that the parts of your sections are not displaced in mounting, or to mount several sections on one slide, the latter should first be smeared over with a very thin layer of a mixture of collodion and oil of cloves, or of glycerine and white of egg, in equal parts : then place the sections on the slide, warm, and immerse the whole slide in a small vessel of turpentine or xylol, leaving it until all the paraffin is dissolved. In order to make a permanent preparation, -remove the paraffin with turpentine or xylol, as above, draw off the turpentine, place a drop of Canada balsam on a cover- glass and very gently lower the cover-glass on the object, spreading out the balsam in a thin, even layer. Before long, the balsam will have set quite hard, and the sections may be preserved for an indefinite period : the balsam will set more quickly if you leave your preparations over the water-bath for a short time. Remember that object, razor, slide, and cover must be kept free from water, the presence of which, from the stage of dehydration onwards, is fatal to success. Examination of Compound Tissues. Examine the following sections, prepared as described above, first with the low, and then with the high power, 1 If you are working in a properly furnished laboratory you will probably learn how to cut sections with a microtome, or section- cutting machine, which gives much better results and is absolutely necessary when a complete series of sections of the same object is required. 140 THE FROG CHAP, vm noting the parts enumerated in each case, as well as the structure of the nuclei (nuclear membrane, chromatin, and nucleoli) . 1. Vertical Section of Skin (Fig. 38). a. Epiderm, stratified epithelium, divisible into outer (horny] and inner (Malpighian) layers. b. Derm, connective-tissue fibres, blood-vessels and pig- ment-cells. c. Cutaneous glands with their ducts. The apertures of the ducts on the surface you will probably have noticed already in your preparation of the epithelial cells of the skin (p. 124). Sketch. 2. Transverse Section of Intestine (Fig. 39). a. Mucous membrane : a superficial epithelial layer of columnar cells, with goblet-cells amongst them ; and a deeper connective- tissue layer, enclosing vessels and nerves. b. Muscular layer : an external longitudinal, and an internal circular layer of unstriped muscular fibres. c. Peritoneal layer. This is very thin, and a" careful examination of good preparations is required in order to make out its structure (p. 131). Sketch. 3. Transverse Section of Stomach (Fig. 40). — After recog- nising the layers as above, note : — a. The gastric glands, and b. the muscularis mucosce. Sketch. 4. Sections of Pancreas (Fig. 41). a. Lobules, separated by connective-tissue, and each consisting of gland-cells with secretion-granules towards their inner ends ; b. ducts. Sketch. 5. Sections of Liver (Fig. 42). a. Polyhedral gland-cells, arranged in columns ; b. bile- passages and ducts ; c. blood-capillaries and vessels. Sketch. CHAPTER IX THE FROG (continued) : RESPIRATION AND EXCRETION IN the fifth chapter it was pointed out that a continual waste of substance goes on in the frog's body, the lost material taking the form of three chief waste-products or products of excretion, water, carbon dioxide, and urea. It was further stated that these substances are got rid of by means of the lungs, the kidneys, and the skin. The Organs of Respiration. — At their anterior ends the two lungs open into a small, thin-walled chamber (Fig. 44, I. tr. c), which, as it corresponds both with the larynx or organ of voice, and the trachea or windpipe in our- selves, is called the laryngo-tracheal chamber : it com- municates with the pharynx through the glottis (gl). The walls of the chamber and the edges of the glottis are supported by cartilages (ar) . The structure of the lung is best made out by distend- ing it with air, and then placing it in formaline or spirit until thoroughly hardened : its walls contain so much elastic tissue that if cut when fresh it contracts immensely, and its structure is then difficult to see. The inner surface of the lung is raised up into a con- plex network of ridges (A, r. Ing), which project into the interior and produce the appearance of an irregular honeycomb. All these ridges are abundantly supplied 141 142 THE FROG CHAP. with blood-vessels fed by the pulmonary artery, the blood being carried away by the pulmonary vein. The main substance of the lung is made of connective- tissue containing elastic fibres and unstriped muscle, and traversed by a network of capillaries. Its cavity is lined by a layer of pavement epithelium, and its outer surface is covered with peritoneum. Respiratory Movements. — In breathing, the frog depresses the floor of the buccal cavity (Fig. 45, A), and FIG. 44. — The respiratory organs of the Frog from the ventral aspect ; B, the laryngo- tracheal chamber in longitudinal section, with the right lung. ( X i£.) ar. the arytenoid, or principal cartilage of the larynx ; b. hy. body of hyoid ; gl. glottis ; 1. Ing. left lung ; 1. tr. c. laryngo-tracheal chamber ; p. c. hy. posterior horn of hyoid ; r. In,",, right lung, laid open in A to show its internal surface ; v. cd. vocal cord. (After Howes.) the mouth being kept shut, air is drawn in through the nostrils. The floor of the mouth is then raised (B) by muscles attached to the hyoid. At the same time the anterior end of the lower jaw presses upon the movable premaxillae (pmx), the upward processes of which (p. 43, Figs. 8 and 9, PMX) act upon certain cartilages in connection with the external nostrils in such a way as to produce closure of these apertures (Fig. 45, B). The gullet (gul) is so contracted, except during the act of swallowing, as to be practically closed. Thus when the floor of the mouth is raised, the air contained in it can IX RESPIRATORY MOVEMENTS 143 escape in one way only, viz., through the glottis into the lungs. Thus inspiration, or breathing-in, is produced by the buccal cavity acting as a force-pump ; the lowering of its floor draws in air through the nostrils, the raising of its floor forces the imprisoned air into the lungs. ^ ln£ FIG. 45. — Diagrams illustrating the respiratory movements of the Frog ( X 2). In A the floor of the mouth is depressed and air is being drawn in through the nostrils ; in B the floor of the mouth is raised, the nostrils are closed, and air is being forced into the lungs. e. n. external nostril; g/. glottis ; gul. gullet ; i. n. internal nostril; Ing. lung; olf. s. olfactory sac ; pmx. premaxilla ; tng. tongue. Expiration, or breathing-out, is due to a contraction of the elastic lungs, accompanied by a slight lowering of the tip of the lower jaw : the latter movement releases the premaxillae and thus opens the external nostrils. Respiration. — By the alternate movements of inspira* 144 THE FROG CHAP. tion and expiration, fresh air passes into the lungs at regular intervals, while part of the air already contained in them is expelled. Now we saw, when studying the blood (p. 79), that dark purple blood drawn from a vein becomes bright scarlet when exposed to air, and we subsequently learnt (p. 107) that this change is due to the absorption of oxygen by the red corpuscles. The blood brought to the lungs by the pulmonary artery is, as we have seen (p. 94), non-aerated, being the impure blood returned by the three caval veins to the right auricle. When this blood is pumped into the capillaries of the lungs it is separated from the air con- tained in those organs only by the extremely thin walls of the capillaries themselves and the equally delicate pavement epithelium lining the lungs (p. 142, and Fig. 23, Cp. Ing, Ep. Ing). Under these circumstances an inter- change of gases takes place between the air in the lungs and the gases dissolved in the blood : the haemoglobin of the red corpuscles absorbs oxygen, and the carbon dioxide in the blood, derived from the waste of the tissue, is given off into the cavities of the lungs. The blood in the pulmonary capillaries thus becomes aerated and is returned as red blood to the left auricle : at the same time it loses carbon dioxide, together with a certain amount of water, and these waste substances are expelled from the body with the expired air. Voice. — It was mentioned above (p. 141) that the glottis and laryngo-tracheal chamber are supported by cartilages. The largest of these are a pair of semilunar arytenoid cartilages (Fig. 44, ar), which bound the glottis to right and left. The mucous membrane on the inner or adjacent faces of the arytenoids is raised into a pair — right and left — of horizontal folds, the vocal cords (v. cd). By means of muscles these folds can be stretched and relaxed, and can be brought into either a ix KIDNEYS 145 parallel or a divergent position. When they are parallel the air, passing to and from the lungs, sets their edges in vibration and gives rise to the character- istic croak, the pitch of which can be slightly altered by stretching or relaxing the cords. Structure of the Kidneys. — The form and situation of the kidneys (Figs. 3, 4, 5, and 7, kd) have already been referred to. Each is a flattened organ of a deep reddish- brown colour, its inner edge nearly straight but for one or two notches, its outer edge curved. Its ventral face ur.tu m.cft Ur jber* 71 St FIG. 46. — Transverse section of Frog's kidney, (x 45.) gl. glomerulus ; m. cp. Malpighian capsule ; nst. nephrostome (see pp. 98 and 601) ; per. peritoneum covering ventral face of kidney ; per', fold of peritoneum supporting its outer border ; per" . fold supporting its inner border ; Ur. ureter ; ur. tu. urinary tubules. (After Marshall and Bles.) is covered by peritoneum (Fig. 46, per), continued on the one hand into the parietal layer (Fig. 5, p. per) of that membrane, on the other into the mesentery (mes) ; its dorsal face is bathed by the lymph of the sub-vertebral sinus (sv. ly. s). From the posterior end of its outer edge a delicate tube, the ureter (Figs. 3, 4, and 7, ur), passes backwards and opens into the cloaca in its dorsal wall. On its ventral face is a singular yellowish-white stripe, the adrenal body, an organ of imperfectly known function (Fig. 7, between the lines from kd and ts). PRACT. ZOOL. T 146 THE FROG CHAP. ur.tit A thin section shows the whole kidney to be made up of a tangled mass of microscopic tubes (Fig. 46, ur. tu), so twisted together that any section cuts them in various planes, some transversely, when they appear as circles, others longitudinally or obliquely. Amongst these urinary tubules or nephridia, as they may be called, are seen globular sacs, the Malpighian capsules (m. cp}, each having in its interior a little rounded bunch, known as the glomerulus (gl). Very accurate examin- ation of numerous sec- tions, as well as of teased-out specimens, shows that each Mal- pighian capsule (Fig. 47, m. cp) is connected with a urinary tubule (ur. tu), to which it forms a blind, bulb- like end. The tubule itself winds through the substance of the kidney, is joined by other tubules, and finally discharges into the ureter (ur) . The tubules are lined with somewhat cubical cells of FIG. 47. — Diagram of a single urinary tubule with its blood-vessels, to illustrate the structure of the Frog's kidney. af. v. afferent vessel of glomerulus ; cp. capil- lary network of kidney ; ef. v. efferent vessel of glomerulus ; gl. glomerulus ; m. cp. Malpighian capsule, showing epi- thelium (which in reality also covers the glomerulus) ; nst. ciliated nephrostome ; r. a. renal artery ; r. pi. v. renal portal vein ; r. v. renal vein ; ur. ureter ; ur. tu, ur. tu', ur. tu", ur. tu'", different por- tions of urinary tubule, showing epithelium and cilia. ix EXCRETION 147 glandular epithelium, which, in some parts (ur. tu, ur. tu"), are ciliated. The Malpighian capsules are lined with flattened cells of pavement epithelium. The arrangement of the blood-vessels is peculiar. Like other organs, the kidney is permeated by a network of capillaries (cp) which form a close mesh between the urinary tubules, so that the cavity of the tubule is separated from the blood only by the thickness of the gland-cells and of the capillary wall. The capillary net- work is supplied partly by the renal arteries (r. a), partly by the renal portal vein (r. pt. v), and is drained by the renal veins (r. v). It is in the behaviour of the renal arteries that tbe chief peculiarity of the kidney-circulation lies. On entering the kidneys they break up into smaller and smaller arteries, but each of the ultimate branches (af. v), instead of discharging into the general capillary network, passes to a Malpighian capsule, in the interior of which it breaks up into a little bunch of coiled capil- laries, the glomerulus (gl) . From this the blood is carried off by a minute vessel (ef. v) by which it is poured into the general capillary network and finally discharged into the renal vein (r. v). Renal Excretion. — While circulating through the glomerulus, water and certain soluble matters are separated from the blood and make their way into the Malpighian capsule and thence into the urinary tubule. As the blood circulates through the general capillary network, the gland-cells of the tubules excrete, out of the materials brought to them by the blood, the nitro- genous waste matter urea, in the formation of which the liver plays an important part ; it is discharged from the cells into the cavity of the tubule, where it is dissolved in the water separated out of the glomerulus. In this way the urine is formed. Accumulating in the tubules, it makes its way into the ureter and thence drop L 2 148 THE FROG CHAP. by drop into the cloaca, whence it is either expelled at once, or stored for a time in the bladder. Note that the formation of urine is a process of secre- tion of a similar nature to the secretion of gastric juice, bile, &c. The fluid secreted is, however, of no further use to the animal, and would, in fact, act as a virulent poison if retained in the system. It is therefore got rid of as soon as possible. Secretions of this kind, consist- ing not of useful but of harmful or waste matters, are distinguished as excretions. Bile is also in part an excretion as it contains pigments due to the disintegration of haemoglobin, and thus by its means the effete colouring matters of the blood are passed into the intestine and got rid of. Pulmonary and Cutaneous Excretion. — The lungs, besides being organs of respiration, take their share in excretion, since they get rid of the important waste pro- duct, carbon dioxide, together with a considerable quantity of water. Similar functions are discharged by the skin, which is also an organ both of respiration and of excretion. Interchange of gases takes place between the outer air and the blood in the capillaries of the derm : the carbon dioxide of the non-aerated blood brought to the skin by the cutaneous artery (p. 93 and Fig. 23) is exchanged for oxygen, and the blood, in the aerated .condition, is returned by the musculo-cutaneous vein to the heart, Moreover, the cells of the cutaneous glands separate water and other constituents from the blood, and the fluid thus formed is poured out on the surface of the body. Here it serves to keep the skin moist, and is finally lost, either by evaporation or by mingling with the water in which the frog is immersed. The cutaneous secretion has also poisonous properties, and so probably serves as a defence against some of the animal's enemies. Summary of the Processes of Nutrition. — We are ix NUTRITION 149 now in a position to understand the general features of the whole complicated series of processes which have to do with the nutrition of the frog, which are collectively spoken of as metabolism. These processes are illustrated in the diagram (Fig. 23, p. 91), which should be constantly consulted in connection with the following summary. All parts of the body are placed in communication with one another by means of the blood-vessels, through which a constant stream of blood is flowing in a definite direction. In all parts, waste of substance (destructive metabolism) is continually going on, and the waste products — water, carbon dioxide, and some nitrogenous substances which ultimately take the form of urea — are passed either directly into the blood, or first into the lymph and thence into the blood. At the same time the cells withdraw nutrient materials from the blood, and assimilate them, i.e., form new living substance, whereby the waste is made good, and the tissues adequately nourished (constructive meta- bolism). Oxygen is also withdrawn from the blood; like the air supplied to a fire, it is essential to the oxidation or low temperature combustion with which the waste of tissues is associated. By the withdrawal of its oxygen the haemoglobin of the blood alters its colour from scarlet to purple. Thus the blood as it passes through the body is constantly being impoverished by the withdrawal of nutrient matters and of oxygen, and as constantly being fouled by the discharge into it of waste products. It reaches the capillaries of an organ as bright red, aerated blood, and leaves it as purple, non-aerated blood. These changes, by which the blood loses nutrient matters and oxygen and gains waste products, takes place all over the body. The converse processes by 150 THE FROG CHAP. which nutrient matters and oxygen are aDsorbed and waste products got rid of are carried on in certain definite portions of the circulatory system. In the walls of the enteric canal (Fig. 23, Ent. C), the products of digestion pass into the blood (Cp. Ent. C), or in the case of fats, first into the lacteals and ulti- mately into the blood. In this way the due proportion of nitrogenous and other food-materials is kept up. In passing through the capillaries of the lungs (Cp. Lng), carbon dioxide is exchanged for oxygen and a certain amount of water is given off. In the capillaries of the skin (Cp. Sk), a large quantity of water and smaller proportions of other waste matters are got rid of. In the kidneys (Cp. Kd), a considerable quantity of water, together with the bulk of the urea, are removed from the blood and finally expelled from the body. Note that all these changes are produced by the special activity of particular groups of epithelial cells which, however alike they may be in general appear- ance and structure, have a marvellous selective faculty peculiar to themselves. Like all other parts of the body, they are constantly undergoing the usual processes of waste and repair, withdrawing nutrient matters and oxygen from the blood, and passing waste matters into it. ' But, in addition to the ordinary processes of nutrition, each particular group of cells has the power of withdrawing a specific substance from the blood or of passing substances into it. Thus the epithelial cells of the enteric canal (Ep. Ent) pass in digested food, those of the skin (Ep. Sk) and Malpighian capsules (Mlp. Cp) withdraw water, those of the urinary tubules (Ep. Ur. T) urea, and so on. Similarly, the various gland-cells, such as those of the liver (Lvr. C), pancreas (Pn. C), gastric and cutaneous (Cu. Gl) glands, with- draw specific substances, or secretions, which are ix NUTRITION 151 discharged on the free surface of the epithelium and serve various purposes. We see that the "blood loses — (i), nutrient matters and oxygen all over the body ; (2), water in the skin, lungs, and kidneys; (3), carbon dioxide in the lungs and skin; (4), urea, principally in the kidneys; and (5), various substances in the glands. It gains (i), waste products all over the body ; (2), nutrient matters in the enteric canal; (3), liver-sugar (p. 135) in the liver; (4), oxygen in the lungs and skin. It is therefore richest in oxygen and poorest in carbon dioxide as it leaves the lungs and skin, i.e., in the pulmonary and musculo- cutaneous veins ; richest in nutriment as it leaves the enteric canal, i.e., in the portal vein ; poorest in urea as it leaves the kidneys, i.e., in the renal veins ; poorest in water as it leaves the skin and kidneys, i.e., in the cutaneous and renal veins. In this way a single closed system of pipes not only supplies all parts of the body with everything necessary for their sustenance, but serves also as a drainage system to carry away their various waste products. Notice that we must distinguish between the nutrition, respiration, and excretion of the frog as a whole, and of its various parts. Every one of the thousands of cells, fibres, &c., in the entire body is nourished, breathes, and excretes, taking its nourishment and oxygen directly from the blood, and discharging its waste products into it. What are called the organs of nutrition and respiration are special portions of the body set apart for taking in fresh supplies of food or of oxygen for the organism as a whole, such supplies being finally distributed by the blood-system. Similarly, what are called the organs of excretion are special portions of the body by which the waste products, collected by the blood from all parts of the organism, are finally discharged. 152 THE FROG CHAP. Evolution of Heat. — The oxidation of the tissues, like that of coal or wood in a fire, is accompanied by a rise in temperature. But in the frog, as in other cold- blooded animals, the evolution of heat is never sufficient to raise the temperature of the body more than very slightly above that of the surrounding medium. In warm-blooded animals, such as ourselves, the tempera- ture is regulated, according to the season, by a greater or less evaporation of water from the surface of the body. In the frog this is not the case : the temperature of the animal is always nearly the same as that of the air or water in which it lives, and hence the frosts of winter would be fatal to it, but for the habit of hibernation (p. 8). Death and Decomposition. — The decomposition under- gone by a dead frog (p. u) may be looked upon as an excessive process of waste unaccompanied by repair. Owing to the action of certain microscopic plants known as Bacteria, which will be referred to again in Part II. (p. 257), the proteids undergo oxidation, amongst the principal products of which are water, carbon dioxide, ammonia, and certain gases of evil odour, such as sulphuretted hydrogen and ammonium sulphide. Most of the gases escape into the air, while the ammonia is finally converted into nitrous and nitric acids. These, combining with certain substances in the soil, give rise to salts called nitrates and nitrites, which furnish one of the chief sources of the food of plants. PRACTICAL DIRECTIONS The Organs of Respiration and of Voice. — Pin down a frog in the usual way (pp. 31 and 32), remove the heart, and make out the precise relation of the lungs, first distending them with air through the glottis. The specimen already used for the dissection of the vascular system or alimentary canal will serve the purpose. Harden thoroughly in for- maline or spirit and note (Fig. 44) the laryngo-tracheal ix PRACTICAL DIRECTIONS 153 chamber, which communicates on the one hand with the pharynx through the glottis, and on the other with both lungs. Observe also the posterior horns of the hyoid which embrace the glottis, and then separate them from the laryngo-tracheal chamber, so as to remove the latter, together with the lungs, from the body. Then dissect off what remains of the mucous membrane of the pharynx around the glottis, and notice the small laryngeal muscles in connection with the laryngo-tracheal chamber : remove these, and pin the respiratory organs down under water, dorsal surface of the lungs uppermost, by means of a pin through each lung. Cut away the ventral wall of one lung, so as to expose the cavity and its connection with the laryngo-tracheal chamber. You will very probably find some parasites in the lungs — small worms called Rhabdonema nigrovenosum belonging to the group of Nematode worms. Note : — 1 . The two arytenoid cartilages, and a ring-shaped cartilage surrounding the base of the lungs. 2. The network of ridges on the inner surface of the lungs. Examine with a lens. Sketch. 3. The vocal cords. Observe these first in their natural position, and then with the scissors cut through the laryngo- tracheal chamber along the line of the glottis so as to divide it into right and left halves and thus expose the vocal cords from their surface. Sketch. The Kidneys. — a. Examine these organs in situ (Figs. 3, 4, and 7) and note : — 1. Their form and position, and the relations of the peri- toneum, which covers them on the ventral side only. (See Fig. 5.) 2. The ureters (their openings into the cloaca may be seen at a later stage). 3. The yellowish adrenals. Sketch. b. Examine under the microscope a transverse section of the kidney, prepared as directed on p. 136, and make out (Fig. 46) :— 1. The urinary tubules, cut through in various planes. 2. The Malpighian (or Bowman's] capsules and their glomeruli. 3. Blood-capillaries and vessels. 4. The nephrostomes (p. 98, Figs. 23, 46, and 47, and see Part II, p. 601). Sketch a portion under the high power. Compare with a section of kidney in which the blood-vessels have been injected with coloured gelatine (p. 101). CHAPTER X THE FROG (continued) : THE NERVOUS SYSTEM IN a machine of human construction, such as a steam- engine, the proper working of the whole depends, providing the parts of the machine itself are in good order, upon two things — the stoking or regulation of the fires, and the turning of certain cocks and levers by the engineer. In that very complex machine the frog, we have already studied what corresponds to stoking, viz., feeding and breathing. We must now direct our attention to what may be considered roughly to correspond with the work of the engineer — the means by which the whole complex machinery is kept under control, and its various parts made to work together to a common end. How does it come about, for instance, that the various digestive glands begin to secrete actively as soon as food is taken into the enteric canal ? How is it that a touch on any part of the body, or even the sight of an enemy, is followed instantaneously by a series of vigor- ous muscular movements so ordered as to facilitate escape from the source of danger ? In the fourth chapter (p. 62) we got so far as to learn that muscular contractions are induced by nervous im- pulses travelling from the brain or spinal cord, along the CHAP, x SPINAL CORD 155 nerves, to the muscles. It may therefore be inferred that the controlling apparatus, by which the functions of the body are regulated, is lodged in either the brain or the spinal cord, or in both. Divisions of the Nervous System. — The nervous system is divisible into (i) the central nervous system, consisting of the brain and spinal cord (Figs. 6 and 7), and (2) the peripheral nervous system, consisting of the nerves which pass from the central nervous system to the various parts of the body. The nerves are divisible into (i), cerebral nerves (Fig. 53), arising from the brain, (2), spinal nerves (Fig. 51), arising from the spinal cord, and (3), sympathetic nerves (Fig. 51). The Spinal Cord. — In form the spinal cord (Figs. 6 and 7, sp. cd) is irregularly cylindrical. It is continuous in front with the brain, and tapers off posteriorly into a fine thread-like portion, the filum terminate (/. t}} while opposite the fore-limbs, and again just anteriorly to the filum terminale, it presents an enlargement : these are known respectively as the brachial and sciatic swellings. Along its dorsal surface runs a delicate longitudinal line, the dorsal fissure (Fig. 48, d. /), and a distinct groove, the ventral fissure (v. /), extends along its lower surface. The cord is covered with a delicate pigmented mem- brane known as the pia mater (p. w), and the neural canal in which it lies is lined with a stout, tough mem- brane, the dura mater (Fig. 52, d. m). Between the two is a space filled with a lymphatic arachnoid fluid, which, like the pericardial fluid, preserves the contained organ from shocks. Examination of a transverse section of the cord under a low power will show that the dorsal fissure is an extremely narrow vertical wall formed by an exten- sion inwards of the pia mater. The ventral fissure is a distinct cleft. Thus the two fissures divide the cord 156 THE FROG CHAP. into paired half -cylinders, right and left, joined in the middle by a narrow bridge. This bridge is traversed from end to end by a very narrow longitudinal canal, the central canal (c. c), lined by epithelium, so that the cord is not a solid cylinder, but a tube with an extremely narrow cavity and excessively thick walls. The section also shows that the cord is not homo- geneous, but is composed of two different substances. Its outer part is pure white and shining in the fresh dJt FIG. 48. — Transverse section of spinal cord of Frog, (x 35.) c. c. central canal ; d. /. dorsal fissure ; d. h, dorsal horn of grey matter ; d.f. fibres of dorsal root of spinal nerve ; nv. c. nerve-cells of dorsal horn ; nv.c' nerve-cells of ventral horn ; p. w. pia mater ; v. f. ventral fissure ; v. h. ventral horn of grey matter; y. r. fibres of ventral root of spinal nerve ; w. m. white matter. (After Howes.) cord, and is hence called the white matter (w. m). Its internal substance has a pinkish colour when fresh, and is called the grey matter. The grey matter has a squarish outline in transverse section. It surrounds the central canal, and is continued upwards and down- wards, forming what are called the dorsal (d. h} and ventral (v. h) horns of the grey matter. The Brain. — Anteriorly the spinal cord passes in- sensibly into the brain (Fig. 49), which is of somewhat BRAIN 157 greater diameter than the cord, and is made up of several very distinct parts or divisions. The hinder- most division is called the bulb or medulla oblongata (Med. obi] ; this appears to be simply a widening of the spinal cord (Sp. cd), except that on its dorsal surface is a tri- angular body (D, ch. plx2), of a reddish colour in the fresh condition, called the posterior choroid plexus ; it is simply a thickening of the pia mater containing abundant blood-vessels. The choroid plexus forms a kind of lid to a triangular cavity (A and D, i>4) excavated in the dorsal region of the medulla oblongata, and called the fourth ventricle. The apex of the cavity, which is directed backwards, opens into the central canal of the spinal cord (Fig. 50, t;4, c. c), and the fourth ventricle is to be looked upon simply as the anterior part of the central canal which has become widened out and is covered only by a thickened portion of the pia mater. The fourth ventricle is bounded in front by a narrow ledge of nervous matter (Fig. 49, Cb), which would be hardly worthy of being considered as a special division of the brain but for the fact that the corresponding part in many animals — e.g., dogfish, rabbit, man — is a large and important structure. It is called the cerebellum. In front of the cerebellum comes a pair of rounded elevations, the optic lobes (Figs. 49 and 50, Opt. I). Each contains a cavity, the optic ventricle (Opt. v), communi- cating with a narrow median passage, the iter (i), which is continuous behind with the fourth ventricle. The bulb is continued forwards beneath the optic lobes as the crura cerebri (Cr. C). In front of the optic lobes is an unpaired struc- ture, the diencephalon or 'tween-brain (Di). On its upper surface is a small rounded vascular body, the anterior choroid plexus (Fig. 49, D, ch. plx1), formed, Clfl FIG. 49. — Brain of Frog. A, from above ; B, from below ; C, from the sick (X 4) ; D, in longitudinal vertical section, (x 5.) Cb. cerebellum ; Cer. H. cerebral hemispheres ; ch. plxl anterior and ch. plx.' posterior choroid plexus (removed in A) ; com. transverse bands of nerve-fibre! or commissures connecting the left and right halves of the brain ; Cr. C. crun 158 CHAP. X BRAIN 159 Clf.v v*. fourth ventricle ; I — X, cerebral nerves ; / Sp. 2 Sp. first and second spinal nerves. (A — C, after Gaupp ; D, from Wiedersheim's Comp. Anatomy, after Osborn.) like the posterior choroid plexus, of a thickening of pia mater, containing numerous blood-vessels. It helps to roof over a narrow slit- like cavity communicating with the Her, ihe third ventricle (v3), the sides of which are formed by thickenings of nervous matteV, the optic thalami (Di), connected by transverse bands of nerve- fibres (D, com). On the ventral surface the dien- cephalon is continued into a funnel-like extension, the infundibulum (Fig. 49, inf), to which is attached a rounded structure, the pituitary body (pit). On the dorsal surface, just behind the choroid plexus, is , the delicate stalk (pin) of the pineal body — the vestige of a sensory apparatus between the skull and the skin, which in some lizards, for example, has the structure of a small eye situated on the top of the head, and which was probably functional in the an- cestors of the frog. We shall meet with other examples of such vestigial organs in the course of our studies. \-Sp.cd FIG. 50. — Diagrammatic horizontal section of Frog's brain. c. c. central canal ; Cer. H. cerebral hemisphere ; Di. diencephalon ; for. M. foramen of Monro ; i. iter ; Lat. v. lateral ventricle ; Med. obi. medulla oblongata; Nv. i, olfactory nerve ; Olf. L. olfactory lobe ; Olf. v. olfactory ventricle ; Opt. I. optic lobe ; Opt. v. optic ventricle ; Sp. cd. spinal cord ; v. 3, third ventricle ; v. 4, fourth ventricle. (After Ecker & Wie- dersheim.) 160 THE FROG CHAP. In front of the 'tween-brain comes a pair of long, oval bodies, wider behind and narrower in front. These are the cerebral hemispheres (Cer. H). Each contains a cavity, the lateral ventricle (Lat. v), which communi- cates with the third ventricle by a small aperture, the foramen of Monro (for. M) . Lastly, each cerebral hemisphere is continued forwards by a rounded olfactory lobe (Olf. I), which is fused with its fellow of the opposite side, the single mass lying in the posterior compartment of the girdle-bone (p. 42). The lateral ventricles are continued forwards into the olfactory lobes, forming the small olfactory ventricles (Fig. 50, Olf. v). The brain, like the spinal cord, is formed of grey and white matter, but differently arranged. In the olfactory lobes, cerebral hemispheres, and 'tween-brain the white matter is internal, and the grey forms a thin outer layer or cortex. In the optic lobes and medulla the grey matter is mainly around the ventricles, and the white matter more external. Like the spinal cord, the whole brain is covered with pia mater, densely pigmented in the region of the optic lobes, and the cranial cavity in which it is contained is lined with dura mater. The Spinal Nerves. — The spinal nerves arise symmetri- cally from the spinal cord. on the two sides of the body, j and pass out from the neural canal through the inter- vertebral foramina (p. 38). There are altogether ten pairs of spinal nerves in the adult frog (Fig. 51, / — X), each of which on leaving the neural canal divides into a smaller dorsal and a larger ventral branch (Figs. 52 and 53). The first pair leaves the cord through the intervertebral foramina between the first and second vertebra. Each passes at first directly I outwards, its large ventral branch, known as the hypo- 1 SPINAL NERVES 161 glossal, turning forwards, and going to the muscles of the tongue (Fig. 51, 7, Fig. 53, i Sp). The second pair (Fig. 51, //) is very large ; it emerges between the second and third verte- brae, and each is soon joined by the small third nerve (///), which emergesbet ween the third and fourth vertebrae, as well as by a small branch or two from the first, thus forming a simple network or plexus— i\ie brachial plexus (br. pi), from which pass off nerves to the fore-limb supplying both skin and muscles. The fourth, fifth, and sixth nerves take a very similar course. The fourth (IV) emerges bet ween the fourth and fifth vertebrae, the fifth (V) between the fifth and sixth, and the sixth (VI) between the sixth and seventh. They all pass obliquely back- wards, and supply the walls of the body, being distributed to both skin and muscles. PRACT. ZOOL. sci.pl FIG 51. — The ventral branches of the spinal nerves and the sympathetic of the Frog, ventral view : shown on the right side of the animal only. ( x 2.) / — X, spinal nerves ; Ao. systemic arch ; br. pi. brachial plexus ; C. calcareous bodies which surround the spinal ganglia ; D. Ao. dorsal aorta ; fern. femoral nerve ; II. A. iliac artery ; set. sciatic nerve ; set. pi. sciatic plexus ; Sk. skull; Sp. A. splanchnic artery; Sy. sympathetic cord ; Sy. c. communi- cating branches between the sympa- thetic and spinal nerves ; Sy. g. sympathetic ganglia ; Ust. urostyle ; V — Fy, centra of vertebrae ; Vg. vagus nerve, with its ganglion. (After Gaupp, slightly altered.) M 162 THE FROG CHAP. The seventh, eighth and ninth nerves supply the muscles and skin of the hind-limbs. The seventh' (7/7) leaves the neural canal between the seventh and eighth vertebrae, the eighth (77/7) between the eighth and ninth, and the ninth (IX) between the ninth vertebra and the urostyle. They all pass almost directly back- wards, and are united with one another by oblique cross branches so as to form the sciatic plexus (sci. pi), from which are given off, amongst others, two nerves to the leg, the largest of which, the sciatic nerve (sci), being that already mentioned in the chapter on the muscular system (p. 62). The tenth (X) is a very small nerve. It emerges through a small aperture in the side of the urostyle (p. 39), and supplies the cloaca, urinary bladder, and adjacent parts. It is connected by cross -branches with the ninth. It will be noticed that while the large ventral branch of the first spinal nerve — the hypoglossal — supplies muscles only, and is therefore a motor nerve, all the others go to both muscles and skin, and are therefore both motor and sensory, or mixed nerves. They all branch out in a complex manner, and are traceable to the remotest parts of the body. The Sympathetic Nerves. — On either side of the dorsal aorta is a very delicate nerve, having at intervals little ; swellings called ganglia, each of which is connected with a spinal nerve by a communicating branch (Figs. 51 and < 53, Sy, Sy. g, Sy. c). In front of the point where the' dorsal aorta (D, Ao) is formed by the union of the two systemic trunks (Ao), these two sympathetic nerves, as they are called, are continued forward, one on either side of the vertebral column, towards the head, when they enter the skull and become connected with certain of the cerebral nerves. x SPINAL AND CEREBRAL NERVES 163 Each sympathetic nerve has altogether nine or ten ganglia, each connected with one of the spinal nerves, and from the ganglia branches are given off which supply the heart and blood-vessels, the stomach, in- testine, liver, kidneys, reproductive organs, and urinary bladder. Origin of the Spinal Nerves. — The mode of origin of the nerves from the spinal cord is peculiar and character- istic. Traced, towards the cord, each nerve is found, on reaching the intervertebral foramen from which it FIG. 52. — Transverse section through the vertebral column and spinal cord of the Frog, to show the mode of origin of the spinal nerves. ( X 6.) c. c. central canal ; en, centrum with periosteum ; d. f. dorsal fissure ; d. m. dura mater ; d, r. dorsal root ; g. m. grey matter ; gn. ganglion of dorsal root ; n. a. neural arch ; n. sp. neural spine ; p. m. pia mater ; t, nerve trunk with its sheath (ventral branch) : t'. dorsal branch : Tr. pr. transverse process ; v. f. * " ' " • ' • " c Howes.) ventral fissure ; v, r. ventral root ; w. m, white matter. (After Howes.) emerges, to divide into two — a dorsal root which springs from the dorsal, and a ventral root which arises from the ventral region of the cord (Fig. 52, d.r, v.r). The dorsal root is distinguished from the ventral by being dilated into a. ganglion (gn). In Fig. 51 these ganglia lie hidden within certain calcareous bodies (C) in this region. Cerebral Nerves. — There are ten pairs of cerebral nerves, some of which are purely sensory, some purely motor, some mixed. The first or olfactory nerves (Fig. 49, /) arise from the olfactory lobes, and pass through two holes in the trans- 164 THE FROG CHAP. verse partition of the girdle-bone. Each is distributed to the mucous membrane of the nasal sac or organ of smell of the same side, and is purely sensory. The second or optic (II) is a large nerve which springs from the ventral surface of the 'tween brain. At their origin the right and left optic nerves have their fibres intermingled, forming a structure something like a St. Andrew's Cross and called the optic chiasma (opt. ch), the other limbs of the cross passing upwards and back- wards to the optic lobes. The optic nerve makes its exit from the brain-case through the optic foramen, and is distributed to the retina, a delicate membrane which lines the eye-ball, and is, as we shall see, the actual organ of sight. This nerve also is purely sensory. The third or oculomotor (III) is a small nerve arising from the crura cerebri beneath the optic lobes. It passes through a small hole in the side of the skull behind the optic foramen, and supplies four out of the six muscles by which the eye-ball is moved (p. 186), and is purely motor. The fourth or pathetic (IV) is a very small nerve leaving the dorsal surface of the brain between the optic lobes and the cerebellum, and making its exit from the skull above the third nerve. It is also purely motor, supplying one of the muscles of the eye — the superior oblique. The fifth or trigeminal (Figs. 49 and 53, V) is a large nerve arising from the side of the medulla oblongata. Its root dilates to form a ganglion, the Gasserian ganglion, and leaves the skull by the large aperture noticed in the pro-otic bone. It owes its name to the fact that it soon divides into three main branches ; one, the ophthalmic (Fig. 53, F1), going to the skin of the snout ; another, the maxillary (F2), to the upper lip and lower eyelid; and the third, or mandibular (F3), CEREBRAL NERVES 165 to the muscles and skin of the lower jaw. The trigeminal is a mixed nerve. The sixth or abducent (Fig. 49, VI) is a very small motor nerve arising from the ventral aspect of the 5.sp FIG. 53. — Dissection of the head and anteriorpart of the body of the Frog from the left side, to show the distribution of the fifth, seventh, ninth, and tenth cerebral nerves, as well as of the hypoglossal and part of the sympathetic, (x i£.) Ao. systemic arch ; br. pi. brachial plexus ; co. columella ; D. ao. dorsal aorta ; du. duodenum ; H. heart ; Hy. body of hyoid ; Hyl. anterior, and Hy2. posterior horns of hyoid ; L. lung ; N. olfactory sac ; On. orbit ; Put. pulmonary artery ; Sp. A. splanchnic artery ; st. stomach ; Sy. sympathetic ; II. cut end of optic nerve ; v\ ophthalmic, V'2. maxillary, and V*. mandibular branch of trigeminal (V) • VII1 palatine, and VIP* hyomandibular branch of facial; IX. glosso- pharyngeal ; X. vagus ; Xcd. cardiac, Xgas. gastric, Xlar. laryngeal, and Xpul. pulmonary branch of vagus ; / Sp. first spinal nerve (hypoglossal) ; 2 sp. — j sp. second to fifth spinal nerves. (After Howes, slightly modified.) bulb, and supplying one of the muscles of the eye-ball called the posterior rectus. The seventh or facial nerve (Figs. 49 and 53, VII) arises just behind the fifth and soon joins the Gasserian ganglion. Both it and the sixth leave the skull by the same aperture as the fifth. It divides into two branches, 166 THE FROG CHAP. one of which, the palatine (Fig. 53, VII1), supplies the mucous membrane of the roof of the mouth, and the other, or hyomandibular (VII2), sends branches to the skin and muscles of the lower jaw and to the muscles of the hyoid. It is a mixed nerve. The eighth or auditory nerve (Fig. 49, VIII) arises from the medulla just behind the seventh, passes through an aperture in the inner wall of the auditory capsule, and is distributed to the auditory organ or membranous labyrinth (see Figs. 49 and 53). It is the nerve of hearing, and is purely sensory. The ninth or glossopharyngeal (Figs. 49 and 53, IX) arises behind the auditory nerve. It sends a branch to join the facial, and supplies the mucous membrane of the tongue and pharynx as well as certain small muscles connected with the hyoid. It is also a mixed nerve. The tenth or vagus (Figs. 49 and 53, X) is a large nerve arising in common with the ninth, and dilating, shortly after leaving the skull, into a vagus ganglion. It supplies the larynx (Xlar), the heart (Xcd), the lungs (Xpul), and the stomach (Xgas), and is there- fore often known as the pneumogastric. It has thus an extraordinarily wide ' distribution, being in fact the only cerebral nerve which supplies parts beyond the head. It is a mixed nerve, and contains many motor fibres, but its branches — some of which have to do with the regulation of the heart's contraction and with respiration — are better described as efferent and afferent than as motor and sensory : the meaning of these terms will be explained later on. The ninth and tenth nerves leave the skull close together through the aperture noticed in the exoccipital bone. The sympathetic nerve (Sy) extends forwards from its junction with the first spinal nerve, joins the vagus, and finally ends anteriorly in the Gasserian ganglion. X NERVE-FIBRES AND CELLS 167 Microscopic Structure of Nervous Tissue.— Examination of a piece of nerve under the microscope shows it to be composed, like striped muscle, of cylindrical fibres, bound together by connective-tissue . The latter is much more abundant than in muscle, and in particular forms a thick sheath round the nerve which must be torn off before the nerve-fibres are reached. Each fibre (Fig. 54) is a cylindrical cord in which three parts can be distinguished. Running along the axis of the fibre is a delicate protoplasmic strand, the neuraxis or axis-fibre (nx). Around this is a sheath formed of a fatty substance and known as the medullary sheath (m. s) ; * and, finally, investing the whole fibre is a delicate, structureless membrane, the neuro- lemma (ne) . At intervals the medullary sheath is absent, and a node is produced, where the fibre consists simply of the neuraxis covered by the neurolemma. In the ganglia are found, not only nerve-fibres, but nerve-cells (Fig. 54) : these are cells of a relatively large size, each with a large nucleus and nucleolus. In the spinal ganglia (B) the cell-body is produced into two pro- cesses, which come off from the cell by a common stalk. One of these processes (the axon) becomes the neuraxis of a peripheral nerve-fibre ; the other (dendron) is also a protoplasmic process which passes into the spinal cord and sends off branches, each branch finally ending in a complicated branch-work or arborisation, which is interlaced, but not actually continuous, with a similar arborisation arising from a nerve-cell in the spinal cord or brain (Fig. 55). The cell with its processes is spoken of as a neurone. The white matter of the brain and spinal cord consists of nerve-fibres, those in the cord having a longitudinal 1 The medullary sheath may be absent in certain nerve-fibres (e.g., in the sympathetic and olfactory nerves). 168 THE FROG direction ; the grey matter contains numerous " multi- polar" nerve-cells, each giving off numerous dendrons i ne ms 713C' rns FIG. r. 54. — A, nerve-cell from the grey matter of the spinal cord of a Frog, and the nerve-fibre arising from it (neurolemma not shown) ; B, cell from the ganglion of a dorsal root, (x about 200.) j. neurolemma; MM. nucleus ; nx. neuraxis ; ms. medullary sheath. (After • u^,,,r.^ \ . Howes.) and one axon, which is continuous with a neura (Figs. 48 and 54, A) ; they are enclosed in a tissue formed partly of the axis-fibres of nerves which enter x REFLEX ACTION 169 the grey from the white matter, losing their medullary sheath as they do so, and partly of a delicate supporting tissue called neuroglia, in which the other elements are imbedded. Functions of the Nervous System : Reflex Action. — In the fourth chapter you learned that a muscle may be made to contract by a stimulus applied either to the muscle itself or to its nerve. You are now in a position to pursue the subject of the control of various parts of the body by the nervous system a little further. A frog is either decapitated or pithed, i.e., the medulla oblongata is severed and the brain destroyed : there can be thus no question either of sensation or of voluntary action on the frog's part. It is then hung up by a hook or string, so that the legs are allowed to hang freely. If one of the toes is pinched with the forceps, the foot will be drawn up as if to avoid the pinch ; or, if some very weak acid be applied to a toe, the foot will again be withdrawn, being raised every time it is touched with the acid with the regularity of a machine. Again, if acid be applied to various parts of the body, the foot of the same side will immediately try to rub off the irritating substance ; or if that foot be held down, the other will come into play. Movements of this kind are called reflex actions : the stimulus applied to the skin is transmitted by sensory nerve-fibres to the spinal cord, where it is, as it were, reflected in another form, and passed along motor fibres to one or more muscles, causing them to contract (p. 60). As already stated, the spinal nerve-trunks are mixed, i.e., contain both sensory and motor fibres, which, how- ever, cannot be distinguished from one another structur- ally. It has been found by numerous experiments that as the nerve approaches the spinal cord these two sets of fibres separate from one another, the sensory fibres 170 THE FROG CHAP. v, passing into the cord by the dorsal root, the motor by the ventral root. As a consequence of this, if the dorsal root be~~cut and its proximal or central end— i.e., the end in connection with the cord — stimulated, muscular contraction will follow just as if the stimulus had been applied to the skin supplied by the nerve in question. If the other cut end— the distal or peri- pheral end — be stimulated, there is no result. Orrthe c.corl FIG. 55. — Diagram illustrating the paths taken by the nervous impulses. c. c. central canal ; col. collaterals ; c. cort. cell in rind or cortex of the cerebral J. fibre ; S. skin ; s. /. sensory fibre ; sp. c. spinal cord ; v. c. cells in ventral horn I of grey matter ; v. r. ventral root ; w. m. white matter. The arrows indicate the direction of the impulses. other hand, if the ventral root be cut and its distal end stimulated, the muscles supplied by it will contract ] while stimulation of the proximal end produces nc result. Very accurate observations have shown that the con nection between the motor and sensory fibres is follows (Fig. 55). A motor fibre (m. f) is traceabl from the nerve-trunk through the ventral root (v. r into the white matter ; and then, its medullary sheati x REFLEX ACTION 171 being lost, passes into the ventral horn of the grey matter, its neuraxis being directly continuous with the axis-fibre process of one of the large motor nerve-cells (v. c) : the remaining processes of these cells simply branch out in the neuroglia. The sensory fibres (s. f) are traceable into the dorsal root (d. r) ; in passing through the ganglion of the root (g) they are found to be continuous with its simple (" bipolar ") nerve-cells (g. c), and then pass into the cord. Instead, however, of entering the grey matter at once, they pass forwards as well as backwards for some distance, along the white matter of the cord, giving off numerous branches, or collaterals (col), which, losing their medullary sheaths, nter the dorsal iiorn of the grey matter and branch out nto a complex series of fine fibres which interlace with ;he similar arborisations of the nerve-cells in the cord p. 167). The path of a nervous impulse known as the reflex arc or chain will now be obvious. The stimulus applied to he skin (Fig. 55, S) is- conducted by a sensory fibre to the nerve-trunk and by the dorsal root to a cell in the ganglion and thence to the spinal cord ; it then masses along the white matter of the cord, enters the grey matter, and is conducted by the collaterals to the nerve-cells of the ventral horn, either directly, or in- iirectly after passing through the cells of the dorsal lorn : from one of the cells of the ventral horn it is con- ducted by an axon which becomes the neuraxis of a lerve-fibre, which, leaving the cord by a ventral root, masses along the nerve-trunk and finally goes to a muscle M) as a motor fibre. It will be noticed that a single stimulus applied to he skin may result in the contraction of numerous miscles — as, e.g., when the application of a drop of icid to the toe causes the lifting of the leg — and that the 172 THE FROG CHAP. movements are of such a nature as to withdraw the part stimulated from the irritating substance. Moreover, as shown by the experiment of applying acid to various parts of the body, the movements are varied according to circumstances ; if one leg is prevented from rubbing off the irritating substance, the other immediately comes into play. Obviously, then, a simple stimulus reaching the spinal cord may be transmitted to numerous motor cells of the ventral horn, and through these to numerous motor nerves, the particular nerves affected differing according to circumstances (compare Fig. 55). The spinal cord, therefore, is able, in response to a stimulus reaching it by a sensory nerve, to originate motor impulses causing complex muscular movements so adjusted as to serve definite purposes. Without such external stimulus, however, the spinal cord of a brain- less frog is quite inactive, and the body of the animal will remain without movement until it dries up or decomposes. In the uninjured frog, i.e., the frog with its brain intact, the case is very different. The animal no longer acts like an unintelligent machine, each stimulus producing certain inevitable movements and no others ; but a single stimulus may produce varied movements, the nature and direction of which cannot be predicted. The frog will probably give a series of leaps, but thej number and extent of these vary according to circum- stances ; its movements are voluntary, not reflex. This is explained by the fact that certain nerve-fibres of the cord pass forwards to the brain, and that the nerve-cells in the grey matter of the cord are in com- munication— owing to the interlacing of their branching! processes with those of the collaterals — with similar cells in the grey matter of the brain (Fig. 55, m. c, c. g, cort). In certain of these brain-cells (c. cort), voluntary- REFLEX AND VOLUNTARY ACTION 173 impulses originate and exercise a controlling effect upon the cells of the spinal cord, so that these latter do not constitute, as in the brainless frog, a machine every movement of which can be accurately predicted. Moreover, it can be shown by experiment that the process of originating voluntary impulses is not per- formed by the whole brain, but is confined to the cerebral hemispheres. If the hemispheres and optic lobes are removed so as to leave nothing but the bulb and cerebellum, the frog no longer lies in any position in which it may be placed, exhibiting no movements beyond the beating of the heart, as is the case when the whole brain is 'removed. It sits up in the ordinary attitude, breathes, swallows food placed in the mouth — while making no attempt to feed itself, turns over and its up if placed on its back, and swims if placed in water. If left alone, however, it remains in the sitting posture till it dies. Hence the bulb and cerebellum are evidently concerned with the co-ordination of mus- ular movements, but have no power of originating impulses. If the optic lobes as well as the medulla oblongata and cerebellum are left, the animal is affected light, is able to perform complex balancing move- ments, and will even croak when stroked in a particular way. There is still, however, no voluntary action ; without the application of stimuli, the animal sits motionless until it dies. To sum up in the language of the illustration with which this chapter was begun, comparing the frog with ,n engine of human construction : — the grey matter of the brain may be compared with the engineer ; much of the work of the engine may go on without him, certain Severs, valves etc., acting automatically ; but it is only by his controlling intelligence that the whole mechanism is adapted to the circumstances of the moment. 174 THE FROG CHAP. So far we have considered the nervous system only in its relations to the skin or general surface of the body and to the muscles or organs of movement. The" other parts of the body are, however, under nervous control. It has been mentioned that the heart continues to beat in a frog when the brain has been removed : not only so, but it pulsates with perfect regularity whe: removed from the body. This is due to the fact tha the muscles of the heart, like the cilia of ciliated epithi Hum, have the power of contracting rhythmically quit independently of the nervous system, although th heart contains nerve-cells which were formerly supposed to serve as a special nervous system for this organ originating all its motor impulses. It is, howeve: under the control of the central nervous system. W< have seen that it is supplied by a branch of the vagus when this is stimulated, the heart stops in the dilate< state and begins to beat again only after a certain interval. A feeble stimulus to the vagus will not actually stop the heart, but will diminish the rate and the strength of its contractions and consequently the amount of blood propelled through the body. The vagus is accompanied by a branch of the sympathetic which has an exactly opposite effect, i.e., stimulation of it accelerates the heart's action. In this way, the general blood-supply of the body is regulated by the central nervous system. The blood-supply of the various parts and organs is regulated by the vaso-motor nerves. These are traceable through the sympathetic into the spinal cord by the ventral roots : distally they send branches to the mus- cular coat of the arteries. Under ordinary circumstances a constant succession of gentle stimuli pass along these from a group of nerve-cells in the medulla oblongata and, as the result, the arteries are ordinarily in a state AFFERENT AND EFFERENT NERVES 175 of slight contraction. By various circumstances these stimuli may be diminished for any given artery and at the same time stimuli pass down another kind of vaso-motor fibres : the artery will then dilate and the blood-supply of the organ to which it is distributed will >e temporarily increased. For instance, the presence of :ood in the stomach acts, through the central nervous system, upon the cceliac branch of the splanchnic artery, causing a dilatation of its capillaries and promoting an ncreased secretion of gastric juice. The secretion of other glands is regulated in a similar way. In some cases, however, it has been proved that the nerves of a gland do not act simply by producing dilatation of the capillaries, but have a direct effect upon the gland-cells, causing an increased secretion. You will thus note that there are nerve-fibres carrying mpulses to the central nervous system which have nothing to do with sensation, and fibres carrying im- >ulses from the central nervous system which have nothing to do with motion, but result in increased secretion or in stoppage of motion. It is therefore best o use the term afferent (which includes sensory) for a nerve carrying an impulse to the brain and spinal cord, and efferent (including motor) for one carrying an impulse in the other direction (p. 166). PRACTICAL DIRECTIONS I. The Central Nervous System (Fig. 6). — Lay bare the )rain and spinal cord as directed on p. 34, noting the dura nater and pia mater : the latter is densely pigmented over )arts of the brain. The specimen on which this operation las already been performed will do, if the dissection has Deen done carefully. Observe the origins of the cerebral and spinal nerves, toting the long dorsal and ventral roots of the latter (compare 176 THE FROG CHAP. Fig. 52) which pass backwards for some distance before making their exit from the neural canal ; and also the ganglia on the dorsal roots, lying just outside the canal and each hidden in a whitish calcareous body in this region (Fig. 51, C). (The ganglia, however, can be more easily made out at a later stage.) Then sever the nerves very carefully from the brain and spinal cord and remove the whole central nervous system from the neural canal : it is best examined after hardening in formaline or spirit. Lay it in a dissecting-dish, under water, and make out its several parts as follows : — a. The spinal cord. 1. Note its cylindrical form, the brachial and sciatic swellings, the filum terminale, and the dorsal and ventral fissures. 2. Examine a transverse section of the spinal cord, prepared as described on p. 136, under the low power of the microscope, and make out the dorsal and ventral fissures, the central canal, and the relations of the grey and white matter (Figs. 48 and 52). Sketch. b. The brain (Fig. 49). Beginning from the posterior end of the brain, where it passes into the spinal cord, make out its several divisions as follows : — 1 . The bulb or medulla oblongata, with the posterior choroid plexus on its dorsal side : remove the latter, and notice that it roofs over the cavity of the fourth ventricle. 2. The small ledge-like cerebellum. 3. The two rounded optic lobes, and the crura cerebri j beneath them. 4. The diencephalon, formed of a right and a left optic thalamus. On its dorsal side is the anterior choroid plexus, roofing in the third ventricle ; and on its ventral side the infundibulum, from which the pituitary body easily becomes detached ; and, more anteriorly, the optic chiasma. 5. The cerebral hemispheres, continuous in front, with 6. The olfactory lobes, which are fused in the middle line. Sketch the whole central nervous system from above, and also the brain from below and from the side. With the small scissors or a sharp scalpel, snip off a small piece of the wall of the hemisphere and optic lobe of one side — say the left, so as to expose the lateral ventricle and the optic ventricle (Fig. 50). Then with a sharp scalpel divide the whole brain into two by a longitudinal vertical PRACTICAL DIRECTIONS 177 cut very slightly to the left of the middle line, so as to reduce it to a longitudinal section (Fig. 49, D). Examine the cut surface of the right side under water, and make out as much as possible of the commissures and of the relations of the ventricles : — viz., the fourth ventricle, the iter and optic ventricle, and the third ventricle, which communicates with the lateral ventricle through the foramen of Monro. Sketch. II. The Peripheral Nervous System. a. The spinal nerves. Fasten out a frog with the ventral side uppermost, and remove the heart, enteric canal, reproductive organs, kidneys, and lungs with great care, leaving behind most of the systemic trunk and the dorsal aorta (Fig. 51). (One of the specimens already dissected will probably serve the purpose if the previous directions have been accurately followed.) Note the spinal nerves passing outwards from the vertebral column on either side, and the calcareous bodies close to their points of exit, covering up the ganglia of the dorsal roots (p. 163). If the centra of the vertebrae are removed, the nerve-roots and their origins from the spinal cord can be made out : the removal of the centra is rendered easier if the frog is first decalcified by being placed in 10 per cent, nitric acid for twenty-four hours and then thoroughly washed in running water. Confine your attention to the large ventral branches of the ten pairs of spinal nerves, as described on pp. 160 — 162. b. The sympathetic nerves (Figs. 51 and 53). Examine the systemic trunk and dorsal aorta carefully with a lens. Closely connected with it will be seen on either side a sympathetic nerve-cord, covered by pigmented con- nective-tissue. Carefully dissect the cord away from the aorta, and note the ganglia and the branches (rami com- municantes) connecting them with the spinal nerves. Sketch the spinal nerves and sympathetic. c. The cerebral nerves (Fig. 53). The dissection of these in a frog is not an easy task for a beginner, and it is best to examine those of a larger animal (e.g., dogfish) before attempting it (see Part II, p. 479)- The origin of some of the nerves from the brain and the apertures through which certain of them pass out from the skull, have already been seen. III. The Microscopic Structure of Nervous Tissue. a. Examine your transverse section of the spinal cord PRACT. ZOOL. N 178 THE FROG CHAP, x (Fig. 48) under the high power of the microscope, and observe — 1. The nerve-cells, present in the grey matter only (com- pare Fig. 54, A). Note their branched form and their nuclei ; the larger motor cells are seen in the ventral horns of the grey matter. Sketch. 2. The nerve-fibres, in both grey and white matter, cut across transversely as well as in other directions, and each, showing a deeply-stained central neuraxis. Sketch. b. Tease up a fresh spinal or sympathetic ganglion in salt solution, and stain with methyl-green. Compare the form of the nerve-cells with those in the spinal cord (Fig. 54, B). Sketch. c. Cut off a very small piece of any fresh nerve (e.g. sciatic), and tease it out carefully, in a longitudinal direction, in salt solution. Note that it is made up of cylindrical, unbranched nerve-fibres, bound together by connective- tissue. Examine a single fibre under the high power (Fig. 54, A), and make out the neurolemma, the medullary sheath, and the nodes ; at the nodes, the neuraxis can also be seen. Sketch. Tease out another piece of fresh nerve in chloroform, so as partially to dissolve the medullary sheath, and note the central neuraxis. Sketch. Tease out in glycerine a piece of nerve which has been treated with a i per cent, solution of osmic acid in water for an hour or two and then well washed in water. The medullary sheath will appear nearly black, and the neuro- lemma, as well as the nodes, can be plainly seen. Sketch. Reflex Action. The experiment described on p. 169 should be seen. CHAPTER XI THE FROG (continued} : THE ORGANS OF SPECIAL SENSE IN the previous chapter you have learnt how the nervous system controls the various functions of the body, and how voluntary action is absolutely dependent upon the connection of the brain, through the spinal cord, with the nerves. Obviously, in order that the power of voluntary action should be of full use to its pos- sessor, some means of communication with the external world is not only desirable but necessary ; the frog, in order to adjust its actions to the circumstances in which it from time to time finds itself, must be able to distinguish friends from enemies, suitable from unsuit- able food, darkness from light, heat from cold. The avenues of communication between the animal and its surroundings are, as in ourselves, the senses of touch, taste, smell, sight, and hearing. The sense of touch, including that of temperature, is lodged in the whole extent of the skin, which, as you* have already learnt, is abundantly supplied with sensory nerves. Many of the nerves terminate in connection with (what are known as tactile cells— large flattened cells arranged in groups just below the epiderm and around which the ultimate fibres of the sensory nerves are distri- buted. Stimuli applied to the skin, either by direct 179 N 2 180 THE FROG CHAP. touch or by the heat of the sun, are transmitted to the tactile cells and thence through the sensory nerves to the brain. Notice that the stimulus is transmitted to the nerve-ends through the epithelial cells of the skin ; if the skin be wounded and a stimulus applied directly to the tactile cells or the nerves, the sensation is one, not of touch, but of pain. The sense of taste is lodged in the mucous membrane of the mouth, especially on the tongue and in the neigh- bourhood of the vomerine teeth, but extending also as far back as the gullet. Certain ot the epithelial cells have an elongated form and are arranged in groups known as taste-buds, to which the fibres of the ninth and palatine branch of the seventh cerebral nerves, or nerves of taste, are distributed ; on the tongue these taste- buds are situated on papilla of the mucous membrane. In this case the stimulus is supplied, not by direct touch or by alteration of temperature, but by the contact of sapid or tasty substances. As before, the stimulus is applied to epithelial cells, and by them transmitted to the nerves and so to the brain, when the sensation of taste becomes manifest. Thus, just as common sensation may be abolished, in any part of the body, in three ways — by destruction of the skin, by cutting the sensory nerve, or by destroying the cerebral hemispheres — so the sense of taste is lost if either the mucous membrane of the mouth is injured or if the glossopharyngeal and palatine nerves are cut, or, again, if the cerebral hemi- spheres are destroyed. The sense of smell is lodged in the nasal or olfactory sacs, which are enclosed in the olfactory capsules of the skull and separated from one another by a partition, the nasal septum. Each sac has two apertures, the external nostril, opening on the surface of the snout, and the internal nostril, opening into the mouth (p. 17). The XI NOSE AND EYE 181 sacs are lined by a delicate mucous membrane, some of the epithelial cells of which are of the ordinary columnar type, while others are extremely slender and produced into delicate processes at their free ends. With these latter the fibres of the olfactory nerves are connected, and they are distinguished as olfactory cells (Fig. 56). As the epithelial cells of the skin are affected by direct contact or by heat, so the olfactory cells are affected by the minute particles given off from odorous bodies : the contact of these particles acts as a stimulus, which, being trans- mitted by the" olfactory nerves to the brain, gives rise to the sense of smell. This sense can be destroyed — as in the case of feeling and taste — either by destruction of the olfactory mucous membrane or by cutting the olfactory nerves, or by destroying the brain. The organ of sight or eye of the frog is so similar in structure to that of man that the reader may be referred for details both of structure and of function to the text-books of Physiology, and it will only be necessary to give a brief outline here. Each eye (Fig. 57) is a nearly glo- bular organ, and when removed from the orbit and cleaned by dissecting away the attached muscles, etc., two regions can be distinguished in it — an opaque portion of a dark bluish colour which forms some two-thirds of the entire globe and is hidden within the orbit in the entire animal ; and a clear, transparent por- tion which is directed outwards and freely exposed between the eyelids in the living frog. The outer coat FIG. 56. — Epithelial cells of the olfac- tory mucous mem- brane of an Am- phibian, highly magnified. E. interstitial cells ; R. olfactory cells. (From Wie- dersheim's Comp. Anatomy.) 182 THE FROG CHAP. of the concealed portion of the eyeball is the sclerotic (scl) and is formed of cartilage in the frog ; the dark colour is due to the presence of a layer of black pigment which forms one of the internal coats ; this will be referred to hereafter. Entering the sclerotic on its inner side, i.e., the side next the brain-case, will be seen the cut end of set chd- FIG. 57. — The eye of the Frog. A, as seen in the living animal. B, in median longitudinal section, through the plane of the optic nerve. C and D, the two halves of the eye, bisected equatorially ; the inner half (C) showing the blind spot. (X 3$.) aq. c. aqueous chamber ; b. s. blind spot ; c. cornea ; chd. choroid ; cj. conjunctiva ; e. I. upper eyelid ; e. I', lower eyelid (nictitating membrane) ; ir. iris ; 1. crystal- line lens ; op. n. optic nerve ; r. retina ; scl. sclerotic; v. c. vitreous chamber. (After Howes.) the optic nerve. The transparent, exposed portion of the eyeball is the cornea (c), a superficial thin layer of which, or conjunctiva (cj}, is continuous with the lining of the eyelids and thus with the skin covering the head ; through it can be seen the coloured part of the eye, or iris (ir), with a black spot — really a hole — in its centre, the pupil. XI EYE 183 The interior of the globe (v.c) is filled with a colour- less, transparent jelly, the vitreous humour, surrounding which, everywhere but on its external face, is a thin semi-transparent membrane, reddish when perfectly fresh, but becoming grey soon after death ; this is the retina (r). Between the retina and the sclerotic is a vascular membrane called the choroid (chd), the inner face of which, i.e., that in contact with the retina, is coloured black. It is this layer of black pigment which gives the dark tint to the semi-transparent sclerotic in the entire eye ; strictly speaking, it belongs to the retina, but actually it adheres to the choroid and appears like the innermost layer of that coat. The retina is readily detachable from the choroid, but at the place where the optic nerve enters (blind spot, b. s) it becomes continuous with the fibres of the latter, which pass through the sclerotic and choroid. The choroid is made up of connective-tissue and contains numerous vessels as well as pigment-cells. Lying just internal to the pupil is a nearly globular body, perfectly transparent when fresh, the crystalline lens (I) ; it is kept in place by a delicate membrane, the capsule of the lens. The iris, which covers the outer face of the lens except where it is perforated by the pupil, is covered on its inner surface with black pigment, and is continuous all round its outer margin with the choroid. Between the iris and the cornea is a space> the aqueous chamber of the eye (aq. c), which contains a watery fluid, the aqueous humour. The main cavity of the eyeball, containing the vitreous humour, is called the vitreous chamber. The actual relations of these parts in the entire eye are best grasped in a vertical section, such as is repre- sented in Fig. 57, B. The main part of the eyeball forms a chamber, enclosed by the sclerotic, darkened internally 184 THE FROG CHAP. by the choroid and lined by the retina. Into the outer side of this dark chamber is let a transparent window, the cornea ; behind which, and separated from it by a space containing the aqueous humour, is a .vertical curtain, the iris, perforated by an aperture, the pupil. Behind the iris and in close contact with it is the lens, and filling the whole of the dark chamber between the lens and iris and the retina is the vitreous humour. The whole eye thus has the structure of a camera ob- scura. The cornea, aqueous humour, lens, and vitreous humour are a series of lenses, so arranged that the rays of light from an external object are refracted and brought to a focus on the retina, where they form a greatly diminished and inverted image of the object- The iris is provided with muscles, by means of which the pupil can be enlarged or diminished ; it therefore acts as a diaphragm and regulates the amount of light entering the eye. Attached to the capsule of the lens are delicate muscles, by means of which the focus of the entire apparatus can be altered to some extent according to whether the object viewed is nearer to or farther from the eye. This arrangement for accommoda- tion is, however, much less highly developed in the frog than in man and the higher animals, in which the relatively smaller lens is flatter and distinctly biconvex in form. Thus the various parts of this complicated organ are so adjusted as to bring the images of external objects to an accurate focus on the back part of the interior of the eyeball, i.e., on the retina. A vertical section of the retina (Fig. 58) reveals a very complex structure. On its inner face, i.e., the surface in contact with the vitreous humour, is a layer of nerve-fibres (n.f), formed by the ramifications of the optic nerve, which, passing through the sclerotic and choroid, perforates the retina, and spreads out over its XI RETINA 185 inner surface. Next comes a layer of nerve-cells (g), and then several layers of fibres and nuclei (gr, nc) ; and finally, forming the outer surface of the retina proper, is a layer of delicate, transparent bodies called, from their form, the rods (r) and cones (c) ; these are known from their development to be modified epithelial cells. The whole of these structures are supported by a complex framework of connective-tissue. In close con- tact with the outer or free ends of the rods and cones is a layer of cells the protoplasm of which is filled with a dense black pigment. It is this pigment-layer (p. ep) which, as we have seen, is often counted as part of the choroid. In spite of its complex structure, the retina is not much more than Jth mm. (T^th inch) thick, and is perfectly transparent. Hence, when an image is formed on it, the rays of light easily penetrate its whole thick- ness until they are stopped by the opaque layer of pigment. The rays can thus stimulate the rods and cones, and the stimulus is trans- mitted through the layers of nuclei and nerve-cells to the fibres of the optic nerve, along which it is con- veyed to the brain. Thus the actual organ of sight is not the eye as a whole, but the retina : all the rest is to be looked upon as an accessory apparatus for focussing and for regulating the admission of light. FIG. 58. — Vertical section of Frog's retina, (x 300.) c. cones ; g. layer of nerve-cells ; gr, gr'., outer and inner granu- lar layers ; nc, nc', outer and inner nuclear layers ; n. /. nerve-fibre layer ; p. ep. pigment- epithelium ; r. rods. (After Howes.) 186 THE FROG CHAP As with the other sense-organs, sight may be destroyed by injury to the retina or actual organ of sight, by cut- ting the optic nerve, or by destroying the brain. But unlike the other sense-organs already considered, that of sight has a complex accessory or focussing apparatus in connection with it, and vision may also be rendered impossible by injury to the cornea or lens. It is an obvious advantage to an organ of sight such as the frog's that it should be capable of movement in any direction, so as to allow the light from any object to enter the pupil. As a matter of fact, the animal can direct its gaze through a very wide range by means of eight muscles connected with the eyeball in the orbit. One of these, the levator bulbi, raises the whole eye, causing it to project further on the surface of .the head. Another, the retractor bulbi, withdraws it, causing it to bulge into the mouth. Four others (compare p. 164 and Fig. 126), the superior, inferior, anterior, and posterior recti, rotate it respectively upwards, down- wards, forwards, and backwards. And, finally, two oblique muscles, the superior and inferior, produce a rota- tion along an axis joining the optic nerve with the middle of the cornea. The conjunctiva, which covers the outer surface of the eye and lines the eyelids, is kept moist by the secre- tion of a lacrymal gland, known as the Harderian gland, situated between the eyeball and the orbit in the antero- ventral region. The excess of this secretion is carried away into the olfactory chamber by means of a tube, the naso-lacrymal duct. Each organ of hearing (which also serves for equili- bration), like the organ of sight, consists of an essential portion and an accessory apparatus. The essential organ of hearing is a structure called the membranous labyrinth, contained within the auditory capsule of the xi EAR 187 skull (Fig. 10), and consisting of a kind of bag of very peculiar and complicated form (Fig. 59). It is made up, in the first place, of the vestibule — two somewhat ovoid sacs separated by a constriction : the dorsal one is called the utriculus (ut), the ventral the sacculus (s), and from the latter a small process, the cochlea (cc), projects backwards. With the FIG. 59. — The membranous labyrinth of the Frog's ear. A, outer, and B, inner view, (x 6.) am', am", am'", ampulla of the semicircular canals ; a. s. c. anterior semicircular canal ; cc. cochlea ; h. s. c. horizontal (external) semicircular canal ; p. sc. posterior semicircular canal ; r. ut. recess of the utriculus ; s. sacculus ; ut utriculus ; VIII. branches of auditory nerve. (After Howes.) utriculus are connected three tubes, called, from their form, the semicircular canals, each of which opens into the utriculus at either end. One of them, the anterior canal (a.s.c), is directed forwards ; another, the posterior canal (p.s.c), backwards ; both these are vertical in position and are united to one another at their adjacent ends. The third, the external canal (k.s.c), 188 THE FROG CHAP. is directed outwards and has a horizontal position. Each canal has one end dilated into a bulb-like swelling or ampulla (am] ; those of the anterior and external canals are at their anterior ends, while that of the pos- terior canal is at its posterior end. The whole of this apparatus is filled with a fluid, the endolymph, in which are contained calcareous particles, the otoliths or ear-stones. It is made of connective- FIG. 60. — Longitudinal section through an ampulla. :. e. auditory epithelium ; a. h. auditory hairs ; c. part of semicircular canal ; c. r. acoustic spot and ridge ; c. t. connective-tissue ; e. epithelium ; n. nerve ; M. junction with utriculus. (From Foster and Shore's Physiology.) tissue and lined with epithelium, the cells of which are cubical for the most part ; but in certain places the wall is thickened, forming what are called acoustic spots, of which there is one to each ampulla, situated on a ridge (Fig. 60), while others occur in the utriculus and sac- culus. On these acoustic spots the epithelial cells are greatly elongated, and produced at the surface intc delicate processes called auditory hairs : to these cell: the fibres of the auditory nerve are distributed. xi EAR 189 The membranous labyrinth does not fit tightly into the cavity of the auditory capsule in which it is con- tained, a space being left between it and the surrounding bone and cartilage (Fig. 10) . This space is filled by loose connective-tissue and a fluid called perilymph, by which the membranous labyrinth is surrounded and protected from shocks. As you learnt in studying the skull, the outer wall of the auditory capsule is perforated by a small aperture, the fenestra ovalis (Fig. 10, fen. ov), in which is fixed the stapes (stp), a small nodule of cartilage connected with a bony rod or columella (col), the cartilaginous hammer-shaped outer end of which, or extra-columella, is fixed to the inner side of the tympanic membrane (tymp. memb). The columella lies in the tympanic cavity (tymp. cav), which is bounded externally by the tympanic membrane, internally by the auditory capsule, and at the sides chiefly by muscles and connec- tive-tissue ; while below it communicates with the pharynx by the Eustachian tube (eus. t). When sound-waves impinge on the tympanic mem- brane, the vibrations to which they give rise are trans- mitted by the columella to the stapes, and so to the perilymph. Thence they are communicated to the endolymph and act as stimuli to the auditory cells of the acoustic spots, and the impulses being carried to the brain by the auditory nerve, give rise to the sensation of sound. Whether or not all the acoustic spots are truly auditory in function is not known : it seems that the semicircular canals are really concerned with the sense of direction and velocity and with the maintenance of equilibrium. The sense of sound can be destroyed by injury to the membranous labyrinth, by cutting the auditory nerve, by destroying the brain, or — to a great extent at least — by injury to the tympanic membrane or columella. 190 THE FROG CHAP. Notice that the general plan of all the sensory organs, those of the skin, eye, and ear, is the same. They con- sist of certain peculiarly modified epithelial cells, specially sensitive to impulses of particular kinds, and in com- munication, by means of an afferent nerve, with nerve- cells of the brain. The three things — sensory cell, afferent nerve, and brain — form a chain, every link of which is necessary for the performance of the sensory function, so that the particular sense in question may be destroyed, not only by destruction of the sense-organs in the strict sense, but also by section of the afferent nerve or by destruction of the brain. General Physiology — Summary. — Before going on to the next chapter it will be as well to take a final glance at the physiological processes of the frog as a whole (com- pare Fig. 23). The enteric canal is the manufactory in which the raw material of the food is worked up into a form in which it can be used by the various parts of the body. The circulatory organs are the communicating system by which the prepared food is taken to all parts ; and they also form a drainage system by which waste matters are collected from all parts and finally ejected by the three main sewers, the skin, lungs, and kidneys. The skin and lungs, besides getting rid of waste matters, serve for the supply of oxygen — a necessary form of gaseous food. The central nervous system forms a sort of headquarters' staff by which the entire body is con- trolled, the means of communication being the nerves, and the muscles the executive by which the orders from headquarters are executed. And finally the sense-organs may be looked upon as the various branches of an intelligence department by which the headquarters are informed of what is going on outside. xi PRACTICAL DIRECTIONS 191 PRACTICAL DIRECTIONS The Organs of Special Sense I. Olfactory organ. Notice again the external and internal nostrils. Then remove the skin covering the snout, dissect off the nasal bones, and open up the olfactory sacs. Note, using a lens, the pigmented olfactory epithelium lining these, and make out the olfactory nerves and nasal septum. Compare with a transverse section through the sacs. Sketch. II. Eye. a. Notice agam the eyelids, iris, and pupil. Then remove the skin covering the head so as to expose the nearly globular eyeballs, lying in the orbits. Examine with a lens. In the antero-ventral region of the orbit make out the Harderian gland, and the eye-muscles passing from the walls of the orbit to the eyeball. The four recti and two oblique muscles can be more easily seen on a larger animal, and directions will be given for their examination in Part II (see Fig. 126) ; but if you make a dissection of them in the frog, you should note at the same time the levator and retractor bulbi, the latter underlying the eyeball, and the former situated internally to the recti muscles. b. Remove the eyeball from a freshly-killed specimen, noticing as you do so the optic nene, which is surrounded by the recti and retractor bulbi muscles : dissect away these muscles and note the cartilaginous sclerotic, the cornea, iris, pupil, and the cut end of the optic nerve. c. Divide the eyeball into an inner and an outer hemi- sphere by a rapid cut with scalpel or scissors taken vertically, midway between the cornea and the optic nerve, through the vitreous chamber. Place them both in a watch-glass or small dissecting-dish, under water, and examine with a lens (compare Fig. 57). In the inner hemisphere note the vitreous humour, retina, pigmented choroid, and blind spot or entrance of the optic nerve ; and in the outer hemisphere, the crystalline lens and the margin of the retina, or or a serrata. Sketch. Remove the lens, and notice the iris — continuous with the choroid, the pupil, and the aqueous chamber. d. Examine sections through the wall of the inner hemi- sphere of the eyeball, prepared as directed on p. 136, first 192 THE FROG CHAP, xi under the low and then under the high power of the micro- scope. Note : — 1. The cartilaginous sclerotic. 2. The choroid, enclosing pigment-cells and blood-vessels. 3. The retina (Fig. 58), composed of a number of layers : notice the pigment epithelium, the rod- and cone-layer and the various other layers of the retina, the innermost of which is composed of nerve-fibres continuous with the optic nerve. Sketch. The anatomy of the eye can be more easily made out by dissecting that of an ox or sheep, which is essentially similar to that of the frog, and directions for the examination of which will be given in Part II. (p. 556). III. Auditory organ. Notice again the tympanic membrane and tympanic ring, and then carefully cut away the former so as to expose the tympanic cavity. Observe the Eustachian tube, the fenestra ovalis, and the relations of the stapes, columella, and extra- columella (Fig. 10). The essential part of the auditory organ (membranous labyrinth) is very small in the frog, and can be more satis- factorily studied in a good -sized fish (e.g., dogfish or cod). Directions for the preparation of the membranous labyrinth of the former will be given at a later stage (p. 479), but if you have sufficient time and patience to dissect it out in the frog, proceed as follows : — Place the head of a large frog in nitric acid (about 10 per cent.) until the bone is dissolved. Wash well in water so as to remove the acid, and dissect away the muscle, etc., from the auditory capsule until the latter is thoroughly exposed. Then with a sharp scalpel slice away the roof of the capsule until the cavity it contains is seen. Proceed now with great caution, removing the cartilage and decalcified bone, bit by bit, until the cavity is sufficiently enlarged to bring the membranous labyrinth into view (compare Figs. 10 and 59). Observe the utriculus, sacculus, otoliths, and the three semicircular canals with their ampulla?. Sketch. CHAPTER XII THE FROG (continued) : REPRODUCTION AND DEVELOPMENT So far we haVe considered those parts and organs of the frog which have to do with its welfare as an indivi- dual. We have now to consider the organs which are connected with the welfare of the frog as a race, that is, with the propagation of its kind. The position of the reproductive organs has already been seen (pp. 23 and 25) : they must now be examined in more detail. The essential part of these organs in each sex is a pair of bodies known as gonads, called in the male spermaries or testes, and in the female ovaries. Reproductive Organs of the Male. — The spermaries (Fig. 3, r. spy, Fig. 5, spy, and Fig. 7, is) are a pair of ovoid bodies, each attached by a fold of the peri- toneum to the corresponding kidney, and having con- nected with it a fat-body (cp. ad). From the inner margin of each spermary spring a number of delicate tubes, the efferent ducts (Fig. 61, q), which run in the fold of the peritoneum to the kidney. Entering this organ near its inner edge, they open into a longitudinal tube (L) from which transverse tubes pass horizontally across the kidney to open into the ureter (Ur). The milt, or spermatic substance (p. 9). is thus carried off |by the same duct as the urine ; the ureter in the male is PRACT. ZOOL. 193 O 194 THE FROG therefore often called the urinogenital duct. On the outer side of the ureter, and communicating with it by numerous short ducts, is a glandular body, the seminal vesicle (Figs. 3 and 7, vs.sm), which serves to store up the spermatic substance. The spermary itself contains a narrow, irregular, central cavity, from which the efferent ducts proceed TJr FIG. 61. — Spermary and kidney of "edible Frog" showing the relations of the 1 efferent ducts (x about 3 ; semidiagrammatic) . C, transverse tubes in kidney ; Ho. spermary ; L. longitudinal tube ; N. kidney ; q. efferent ducts of spermary; Ur. ureter Wiedersheim's Comp. Anatomy.) . . (urinogenital duct). (From and into which open a number of short tubes or crypts (Fig. 62, A). These are lined with epithelium (t. e), the cells of which divide and subdivide, forming groups of smaller cells. Each of the latter undergoes a remark- able change, becoming converted into a rod-like body, produced into a long thread, which performs lashing movements, very much like those of the cilia in ciliated epithelium. These bodies are called sperms or sperma- XII REPRODUCTIVE ORGANS 195 tozoa (Fig. 62, A, sp, and B) ; in spite of their peculiar form, they are cells, the rod-like portion, or head, being the nucleus, and the delicate vibratile part, or tail, the protoplasm. In the breeding season the cavities of the testes are full .of sperms floating in a fluid. Thus the spermatic fluid, like the blood, owes its distinctive character to the cells floating in it. FIG. 62. — A, transverse section of a crypt of the Frog's spermary. (x about 350.) B, stages in the development of the sperms. ( x 500.) sp. bundles of sperms ; i.e. germinal epithelium. (A, after Blomfield ; B, after Howes. Reproductive Organs of the Female. — Each ovary (Fig. 4, /. ovy) is a greatly folded sac with thin walls and a large cavity divided up by partitions. It is attached to the dorsal body- wall by a fold of peritoneum. As we have seen (p. 25), its surface is studded all over with little rounded projections, each of which is an ovisac, and contains an egg. The egg or ovum (Fig. 63) is a large globular cell with a clear nucleus (nu) containing numerous nucleoli (nu'), and having its proto- plasm (pr) full of yolk-granules — grains of proteid material which serve as nutriment for the growing embryo. It O 2 196 THE FROG CHAP. becomes covered with a delicate egg-membrane. By the time the egg is mature a superficial deposit of pigment takes place over one hemisphere. In the young condition all the epithelial cells forming the walls of the ovary are alike, but as the organ reaches maturity, certain of them (o) enlarge and give C.I b.v. FIG. 63. — Transverse section of Frog's ovary. (X 30.) b. v. blood-vessels ; c. t. connective-tissue ; ep. outer layer, and ep' inner layer of epithelium ; ep", outer layer of ovisac, continuous with ep' ; ep'", germinal I epithelium, derived from ep. ; ep'*', follicular epithelium, derived from ep. ; nu. nuclaus of ovum ; nu't nucleoli ; o. young ovum ; pr. protoplasm of ovum I containing yolk-granules. (After Marshall.) rise to the ova, while others form an investment or j follicle (ep""} for each ovum. The oviduct (Fig. 4, 1. ovd, r. ovd), as you have seen, I is a long and greatly convoluted tube lying above or I dorsal to the ovary. Its anterior end narrows consider- I ably, runs parallel with the gullet, passes to the outside I of the root of the lung, and then opens into the coelome j by a small aperture (r. ovd'). The greater part of the oviduct is about as wide as the small intestine, and is I thick-walled and lined with gland-cells, which secrete I xii FERTILISATION 197 the jelly surrounding the eggs when laid (p. 9). Posteriorly it suddenly dilates into a wide, thin-walled chamber (r. ovd"} which opens into the dorsal wall of the cloaca. Notice that there is thus no connection between the generative organs and the kidneys in the female such as occurs in the male, the ureters serving as renal ducts only. In the breeding season the ovisacs burst and set free the eggs into the coelome, whence by some means or other they find their way into the small openings of the oviducts, and so into these tubes, when each becomes surrounded by a little sphere of jelly secreted by the gland-cells. Passing down the oviducts the eggs accu- mulate in the dilated extremities, which they distend enormously, so that, just before laying, the abdomen of a female frog is nearly filled by these two great egg- reservoirs ; the ovaries, having lost so many of their eggs, are correspondingly reduced in size, and all the other organs are squeezed out of place. Fertilisation. — The eggs are now laid, and imme- diately the spawn is passed from the oviducts of the female into the water, the male sheds over them a quan- tity of spermatic substance (p. 9). The sperms, swim- ming actively through the water, enter the spheres of jelly and come into contact with the eggs. A single sperm then penetrates the egg-membrane of an ovum, loses its tail, and its head coming into contact with the nucleus of the egg, fuses or conjugates with it, so that a single nucleus is formed by the union of the egg- nucleus with the sperm-nucleus. We may speak of conjugating cells in general as gametes, the sperm in this case being the male gamete, and the ovum the female gamete, the body formed by the fusion of two gametes being known as a zygote. This process is known as fertilisation or impregnation. 198 THE FROG CHAP, xn Without it, as we have seen, the egg is incapable of development ; after it has taken place, the egg — or more strictly, the oosperm, since it is now an ovum plus a sperm — is potentially a young frog, since, if left undisturbed in water, it will in course of time give rise to a tadpole, which in its turn will change into a frog. It must be remembered in the first place that the fertilised egg is a single cell, comparable with a blood- corpuscle or an epithelial cell. It is, however, peculiar in two respects ; first in having its nucleus derived in part from a sperm, so as to contain matter from both the male and the female parent ; and secondly in having its protoplasm distended with yolk-granules to such an extent that, instead of being a minute body visible only under the microscope, it is easily visible to the naked eye. The yolk is not equally distributed : on one hemi- sphere it is less abundant than elsewhere, and it is this more protoplasmic hemisphere which is superficially blackened by a layer of pigment, and which always floats upwards in the water when the egg is laid. Segmentation of the Oosperm. — Almost directly after being laid and fertilised the egg undergoes a remark- able change. A furrow appears all around it, as if made with a blunt instrument, and deepening gradually, at last divides the oosperm into two hemispheres in con- tact with one another by their flat faces (Fig. 64, A). The examination of sections shows that this process is preceded by the division of the nucleus into two ; its final result is the division of the originally one-celled egg into two cells. Now if you refer to Chapter VIII, you will be reminded of the fact that the epithelial cells of the skin multiply by a similar process of simple fission, or division into two ; the nucleus in each case dividing first, and afterwards the protoplasm. FIG. 64. — Development of the Frog. A— F, segmentation ; G, overgrowth of ectoderm; H, I, establishment of germinal layers ; J, K, assumption of tadpole-form and establishment of nervous system, notochord, and enteric canal ; L, newly hatched tadpole, (x 10.) bl. cccl. segmentation cavity ; blp, blp', blastopore ; bri, br^, gills ; br. a. branchial arches ; e, eye ; eel. ectoderm ; end. endoderm ; ent. enteron /. br. fore- brain ; h. br. hind-brain ; m. br. mid-brain ; md. f. medullary fold ; md. gr. medullary groove ; mcs. mesoderm : mg. large lower cells ; mi. small upper cells ; nch. notochord ; n. e. c. neurentenc canal ; pcdm. proctodaeum ; pty. invagina- tion of ectoderm which will form the pituitary body ; ret. rudiment of rectum ; sk. sucker ; sp. cd. spinal cord ; stdm. stomodasum ; t. tail ; yk. yolk-cells ; yk. pi. yolk-plug which fills the blastopore. (From Parker and HaswelTs Zoology. A — D, F H, and J from Ziegler's models ; E, I, K, and L after Marshall.) 199 200 THE FROG CHAP. The furrow which effects this division of the oosperm passes through both black and white poles, so that each of the two cells formed is half black and half white. Soon a second furrow is formed at right angles to tke first, being, like it, meridional, i.e., passing through the poles (Fig. 64, B). It divides what we must now call the embryo into four cells, each half black and half white. A third furrow is then formed, passing round the equator, but nearer the black than the white pole (C). It therefore divides the embryo into eight cells, four upper black and four lower white, the latter being obviously the larger. This process of division or segmentation of the oosperm continues, the black cells dividing more rapidly than the white, so that before long the embryo consists of a mass of cells, the polyplast, somewhat resembling a mulberry, a:id therefore often called a morula, one hemisphere being composed of small cells containing much proto- plasm, and little yolk, and externally pigmented (D — F, mi), and the other of larger cells containing little proto- plasm and much yolk, and not pigmented (mg). As segmentation goes on, a cavity appears in the interior of the embryo ; it is called the segmentation-cavity (E, bl. ccel), and is due to the fact that the work of segmenta- tion produces a waste of substance which there is at present no means of making good, so that for a timej while the size of the embryo remains the same, its bulk diminishes, some of its substance being used up ; in other words, it feeds on itself. Segmentation now proceeds to such an extent that the j black cells become too small to be seen except with aj lens of tolerably high magnifying power, so that with? the amount of magnification used in Fig. 64, G, thej black hemisphere shows no division into cells. At the] same time the black hemisphere gradually encroaches on XII DEVELOPMENT 201 the white until only a small circular space — the blastopore, filled by a mass of yolk-cells — the yolk-plug, is left uncovered (G, H, yk. pi). Development of the Chief Organs.— In the mean- time a second cavity appears in the interior of the embryo, and as it increases in size, the segmentation- cavity undergoes a proportional diminution and finally disappears. This new cavity, or enteron(Figs. 64, 1, and 65 mes FIG. 65. — Transverse sections through the body of a Frog embryo. A, with open medullary groove (compare Fig. 64, J). B, with closed medullary groove (compare Fig. 64, L.) - ( x about 30.) c^coel. coelome; ccel1, prolongation of ccelome into mesodermal segments (pr. v); ect. ectoderm ; end. endoderm; ent. enteron; mes, msd. mesoderm ; mes1,sotn. outer paristal, and mes2, spl. inner visceral layer of mesoderm ; md. f. medullary fold; md. gr. medullary groove; nch. notochord ; pr. v. mesodermal segment; sg. d. primary kidney duct (pronephric duct) ; sp. c. medullary cord; yk. yolk- cells. ( \fter Marshall.) ent), is the rudiment of the enteric canal : it begins to be formed as a narrow slit at the edge of the blastopore through which it at first communicates with the exterical but it soon becomes closed : at present there is neither mouth nor anus. By this time the cells of which the embryo is com- posed have assumed different forms and become arranged in a very definite manner. First of all there are several layers of small pigmented cells, derived from the black 202 THE FROG CHAP. cells of earlier stages, which cover the whole embryo with the exception of the yolk-plug : these constitute the ectoderm (Fig. 64, G, H, I, ect}. Forming the roof of the enteric cavity are other layers of cells derived from the yolk-cells of earlier stages and forming the endoderm (I, end) : the floor of the enteron is at present formed of unaltered yolk-cells. Between the ectoderm and the yolk-cells on the lower pole are several layers of small cells which gradually spread until they form a complete layer between the ectoderm and the endoderm : these constitute the mesoderm (I, mes), The ectoderm, endoderm, and mesoderm are known as the three embryonic tissues or germinal layers. While these processes have been proceeding, a longi- tudinal groove appears on the dorsal surface of the embryo : this is the medullary or neural groove, and is bounded by a pair of medullary folds (Figs. 64, H, J, and 65, md. gr, md. /). The medullary groove extends back- wards to the blastopore, which now marks its posterior end. As development goes on, the folds approach one another and unite, so as to convert the groove into a canal ; the cells lining this canal give rise to the central nervous system (Fig. 64, K ; compare Part II, Chapter XIII), and its cavity becomes the central canal of the spinal cord and the ventricles of the brain. The anterior end of the hollow medullary cord thus developed i enlarges to form three vesicles, one behind the other, and known respectively as the fore-brain, mid-brain, and hind-brain, each containing a ventricle. The fore-brain eventually gives rise to the diencephalon and to the cerebral hemispheres, produced in front into the olfactory lobes ; the mid-brain to the optic lobes and crura cerebri ; and the hind-brain to the medulla oblongata j and cerebellum. It will be seen (Fig. 65) that the! medullary groove is lined by ectoderm and that there- DEVELOPMENT 203 ore the whole central nervous system is ectodermic in >rigin ; and this is also true of the nerves. A longitudinal thickening of the* endoderm on the dorsal side of the enteron becomes constricted off as a solid rod of cells, lying between the medullary cord and the enteron. This is the notochord Figs. 64, K, and 65, nch) ; it forms the primary axial skeleton around which the vertebral column is subse- quently developed. The mesoderm is for some time a solid mass occupying all the space between the ectoderm and endoderm, so that there is no body-cavity. But at a later stage a cavity appears dividing the mesoderm into wo layers/ one in contact with the ectoderm (parietal ayer, Fig. 65, mes1, sow), the other with the endoderm visceral layer, mes2, spl) . The space between them is the ccelome (c, coel). The dorsal part of the mesoderm on ither side of the medullary cord and notochord becomes divided transversely, and gives rise to muscle-segments or myomeres (Figs. 64, L, and 65). The embryo soon begins to elongate in a definite direc- tion (Fig. 64, J) : its dorsal surface, still marked by the medullary groove, which now soon becomes closed, is slightly concave, its ventral surface very convex. The ^lastopore closes up, the yolk-plug becoming entirely covered by ectoderm ; but for a short time the medullary canal and enteron still communicate by the neurenteric canal (K, ri.e.c) . This soon disappears, and at the hinder end of the embryo a conical outgrowth forms the rudi- ment of the tail (t) . The opposite end is rounded, and on its ventral surface appears a little half-moon-shaped groove, the rudiment of the sucker by which the tadpole attaches itself to weeds (J and L, sk) ; this afterwards becomes subdivided into two (Fig. 66, C). Just above the sucker a depression — the mouth-pit or stomodceum (L, stdm) — makes its appearance, and is the 204 THE FROG CHAP. first indication of the mouth. A similar depression below the tail — the proctodceum (pcdm) — marks the anus : both at first are mere blind pouches and have no communication with the enteric cavity. The head- region is further marked by two pairs of vertical ridges separated by depressions : the ridges are the branchial or gill arches (J, br. a), and the depressions the branchial clefts. Further elongation takes place (Fig. 64, L), the head becomes distinctly marked off, the tail extends consider- ably beyond the anus, a thickening appears which marks the position of the eye (e), and a depression that of the ear (just above br2 in the figure) ; and from each branchial arch arises a little tuft, the rudiment of one of the external gills (br1, br2). In this condition the tad- pole is hatched : it is still unable to feed, the stomodseum not yet being in communication with the enteric cavity ; it is nourished therefore entirely by the yolk with which a large portion of the body is still filled. Up to the stage shown in Fig. 64, I, the cells of which the tadpole is composed, although distinguishable into ectoderm, endoderm, and mesoderm, are all morel or less similar : there are no muscles, no cartilages, bones, or connective-tissue. But shortly after the stage! referred to, the permanent tissues begin to be formed : I the outer ectoderm cells take on the form of epiderm, ; the endoderm cells become the epithelium of the enteric canal, and from offshoots of the latter — all of course lined by endoderm — are formed the lungs, the liver, the : ! pancreas, and the urinary bladder. The mesodermi undergoes much more extensive changes giving rise to the connective-tissue, cartilage, bone (aftei ! metamorphosis), and muscle, as well as to other parts— i.e., to by far the greater portion of the permanent tissues ! i In certain places the embryonic mesoderm cells, hitherto j xii DEVELOPMENT 205 in close contact, separate from one another and become Dranched, while between them appears intercellular substance with fibres crossing one another in various directions : in this way the connective-tissue of the adult 3ody is produced. The various parts of the skeleton first arise by the conversion of portions of the mesoderm into cartilage. The cells retreat from one another and between them a clear substance, the matrix, makes its appearance. For a considerable time almost the whole skeleton consists of cartilage, but after metamorphosis much of this tissue is replaced by bone developed from a layer of cells on :he inner surface of the perichondrium. The bones thus iormed in connection with cartilage are replacing bones : investing bones arise in connective-tissue which has no direct relation to the cartilage (compare p. 43). In the place where the voluntary muscles are to appear the mesoderm cells elongate, their nuclei multiply by fission, and their protoplasm gradually becomes con- verted into the striated substance so characteristic of the adult muscular tissue. These examples will suffice to illustrate the fact, which further study would show to be true, that all the perma- nent tissues are either — like epithelium — formed entirely of actual cells, or are — like connective-tissue and striped muscle — derived from cells. The entire embryo in its earliest condition — the oosperm stage — is a single cell, which multiplies repeatedly by simple fission, forming a group of cells : and these, by assuming various forms and undergoing various changes, give rise to all the complex tissues— differing from one another both structurally and functionally — of the adult animal. So that every cell, fibre, or what not in the frog is a linear descendant, through repeated fission, of the oosperm ; and the oosperm itself is the product of the fusion of two cells, 206 THE FROG ci one — the ovum — derived from the female, the other — the sperm — from the male parent. Thus in passing from the oosperm to the adult animal there is a gradual structural or morphological differentiation accompanied by a differentiation of function or division of physiological labour. The expression " division of physiological labour " was invented by the great French physiologist, Henri Milne- Edwards, to express the fact that a sort of rough corre- spondence exists between lowly and highly organised animals and plants on the one hand, and lowly and highly organised human societies on the other. In primitive communities there is little or no division of labour : every man is his own butcher, baker, soldier, doctor, etc., there is no distinction between " classes" and " masses/' and each individual is to a great extent independent of all the rest . Whereas in complex civilised communities society is differentiated into politicians, soldiers, professional men, mechanics, labourers, and so on, each class being to a great extent dependent on every other. Development of other Organs — Structure and Metamor- phosis of the Tadpole — Metamorphosis. — A sketch of the further development of the tadpole and of its metamor- ' phosis has already been given (pp. 9-11, Fig. i), but it is now necessary to add a few details to those already mentioned. A third pair of branchial arches appear behind the two already noticed, and on each a third external gill. The first two pairs increase greatly in size and all the gill become branched (Fig. 66, A), The four branchia clefts communicate with the pharynx, and a current of water enters the mouth and passes out by these clefts, thus providing the gills with a constant supply of xii DEVELOPMENT 207 aerated water for purposes of respiration. A thin median fold of the skin, the tail-fin (Fig. i), arises all round the tail, both above and below, and the tail now forms a powerful swimming organ. The stomo- daeum and, at a still earlier period, the proctodaeum (p. 204) open into the enteric cavity (Fig. 66, B — E), so that there is now a complete enteric canal : it grows much faster than the body generally, and becomes coiled like a watch-spring (D, E). The mouth is very small, and is bounded by lips beset with little horny projections or papillae and provided with a pair of horny jaws (C — E), with v/hich the tadpole now browses upon the water- weeds which form its staple food. It is on account of this diet that the intestine is of such great proportionate length : vegetable diet contains less nourishment, bulk for bulk, than animal, and a longer intestine is required to extract all the nutriment from it. Soon the external gills show signs of shrivelling, and on the inner portions of the branchial arches internal gills (Fig. 66, D1) are developed, like those of a fish. A fold of skin appears on either side in front of the gills and gradually grows backwards, covering the branchial clefts and the external gills, which latter soon disappear entirely ; this is the operculum or gill-cover (B, C) ; it is I similar to that of an ordinary fish, such as a cod or a perch, except that it is not supported by bone. The current of water from the pharynx now, of course, makes its final exit by a wide cleft between the edge of the operculum and the flank. The operculum gradually ! extends, and its free edge unites with the wall of the body posterior to the gills, so that the originally large aperture through which the water from the pharynx passes is much reduced. After some time a complete union of the operculum with the flank takes place on the right side, the opercular aperture becoming closed : 208 THE FROG CHAP. on the left side the spout-like aperture remains open for some time longer, and the water from both sides passes out through it (D). All this time the tadpole is to all intents and purposes a fish ; apart from the possession FIG. 66. — Stages in the later development of the Tadpole, (x 7|.) A, stage with external gills ; B, stage showing the formation of the operculum ; C, later stage, in which the external gills are disappearing ; D, stage with in- ternal gills and budding limbs ; D1, dissection of same stage to show the heart, gills, and lungs. E, later stage, in which the limbs are further differentiated. (After Howes.) of gills and a tail-fin, in the structure of the circulatory organs and various other parts it is more like a fish than a frog. The lungs (D1) become functional, and the tadpole is now truly amphibious, rising periodically to the surface, to breathe air. At a later stage, however, the gills ii METAMORPHOSIS 209 radually disappear, the single opercular aperture closes, nd thenceforth respiration is purely aerial. In the meantime the limbs are developed. The hind- mbs appear as little rounded buds, one on either side f the root of the tail (D) : they gradually elongate and ivide into thigh, shank, and foot (E). The fore-limbs ppear beneath the operculum and are therefore hidden first (E) ; at a later stage each divides into upper rm, fore-arm, and hand, and emerges from its conceal- ment. As the limbs increase, the tail undergoes a regressive shrinking ; its tissues become, as it were, igested, and are carried off by the blood ; so that for a Line the metamorphosing tadpole is nourished largely y its own tail. The mouth widens, the horny jaws and papillae dis- ppear, and teeth are formed. The suckers vanish, and le intestine not only begins to grow less rapidly than body, but even becomes reduced in length and loses s spiral arrangement, while vegetable is exchanged 210 THE FROG CHAP. as the crystalline lens of the eye ; and also the epithelium lining the mouth (stomodaeum) and outer part of the cloaca (proctodaeum) . The endoderm gives rise to the epithelium lining the enteric canal and its various offshoots, including the lungs, urinary bladder, gastric glands, bile and pancreatic ducts, as well as the glandular cells of the liver and pancreas ; and to the notochord. From the mesoderm are developed the various parts which are situated between the ectoderm and endoderm with the exception of the notochord, viz., the connective-tissue, cartilage, bone, striped and un- striped muscles, circulatory organs, and peritoneum, as well as the urinary and reproductive organs and the accessory parts of the sensory organs. PRACTICAL DIRECTIONS Organs of Reproduction. Open a frog in the usual way : cut through the gullet, rectum, and mesentery, and remove all the digestive organs. In the female, take especial care not to injure the roots of the lungs in severing the gullet. One of the specimens you have already dissected and kept in the preservative fluid should still contain the urinogenital organs intact in the case of the male : in the female, the ovaries and greater part of the oviducts will have been removed, but the relations of the two ends of each oviduct can still be made out. Examine the reproductive organs, which will now be freely exposed, under water — or in the case of the female, in i per cent, salt solution. I. Male Organs. (Figs. 3, 5, 7, and 61.) 1 . Notice again the spermaries or testes, each supported by a fold of peritoneum connecting it with the corresponding kidney ; the efferent ducts ; the branched fat-body ; the ureter (urinogenital duct] ; and the seminal vesicle. Sketch after examining the cloaca (see next page) . 2. Tease up a bit of the spermary of a recently-killed frog in salt-solution, and notice the form and movements of the sperms (Fig. 62, B). Sketch. xii PRACTICAL DIRECTIONS 211 3. Examine under the microscope a transverse section of the spermary, prepared as directed on p. 136, and note (Fig. 62, A) the numerous seminal tubes or crypts which open into it. Under the high power, observe the epithelial cells (germinal epithelium) lining the tubes, and their subdivision into smaller cells, which eventually give rise to the sperms, the tails of which project freely into the cavities of the tubes. Sketch. II. Female Organs. 1. Notice again — (a) the ovaries, varying in size and appearance according to the time of year, and each sus- pended by a fold of peritoneum : they are studded all over with ovisacs, each of which contains an egg or ovum, pig- mented when ripe ; and (b) the oviducts (Fig. 4). Trace the convoluted and, glandular middle portion of the oviduct forwards, and make out the anterior thin-walled portion, running parallel to the gullet and opening into the ccelome by a small aperture at the base of the lung ; then trace the middle portion backwards, and notice the thin-walled, dilated, posterior portion, which opens into the cloaca. Sketch after examining the cloaca (see below). If a female frog is examined in the spring, just before the eggs are laid, the ovaries will be seen to be reduced in size, and the posterior portion of the oviduct filled with eggs, each surrounded with a gelatinous coat secreted by the middle portion. 2. Examine under the microscope a section of the ovary, prepared as directed on p. 136, first with the low, and then with the high power (Fig. 63). Note the epithelial cells (including the germinal epithelium) forming the walls of the ovary, and the ovisacs in different stages of development. Each ovisac contains an ovum, surrounded by follicle-cells. In the ova, note the protoplasm and its contained yolk- granules, the nucleus, and the nucleoli. Sketch. Cloaca. Carefully cut through the pelvic symphysis in the middle line with a scalpel and press apart the two innominate bones, so as to expose the ventral surface of the cloaca. Inflate from the vent, and note that the large intestine is continuous with the cloaca and that the urinary bladder opens into it on the ventral side. The entire urinogenital apparatus, together with the cloaca, may now be removed from the body, first cutting through the skin round the vent. Pin down under water, P 2 212 THE FROG CHAP. insert the small scissors into the vent, and slit open the cloaca very slightly to one side of the middle line, so as not to injure the connection between it and the bladder. Notice the openings into the bladder and large intestine respectively, and also : — In the male, the apertures of the ureters (urinogenital ducts) , situated on two small papillae, lying close to one another on the dorsal side of the cloaca : insert a bristle into the ureter. Sketch. In the female, the two small apertures of the ureters, and, just in front of these, the two large apertures of the oviducts, all situated on the dorsal wall of the cloaca. Insert bristles into the ureter and oviduct. Sketch. Impregnation and Development. In the early spring (end of February or beginning of March), look about in ponds and ditches for frogs which have taken to the water for the purpose of laying eggs. This process can be watched more conveniently by catching a few frogs, male and female, and putting them in a large vessel of water or aquarium. If you have been unsuccessful in procuring frogs or frogs' spawn, toads will do equally well : their eggs are laid a few weeks later, and are arranged, each surrounded by its gelatinous envelope, not in clumps, like those of the frog, but in a string, like the beads on a necklace. For purposes of observation, the spawn is best kept in a glass vessel, together with some water-weeds. Put only a small quantity of spawn in one vessel ; if the water begins to get foul, change it at once. Examine the eggs every day with a magnifying-glass — or better, a low power binocular microscope — as development proceeds, and keep some of the tadpoles alive until metamorphosis takes place in June. If you wish to see some of the stages in the development of the frog during other times of the year, you must obtain some preserved eggs and embryos, and, if possible, you should also examine the series of wax models, made on an enlarged scale, which are to be seen in most zoological museums. The following are some of the more important things to be noticed. (Sketch a series of stages) : — Before hatching. I. The unsegmented oosperm. black above, and white below. xii PRACTICAL DIRECTIONS 213 II. Early stages in segmentation, during which the individual cells can be made out by means of a lens. (Fig 6^ A— F.) III. Later stages in segmentation, during which the individual cells cannot be distinguished without the aid of a microscope. Note the gradual enclosure of the white by the black hemisphere, until only a small rounded yolk-plug is left, filling in an aperture — the blastopore — in the black layer or ectoderm, as it is now called. The yolk-plug is continuous with a mass of yolk-cells, enclosed by the ecto- derm, from which the endoderm and mesoderm are developed. (Fig. 64, G— I.) IV. The gradual flattening and elongation of the embryo, and the position of the blastopore at the posterior end of the dorsal region ; • the formation of the medullary folds and groove along the dorsal side, and the closure of the blastopore. (Fig. 64, H-J.) V. The closure of the medullary groove, the formation of the head with its suckers and of the tail with its tail-fin ; as well as the appearance of the eyes, ear-sacs, branchial arches, external gills, and the involutions for the mouth and anus, which, however, do not open until after the tadpole is hatched. (Fig. 64, K— L.) After hatching. VI. The appearance of the horny jaws and papilla, and the further development of the head, external gills, and tail. (Fig. i, i—2, and Fig. 66, A, B.) VII. The formation of the operculum, the closure of the right opercular aperture, and the disappearance of the external gills. The bud-like rudiments of the hind-limbs, and their further development. The fore-limbs remain for a long time hidden beneath the operculum. (Fig. i, 3 — 4, and Fig. 66, C — E.) Metamorphosis. VIII. The gradual change in the form and colour of the head and body ; the widening of the mouth and loss of the horny jaws and suckers ; the appearance of the fore- limbs from beneath the operculum, and the shrinking of the tail. (Fig. i, 5—8.) Dissection of tadpole. Pin down a full-sized tadpole under water in a small dis- secting-dish, with the ventral surface uppermost, inserting small pins through the tail only. Carefully dissect off the 214 THE FROG CHAP, xn ventral body walls, and note the coiled intestine, the heart, the internal gills, &c. (Fig. 66, D, D1, E.) Sketch. It is rather a difficult task, and requires much time, to prepare sections of the early stages of the frog-embryo in order to make out the formation of the ectoderm, endoderm, and mesoderm, the relations of the segmentation-cavity and enteron, and the development of the central nervous system and notochord (Figs. 64, I, K, and 65) ; and directions for the practical study of development are given in Part II, Chapter XIII (p. 615). But if you wish to make the attempt at this stage, proceed as follows : — Take a few eggs every day from the time they are laid until the tadpoles are hatched : place these on a tile or piece of glass, and with a pair of needles dissect off the gelatinous covering. Then carefully transfer the segment- ing eggs or embryos into corrosive sublimate for a quarter to half an hour, and after washing in running water, put them into weak and then strong alcohol ; stain, imbed, and cut sections as directed on p. 136. CHAPTER XIII THE FROG (continued] : MEANING OF THE TERM SPECIES — THE PRINCIPLES OF CLASSIFICATION — EVOLUTION — ONTOGENY AND PHYLOGENY — HEREDITY AND VARIATION — STRUGGLE FOR EXISTENCE — NATURAL AND ARTIFICIAL SELECTION — ORIGIN OF SPECIES THE frog which you have been studying is the only one commonly found in Great Britain. Another kind, . very similar to it, is, like the common frog, abundant in Germany and other parts of the European continent, but is rare in this country, only occurring in some places in the eastern counties ; and various other frogs, differing in certain minor respects from these are found in different parts of the world. This fact is expressed in the language of systematic zoology by saying that there are various species of frogs, belonging to the same genus, which are distinguished from one another by certain definite characteristics as regards form, structure, and colour. According to the system of binomial nomenclature introduced by Linnaeus, each kind of animal receives two names — one, the generic name, common to all the species of the genus ; the other the specific name, peculiar to the species in question. Both generic and specific names are Latin in form, and are commonly Latin or Greek in origin, although frequently modern names of 215 216 THE FROG CHAP. persons or places with Latinised terminations are em- ployed. In giving the name of an animal, the generic name is always placed first, the specific name following it, and being written as a rule with a small letter. Thus the common English frog is called Rana temporaria, and the continental form referred to above, often spoken of as the edible frog, Rana esculenta. You will probably have noticed certain differences in colour and markings in the different individual frogs you have examined, and it is matter of common observa- tion that no two individuals of a species are exactly alike. In the case of human beings and many of the more familiar animals this is very apparent to everyone : in other cases a more careful examination of the indi- viduals is necessary in order to tell them apart ; thus, for instance, the individuals in a flock of sheep appear all alike to the casual observer, but the shepherd can easily distinguish them from one another. These differences we designate individual variations, and it is often difficult to decide whether two kinds of animals should be con- sidered as distinct species, or as varieties of a single species, and no universal rule can be given for determin- ing this point. Among the higher animals, mutual fer- tility is a fair practical test, the varieties of a species (e.g., common pigeon, fowl, dog, horse) usually breeding freely with one another and producing fertile offspring, while distinct species usually do not breed together, or else produce infertile hybrids or mules. Compare for instance the fertile mongrels produced by the union of the various breeds of domestic dog with the infertile mule produced by the union of the horse and ass. But this rule is not without exception, and in the case of wild animals is, more often than not, impossible of applica- tion ; failing it, the only criterion of a " good species " is usually the presence of constant differences from xiii MORPHOLOGY AND PHYSIOLOGY 217 allied species, whereas if there is a complete series of graduations between two forms, they will be considered to form a single variable species. In the previous chapters it will have been evident that an animal may be studied from two chief points of view : first, from the point of view of its structure, and secondly from that of the functions performed by its various parts and the way in which these work together for the welfare of the whole. The branch of zoology dealing with the former is known as morphology, and with the latter physiology. It is evident that a knowledge of morphology is necessary as a preliminary to the study of physiology, and also that, as animals have to be distinguished from one another largely by structural characters, the founda- tions of a scientific zoology must be laid in morphology, which, as we have seen, deals not only with the external characters and the anatomy and histology of the adult animal, but also with the changes undergone during the development of the egg into the adult form, i.e., with embryology. Given a sound knowledge of the anatomy, histology, and embryology of animals, their classification may be attempted : that is, we may pro- ceed to arrange them in groups and sub-groups, each capable of accurate definition. In doing so we must be careful to distinguish between homologous parts, or those which correspond structurally (compare p. 39), and analogous parts, which have merely a similar function, and are of no value for purposes of classification. Thus the fore-limbs of a frog are homologous with— i.e., are formed on a similar plan to — the wings of a bird, although differing from them in function ; they are only analogous to the limbs of an insect, for though the function is similar in both, the structure is widely different. In the same way the wing of the bird is only analogous to that of an insect. 218 THE FROG CHAP. The general method of classification employed by zoologists may be illustrated by reference to the different kinds of frogs already referred to in explanation of the terms genus and species. The common frog (Rana temporaria) is distinguished from the " edible frog" (R. esculenta) by its smaller size and brown colour, by the large dark patch in the tym- panic region, and by the rudimentary character of a pair of inflatable vocal sacs at the sides of the head in the male, which are very large and highly distensible in R. esculenta. On the other hand, these two frogs agree with one another and with all the other species of the genus Rana, in having teeth on the upper jaw, and in not having the transverse processes of the sacral vertebra dilated. Compar- ing all these frogs with our English toads, of which there are two species, the common toad and the rarer ''natterjack/' we find that in them the skin is comparatively dry and covered with glandular warts, the hind-limbs are proportionally shorter, there are no teeth, and the transverse processes of the sacral vertebra are more or less dilated. These differences are so great as not only to necessitate placing the toads in another genus (Bufo), but also to relegate the frogs just mentioned to one family — the Ranidcz (including many genera), and the toads to another — the Bufonidcz. All frogs and toads, however, agree with one another in having no tail in the adult, while the trunk is relatively short and broad, and the hind-limbs are longer than the fore-limbs. They therefore differ fundamentally from such animals as the common English newts and the salamanders, which retain the tail throughout life, and in which the fore- and hind-limbs are of approxi- mately equal size. The differences here are obviously far greater than those between the families mentioned xin CLASSIFICATION 219 220 THE FROG CHAP. Vertebrata, the many-legged, armoured forms in the phylum Arthropoda. Similarly, soft -bodied animals with shells, such as mussels and snails, form the phylum Mollusca ; various worms, such as the earthworm, the phylum Annulata ; polypes and jelly-fishes the phylum C&lenteraia ; the simplest animals, mostly minute, such as Amoeba, the phylum Protozoa. Finally, the various phyla recognised by zoologists together con- stitute the kingdom Animalia. Thus the animal kingdom is divided into phyla, the phyla into classes, the classes into orders, the orders into families, the families into genera, and the genera into species ; while the species themselves are assem- blages of individual animals agreeing with one another in certain definite characteristics. It will be seen that the individual is the only term in the series which has a real existence ; all the others are mere groups, formed, more or less arbitrarily, by man. Thus the zoological position of the common frog is expressed as follows : — Kingdom — ANIMALIA. Phylum — VERTEBRATA. Class — AMPHIBIA. Order — ANURA. Family — Ranidce. Genus — Rana. Species — temporaria . Let us now briefly consider some of the reasons which have led zoologists to adopt the system of classification now in general use. It is obvious that there are various ways in which animals may be classified, and the question arises — Which of these, if any, is the right one ? Is there any xiii CLASSIFICATION 221 standard by which we can judge of the accuracy of a given classification, or does the whole thing depend upon the fancy of the classifiers, like the arrangement of books in a library ? In other words, are all possible classifications of living things more or less artificial, or is there such a thing as a natural classification ? .Suppose we were to try to classify all the members of a given family — parents and grandparents, uncles and aunts, cousins, second cousins, and so on. Obviously there are a hundred ways in which it would be possible to arrange them — into dark and fair, tall and short, curly-haired and straight-haired, and so on. But it is equally obvious that all these methods would be purely artificial, and that the only natural way, i.e., the only way to show the real connection of the various members of the family with one another, would be to classify them according to blood-relationship ; in other words, to let our classification take the form of a genealogical tree. There are two theories which attempt to account for the existence of the innumerable species of living things which inhabit our earth : the theory of special creation and the theory of evolution. According to the theory of creation, all the individuals of every species existing at the present day — the tens of thousands of dogs, frogs, oak-trees, and what not — are derived by a natural process of descent from a single individual, or from a pair of individuals — in each case precisely resembling, in all essential respects, their existing descendants — which came into existence by a process outside the ordinary course of nature and known as creation. On this hypothesis each species of frog is derived from a common ancestral pair which came into existence, independently of the progenitors of all the 222 THE FROG CHAP. other species, at some previous period of the earth's history. Notice that on this theory the various species of frogs are no more actually related to one another than is either of them to a newt, or, for the matter of that, to man- The individuals of any one species are truly related since they all share a common descent, but there is no more relationship between the individuals of any two independently created species than between any two independently manufactured chairs or tables. The words affinity, relationship, &c., as applied to different species are, on the theory of creation, purely metaphorical, and mean nothing more than that a certain likeness or community of structure exists ; just as we might say that an easy chair was more nearly related to a kitchen chair than either of them to a three-legged stool. We see, therefore, that on the hypothesis of creation, the varying degrees of likeness and unlikeness between the species receive no explanation, and that we get no absolute criterion of classification : we may arrange our organisms, as nearly as our knowledge allows, accord- ing to their resemblances and differences, but the relative importance of the characters relied on becomes a purely subjective matter. According to the rival theory — that of Descent or Organic Evolution, with which the name of Darwin is inseparably connected — every species existing at the present day is derived* by a natural process of descent from some other species which lived at a former period i of the world's history. If we could trace back from generation to generation the individuals of any existing; species, we should, on this hypothesis, find their charac- ( ters gradually change, until finally a period was reached at which the differences were so considerable as to xin EVOLUTION 223 necessitate the placing of the ancestral forms in a different species from their descendants at the present day. And in the same way, if we could trace back the species of any one genus, we should find them gradually approach one another in structure until they finally converged in a single species, differing from those now existing, but standing to ail in a true parental relation. It will be seen that, on this hypothesis, the relative likeness and unlikeness of the various species of frogs are explained as the result of their descent with greater or less modification or divergence of character from the ancestral form : and that we get an arrangement or classification in* the form of a genealogical tree, which, on this hypothesis, is a strictly natural one, since it shows accurately the relationship of the various species to one another and to the parent stock. So that on the theory of evolution, a natural classification of any given group of allied organisms is simply a genealogical tree. Now it is evident that the only way in which we could be perfectly sure of an absolutely natural classi- fication of the species of any kind of animal — the frog, for example — would be by obtaining specimens as far back as the distant period when the genus first came into existence. Forming part of the solid crust of the earth are a series of sedimentary or stratfad rocks, and the researches of geologists have shown that these present a general order of succession, the lowest, when undisturbed, being in every case older than the more superficial layers. Imbedded in these rocks are found the remains of various extinct animals in the form of what are called fossils ; and it might perhaps, on first considering the subject, be supposed that, had a process of evolution taken place, we ought to be able to find in the rocks 224 THE FROG CHAP. belonging to the various geological formations a com- plete series of animal remains, representing all the stages in the evolution of the highest from the lowest forms. But owing to various causes which we cannot further consider here, the record of the succession of life on the globe is very imperfect and incomplete in the case of any individual species ; but the evidence furnished by paleontology — as the study of fossils is called — is very important in supplying proofs that there has been a gradual evolution of the higher from the lower forms. Further evidence in the same direction is furnished by morphology and embryology, as will be more apparent to you when you have studied a number of other kinds of animals. You have, however, seen that the frog begins life as a single cell, and that it is possible to trace a series of modifications which gradually convert the unicellular oosperm into a tadpole — which is to all intents and purposes a fish — and that the tadpole subsequently undergoes metamorphosis into the more highly organised frog. According to the theory of recapitulation, these facts indicate that the frog repeats, during its single life, the series of changes passed through by its ancestors in the course of ages. In other words, ontogeny, or the evolution of the individual, is, in its main features, a recapitulation of phylogeny, or the evolution of the race. At the same time you must bear in mind that it is not always an easy matter to determine which characters are of phylogenetic i| significance, and which have been secondarily acquired <| owing to various causes. To take an example : the horny jaws of the tadpole might be taken to indicate j that frogs were descended from ancestors with horny jaws and without teeth, though in all probability these ORIGIN OF SPECIES 225 organs have been secondarily acquired as adaptations in connection with the habits of the tadpole, and have, therefore, unlike the gills, for instance, no ancestral significance. It is obvious that the evolution of one species from another presupposes the occurrence of variations in the ancestral form. As we have seen, such individual varia- tion is of universal occurrence. This may be expressed by saying that heredity, according to which the offspring tends tc resemble the parent in essentials, is modified by variability, according to which the offspring tends to differ from the parent in detail. If, from any cause, any well-marked individual variation is perpetuated, there is produced what is known as a variety of the species, and according to the theory of the origin of species by evolution, such a variety may, in course of time, become a new species. Thus a variety is an incipient species, and a species is a (relatively) permanent variety. One other important factor in evolution must be briefly referred to before concluding this chapter. In order that every species of animal and plant may flourish f certain conditions are necessary. Thus the frog requires, for example, a moist place to live in, and water in which to lay its eggs. For spots presenting the necessary favourable conditions, there is constantly going on a competition between individuals of one species and between the members of different species. The nature of this struggle is well seen when a piece of garden ground is allowed to run to waste. Its surface is soon overgrown by weeds of many kinds, which kill out nearly all the original garden-plants by depriving them of light and food. By and by the more hardy weeds exterminate and replace such weaker forms as may first PRACT. ZOOL. Q 226 THE FROG CHAP. have obtained a footing, till an entirely new set of weeds may take the place of those that first appeared. A struggle for existence goes on on all sides among animals. To begin with, before there is any struggle for existence in the strict sense, there is — particularly in animals which, like the frog, produce eggs in great numbers annually — a very great indiscriminate destruc- tion of ova and young embryos. Only a few of them reach maturity ; a large proportion are destroyed at one stage or other, some failing to reach a spot favourable for their further development, others becoming the food of other animals. But such of the young as are less adapted to escape the various dangers to be encountered, and less fitted to procure the necessary food, are more likely to be destroyed. This is one phase — and the most important, perhaps, of all — of the struggle for existence amongst animals. But there is also a struggle for existence, not only between individual animals of the same kind, but between animals of different kinds. This struggle, in so far as it relates to the competition for food and shelter, is more severe between nearly related species ; for in such a case the food and the favourable conditions required are the same, or nearly so, in the two competitors. Again, a struggle for exist- ence of a constant and severe kind also goes on between carnivorous animals and the animals on which they prey — e.g., between the frog and the insects and worms on which it feeds, and between the snake and the frog ; a* struggle in which the defensive qualities of the prey — such as swiftness, power of eluding observation, or of resisting attack — are opposed to the predatory powers of the attacker. There can be little doubt that, in the long run, such individuals will survive as are best fitted to cope with xm NATURAL SELECTION 227 the conditions to which they are subject. According to Darwin's theory of natural selection, it is assumed that such surviving individuals would transmit their special properties to their progeny, and there would thus be a gradual approximation towards a perfect adaptation of the species to its surrounding conditions by virtue of this " survival of the fittest/' Let us suppose he conditions to change. Gradual alterations in climate and other conditions are known to take place, owing to subsidence or elevation of the land. But conditions might be changed in many other ways : some animal or plant previously used as food might become exterminated, or a new enemy might find its way into the district inhabited by the species. Then such individuals as presented variations which enabled them better to cope with the new surroundings would have the advantage over the others, and would have a much better chance of surviving and leaving progeny. The useful variations thus produced and transmitted to the progeny would tend to increase, generation after generation, until a form sufficiently distinct to be re- garded as a new species had become developed from the original one. That very different varieties of animals and plants can, and have been produced, in a comparatively short time, by man selecting those forms which tend to vary in a desired direction, is a well-known fact. All the breeds and varieties of our domestic animals have been produced by this process of artificial selection ; and " if man can by patience select variations useful to him, why, under changing and complex conditions of life, should not variations useful to Nature's living products often arise, and be preserved or selected ? " It does not come within the scope of the present work Q 2 228 THE FROG CHAP. xm. to discuss either the causes of variability or those which determine the elevation of a variety to the rank of a species : both questions are far too complex to be adequately treated except at considerable length, and anything of the nature of a brief abstract would only be misleading.* * At a later stage the student is recommended to consult Doncaster's Heredity in the Light of Recent Research (Cambridge Manuals of Science and Literature, 1910), which deals with the modern developments of these and other questions, such as Mendelian Heredity. PART II CHAPTER I AMOEBA: UNICELLULAR AND MULTICELLULAR ANIMALS FROM your study of the frog you will have learnt some of the more important facts with regard to the morphology and physiology of a comparatively highly- organised animal, and will have overcome a number of preliminary difficulties in acquiring a knowledge of zoo- logical terminology and technique. You will now, therefore, be in a better position to undertake a syste- matic and comparative examination of a number of other animals — some much less complicated, some more com- plicated, than the frog — working upwards from the simple to the complex forms. In doing so, you must continually bear in mind the deductions in connection with the theory of evolution referred to in the previous chapter. Let us begin with a very instructive animalcule belonging to the genus Amoeba. Amoebae are often found in the slime at the bottom of pools of stagnant water, adhering to weeds and other submerged objects. They are mostly invisible to the naked eye, rarely exceeding J of a millimetre (T^otn inch) in diameter, so that it is necessary to examine them entirely by the aid of the microscope. Though they can be seen and recognised with the low power, the high power is neces- sary for the accurate examination of their structure. FIG. 67. — A, Amoeba quarta, a living specimen, showing granular endoplasm sur- rounded bv clear ectoolasm. and several Dseudonods (t>sd}. some formed of (nu) present in this species (compare Fig. 73). (x 300.) CHAP, i AMCEBA 231 C, Amoeba proteus, a living specimen, showing large irregular pseudopods, nucleus (nu), contractile vacuole (c. vac), and two food-vacuoles (/. vac], each containing a small infusor (see Chapter III) which has been ingested as food. The letter a to the right of the figure indicates the place where two pseudopods have united to enclose the food- vacuole. The contractile vacuole in this figure is supposed to be seen through a layer of granular protoplasm, whereas in D, E, and G it is seen in optical section, and therefore appears clear, (x about 300.) D, an encysted Amoeba, showing cell-wall or cyst (cy), nucleus (nu), clear con- tractile vacuole, and three microscopic plants (diatoms) ingested as food. E, Amoeba proteus, a living specimen, showing several large pseudopods (psd), single nucleus (nu) and contractile vacuole (c. vac), and numerous food-particles embedded in the granular endoplasm. (x 330.) F, nucleus of the same after staining, showing a ground substance containing deeply-stained granules of chromatin, and surrounded by a distinct membrane. (x 1010.) G, Amoeba verrucosa, living specimen, showing wrinkled surface, nucleus (nu), large contractile vacuole (c. vac), and several ingested organisms, (x 330.) H, nucleus of the same, the chromatin aggregated in the centre, (x ioio.) I, Amoeba proteus, in the act of multiplying by binary fission, (x 500.) (From Parker's Biology : A, B, E, F, G, and H after Gruber ; C and I after Leidy ; D after Howes.) Examined under the high power (Fig. 67), the Amoeba appears like a little shapeless blob of jelly, nearly or quite colourless, and closely resembling a colourless blood-corpuscle or leucocyte of one of the higher animals (p. 105). The central part of it, or endoplasm, is granular and semi-transparent — something like ground-glass — while surrounding this inner mass is a border of perfectly transparent and colourless sub- stance— the ectoplasm. One very noticeable thing about the Amoeba is that, like the leucocyte, it is never of quite the same form for long together, owing to the protrusion of pseudopods (psd), by means of which it creeps along slowly. The occurrence of amoeboid movements is alone sufficient to show that it is an organism, or living thing, and no mere mass of dead matter. Moreover, it consists of protoplasm, and encloses a nucleus (C — H, nu) contain- ing chromatin and rendered more apparent by staining. The Amceba is therefore a cell (compare pp. 106 and no). A very important difference is thus at once seen between the Amoeba and the frog : the Amoeba is unicellular, i.e., it consists of a single cell, while the 232 AMCEBA CHAP. frog is, as we have seen, a multicellular animal, built up of innumerable cells which are incapable of an indepen- dent existence for any length of time. Besides the nucleus, there is another structure fre- quently visible in the living Amoeba and not present in the leucocyte. This is a clear, rounded space in the protoplasm (c. vac), which periodically disappears with a sudden contraction and then slowly reappears, its movements reminding one of the beating of a minute colourless heart. It is called the contractile vacuole, and consists of a cavity containing a watery fluid. We must now study the physiology of our animal- cule. First of all, as we have already seen, it is contractile : although it has no muscles, it can move about from place to place. Its movements, like the voluntary movements of the frog (pp. 7, 172), may occur without the application of any external stimulus, i.e., they are spontaneous or automatic ; or they may be induced by external stimuli — by a sudden shock or by coming in contact with an object suitable for^ food. Movements of this latter kind, like those resulting from the stimulation of the nerves in a brainless frog, are the result of the irritability of the protoplasm ; the animalcule is therefore both automatic and irritable, although it possesses neither nerves nor sense-organs. Under certain circumstances an Amoeba temporarily loses its power of movement, draws in its pseudopods, and becomes a globular mass around which is formed a thick, shell-like coat, called the cyst or cell-wall (Fig. 67, D, cy). This is formed by the protoplasm by a process of secretion (p. 130) ; its composition is not known ; it is certainly not protoplasmic, and very probably consists of some nitrogenous substance allied in composition to horn and to the chitin (see Chapters VI — VIII) which forms the external shell of crustaceans, insects, &c. I NUTRITION 233 The formation of the cyst is probably of great im- portance in preserving the animalcule from destruction by drought, so that should the pool in which it is living dry up, it may still remain alive, protected by its shell- like case, until the conditions for its active life are once more restored ; it escapes by the rupture of the cyst, sometimes having first divided into numerous young Amoebae. Very often an Amoeba in the course of its wanderings comes in contact with a still smaller organism of some kind or other. When this happens the Amoeba may be seen to extend itself round the lesser organism until the latter becomes §unk in its protoplasm in much the same way as a marble might be pressed into a lump of clay (Fig. 67, C, a) . The diatom (D) or other organism becomes in this way completely enclosed in a cavity or food vacuole (/. vac], which also contains a small quantity of water necessarily included with the prey. The latter is taken in by the Amoeba as food : so that the Amoeba, like the frog, feeds. It is to be noted that the reception of food takes place in a particular way, viz., by in- gestion — i.e., it is enclosed entire by the organism. When the prey is thus ingested, its protoplasm be- comes digested, any insoluble portions being passed out or egested, as faeces (pp. 8 and 75), from the surface of the Amoeba as it creeps slowly on. Note that all this is done without either ingestive aperture (mouth), digestive cavity (stomach), or egestive aperture (anus) : the food is simply taken in by the flowing round it of protoplasm, digested as it lies enclosed in the proto- plasm, and those portions for which it has no further use are got rid of by the Amoeba flowing away from them. We have seen that the frog possesses certain digestive glands, the function of which is to secrete digestive fluids which have an important chemical action on the food 234 AMCEBA CHAP. swallowed, rendering it soluble and diffusible before it passes through the epithelial cells of the enteric canal into the blood : the gastric juice, for example, has the power of converting proteids into peptones by means of the ferment pepsin (p. 74) ; the digestion here takes place outside the cells, i.e., is extracellular. There can be little doubt that the protoplasm of Amoeba is able to render that of its prey soluble and diffusible by the agency of some substance analogous to pepsin, and that the dissolved matters diffuse through the body of the Amoeba until the latter is, as it were, soaked through and through with them. The process of digestion in Amoeba thus takes place within a single cell, i.e., it is intracellular. It has been proved by experiment that proteids are the only class of food which Amoeba can make use of : it is unable to digest either starch or fats (p. 72). Mineral matters must, however, be taken with the food in the form of a weak watery solution, since the water in which the animalcule lives is never absolutely pure. The Amoeba being thus permeated, as it were, with a nutrient solution, the elements of the solution, hitherto arranged in the form of peptones, mineral salts, and water, become rearranged in such a way as to form new particles of living protoplasm, which are deposited among the pre-existing particles. In a word, the food is assimilated, or converted into the actual living sub- stance of the Amoeba, which must therefore grow, if nothing happens to counteract this formation of new protoplasm. We have seen, however, that work results in a propor- tional amount of waste (p. 66), and just as in the frog or in ourselves, every movement of the Amoeba, how- ever slight, is accompanied by a proportional oxidation or low temperature combustion of the protoplasm, i.e., the constituents of the protoplasm combine with oxygen, EXCRETION AND RESPIRATION 235 236 AMCEBA CHAP. spoken of collectively as metabolism — constructive and destructive (p. 149). Living protoplasm is thus the most unstable of substances ; it is never precisely the same thing for two consecutive seconds ; its existence, like that of a waterfall or a fountain, depends upon the constant flow of matter into it and away from it. It follows from what has been said that if the income of an Amoeba, i.e., the total weight of substances taken in (food plus oxygen plus water), is greater than its ex- penditure or the total weight of substances given out (faeces plus excreta proper plus carbon dioxide), the animalcule will grow : if less it will dwindle away : if the two are equal it will remain of the same weight or in a state of physiological equilibrium. It is evident that Amoeba must also be able t perform the function of reproduction. You have learn that the cells of the frog multiply by simple or binary fission (p. 106) : the nucleus first divides into two, and then the surrounding protoplasm ; and precisely the same thing occurs in Amoeba, the reproduction of which therefore takes place by the simplest method known, without any special reproductive organs. The animalcule simply divides into two Amoebae, each exactly like itself, and in doing so ceases to exist as a distinct in- dividual. Instead of the successive production of offspring from an ultimately dying parent, we have the simultaneous production of offspring by the division of the parent, which does not die, but becomes simply merged in its progeny. There can be no better instance of the fact that reproduction is discontinuous growth. From this it seems that an Amoeba, unless suffering a violent death, is practically " immortal/' since it divides into two completely organised individuals, each of which begins life with half of the entire body of its parent, there being therefore nothing left of the latter to die : I REPRODUCTION 237 it therefore appears certain that " death has no place as a natural recurrent phenomenon " in that organism. In multicellular forms it is only the reproductive cells which are, physically, potentially immortal. It is said that occasionally two Amoebae come into con- tact and undergo fusion, just as the gametes of the frog (sperm and ovum) unite in the processes of fertilisation (p. 197). This process of conjugation — which probably precedes encystment and multiple fission (see p. 233) — will be referred to again in the following chapters, and it is important to bear in mind that reproduc- tion can take place quite independently of such a process. Amoebae may also be propagated artificially. If a specimen is cut into pieces, each fragment is capable of developing into a complete animalcule provided it contains a portion of nuclear matter, but not otherwise. From this it is obvious that the nucleus exerts an in- fluence of the utmost importance over the vital processes of the organism. If an Amoeba does happen to be killed and to escape being eaten it will, like a dead frog, undergo gradual decomposition, becoming converted into various simple substances of which carbon dioxide, water, and ammonia are the chief (p. 152). Death results if the temperature to which an Amoeba is exposed reaches about 40° C., and at freezing point its movements cease entirely and it becomes inert. We thus see that complex organs, composed of various tissues, each consisting of cells of characteristic form, are not necessary in order that the vital functions may be performed : the only essential is nucleated proto- plasm. As we pass from the unicellular Amoeba to the higher multicellular animals we shall find — just as we found in tracing the development of the frog from the unicellular oosperm (p. 205) — that a differentiation of 238 AMOEBA CHAP. structure accompanied by a division of physiological labour becomes more and more marked, some cells giving rise to organs of locomotion, others to organs of reproduction, and so on. But every function necessary for the life of an animal — or a plant — is due in the first instance to protoplasm, and a simple cell, like the] Amoeba, can perform them all. In the next two chapters we shall study certain other unicellular organisms which show an advance on Amoeba in possessing a certain amount of morphological and physiological differentiation. But the structural differentiations, as they are merely parts of one cell, cannot be spoken of as " organs " in the sense in whi we have used the word hitherto, as they are not com- posed of numerous cells. They are, however, organ in the physiological sense, as they perform differen functions. PRACTICAL DIRECTIONS1 Amoeba. Examine a drop of water from the bottom of a pond, with the low power, first putting on a cover-glass, and look for Amoebae : if the water does not contain small particles of sand or mud, place a small piece of thin paper under the edge of the cover so as to avoid crushing the organisms. When you have found a specimen, note — i. The irregular and changing form of the animal, the protoplasm running out into blunt pseudopods. 1 You should, if possible, try to obtain specimens of Amoebae '! and the other fresh-water organisms described in this and the two following chapters for yourself, by collecting stagnant pond-water, I together with a little of the mud at the bottom and some water- weeds, and letting it stand for a few days in a glass jar or bottle. If you are unable to find the organisms you require, they, as well as most of the other animals described in this book, may be obtained from dealers in Natural History objects (see, e.g., the advertisements in Nature}. i PRACTICAL DIRECTIONS 239 2. The granular character of the protoplasm, the granules usually not extending to the periphery, so that a clear ectoplasm can be distinguished from a granular endoplasm. The granules render the flowing movements of the proto- plasm visible. 3. The food-vacuoles in the protoplasm, containing fluid, and often also food-particles. 4. The contractile vacuole, containing fluid, and its rhythmical contractions. 5. The protrusion and retraction of the pseudopods. Sketch a specimen several times at short intervals, noting the direction in which the granules flow. Then put on the high power. Go over §§ i — 5 again, and make a detailed sketch. 6. Look out for specimens undergoing multiplication by binary fission, and also for encysted individuals. 7. Run a little dry carmine or indigo under the cover- glass, and note that the particles can be taken in at all parts of the surface. 8. Treat with methyl-green, or magenta and acetic acid (see p. 121). This will kill the animal, and render the nucleus distinct. 9. Permanent preparations, showing the nucleus, may be made on the slide as follows : — Place a drop of water containing Amcebae on a slide, and soak up with blotting-paper as much of the water as is possible without carrying the Amcebae along with it. Fix (see p. 136) with a drop of absolute alcohol, stain (a staining- fluid called picrocarmine is better than borax-carmine for this purpose), wash carefully with weak and then with absolute alcohol, and add a drop of turpentine or xylol — or better, oil of cloves. Soak off the excess of oil of cloves with blotting-paper, and mount in Canada balsam. CHAPTER II SPILERELLA AND EUGLENA I MONADS AND BACTERIA — DIFFERENCES BETWEEN ANIMALS AND PLANTS- SAPROPHYTES THE rain-water which collects in puddles, open gutters, &c., is frequently found to have a green or red colour. The colour is due to the presence of various organisms- plants or animals — one of the commonest of which is Sphcerella (or, as it is sometimes called, Hcematococcus or Proto coccus) pluvialis. Like Amoeba, Sphaerella is so small as to require a high power for its examination. Magnified three or four hundred diameters it has the appearance (Fig. 68, A) of an ovoidal body, somewhat pointed at one end, and of a bright green colour, more or less flecked with bright red. Like Amoeba, moreover, it is in constant movement, but the character of the movement is very different in the two cases. An active Sphaerella is seen to swim about the field of the microscope in all directions and with considerable apparent rapidity. We say apparent rapidity because the rate of progression is magnified to the same extent as the organism itself, and what appears a racing speed under the microscope is actually a very 240 CHAP. II SPH.ERELLA 241 slow crawl when divided by 300. It has been found that such organisms as Sphaerella travel at the rate of one foot in from a quarter of an hour to an hour ; or, to FIG. 68. — A, Sphcerella pluvialis, motile phase (x about 350). Living specimen, showing protoplasm with chromatophore (chr) and pyrenoids (pyr), cell-wall (c. w) connected to cell-body by protoplasmic filaments, and flagella (ft). The scale to the left applies to Figs. A— D. B, resting stage of the same, showing nucleus (nu) with " nucleolus " (nur) and thick cell-wall (c. w) in contact with the protoplasm. C, the same, showing division of the cell-body in the resting stage into four daughter -eel Is. D, the same, showing the development of flagella and detached cell- wall by the daughter-cells before their liberation from the enclosing mother cell-wall. E, Sphterella lacustris, showing nucleus (nu), single large pyrenoid (pyr). and contractile vacuole (c. vac). F, diagram illustrating the movement of aflagellum ; ab. its base : c, c\ c", different positions assumed by its apex. (From Parker's Biology; E, after Butschli.) express the fact in another and fairer way, that they travel a distance equal to two and a half times their own diameter in one second. In swimming the pointed PRACT. ZOOL. R 242 SPH^ERELLA CHAP. end is always directed forwards, and the forward move- ment is accompanied by a rotation of the organism upon its longer axis. Careful watching shows that the outline of a swim- ming Sphserella does not change, so that there is evidently no protrusion of pseudopods, and at first the cause of the movement appears rather mysterious. Sooner or later, however, the little creature is sure to come to rest, and there can then be seen projecting from the pointed end two excessively delicate colourless threads (Figs. 68, A, fl), each about half as long again as the organism itself : these resemble the cilia on the epithelial cells lining the frog's mouth (p. 109) except that they are few in number, and do not vibrate rhythmically ; they are therefore usually distinguished as flagella. In a Sphaerella which has come to rest these can often be seen gently waving from side to side : when this slow movement is exchanged for a rapid one the whole organism is propelled through the water, the flagella acting like a pair of extremely fine and flexible fins or paddles. Thus the movement of Sphaerella is not amoeboid, i.e., produced by the protrusion and with- drawal of pseudopods, but is ciliary, i.e., due to the rapid vibration of cilia or flagella. By staining and other tests it is shown that Sphaerella, like Amoeba, consists of protoplasm, and that the flagella are simply filamentous processes of the protoplasm. The green colour of the organism is due to the presence of a special pigment called chlorophyll, the sub- stance to which the colour of leaves is due. That this is something quite distinct from the protoplasm may be seen by treatment with alcohol, which simply kills and coagulates the protoplasm, but completely dissolves out the chlorophyll, producing a clear green solution. The solution, although green by transmitted light, is red 1 ii CHROMATOPHORES 243 under a strong reflected light, and is hence fluorescent ; when examined through the spectroscope it has the effect of absorbing the whole of the blue and violet end of the spectrum as well as a part of the red. The red colour which occurs in so many individuals, sometimes entirely replacing the green, is due to a colouring matter closely allied in its properties to chlorophyll, and called hcematochrome. At first sight the chlorophyll appears to be evenly distributed over the whole body, but accurate examina- tion under a high power shows it to be lodged in a structure called a chromatophore (Fig. 68, A, chr), which forms a layer immediately beneath the surface, and in this case is relatively large and urn-shaped. It consists of a protoplasmic substance impregnated with chlorophyll. After solution of the chlorophyll with alcohol a nucleus (B, nu) can be made out ; like the nucleus of Amoeba, it is rendered more distinct by staining. Other bodies which might easily be mistaken for nuclei are also visible in the living organism. These are small ovoidal struc- tures (A, pyr), with clearly defined outlines, occurring in varying numbers in the chromatophore. When treated with iodine they assume a deep, apparently black, but really dark blue colour. The assumption of a blue colour with iodine is the characteristic test of the carbo- hydrate starch (p. 72), as can be seen by letting a few drops of a weak solution of iodine fall upon some ordinary washing starch. The bodies in question have been found to consist of a proteid substance covered with a layer of starch, and are called pyrenoids. In S. pluvialis a distinct contractile vacuole is wanting, but in another species, S. lacustris, this structure can be recognised as a minute space near the anterior or pointed end (Fig. 68, E, c. vac}. R 2 244 SPILERELLA CHAP. There is still another characteristic structure to which no reference has yet been made. This appears at the first view something like a delicate haze round the red or green body, but by careful focussing is seen to be really an extremely thin globular shell (A, c. w), com- posed of some colourless transparent material, and separated, by a space containing water, from the body, to which it is connected by very delicate radiating strands of protoplasm. It is perforated by two extremely minute apertures for the passage of the flagella. Ob- viously we may consider this shell as a cyst or cell-wall, differing from that of an encysted Amoeba (Fig. 67, D) in not being in close contact with the protoplasm. A more important difference, however, lies in its chemical composition. The cyst or cell- wall of Amoeba, as stated in the preceding chapter (p. 232), is very probably nitrogenous ; that of Sphaerella, on the other hand, is formed of a carbohydrate called cellulose, allied in composition to starch, sugar, and gum, and, like starch, having the formula C6H10O5. Many vegetable substances, such as cotton, consist of cellulose, and wood is a modification of the same compound. Cellulose is stained yellow by iodine, but iodine and sulphuric acid together turn it blue, and a similar colour is produced by a solution of iodine and potassium iodide in zinc chloride known as Schulze's solution. These tests are quite easily applied to Sphaerella : the protoplasm stains a deep yellowish-brown, around which is seen a sort of blue cloud, due to the stained and partly-dissolved cell- wall. After leading an active existence for a longer or shorter time, the Sphaerella comes to rest, losing its flagella, and its protoplasm being closely surrounded by the cellulose cell-wall (Fig. 68, B). So that, as in Amoeba, there is an II NUTRITION 245 alternation of an active or motile with a stationary or resting condition. In the matter of nutrition, the differences between Sphaerella and Amoeba are very marked, and indeed fundamental. As we have seen, Sphaerella has no pseudopods, and therefore cannot take in solid food after the manner of Amoeba ; moreover, even in its active condition it is usually surrounded by a cell-wall, which of course quite precludes the possibility of ingest ion. As a matter of observation, also, however long it is watched it is never seen to feed in the ordinary sense of the word. 4 Nevertheless it must take food in some way or other, or the decomposition of its protoplasm would soon bring it to an end. Sphaerella lives in rain-water. This is never pure water, but always contains certain mineral salts in solu- tion, especially nitrates, ammonia salts, and often sodium chloride or common table-salt. These salts can and do diffuse into the water which is a constituent part of the protoplasm of the organism, so that we may con- sider its protoplasm to be constantly permeated by a very weak saline solution, the most important elements con- tained in which are oxygen, hydrogen, nitrogen, potassium, sodium, calcium, sulphur, and phosphorus. It must be remarked, however, that the diffusion of these salts does not take place in the same uniform manner as it would through parchment or other dead membrane. The living protoplasm has the power of determining the extent to which each constituent of the solution shall be absorbed. If water containing a large quantity of Sphaerella is exposed to sunlight, minute bubbles appear in it, and these bubbles, if collected and properly tested, are found to consist largely of oxygen. Accurate chemical analysis 246 SPH^EKELLA CHAP. has shown that this oxygen is produced by the decom- position of the carbon dioxide contained in solution in rain-water, and indeed in all water exposed to the air ; the gas, which is always present in small quantities in the atmosphere, being very soluble in water. As the carbon dioxide is decomposed in this way, its oxygen being given off, it is evident that its carbon must be retained. As a matter of fact it is retained by the organism but not in the form of carbon ; in all probability a double decomposition takes place between the carbon dioxide absorbed and the water contained in its proto- plasm, the result being the liberation of oxygen in the form of gas and the simultaneous production of some extremely simple form of carbohydrate, i.e., some com- pound of carbon, hydrogen, and oxygen with a com- paratively small number of atoms to the molecule. The next step seems to be that the carbohydrate thus formed unites with the ammonia salts or the nitrates absorbed from the surrounding water, the result being the formation of some comparatively simple nitrogenous compound. Then further combinations take place, substances of greater and greater complexity are pro- duced, sulphur from the absorbed sulphates enters into combination, and proteids are formed. From these, finally, fresh living protoplasm arises. From the foregoing account, which only aims at giving the very briefest outline of a subject as yet imperfectly \ understood, it will be seen that, as in Amoeba, the final i result of the nutritive process is the manufacture of protoplasm, and that this result is attained by the form- ation of various substances of increasing complexity. But it must be noted that the steps in this process of constructive metabolism are widely different in the two cases. In Amoeba we start with living protoplasm — that of the prey — which is killed and broken up into ii NUTRITION 247 diffusible proteids, these being afterwards re-com Dined to form new molecules of the living protoplasm of Amoeba. So that the food of Amoeba is, to begin with, as complex as itself, and is first broken down by diges- tion into simpler compounds, these being afterwards re-combined into more complex ones. In Sphaerella, on the other hand, we start with extremely simple com- pounds, such as carbon dioxide, water, nitrates, sul- phates, &c. Nothing which can be properly called digestion, i.e., a breaking up and dissolving of the food, takes place, but its various constituents are combined into substances of gradually increasing complexity, protoplasm, as before, being the final result. To express the matter in another way : Amoeba can only make protoplasm out of proteids already formed by some other organism : Sphaerella can form it out of simple liquid and gaseous inorganic materials. Speaking generally, it may be said that these two methods of nutrition are respectively characteristic of the two great groups of living things. Animals require solid food containing ready-made proteids, and cannot build up their protoplasm out of simpler compounds. Green plants, i.e., all the ordinary trees, shrubs, weeds, &c., take only liquid and gaseous food, and build up their protoplasm out of carbon dioxide, water, and ' mineral salts. The first of these methods of nutrition is conveniently distinguished as holozoic, or wholly-animal, the second as holophytic, or wholly- vegetal. It is important to note that only those plants or parts of plants in which chlorophyll is present are capable of holophytic nutrition. Whatever may be the precise way in which the process is effected, it is certain that the decomposition of carbon dioxide which characterises this form of nutrition is a function of chlorophyll, or, to speak more accurately, of chromatophores, since there 248 SPH^ERELLA CHAP. is reason for thinking that it is the protoplasm of these bodies and not the actual green pigment which is the active agent in the process. Moreover, it must not be forgotten that the decom- position of carbon dioxide is carried on only during daylight, so that organisms in which holophytic nutrition obtains are dependent upon the sun for their very existence. While Amoeba derives its energy from the breaking down of the proteids in its food (see p. 235), the food of Sphaerella is too simple to serve as a source of energy, and it is only by the help of sunlight that the work of constructive metabolism can be carried on (photosynthesis) . This may be expressed by saying that Sphaerella, in common with other organisms containing chlorophyll, is supplied with kinetic energy (in the form of light or radiant energy) directly by the sun. As in Amoeba, destructive metabolism is constantly going on side by side with constructive. The proto- plasm becomes oxidised, water, carbon dioxide, and nitrogenous waste matters being formed and finally got rid of. Obviously, then, absorption of oxygen must take place, or, in other words, respiration must be one of the functions of the protoplasm of Sphaerella as of that of Amoeba. In many green, i.e., chlorophyll - containing, plants, this has been proved to be the case : respiration, i.e., the taking in of oxygen and giving out of carbon dioxide, is constantly going on, but during daylight is obscured by the converse process — the taking in of carbon dioxide for nutritive .purposes and the giving out of the oxygen liberated by its decomposi- tion. In darkness, when this latter process is in abeyance, the occurrence of respiration is more readily ascertained. Owing to the constant decomposition, during sunlight, of carbon dioxide, a larger volume of oxygen than of carbon dioxide is evolved ; and if an analysis were made ii METABOLISM 249 of all the ingesta of the organism (carbon dioxide plus mineral salts plus respiratory oxygen) they would be found to contain less oxygen than the egesta (oxygen from decomposition of carbon dioxide plus water, excueted carbon dioxide, and nitrogenous waste) ; so that the nutritive process in Sphaerella is, as a whole, a process of deoxidation. In Amoeba, on the other hand, the ingesta (food plus respiratory oxygen) contain more oxygen than the egesta (faeces plus carbon dioxide, water, and nitrogenous excreta), the nutritive process being therefore on the whole one of oxidation. This difference is, speaking broadly, characteristic of plants and animals generally ; animals, as a rule, take in more free oxygen than they give out, while green plants always give out more than they take in. Destructive metabolism is, however, manifested not only in the formation of waste-products, but in that of substances simpler than protoplasm which remain an integral part of the organism, viz., cellulose and starch. The cell-wall is probably formed by the conversion of a thin superficial layer of protoplasm into cellulose, the cyst attaining its final thickness by frequent repetition of the process. The starch of the pyrenoids is apparently formed by a similar process of decomposition or de- structive metabolism of protoplasm. We see then that destructive metabolism may result in the formation of (a) waste products and (b) plastic products, the former being got rid of as of no further use, while the latter remain an integral part of the organism. Let us now turn once more to the movements of Sphserella, and consider in some detail the manner of their performance. Each flagellum (Fig. 68, A, fi) is a thread of proto- plasm of uniform diameter except at its distal or free 250 SPELERELLA CHAP, j . end, where it tapers to a point. The lashing movements are brought about by the flagellum bending successively in different directions (F.). Thus the ciliary movement of Sphaerella, like the amoeboid movement of Amoeba, is a phenomenon of contractility. Imagine an Amoeba to draw in all its pseudopods but two, and to protrude these two until they became mere threads : imagine further these threads to contract rapidly and more or less regularly instead of slowly and irregularly ; the I result would be the substitution of pseudopods by flagella, i.e., of temporary slow-moving protoplasmic processes by permanent rapidly-moving ones. To put the matter in another way : in Amoeba the function of contractility is performed by the whole organism ; in Sphaerella it is discharged by a small part only, viz., the flagella, the rest of the protoplasm being incapable of movement. Sphaerella multiplies, after becoming quiescent, in the encysted condition (Fig. 68, C, D) ; as in the active Amoeba (p. 236), its protoplasm undergoes simple or binary fission, but with the peculiarity that the process is immediately repeated, so that four daughter-cells are produced within the single mother-cell-wall. By the rupture of the latter the daughter-cells are set free as the ordinary motile form, acquiring their flagella and detached cell- wall before making their escape (D). Under certain circumstances the resting form dividesj into numerous smaller daughter-cells, and these when liberated are found to have no cell- wall. Sphaerella therefore occurs, in the motile condition, under two dis- tinct forms, i.e., is dimorphic : the larger or ordinary form with detached cell-wall is called a megazooid, the smaller form without a cell-wall a microzooid. The microzooids, which are all similar to one another, are (in S. btitschlii) liberated as gametes which conjugate in ii EUGLENA 251 pairs to form zy gates (pp. 197, 237), each of which becomes surrounded with a cell- wall, and after a time again undergoes multiplication We will now examine, another small organism which is often found in puddles and pools, frequently in. such vast numbers as to give the water a green colour. This organism is known as Euglena viridis. Euglena is also microscopic, its length varying from JT mm. to J mm. The body is spindle-shaped, wide in the middle and narrow at both ends (Fig. 69, A, C — F) : one extremity is blunter than the other, and from it proceeds a single long flagellum (fl) by the action of which the organism swims with great rapidity, the flagellum forming a loop which travels along it. Besides its rapid swimming movements, Euglena frequently performs slow movements of contraction and expansion, something like those of a short worm, the body becoming broadened out first at the anterior end, then in the middle, then at the posterior end, twisting to the right and left, and so on (C — F). These move- ments are so characteristic of the genus that the name euglenoid is applied to them. The organism consists of protoplasm covered with a very delicate membrane or cuticle which is often finely striated, and is to be looked upon as a superficial hardening of the protoplasm. The colour is due to the presence of chlorophyll, which tinges all the central part, the two ends being colourless. It is difficult to make out that the chlorophyll is lodged in a number of distinct chromatophores (ch). In Sphaerella we saw that chlorophyll was associated with starch (p. 243). In Euglena there are, near the middle of the body, a number of grains of paramylum (A, p)t a carbohydrate of the same composition as 252 EUGLENA CHAP. starch (C6H10O5), but differing from it in remaining uncoloured by iodine. Water containing Euglenae gives off bubbles of nu FIG. 69. — Euglcna viridis. A, view of entire organism, showing details of structure (x about 1000). B, anterior end, to show origin of flagellum, etc. (x about 3000). C — F, four views of the living organism, showing the changes of form produced by the characteristic euglenoid movements. G, resting form after binary fission, showing cyst or cell-wall, nuclei, and reservoirs of the daughter-cells. ch. chromatophores ; cv. contractile vacuole ; cy. cyst or cell-wall ; fl. flagellum ; fl' thickening on flagellum ; fl". bifurcated base of flagellum ; gul. gullet ; nc ., nu. nucleus ; ncl. " nucleolus " ; p. paramylum bodies ; r. reservoir ; st. eye-spot or stigma. A, from Doflein ; B, from Doflein, after Wager ; C— G, after Saville Kent. n NUTRITION 253 oxygen in sunlight : as in Sphaerella the carbon dioxide in solution in the water is decomposed in the presence of chlorophyll, its oxygen evolved, and its carbon com- bined with the elements of water and used in nutrition. For a long time Euglena was thought to be nourished entirely in this way, but there is reason for thinking that this is not the case. When the anterior end of a Euglena is very highly magnified, it is found to have the form shown in Fig. 69, A, B. It is produced into a blunt snout-like extremity, at the base of which is a conical depression (gul) : — just the sort of depression one could make in a clay model of Euglena by thrusting one's finger or the end of a pencil into the clay. The bottom of this tube opens into a space, the reservoir (r), from one side of which the flagellum arises by a bifurcated base, and by its continual movement gives rise to a sort of whirlpool in the neighbourhood. By the current thus produced, minute solid food-particles are said to be swept down the tube and forced into the soft internal protoplasm, where they doubtless become digested in the same way as the substances ingested by an Amoeba. That solid particles are so ingested by many unicellular organisms (see e.g., pp. 266 and 272) has been proved by diffusing finely powdered carmine in the water, when the coloured particles were seen to be swallowed in the way described. The depression in question serves therefore as a gullet, and its external aperture or margin (m) as a mouth. Euglena, like Amoeba, may take in solid food, but instead of ingesting it at any part of the body, it can do so only at one particular point where there is a special ingestive aperture or mouth. This is clearly a case of specialisation or differentiation of structure : in virtue of the possession of a mouth and gullet, Euglena is more highly organised than Amoeba. 254 EUGLENA CHAP. It thus appears that in Euglena nutrition is both holozoic and holophytic (p. 247) : very probably it is mainly holophytic during daylight and holozoic in darkness (compare also p. 256). Near the centre or somewhat towards the posterior end is a nucleus (A, nc) with a well-marked " nucleolus " ; 1 at the anterior end is the clear space (r) already referred to, looking very like a contractile vacuole, but which is in reality a non-contractile cavity or reservoir into which the true contractile vacuole (c. v) opens, and which itself communicates with the gullet. In close relation with the reservoir is found a little bright red speck composed of pigment granules and called the eye-spot or stigma (st), its colour being due to haematochrome (p. 243). Recent experiments seem to show that it is specially sensitive to light, and is there- fore a true eye in the sense of a light-perceiving organ, although having no actual visual function. As in Sphaerella a resting condition alternates with the motile phase : the organism loses its flagellum and surrounds itself with a cyst of cellulose (G, cy, see p. 244) t from which, after a period of rest, it emerges to resume active life. Reproduction takes place by simple binary fission of the resting form, the plane of division being always longitudinal (G) . Sometimes each product of division ! or daughter-cell divides again : finally the two, or four, or sometimes even eight daughter-cells emerge from the cyst as active Euglenae. A process of multiple fission (P- 237) nas ^so been described, numerous simple, minute, active bodies or spores being produced, which gradually assume the ordinary form and size. We have seen that typical animal-cells, such as those ii ANIMALS AND PLANTS 255 of the frog (Part I, Chap. VII) are not provided with a cellulose cell-wall and do not contain chlorophyll. It is characteristic, on the other hand, of most plant-cells — which also consist of nucleated protoplasm — that they are surrounded with a cellulose cell-wall, and that, in the case of green plants, they contain chlorophyll. Speaking generally, the nutrition of animals is holozoic, and that of green plants holophytic ; and in correspondence with this difference in the character of the food, most animals have an ingestive aperture or mouth for taking in the solid food, and some kind of digestive cavity, either permanent (stomach) or temporary (food- vacuole) ; they also have, as a»rule, some kind of excretory apparatus. Moreover, animals are usually capable of automatic movement, while in most plants the organism, as a whole, exhibits no automatism, but only the slow move- ments of growth. Let us now apply these definitions to the simple forms described above and see how far they will help us in placing those organisms in one or other of the two " king- doms " (p. 220) into which living things are divided. Amoeba has a cell-wall, probably nitrogenous, in the resting condition ; it ingests solid proteids, its nutrition being therefore holozoic : it has a contractile vacuole : and it performs amoeboid movements. It may therefore be safely considered as an animal. Sphaerella has a cellulose wall that contains chlorophyll and its nutrition is purely holophytic : a contractile vacuole is present in S. lacustris, though not distinct in S. pluvialis : and its movements are ciliary. Euglena has a cellulose wall in the encysted state : in virtue of its chlorophyll it is nourished by the absorption of carbon dioxide and mineral salts, but it may also ingest solid food through a special mouth and gullet : it has a 256 MONADS CHAP. contractile vacuole, and performs both euglenoid and ciliary movements. In both these organisms we evidently have conflicting characters : the cellulose wall and holophytic nutrition would place them both among plants, while from the contractile vacuole and active movements of both genera, and from the holozoic nutrition of Euglena, we should group them with animals. That the difficulty is by no means easily overcome may be seen from the fact that both genera are claimed at the present day by zoologists and by botanists. Another mode of nutrition, which under certain cir- ] cumstances may also be adopted by fiuglena, occurs c.vac FIG. 70. — A Monad (Heteromita rostrata), showing nucleus (nu), contractile vacuole (c. vac), and two flagella (jfl, fl%). (x about 1500). (From Parker's Biology. after Dallinger.) in certain organisms which must now be referred to • very briefly. When animal or vegetable matter is placed in water fl and allowed to stand at the ordinary temperature, the well-known process of decomposition or putref action rey is brought near the mouth, the other tentacles >eing also used to aid in the process. The water-flea is hus forced against the* apex of the hypostome, the mouth expands widely and seizes it, and it is finally passed down into the digestive cavity. Hydrae can often >e seen with their bodies bulged out in one or more >laces by recently swallowed water-fleas. The precise structure of Hydra is best made out >y cutting it into a series of extremely thin sections ind examining them under a high power of the micro- ,cope. The appearance presented by a vertical section hrough the long axis of the body is shown in Fig. 76, A. The whole animal is seen to be built up of cells, each consisting of protoplasm with a large nucleus (B-D, ), and with or without vacuoles. As in the case of nost animal cells, there is no cell- wall. 298 HYDRA CHAP, v The essential feature in the arrangement of the cells is that they are disposed in two layers round the central digestive cavity or enteron (A, ent. cav} and the cavities of the tentacles (ent. cav'}. So that the wall of the body is formed throughout of an outer layer of cells, the ectoderm (ect}y and of an inner layer, the endoderm (end] which bounds the enteric cavity (compare p. 202) Between the two layers is a delicate transparent mem- brane, the mesoglcea, or supporting lamella (msgl). A transverse section (B) shows that the cells in both layers are arranged radially. Thus Hydra is a two-layer or diploblastic animal, and may be compared to a chimney built of two layers o radially arranged bricks with a space between the layers filled with mortar or concrete. Accurate examination of thin sections, and of specimens teased out or torn into minute fragments with needles shows that the structure is really much more compli- cated than the foregoing brief description would indicate The ectoderm-cells are of two kinds. The first anc most obvious (B, ect, and C) are large cells of a conica: form, the bases of the cones being external, their apices , internal. Spaces are necessarily left between their inner or narrow ends, and these are filled up with t second kind of cells (int. c), small rounded bodies which lie closely packed between their larger companions and are distinguished as interstitial cells. J The inner ends of the large ectoderm-cells are con-! tinued into narrow, pointed prolongations (C, m. pr) placed at right angles to the cells themselves and parallel to the long axis of the body. There is thus a layer of these longitudinally-arranged muscle-processes lying immediately external to the mesoglcea (B, m. pr}, They appear to possess, like the axial fibre of Vorticella FIG. 76. — Hydra. A, vertical section of the entire animal, showing the body-wall composed of ecto- derm (ect) and endoderm (end), enclosing an enteric cavity (ent. cav), which, as well as the two layers, is continued (ent. cav') into the tentacles, and opens 300 HYDRA CHAP. externally by the mouth (mth) at the apex of the hypostome (hyp). Between the ectoderm and endoderm is the mesogloea (msgl), represented by a black line. In the ectoderm are seen large (ntc) and small (ntcf) nematocysts ; some of the endoderm- cells are putting out pseudopods (psd), others flagella (fl). Two buds (bd.1, bd.'2) in different stages of development are shown on the left side, and on the right a spermary (spy) and an ovary (ovy) containing a single ovum (ov). (X 20.) B, portion of a transverse section more highly magnified, showing the large ecto- derm-cells (ect) and interstitial cells (int. c) ; two cnidoblasts (cnbl) enclosing nematocysts (ntc) and one of them produced into a cnidocil (cnc) ; the layer of muscle-processes (m. pr) cut across just external to the mesogloea (msgl) ; endoderm-cells (end) with large vacuoles and nuclei (nu), pseudopods (psd), and flagella (fl). The endoderm-cell to the right has ingested a diatom (a), and all enclose minute black granules, (x 100.) C, two of the large ectoderm-cells, showing nucleus (nu) and muscle-process (m. pr). D, an endoderm-cell of H. viridis, showing nucleus (nu), numerous Zoochlorell'ae (chr), and an ingested nematocyst (ntc). E, one of the larger nematocysts with extruded thread, barbed at the base. F, one of the smaller nematocysts. G, a single sperm. (From Parker's Biology : D after Lankester ; F and G after Howes.) (p. 273), a high degree of contractility, the almost instantaneous shortening of the body being due, in great measure at least, to their rapid and simultaneous con- traction. It is probably correct to say that, while the ectoderm-cells are both contractile and irritable, a special degree of contractility is assigned to the muscle- processes, the cells themselves being eminently irritable, the slightest stimulus applied to them usually being followed by immediate contraction of the whole body. Imbedded in and between some of the large ectoderm- cells are found clear, oval sacs (ntc), with very well defined walls, called " thread-cells " or nematocysts. Both in the living specimen and in sections they ordinarily present the appearance shown in Figs. 76, B, and 77, A, but are frequently met with in the condition shown in Figs. 76, E, and 77, B, that is, with a short, conical, tube protruding from the mouth of the sac, armed near its distal end with three recurved barbs besides several! similar processes of smaller size, and giving rise distally to a long, delicate, flexible filament. Accurate examination of the nematocysts shows that the structure of these curious bodies is as follows. Each consists of a tough sac (Fig. 77, A), one end of NEMATOCYSTS 301 FIG. 77. — Hydra. A, a nematocyst contained in its cnidoblast (cnb), showing its coiled filament and the cnidocil (cnc). (x 400.) B, the same after extrusion of the thread, showing the larger and smaller barbs at the base of the thread ; nu. the nucleus of the cnidoblast. C, a cnidoblast, with its contained nematocyst, connected with one of the processes of a nerve-cell (nv. c). (From Parker's Biology: after Schneider.) • which is turned in as a hollow pouch : the free end of the latter is continued into a hollow, coiled filament, ,nd from its inner surface project the barbs. The 302 HYDRA CHAP. whole space between the wall of the sac and the con- tained pouch and thread is tensely filled with fluid. When pressure is brought to bear on the outside of the sac the whole apparatus goes off like a harpoon-gun (B), the compression of the fluid forcing out first the barbed pouch and then the filament, until finally both are turned inside out. It is by means of the nematocysts — the resemblance of which to the trichocysts of Paramcecium (p. 267) should be noted — that the Hydra is enabled to paralyse its prey. Probably some specific poison is formed and ejected into the wound with the thread : in the larger members of the group to which Hydra belongs, such as jelly-fishes, the nematocysts produce an effect on the human skin quite like the sting of a nettle. The nematocysts are formed in special interstitial cells called cnidoblasts (Figs. 76, B, and 77, cnb), and are thus in the first instance at a distance from the surf ace. |c But the cnidoblasts migrate outwards, and so come to lie quite superficially either in or between the large ectoderm-cells. On its free surface the cnidoblast is produced into a delicate pointed process, the cnidocil or "trigger-hair" (cnc). In all probability the slightest touch of the cnidocil causes contraction of the cnidoblast, and the nematocyst, thus compressed, instantly explodes, Nematocysts are found in the distal part of the body, but are absent from the foot or proximal end, where alsc there are no interstitial cells. They are especially! abundant in the tentacles, on the knob-like elevations' of which — due to little heaps of interstitial cells — the} are found in great numbers. Amongst these occur small nematocysts with short threads and devoid of barbs (Fig. 76, A, ntc', and F). In connection with the cnidoblasts small irregulai cells with large nuclei occur (Fig. 77, C, nv. c) ; they are bi ENDODERM 303 upposed to be nerve-cells, and to constitute a rudi- mentary nervous system (compare p. 167). The ectoderm-cells of the foot differ from those of the rest of the body in being very granular (Fig. 76, A). The granules are probably the material of the adhesive secretion by which the Hydra fixes itself, and these cells are therefore glandular (p. 130). The endoderm consists for the most part of large cells which exceed in size those of the ectoderm, and are remarkable for containing one or more vacuoles, some- times so large as to reduce the protoplasm to a thin superficial layer containing the nucleus (Fig. 76, A and B, end). Then, again, their form is extremely variable, ;heir free or inner ends undergoing continual changes of form. This can be easily made out by cutting trans- verse sections of a living Hydra, when the endoderm- :ells are seen to send out long blunt pseudopods (psd) nto the digestive cavity, and now and then to withdraw ;he pseudopods and send out from one to three long, delicate flagella (fl) . Thus the endoderm-cells of Hydra illustrate in a very instructive manner the essential similarity of flagella and pseudopods already referred to ^p. 250). In the hypostome the endoderm is thrown nto longitudinal folds, so as to allow of the dilatation of the mouth in swallowing. Amongst the ordinary endoderm-cells are found long narrow cells of an extremely granular character. They are specially abundant in the distal part of the body, Deneath the origins of the tentacles and in the hypo- •tome, but are absent in the tentacles and in the foot. There is no doubt that they are gland-cells, their secretion being a fluid used to aid in the digestion of the food. In Hydra viridis the endoderm-cells (Fig. 76, D) 304 HYDRA CHAP. contain chromatophore-like bodies (chr) coloured green by chlorophyll (p. 242), the function of which we have already considered (p. 247). It has been proved, how- ever, that these are not actual parts of the endoderm- cells, but are distinct Sphserella-like organisms known as Zoochlorellcz, which are passed on from one generation of the Hydra to another by entering its developing eggs. Such an intimate living-together of two organisms is known as symbiosis. It differs essentially from parasitism (see p. 280), in which one organism preys upon another, the host deriving no benefit but only harm from the presence of the parasite. In symbiosis, on the contrary, the two organisms are in a condition of mutually beneficial partnership. The carbon dioxide and nitro- genous waste given off by the cells of the Hydra serve as a constant food-supply to the Zoochlorella : at the same time the latter by decomposing the carbon dioxide provides the Hydra with a constant supply of oxygen, and also with two important foodstuffs — starch and proteids, which, after solution, diffuse from the proto- plasm of the Zoochlorella into that of the endoderm- cells. The latter may therefore be said to keep the 1 Zoochlorellae constantly manured, while the Zoochlorellae I in return supply them with oxygen and ready-digested food. In the endoderm of H. fusca bodies of an orange or brown colour are present which are devoid of | chlorophyll. Muscle-processes also exist in connection with the endo \ derm-cells, and they are said to take a transverse or circular direction, i.e., at right angles to the similar processes of the ectoderm cells. When a water-flea or other minute organism is swallowed by a Hydra, it undergoes a gradual process of disintegration. The process is begun by a solution of the soft parts due to the action of a digestive v DIGESTION 305 fluid secreted by the gland-cells of the endoderm ; it is apparently completed by the endoderm-cells seizing minute particles with their pseudopods and engulfing them quite after the manner of Amoebae. It is often found that the protrusion of pseudopods during digestion results in the almost complete obliteration of the enteric cavity. It would seem, therefore, that in Hydra the process of digestion or solution of the food is to some extent intracellular, i.e., takes place in the interior of the cells themselves, as e.g., in Amoeba or Paramcecium : it is, however, largely extra cellular or enteric, i.e., is performed in a special digestive cavity lined by cells (pp. 67 and 131) . The ectoderm-cells do not take in food directly, but are nourished entirely by diffusion from the endoderm. Thus the two layers have different functions : the ecto- derm is protective and sensory — it forms the external covering of the animal, and receives impressions from without ; the endoderm, removed from direct communi- cation with the outer wrorld, performs a nutrient function, its cells alone having the power of digesting Ifood. The essential difference between digestion and assimi- lation (p. 234) is here plainly seen : all the cells of Hydra ssimilate, all are constantly undergoing waste, and all mst therefore form new protoplasm to make good the lioss. But it is the endoderm-cells alone which can make of raw or undigested food : the ectoderm has to lepend upon various products of digestion received by liffusion or osmosis from the endoderm. It will be evident from the preceding description that lydra is comparable to a colony of Amoebae in which articular functions are made over to particular lividuals — just as in a civilised community the I PRACT. ZOOL. X 306 . HYDRA CHAP. functions of baking and butchering are assigned to certain members of the community, and not performed by all. Hydra is therefore an example of individuation : morphologically it is equivalent to an indefinite number of unicellular organisms : but, these acting in concert, some taking one duty and some another, form, physio- logically speaking, not a colony of largely independent units (compare p. 277), but a single multicellular individual. Hydra has two distinct methods of reproduction, asexual and sexual. Asexual multiplication takes place by a process of budding. A little knob appears on the body (Fig. 7 A, bdl), and is found by sections to arise from a group of ectoderm-cells ; soon, however, it takes on the character of a hollow outpushing of the wall containing a prolongation of the enteron and made up of ecto- derm, mesoglcea, and endoderm (Fig. 76, A, bdl). In the course of a few hours this prominence enlarges greatly, and near its distal end six or eight hollow buds appear arranged in a whorl (Figs. 75, A, and 76, A bd2). These enlarge and take on the characters of tentacles, and a mouth is formed at the distal end of the bud, which thus acquires the character of a small Hydra (Fig. 75, A, bd3). Finally the bud becomes constricted at its base, separates from the parent, and begins an. independent existence. Sometimes, however, several} buds are produced at one time, and each of these buds again before becoming detached : in this way temporary colonies are formed. But the buds always separate sooner or later, although they frequently begin to feed while still attached. It is a curious circumstance that Hydra can also be multiplied by artificial division : the experiment has v REPRODUCTION 307 been tried of cutting the living animal into pieces, each of which was found to undergo regeneration into a perfect individual (compare p. 268). The sexual organs or gonads (p. 193) are of two kinds, spermaries and ovaries. Both are found in the same individual, Hydra being hermaphrodite or monoecious. The spermaries (Figs. 75, B, and 76, A, spy) are white conical elevations situated near the distal end of the body : as a rule not more than one or two are present at the same time, but there may be as many as twenty. They are perfectly colourless, even in the green and brown species, being obviously formed of ectoderm alone. In the immature condition the spermary consists of a little heap of interstitial cells covered by an investment of somewhat flattened cells formed by a modification of the ordinary large cells of the ectoderm. When mature each of the small internal cells becomes converted into a sperm (p. 194), consisting of a small ovoid head formed from the nucleus of the cell, and of a long vibratile tail formed from its protoplasm (Fig. 76, G). I By the rupture of the investing cells or wall of the spermary the sperms are liberated and swim freely in I the water. The ovaries (Figs. 75, B, and 76, A, ovy) are found [nearer the proximal end of the body, and vary in I number from one to eight. When ripe an ovary is •larger than a spermary, and of a hemispherical form. lit begins, like the spermary, as an aggregation of [(interstitial cells, so that in their earlier stages the sex of the gonads is indeterminate. But while in the spermary each cell is converted into a sperm, in the ovary one ell (Fig. 76, A, ov) soon begins to grow faster than the st, and becomes amoeboid in form, sending out eudopods amongst its companions and ingesting the X 2 308 HYDROID POLYPES CHAP. fragments into which they become broken up, thus continually increasing in size at their expense. Ulti- mately the ovary comes to consist of this single amoeboid ovum and of a layer of superficial cells forming a capsule for it. As the ovum grows, yolk granules (p. 195) are formed in it, and in Hydra viridis it also acquires Zoochlorellse (p. 304). When the ovary is ripe the ovum draws in its pseudopods and takes on a spherical form : the investing layer then bursts so as to lay bare the ovum and allow of the free access to it of the sperms. One of the latter conjugates with the ovum, producing an oosperm (p. 198) or unicellular embryo. The oosperm undergoes segmentation, dividing into a number of cells which constitute a morula or polyplast (p. 200), the outermost cells of which become changed into a hard shell or capsule, which eventually bursts and sets free the embryo. The embryo develops into a Hydra, its cells becoming differentiated into ectoderm and endoderm, the enteron and mouth being formed, and the tentacles budding out around the latter. It was stated on p. 306 that in a budding Hydra the buds do not always become detached at once, but may themselves bud while still in connection with the parent, temporary colonies being thus produced. Suppose the state of things to continue indefinitely : the result would be a tree-like colony or compound j organism consisting of a stem with numerous branchletsl each ending in a Hydra-like zooid. Such a colony would bear much the same relation to Hydra as Carchesium or Epistylis bears to Vorticella. As a matter of fact this is precisely what happens in a great number of animals allied to Hydra and known by the name of Zoophytes or Hydroid polypes. v OBELIA 309 Everyone is familiar with the common Hydroids known as Sertularians of the sea-coast, often mistaken for sea-weeds : they are delicate, much-branched, semi- transparent structures of a horny consistency, the branches beset with little cups, from each of which, during life, a Hydra-like body is protruded. A very convenient genus of Hydroids for our purpose is Obelia, which occurs in the form of a deli- cate, whitish or light-brown, almost fur-like growth on seaweed, the wooden piles of piers, etc. It consists of branched filaments about the thickness of fine sewing- cotton : of these, some are closely adherent to the timber, and serve for attachment, while others are given off at right angles, and present at intervals short lateral branches, each terminating in a bud-like enlargement. The structure is better seen under a low power of the microscope. The organism (Fig. 78) is a colony, consisting of a common stem or axis, on which are borne numerous zooids (compare p. 277). The axis consists of a horizontal portion, resembling a root or creeping stem, and of vertical axes, which give off short lateral branches in an alternate manner, bearing the zooids at their ends. At the proximal ends of the vertical axes the branching often becomes more complex : the offshoots of the main stem, instead of ending at once in a zooid, send off branches of the third order on which the zooids are borne. In many cases, also, branches are found to end in simple club-like dilatations (Bd. i, 2) : these are immature zooids. The large majority of the zooids are little Hydra-like bodies, the polypes or hydranths, each with a hypostome or manubrium and a circlet of about two dozen ten- tacles. Less numerous, and found chiefly towards the proximal region of the colony, are long cylindrical FIG. 78. — Obelia. A, portion of a colony with certain parts shown in longitudinal section ( x 10) ; B, medusa ( x 5) ; C, the same with reversed umbrella ; D, the same, oral aspect. Bd. i, 2, buds ; bis. blastostyle ; coe. coenosarc ; ect. ectoderm ; end. endoderm ; ent. enteric cavity ; g. th. gonotheca ; h. th. hydrotheca ; /. litho- cyst ; m. bd. medusa-bud ; mnb. hypostome or manubrium ; msgl. mesoglcea ; mth. mouth ; p. perisarc ; P. /, 2, 3, polypes ; rod. c. radial canal ; t. tentacle j vl. velum (From Parker and HaswelTs Zoology, reduced.) 310 ! CHAP, v OBELIA 311 bodies or blastostyles (bis), each bearing numerous small lateral offshoots, varying greatly in form according to their stage of development, and known as medusa-buds (m. bd) ; these will be considered presently. Examination under a high power, either of an entire branch or of sections, shows that the polypes have essentially the structure of a Hydra, consisting of a double layer of cells — ectoderm (with nematocysts) and endoderm — separated by a supporting lamella or mesoglcea and enclosing a digestive cavity (ent) which opens externally by a mouth placed at the summit of the hypostome. The mouth is capable of great dilatation and contraction, and accordingly the hypostome appears now conical, now trumpet-shaped. The tentacles, however, differ from those of Hydra in two important respects. In the first place they are solid : the endoderm, instead of forming a lining to a prolongation of the enteron, consists of a single axial row of large cells with thick cell-walls and vacuolated protoplasm. Then in the position of the muscle-pro- cesses of Hydra there is a layer of spindle-shaped fibres many times longer than broad, and provided each with a nucleus. Such muscle-fibres are obviously cells greatly extended in length (p. in), so that the ectoderm-cell of Hydra with its continuous muscle-process is here represented by an ectoderm-cell with an adjacent muscle- cell. We thus get a partial intermediate layer of cells between the ectoderm and endoderm, in addition to the gelatinous mesoglcea ; and so, while a hydroid polype is, like Hydra, diploblastic (p. 298), it shows a tendency towards the assumption of a three-layered or triploblastic condition (compare p. 202). The part of the stem and branches continuous with the bases of the polypes, which is known as the ccenosarc, is formed of the same layers and contains a cavity con- 312 OBELI A CHAP. tinuous with those of the hydranths : thus the structure of a hydroid polype is, so far, simply that of a Hydra in which the process of budding has gone on to an indefinite extent and without separation of the buds. There is, however, an additional layer added for protective and strengthening purposes. It is evident that such a colony would, if formed only of soft ectodermal and endodermal ceils, be so weak as to be hardly able to bear its own weight even in water. To remedy this a layer of transparent, yellowish substance of horn-like consistency, called the perisarc, is developed outside the ectoderm of the ccenosarc, extending on to the branches and continuous with a glassy, cup-like investment, or hydrotheca, around the base of each polype, and with a transparent case, or gonotheca, enclosing each blastostyle. Each hydrotheca (h. th) has the form of a vase or wine-glass, and is perfectly transparent and colourless. A short distance from its narrow or proximal end, it is produced inwards into a sort of circular shelf (sh) , perforated in the centre : upon this the base of the polype rests, and through the aperture it is continuous with the common stem. When irritated — by a touch or by the addition of alcohol or other poison — the polype undergoes a very marked con- traction : it suddenly withdraws itself more or less completely into the theca, and the tentacles become greatly shortened and curved over the manubrium (P. 2). At the base of each zooid or branch the perisarc presents several annular constrictions, giving it a ringed appear- ance : for the most part it is separated by an interval from the ccenosarc, but processes of the latter extend outwards to it at irregular intervals, and at first (Bd. 2} the two layers are in close apposition. It is this layer which, when the organism dies and decays, is left as a semi-transparent, branched structure v STRUCTURE 313 resembling the living colony in form except that polypes, blastostyles, and medusa-buds are wanting. The peri- sarc is therefore a supporting organ or skeleton, not, like our own bones, formed in the interior of the body (endoskeletori) , but, like the shell of a crayfish or lobster, lying altogether outside the soft parts (exo skeleton) . As to the mode of formation of the perisarc : — we saw that many organisms, such as Sphaerella and Amoeba, are able to form a cyst or cell-wall, by secreting or separating from the surface of the protoplasm a succession of layers either of cellulose or of a trans- parent horn-like substance (pp. 232 and 244). But Amoeba and Sphaerella are unicellular, and are there- fore free to form this protective layer at all parts of their surface. The ectoderm-cells of Obelia, on the other hand, are* in close contact with their neighbours on all sides and with the mesoglcea at their inner ends, so that it is not surprising to find the secretion of skeletal substance taking place only at their outer ends. As the process takes place simultaneously in adjacent cells, the result is a continuous layer common to the whole ectoderm instead of a capsule to each individual cell. It is to an exoskeletal structure formed in this way, i.e., by the secretion of successive layers from the free faces of adjacent cells, that the name cuticle is in strictness applied in multicellular organisms. In the blastostyles both mouth and tentacles are absent, the zooid ending distally in a flattened disc : the hydrotheca of a polype is represented by the gono- theca (g. th), which is a cylindrical capsule enclosing the whole structure, but ultimately becoming ruptured at its distal end to allow of the escape of the medusa-buds. These latter are, in the young condi- tion, mere hollow offshoots of the blastostyle : when fully developed they have the appearance of saucers 314 OBELIA CHAP. attached by the middle of the convex surface to the blastostyle, produced at the edge into sixteen very short tentacles, and having a blunt process, the manubrium, projecting from the centre of the concave surface. They are ultimately set free through the aperture in the gono- theca as little medusce or jelly-fish (B-D). The structure of a medusa must now be described in some detail. The saucer-shaped "bell" or umbrella (Figs. 78, B-D, 79, and 80) is formed of a gelatinous end.fam FIG. 79. — Dissection of a Medusa (X25) with rather more than one-quarter of the I umbrella and manubrium cut away (diagrammatic) . The ectoderm is dotted, the i endoderm striated, and the mesogloea black, circ. c. circular canal ; end. lam. I endoderm-lamella ; gon. gonad ; /. lithocyst ; mnb. manubrium ; mth. mouth ; rod. c. radial canal ; vl. velum. substance (Fig. 80, D, msgl) covered both on its inner surface, or sub-umbrella, and on its outer surface, or ex-umbrella, by a thin layer of delicate cells (ect). The clapper-like manubrium (mnb) is formed of two layers of cells, precisely resembling the ectoderm and endoderm , of Hydra, and separated by a thin mesoglcea ; it is hollow, its cavity (ent. cav) opening below, i.e., at its distal or free end, by a four-sided aperture, the mouth , v MEDUSAE 315 (nith), used by the medusa for the ingestion of food. Very commonly as the medusa swims the umbrella becomes turned inside out, the sub-umbrella then forming the convex surface and the manubrium spring- ing from its apex (Fig. 78, C). At its upper (attached or proximal) end the cavity of the manubrium is con- tinued into four narrow, radial canals (Figs. 78, B, D, and 79, rad. c, and Fig. 80, D, and D', rad) which extend through the gelatinous substance of the umbrella at equal distances from one another, like four meridians, and finally open into a circular canal (dr. c) which runs round the edge of the umbrella. By means of this system of canals the food, taken in at the mouth and digested in the manubrium, is distributed to the entire medusa. The canals are lined by a layer of cells (Fig. 80, D and D', end) continuous with the inner layer or tndoderm of the manubrium ; and extending from one canal to another, in the gelatinous substance of the umbrella, is a delicate sheet of cells, the endoderm-lamella (D', end. lam). The edge of the umbrella is produced into a very narrow fold or shelf, the velum (Fig. 79, vl, Fig. 80, v), and gives off the tentacles (/), which are sixteen in number in the newly-born medusa, very numerous in the adult. At the bases of eight of the tentacles — two in each quadrant — are minute globular sacs (/), each containing a cal- careous particle or lithite. These are the marginal sense- organs or lithocysts : they were formerly considered to be organs of hearing, and are hence frequently called otocysts " : in all probability their function is to guide the medusa by enabling it to judge of the direction in which it is swimming. The marginal organs in this case |!may therefore be looked upon as organs of the sense of Direction or of equilibration, and may be spoken of as statocysts (compare p. 189). The velum consists of a A' FIG. 80. — Diagrams illustrating the derivation of the medusa from the hydranth. In the whole series of figures the ectoderm (ect) is dotted, the endoderm (end) striated, and the mesogloea (msgl) black. A, longitudinal section of a simple polype showing the tubular body, with enteric cavity (ent. cav), hypos tome (hyp), mouth (mth), and tentacles (t). A', transverse section of the same through the plane a b. B, the tentacular region is extended into a hollow disc. C, the tentacular region has been further extended and bent into a bell-like form, 316 CHAT, v MEDUSA 317 the enteric cavity being continued into the umbrella (ent. cav') ; the hypostome now forms a manubrium (tnnb). C', transverse section of the same through the plane a b, showing the continuous cavity (ent. cav) in the umbrella. D, fully formed medusa ; the cavity in the umbrella is reduced to the radial (rad) and circular (dr. c.) canals, the velum (v) is formed, and a double nerve- ring (nv . nv') is produced from the ectoderm. D', transverse section of the same through the plane a b, showing the four radial canals (rad) united by the endoderm- lamella (end. lam), produced by partial obliteration of the continuous cavity (ent. cav') in C. (From Parker's Biology). middle layer of mesogloea with ectoderm on either side : there is no extension of endoderm into it. The tentacles, like those of the polype, are formed of a core of endoderm covered by ectoderm, which encloses numerous stinging- capsules. At first sight there appears to be very little re- semblance between a medusa and a polype, but it is really quite easy to derive the one form from the other. Suppose a simple polype or Hydra-like body with four tentacles (Fig. 80, A, A') to have the region from which the tentacles spring pulled out so as to form a hollow, transversely extended disc (B). Next, suppose this disc to become bent into the form of a cup with its concavity towards the hypostome, and to undergo a great thicken- ing of its mesogloea. A form would be produced like C, i.e., a medusa-like body with umbrella and manu- brium, but with a continuous cavity (C', ent. cav'} in the thickness of the umbrella instead of four radial canals. Finally, suppose the inner and outer walls of this cavity to grow towards one another and meet, thus obliterating the cavity, except along four narrow radial areas (D, rad) and a circular area near the edge of the umbrella (dr. c) . This would result in the substitution for the continuous cavity of four radial canals opening on the one hand into a circular canal and on the other into the cavity of the manubrium (ent. cav), and connected with one another by a membrane — the endoderm-lamella (D', end. lam) — indicating the former extension of the cavity. It follows from this that the inner and outer layers of 318 OBELI A CHAP. the manubrium are respectively endoderm and ectoderm : that the gelatinous tissue of the umbrella is an immensely thickened mesoglcea : that the layer of cells covering both inner and outer surfaces of the umbrella is ecto- dermal : and that the layer of cells lining the system of canals, together with the endoderm-lamella, is endo- dermal. Thus the medusa, the polype, and the blastostyle are similarly constructed or homologous structures (p. 217), and the hydroid colony is trimorphic (compare p. 250), bearing zooids of three kinds. In some allied forms this individuation may go still further, the zooids being of very various forms and performing diverse functions : such a colony is said to be polymorphic. Sooner or later the medusae separate from the hydroid colony and begin a free existence. Under these cir- cumstances the rhythmical contraction — i.e., contraction taking place at regular intervals — of the muscles of the umbrella causes an alternate contraction and expansion of the whole organ, so that water is alternately pumped out of and drawn into it. The obvious result of this is that the medusa is propelled through the water by a series of jerks. The movement is due to the contraction of the muscle-processes and muscle-fibres of the sub-umbrella and velum, some of which differ from the similar structures in the polype in exhibiting a delicate transverse striation. There is still another important matter in the structure of the medusa which has not been referred to. At the junction of the velum with the edge of the umbrella there lies, immediately beneath the ectoderm, a layer o: peculiar, branched cells, containing large nuclei anc produced into long fibre-like processes. These nerve- cells (pp. 167 and 303) are so disposed as to form a double v MEDUSA 319 ring round the margin of the bell, one ring (Fig. 80, D, nv) being immediately above, the other (nvr) immediately below the insertion of the velum. An irregular network of similar cells and fibres occurs on the inner or concave face of the bell, between the ectoderm and the layer of muscle-fibres. The whole constitutes the nervous system of the medusa ; the double nerve-ring is the central, the network the peripheral nervous system (p. 155). Some of the processes of the nerve-cells are connected with ordinary ectoderm-cells, which thus as it were con- nect the nervous system with the external world : others, in some instances at least, are probably directly connected with muscle-fibres. We thus see that while the manubrium of a medusa has the same simple structure as a polype, the umbrella has undergone a very remarkable differentiation of its tissues. Its ordinary ectoderm-cells, instead of being large and eminently contractile, form little more than a thin cellular skin or epithelium (p. 109) over the gelatinous mesogloea : they have largely given up the function of contractility to the muscle-processes or fibres, and serve merely as a protective and sensitive layer. Similarly the function of automatism, possessed by the whole body of Hydra, is made over to the group of specially modified ectodermal cells which constitute the central nervous system. If a Hydra is cut into any number of pieces each of them is able to perform the ordinary movements of expansion and contraction, but if the nerve-ring of a medusa is removed by cutting away the edge of the umbrella, the rhythmical swimming movements stop dead : the umbrella is in fact perma- nently paralysed. It is not, however, rendered incapable of movement, for a sharp pinch, i.e., an external stimulus, causes a single contraction, showing that the muscles still retain 320 OBELIA CHAP. their irritability. But no movement takes place without such external stimulus, each stimulus giving rise infalli- bly to one single contraction : the power possessed by the entire animal of independently originating movement, i.e., of supplying its own stimuli, is lost with the central nervous system (compare p. 172). Another instance of morphological and physiological differentiation is furnished by the marginal sense-organs situated at the base of the tentacles (p. 315). The polype and medusa are respectively nutritive and reproductive in function, the reproductive zooids becoming detached and swimming off to found a new colony elsewhere : the polypes are purely nutritive zooids ; the medusae, although capable of feeding, are specially distinguished as reproductive zooids. Hanging at equal distances from the sub-umbrella, in immediate relation with the radial canal, are four ovoid gonads (Fig. 79, gon), each consisting of an outer layer of ectoderm continuous with that of the sub-umbrella, an inner layer of endoderm continuous with that of the radial canal and enclosing a prolongation of the latter, and of an intermediate mass of cells which have become differentiated into ova or sperms. As each medusa bears organs of one sex only (spermaries or ovaries, as the case may be), the individual medusae are dioecious, and not, like Hydra, monoecious. It will be noticed that the gonad has the same general structure as an immature zooid — an out-pushing of the body-wall consisting of ectoderm and endoderm, and containing a prolongation of the enteric cavity. The medusae, when mature, become detached and swim away from the hydroid colony. The sperms of the males are shed into the water and carried to the ovaries of the females, where they fertilise the ova, converting them, as usual, into oosperms. ALTERNATIONS OF GENERATIONS 321 The oosperm undergoes segmentation, forming a poly- plast or morula (p. 200) : ectoderm and endoderm become differentiated, and the ectoderm-cells acquire cilia, by means of which the embryo now swims freely in the water. An enteron appears in the endoderm, and in this stage the embryo, which has an elongated form, is known as a planuta. It then loses its cilia and settles down on a rock, shell, sea-weed, or other sub- marine object, assuming a vertical position, with its broader end fixed to the support. The attached or proximal end widens into a disc of attachment, a dilatation is formed a short distance from the free or distal end, and a thin cuticle is secreted from the whole surface of the ectoderm. From the dilated portion short buds arise in a circle : these are the rudiments of the tentacles : the narrow portion distal to their origin becomes the hypostome. Soon the cuticle covering the distal end is ruptured so as to set free the growing tentacles : an aperture, the mouth, is formed at the end of the hypostome, and the young hydroid has very much the appearance of a Hydra with a broad disc of attachment, and with a cuticle covering the greater part of the body. Extensive budding next takes place, the result being the formation of the ordinary hydroid colony. Thus from the oosperm or impregnated egg-cell of the medusa the hydroid colony arises, while the medusa is produced by budding from the hydroid colony. We have what is called an alternation of generations, the asexual generation or agamobium (hydroid colony) giving rise by budding to the sexual generation or gamobium (medusa), which in its turn produces the agamobium by a sexual process, i.e., by the conjugation of ovum and sperm (compare p. 287). This form of alternation of generations is known as metagenesis. PRACT. ZOOL. v 322 CCELENTERATA CHAP. Hydra and Obelia both belong to the simplest class — the Hydrozoa — of the phylum Coelenterata : this phylum includes all the polypes or zoophytes, the jelly-fishes, and the anemones and corals. In all there is an ectoderm and an endoderm, separated by a mesoglcea, which may consist, as in Hydra, of a structureless membrane containing no cells, or may be gelatinous as in the medusa, and may even contain cells, thus assuming more the character of an intermediate cell- layer or mesoderm. There is no body-cavity or ccelome (p. 20) surrounding the digestive cavity or enteron, and tentacles are present round the mouth. Organs of offence occur in the form of thread-cells or nematocysts. In all the higher phyla a definite mesoderm is developed in the embryo in addition to the ectoderm and endoderm (triploblastic condition), and in nearly all cases there is a definite cavity or coelome present in the mesoderm : hence all these animals are often included together as the Coelomata. PRACTICAL DIRECTIONS Hydra. Examine some living Hydrae in a vessel of water, with the naked eye or with a pocket lens, and note the differences in form according to the degree of contraction. The animal is usually attached to foreign bodies (weeds, &c.) at one end, and at the other end a number of tentacles (usually six to eight) are given off. In the expanded state the body and tentacles are greatly elongated and thread- like, while when contracted the body is more globular, and the tentacles appear like small knobs. Note the brown colour in H. fusca, and the green colour in H. viridis. Observe the method of seizing food. Place a specimen on a. slide in a drop of water, together with a small piece of water-weed or paper to prevent crushing, and then put on a cover-glass. Wait till the animal is fully expanded, and then examine with the low power. Note (Fig. 75) : — i. The body, enclosing the digestive cavity or enteron, which opens by the mouth at the distal end of the animal, at the PRACTICAL DIRECTIONS 323 summit of a conical hypostome. At the proximal end is the foot, or disc of attachment. 2. The tentacles, arranged in a single circlet or whorl around the base of the hypostome. They are hollow, and their cavities communicate proximally with the general digestive cavity of the body. On their surface are a number of small knobs. 3. The contractions of the body and tentacles. 4. The structure of the body-wall, which is made up of (a) an outer layer of colourless cells (ectoderm), and (b) an inner layer (brown in H. fusca, and green in H. viridis) of cells (endoderm) lining the digestive cavity. Between these two layers is a thin gelatinous non-cellular supporting lamella or mesoglcea, not easily seen with the low power. (The tentacles have a similar structure, the details of which cannot be made out with the low power.) Sketch. Put on the high power and examine a tentacle, focussing on to the surface as well as deeper, so as to get an optical section (see p. 558 and Fig. 76, A). Note : — 5. The relations of the ectoderm, endoderm, and support- ing lamella, and the nuclei of the ectoderm and endoderm cells. 6. The structure of the ectoderm : — (a) large conical cells with their broader ends outwards, arranged in a single row, and differing in form according to the state of contraction. The spaces between the inner narrower ends of these are filled up with (b) smaller rounded interstitial cells (absent on the foot) ; (c) thread-cells or nematocysts (Fig. 77) — oval capsules containing a spirally-wound thread, developed within certain of the interstitial cells called cnidoblasts, and when fully formed, found imbedded in or between the large ectoderm-cells ; they are much more numerous on the tentacles than on the body, causing the knobs referred to above. Each cnidoblast gives rise to a small process — the trigger-hair or cnidocil, which projects from the surface. Notice the discharged thread-cells, and observe that each consists of a flask-like base (to which part of the protoplasm and the nucleus of the burst cnidoblast usually remains attached) and a long filament, with three large and several smaller spines or barbs at its proximal end. (Smaller thread-cells, with thicker threads and no spines, are also present ; some of these have long, spirally-coiled threads, others shorter, straight threads. These can be seen later on.) 7. The endoderm, consisting of a single layer of large amoeboid cells, which in H. viridis contain green Zoo- Y 2 324 HYDRA CHAP. tf (p. 304). Note the currents in the tentacle?, which are produced by long vibratile flagella present on many of the endoderm-cells. 8. The thin transparent supporting lamella. Sketch. 9. Treat a specimen with methyl-green. A slight pressure on the cover-glass will crush the animal, and render the interstitial cells and thread -cells especially distinct. Note also other isolated cells of the ectoderm and endoderm. Sketch. 10. Examine a specimen with buds in different stages of development, and note as much as possible of the mode of asexual reproduction by gemmation. Sketch. 11. If none of your specimens bears sexual organs, try to procure a mounted preparation which shows them, and examine first with the low, and then the high power. Note — (a) the spermaries — several conical swellings near the bases of the tentacles ; they are covered with large ectoderm- cells, and contain numerous interstitial cells, each of which eventually gives rise to a sperm with a " head " and vibratile " tail." These are discharged at the apex of the cone, which when ripe may be ruptured by a slight pressure on the cover-glass, (b) The ovaries (sometimes only one), generally situated nearer the proximal end of the body. They are larger than the spermaries and more spherical, but at first have a similar structure. When ripe a single ovum is found in each. Sketch. Place some Hydras in a watch-glass with a very small amount of water, and when they have expanded, pour quickly over them a warm saturated solution of corrosive sublimate in alcohol or water. Wash several times with weak alcohol, stain for a few minutes with borax-carmine or hoematoxylin, and wash with weak and then with stronger alcohol. Place in absolute alcohol for a few minutes, and afterwards in turpentine, xylol, or oil of cloves ; mount in balsam. Work through §§ 5 — 8 again, noting especially the characters of the various cells and their nuclei, as well as — 12. The contractile processes coming off from the inner ends of the large ectoderm-cells (Fig. 76, C). These extend longitudinally, and lie against the outer surface of the supporting lamella. Sketch. Examine transverse sections through the body or tentacles (Fig. 76, B) prepared as directed on p. 136, after killing and fixing the specimens as above. Work through v PRACTICAL DIRECTIONS 325 §§ 6 — 8 again, noting the various cells and their nuclei, &c. Observe especially — 13. (a) The contractile processes of the ectoderm-cells, which will be cut across transversely, so as to appear as dots just outside the supporting lamella ; (b) the amceboid and vacuolated character of the endoderm-cells. (Special methods of preparation are necessary in order to show the flagella.) Sketch. Obelia.1 If possible, examine first alive, and then kill and stain as directed in the case of Hydra. 1. Examine under the low power and note : — (a) The polypes, with their tentacles and hypostome, expanded and contracted ; and the immature polypes. (b) The blasiostyles and medusa-buds, (c) The ccenosarc and the perisarc, hydrothecce, and gonothecce (Fig. 78). Sketch. 2. Then stain, put on the high power and make out the minute structure of the polypes, noting the — (a) Mouth, Cb) enter on, (c) ectoderm, supporting lamella, and endoderm (solid in the tentacles). On the blastostyles examine the medusa-buds. (If you wish to make permanent preparations, mounted in balsam, use the method given for Hydra.) Sketch an optical section of a polype and blastostyle. Sections of an entire branch, prepared in the usual way (see p. 136), should also be made. Select for examination those which pass as nearly as possible through the vertical and transverse axes of a polype, and compare with your sections of Hydra. 3. Place a medusa (Figs. 78 B — D and 79) on a slide with the sub-umbrella surface uppermost, stain, and mount carefully in glycerine. Note — (a) The umbrella, (b) manubrium and mouth, (c) tentacles, (c) radial and circular canals, (d) velum, (e) gonads, and (/) lithocysts (often difficult to recognise in preserved speci- mens). Sketch. 1 Specimens living or preserved, both of the colonial and medusa-stage of Obelia or some other Hydroid (as well as other marine animals described in this book) can be obtained from any Marine Biological Laboratory ; or the fresh-water Cordylophora will answer the purpose as far as the colony is concerned, but it has no medusa-stage. CHAPTER VI THE EARTHWORM : NEREIS — CHARACTERS OF THE PHYLUM ANNULATA THE general form and appearance of an earthworm are familiar to everyone. In this country there are a number of different species of earthworms belonging to several genera, the commonest of which are Lumbricus and Allolobophora ; bat the differences between these are of minor importance to the beginner, and any one of the common forms will serve our purpose. Earthworms burrow into the soil and live on decaying leaves and other organic matter, which they swallow to- gether with a considerable quantity of earth. This earth, mingled with the undigested portions of the food, is passed from the body on to the surface of the ground in the form of the well-known little heaps or " castings " which you must have noticed in gardens and fields, especially after rain, v/hen the worms come more frequently to the surface. In this way a quantity of finely divided earth, mixed with the faeces of the worms, is constantly being spread out on the surface of the soil, and Darwin calculated that on an average a layer of earth about one-fifth inch in thickness, or about ten tons an acre, is thus brought to the surface in the course of a year. Earthworms are therefore good friends to the 326 CHAP, vi THE EARTHWORM 327 gardener and agriculturist, as they are continually ploughing and manuring the soil, and in doing so they gradually cover up stones and other objects lying on the surface. The body of the earthworm is long and narrow, approximately cylindrical in shape, and bilaterally symmetrical (p. 296) : in the common forms it reaches a length of about six inches. Anteriorly it is bluntly pointed, while more posteriorly it is somewhat flattened, its greatest diameter being reached at about a third of the entire length from the anterior end. In the ordinary creeping movements of the animal, which are effected by the alternate contraction and extension of its body, the anterior end is directed forwards. The colour is pinkish in most species, and is paler on the lower or ventral than oh the upper or dorsal side. The surface of the body is distinctly marked by trans- verse annular grooves into body-segments or metameres (Fig. 82), the number of which is about 150, more or less : the segments are rather longer towards the anterior end than they are further back. At the extreme anterior end is a small finger-shaped head-lobe or prostomium, which overhangs the mouth, situated on the antero- ventral surface of the next segment, which is therefore called the peristomium, and is counted as the first metamere. The anus is a slit-like aperture on the hinder surface of the last or anal segment. The earth- worm is thus a metamerically segmented animal, and the segments are serially homologous with one another (P- 39)- In adult worms a prominent glandular swelling is noticeable on the dorsal and lateral surfaces of the body ^ extending through from five to ten segments beginning at about the thirtieth ; this is known as the clitellum, and, as we shall see, it is important in the process of impreg- 328 THE EARTHWORM CHAP. nation and in forming a case or cocoon for the eggs. On the ventral part of this region are some small glandular swellings, which are more conspicuous in young worms before the clitellum is developed. The whole of the body is invested with a delicate, iridescent membrane or cuticle (p. 313), formed as a secretion of the epiderm or outer epithelial layer of the body (p. 128). Every segment, except the first and the last, is provided with eight small cuticular spines or set a (Fig. 81, set) — slightly curved bodies with tapering ends, composed of a horn-like substance called chitin (compare p. 232) — each of which is developed in a small sac formed as an involution of the epiderm and is provided with muscles by means of which it can be protruded and retracted. These setae are arranged in couples, forming two double rows along each latero-ventral region of the body, and their points can be distinctly felt on drawing the worm through the fingers : they serve to prevent the animal from slipping backwards as it moves along on the surface of the ground or in its burrows. We have seen that the earthworm takes in its food, together with quantities of earth, by the mouth, and after retaining it for a longer or shorter time in the body expels it by the anus. It is obvious, there- fore, that there must be some kind of digestive cavity into which the food passes by the mouth, and from which effete matters are expelled through the anus. Sections (Fig. 81) show that this cavity is not a mere space excavated in the interior of the body, but a definite tube, the enteric or alimentary canal (p. 23), which passes in a straight line from mouth to anus, and- is separated in its whole extent from the walls of the body by a wide space, the body-cavity or ccelome (ccel), as in the frog (p. 20). So that the general structure of the earthworm might be imitated by taking a wide tube, stopping the ends of it vi TRANSVERSE SECTION 329 with corks, boring a hole in each cork, and then insert- ing through the holes a narrow tube of the same length as the wide one. The outer tube would represent the body-wall, the inner the enteric canal, and the cylindrical space between the two the coelome. The inner tube would communicate with the exterior by each of its ends, representing respectively mouth and anus ; the space between the two tubes, on the other hand, would have no such communication with the outside. A transverse section of the body has, therefore, the general character of two concentric circles. It will be remembered that a transverse section of Hydra has the character of two concentric circles, formed respectively of ectoderm and endoderm (Fig. 76, p. 298), the two layers being, however, only separated by the thin mesoglcea. At first sight, then, it seems as if we might compare the earthworm to a Hydra in which the ectoderm and endoderm, instead of being in contact, were separated by a wide interval ; we should then compare the body-wall of the earthworm with the ectoderm of Hydra, and its enteric canal with the endoderm. But this comparison would only express part of the truth. A thin transverse section (Fig. 81) shows the body- wall of the earthworm to consist of several distinct layers. Outside is a thin transparent cuticle (cut) show- ing no structure beyond a series of intersecting oblique lines. Next comes a layer of epithelium, the epiderm or deric epithelium (epid). Within this is a very thin connective-tissue layer representing the derm (p. 128), and a double layer of muscle-fibres by means of which the movements of the body are produced — an outer, in which the fibres extend transversely round the body (circ. mus), and a much thicker inner layer consisting of. longitudinal fibres, in section arranged like the barbs of 330 THE EARTHWORM CHAP. a feather on a central axis (long. mus). Finally, within the muscular layer and lining the ccelome is a thin peritoneal membrane (parietal layer, compare p. 26), on dors.v typh neph nephrost n.co set FIG. 81. — Lumbricus, semidiagrammatic transverse section of the middle region of the body, (x about 18.) circ. mus. layer of circular muscular fibres ; cod. coelome ; cut. cuticle ; dors. v. dorsal vessel ; epid. epiderm ; ext. neph. nephridiopore ; hep. layer of yellow cells ; long. mus. longitudinal muscles ; neph. nephridium (shown entire) : nephrost. nephrostome ; n. co. nerve cord ; set. setae ; sub. n. vess. sub- neural vessel ; typh. typhlosole ; vent. v. ventral (sub-intestinal) vessel. (From Parker and Haswell's Zoology, after Marshall and Hurst.) the inner surface of which is a very thin layer of cells, the ccelomic epithelium. A transverse section of the intestine shows an inner layer of ciliated, columnar enteric epithelium (compare p. 109), a very thin middle layer composed of muscle- vi TRANSVERSE SECTION 331 fibres and connective-tissue, and an outer layer of large yellow cells, the function of which is said to be excretory, and which correspond to a special development of the ccelomic epithelium covering the visceral layer of the peritoneal membrane which invests the intestine. We are now in a better position to compare the trans- verse section of the Hydra and the earthworm. The epiderm of the earthworm being the outermost cell-layer is to be compared with the ectoderm of Hydra, and its cuticle with the layer of the same name which, though absent in Hydra, is present in the stem of hydroid polypes, such as Obelia (viz., the perisarc). The enteric epithelium of the earthworm, bounding as it does the digestive cavity, is clearly comparable with the endoderm of Hydra. So that we have the double layer of muscle-fibres and the two layers of peritoneum not represented in Hydra, in which their position is occupied merely by the mesogloea. The muscle-fibres are not of the striped kind, like those in the corresponding position in the frog (p, 112). But it will be remembered that in medusae there is sometimes found a layer of separate muscle-fibres between the ectoderm and the mesogloea, and it was pointed out (p. 311) that such fibres represented a rudimentary intermediate cell-layer or mesoderm. We may therefore consider the muscular layer and the peritoneum of the earthworm as mesoderm, and we may say that in this animal, a^ln the frog (p. 203, and Fig. 65), the mesoderm is divisible into an outer or parietal layer, and an inner or visceral layer. The parietal layer is in contact with the ectoderm or deric epithelium, and with it forms the body-wall ; the visceral layer is in contact with the endoderm or enteric epithelium, and with it forms the enteric . canal. The 332 THE EAKTHWORM CHAP. coelome separates the parietal and visceral layers from one another, and is lined throughout by crelomic epithelium. The relation between the diploblastic polype and the triploblastic worm may therefore be expressed in a tabular form as follows : Hydroid. Earthworm.1 Cuticle Cuticle. Ectoderm Deric epithelium or epiderm. ( Connective-tissue and mus- Parietal cle-fibres. layer 1 Peritoneum with its ccelomic I epithelium (parietal layer). ! Peritoneum with its ccelomic epithelium (visceral layer). Connective-tissue and mus- cle-fibres. Endoderm .... Enteric epithelium. Mesoderm . (rudimen- tary) Strictly speaking this comparison does not hold good of the anterior and posterior ends of the worm : at both mouth and anus the deric passes insensibly into the enteric epithelium, and the study of development shows that the cells lining both the anterior and posterior ends of the canal are ectodermal (compare pp. 204 and 207). For this reason the terms deric and enteric epithelium are not mere synonyms of ectoderm and endoderm respectively. It is important that you should, before reading further, understand clearly the general composition of a triplo- 1 It will be seen that the relations of these layers in the earth- worm and frog are similar, except that in the latter the cuticle is wanting (compare Figs. 5, 38, and 39). vi CCELOME 333 blastic animal as typified by the earthworm, which may be summarised as follows. It consists of two tubes formed of epithelial cells, one within and parallel to the other, the two being continuous at either end of the body where the inner tube (enteric epithelium) is in free com- munication with the exterior ; the outer tube (deric epithelium) is lined by a layer of connective-tissue and muscle-fibres, within which is a thin peritoneum lined by coelomic epithelium, the three together forming the body- wall ; the inner tube (enteric epithelium) is covered externally by a layer of muscle-fibres and connective- tissue and a thin peritoneum covered by ccelomic epithelium, which form with it the enteric canal ; lastly, the body-wall and enteric canal are separated by a considerable space, the ccelome. The enteric canal is not, as might be supposed from the foregoing description, connected with the body-wall only at the mouth and anus, but is supported in a peculiar way. There is no dorsal mesentery as in the frog (p. 27), but a series of transverse vertical par- titions or septa (Figs. 82 and 83) extends right across the body-cavity, each being perforated by the canal. The septa are regularly arranged and correspond in position with the external grooves by which the body is divided into metameres. Thus the transverse or metameric segmentation affects the ccelome as well as the body-wall, the former being divided up into a series of chambers, which, however, communicate with one another ventrally, where the septa are incomplete (Fig. 83, n. a). Each septum is composed of a sheet of connective-tissue and muscle-fibres, and is covered on both sides by ccelomic ipithelium. The ccelome communicates with the exterior by a series of dorsal pores situated in the grooves between all the segments except about the first ten. 334 CHAP, vi ENTERIC CANAL 335 The digestive canal is not a simple tube of even calibre throughout, but is divisible into several portions. The mouth is bounded by a soft lip and leads into a small bnccal cavity, which communicates with a thick- walled pharynx (Figs. 82 and 83, ph), extending through about five segments and connected with the body-wall by a number of radially arranged muscle-fibres, the septa being absent in this region. When the worm feeds, the buccal cavity i§ everted, and the muscles serve to draw it and the pharynx back again, as well as to dilate the pharynx. The latter is followed by a narrow gullet or oesophagus (ces, ce), extending through about eight segments, and provided at about the middle of its course with a pair of lateral pouches (Fig. 83, ce. g), with each of which, in Lumbricus, two yellowish cesophageal or cdlciferous glands communicate posteriorly : these contain a calcareous substance which may neu- tralise the organic acids present in the food, swallowed. The pouches open into the gullet (Fig. 82, oes. gl), which passes posteriorly into a dilated, thin-walled receptacle, the crop (cr), and this, again, communicates posteriorly with a large gizzard (giz) with thick and muscular walls, which in about the 2oth segment communicates with the intestine (int). The intestine has a similar char- acter throughout, and extends from the gizzard to the anus ; it is ciliated and its dorsal wall is folded inwards so as to produce a longitudinal ridge or typhlosole (Fig. 81, typh), which serves to increase the absorptive surface and in the interior of which the yellow cells are very numerous : it is wanting in the last segment. Certain of the cells lining the enteric canal, and especially those along the typhlosole, are very granular, and, like the endoderm cells of the hypostome of Hydra (P- 3°3)> are to be considered as unicellular glands. They secrete a digestive juice which — mixing with the 336 THE EARTHWORM CHAP. various substances containing organic acids taken in by the mouth and said to be neutralised by the calcareous , secretion of the cesophageal glands — dissolves the proteids and other digestible parts so as to allow of their absorp- tion. It is very probable that the process is purely extra-cellular or enteric, the food being dissolved and rendered diffusible entirely in the cavity of the canal (p. 305). By the movements of the canal — caused partly by the general movements of the body and partly by the contraction of the muscles of the canal and septa aided by the action of the cilia — the contents are gradu- ally forced backwards, and the earth and other indigestible matters expelled at the anus. The ccelome is filled with a colourless transparent ccelomic fluid in which are suspended amoeboid corpuscles or leucocytes like those of the frog's blood and lymph (p. 105). The function of this ccelomic fluid is probably to distribute the digested food in the enteric canal to all parts of the body. In Hydra, where the lining wall of the digestive cavity is in direct contact with the simple wall of the body, the products of digestion can pass at once by diffusion from endoderm to ectoderm ; but in the present case a means of communication is wanted between the enteric epithelium and the comparatively complex and distant body- wall. The peptones and other products of digestion diffuse through the enteric epithelium into the ccelomic fluid, and by the continual movement of the latter — due to the contractions of the body-wall — are distributed to all parts. Thus the ex- i ternal epithelium and the muscles, as well as the nervous system, reproductive and other organs not yet described, are wholly dependent upon the enteric epithelium for their supply of nutriment. The earthworm, like the frog, possesses a series of i VASCULAR SYSTEM 337 blood-vessels, containing red blood, the whole of which ,form a single elosed vascular system, there being no communication between them and any of the other cavities of the body. The main trunks have a longi- tudinal direction, the chief ones being a large dorsal vessel, running along the dorsal surface of the enteric canal, and a» ventral or sub-intestinal vessel, below the canal (Figs. 81, dors, v, vent, v, and 83), In addition to these there are three smaller longitudinal trunks in relation to the nerve-cord, which, as we shall see, extends along the ventral side of the coelome : these are a median subneural and two lateral neural vessels (Figs. 81 and 83). All these longitudinal trunks give off branches to the various parts of the body, and certain of them are connected with one another by a series of pairs of lateral commissural vessels : in the region of the gullet there are five (Lumbricus) or six (Allolobo- phora) pairs of large vessels connecting the dorsal and subintestinal trunks ; and the dorsal and subneural trunks are also connected in each segment all along the body by a pair of smaller commissural vessels, running in the inner surface of the body- wall (Fig. 83). Notice that there is here no heart and no distinction into arteries and veins, as in the frog (p. 27). The vessels gradually divide up into smaller and smaller branches in the various parts of the body, and then again unite to form larger and larger vessels which eventually open into one or other of the main trunks. The circulation of the blood is effected by the rhyth- mical, peristaltic contraction (p. 75) of certain of the larger vessels : thus the dorsal trunk contracts from behind forwards, and the large commissural vessels — often spoken of as " hearts " — which connect it anteriorly with the sub-intestinal trunk, from above downwards, PRACT. ZOOL. 7 338 THE EARTHWORM CHAP. so that the blood passes forwards in the dorsal, back- wards in the ventral vessel. The series of vessels of FIG. 83. — Blood-vessels of Alloldbophora. Dissections from the left side of — A, the twelve anterior segments (x 5), and B, three segments in the intestinal region (X 10.) The arrows indicate the course of the circulation. (From Howes's Atlas.) be, buccal sac ; c. I, nerve-cord ; cm, commissural vessels connecting the dorsal and subneural vessels ; cm', neural commissural vessels ; e. m, mesentery, and ex, vessel of nephridium ; g. c, cerebral ganglion ; g. n, ganglionic swelling of nerve-cord ; h, commissural vessels (" hearts ") connecting the dorsal and sub- intestinal vessels anteriorly ; i, intestine ; i. I, lateral intestinal vessels ; i. s, dorsal (supra-intestinal) vessel ; m. s, septum ; n. a, ventral perforation of septum; n. I, lateral neural vessel ; n. s,_ sub-intestinal vessel ; n. s', subneural vessel ; ce, gullet ; c;tr»mp (fl\ ranged, as well as the different portions liePU of the tubule. ( a. nephrostome ; b, about 30.) b, b, slender portion of vi NEPHRIDIA 341 of the tubule is situated. The nephrostome opens into a long and slender transparent part of the tube, lined with ciliated cells in part of its course and extending along the first and second loops (b) ; this part is succeeded by a wider, ciliated portion in the second loop (c), which communicates with a still wider portion (d) lined by granular, non-ciliated, glandular cells, also lying in the second loop ; the glandular portion opens into a much wider muscular part of the tube (e), which constitutes the third loop and communicates with the exterior by a small pore — the nephridiopore — near the outer seta of the inner couple. The muscular part is lined by an epithelium, while the rest of the nephridium is formed of a single row of hollowed cells, set end to end, like a series of drain-pipes, so that their cavity is intracellular, not intercellular. Thus the nephridia, which are abundantly supplied with blood-vessels, are lined in part by gland-cells and in part by cilia which work towards the exterior. Water and nitrogenous waste from all parts of the body pass by diffusion into the blood and are conveyed to the nephridia, the gland-cells of which withdraw the waste products and pass them into the cavities of the tubes, whence they are finally discharged from the body. The granular yellow ccelomic cells on the wall of the intestine also appear to contain excretory products,1 which become set free in the body-cavity and are thence got rid of by means of the nephridia. It will be noticed that a certain amount of loss of the ccelomic fluid must take place through the dorsal pores as well as through the nephridia. In discussing the hydroid polypes we found that one of the most important points of difference between the 1 It is probable that in primitive forms the whole ccelomic epithelium was excretory in function. 342 THE EARTHWORM CHAP. locomotive medusa and the fixed polype was the presence in the former of a well-developed nervous system (p. 319) consisting of an arrangement of peculiarly modified cells, to which automatic action was seen to be due. It is natural to expect in such an active and otherwise highly- organised animal as the earthworm a nervous system of a considerably higher degree of complexity than that of a medusa. The central ner- vous system (Figs. 82, 83, and 85) con- sists of two parts, -com the brain and the ventral nerve - cord. The brain consists of a pair of white pear-shaped swell- ings or ganglia situ- ated on the dorsal side of the buccal is con- the FIG. 85. — Anterior portion of nervous system of saC, where it Lumbricus. (x about 8.) .... cer. gang, cerebral ganglia or brain ; com. 0350- tinUeCL into phageal connectives ; ne. co. ventral nerve- , —,, cord ; prost. prostomium. (From Parker and pharynx. 1116 Veil- tral nerve-cord is a longitudinal band consisting of two cords surrounded by a common sheath extending along the whole middle ventral line of the body, internally to the longitudinal muscular layer, from the third to the anal segment, and slightly swollen in each segment. The brain is connected with the anterior end of the ventral nerve-cord by a pair of nervous bands, the pharyngeal connectives which pass respec- tively right and left of the buccal sac and thus form a nerve-collar : the cord is also primarily paired, vi NERVOUS SYSTEM 343 It is to be noted that one division of the central nervous system — the brain — lies altogether above and in front of the enteric canal, the other division — the ventral nerve-cord — altogether beneath it ; and that, in virtue of the union of the two divisions by the pharyn- geal connectives, the enteric canal perforates the nervous system. Both brain and cord are composed of delicate nerve-fibres and nerve-cells, the latter being situated in the ventral and lateral regions of the cord along its whole length, so that there is here hardly any distinction into ganglia and connectives, although the swellings are often spoken of as ganglia. Along the dorsal side of the cord are three transparent tube-like structures, known as giant- fibres, the function of which is not known (Fig. 81). The whole cord is enclosed in a sheath consisting of 'connective-tissue and muscular fibres. The peripheral nervous system consists of a number of nerves, both sensory and motor (p. 162), which arise from the central nervous system and supply the various parts of the body. From the brain a number of nerves are given off to the prostomium, and from each gan- glionic enlargement two pairs of nerves can be traced into the body-wall, while between these enlarge- ments one pair is given off which supply mainly the septa. Comparing the nervous system of the earthworm with that of a medusa, it is important to notice the con- centration of the central nervous system in the higher type, and the special concentration at the anterior end of the body to form a brain. When, again, we com- pare the central nervous system of the earthworm with that of the frog (pp. 28 and 155) several important points of difference are noticeable. In the former it lies freely in the coelome, and, with the exception of the brain, is situated on the ventral side of the body ; while in the frog it is enclosed in 3, neural canal and is dorsal 344 THE EARTHWORM CHAP. in position. The brain of the frog is a complicated structure, and the whole nervous system is hollow, there being ventricles in the brain and a central canal in the spinal cord ; while in the earthworm the brain consists merely of a pair of cerebral ganglia, and it and the ventral cord are solid. The whole nervous system is capable of originating automatic action. It is a well-known fact that if the body of an earthworm is cut into several pieces each performs independent movements ; in other words, the whole body is not, as in the higher animals, paralysed by removal of the brain (p. 172). There can, however, belittle doubt that complete co-ordination, i.e., the regu- lation of the various movements to a common end, is lost when the brain is removed. The earthworm is devoid of organs of sight or hearing. It exhibits sensitiveness to bright light, which may be due to direct action on the central parts of the nervous system. The sense of hearing appears to be absent ; but a faculty analogous to taste or smell enabling the animal to distinguish between different kinds of food is well developed. Groups of narrow sensory cells in the epiderm, which are most abundant on the prostomium and peristomium, have probably to do with this faculty. There are two matters of general importance in con- nection with the structure of the earthworm to which special attention must be drawn. Notice in the first place how in this type, far more than in Hydra, we have, as in the frog, certain definite parts of the body set apart as organs (p. 30) for the performance of particular functions : it is clear that differentiation of structure and division of physiological labour play a far more obvious and important part than vi REPRODUCTIVE ORGANS 345 in any of the lower organisms described in the five previous chapters. Notice in the second place the vastly greater com- plexity of microscopic structure, the body being divisible into tissues (p. 118), each clearly distinguishable from the rest. We have epithelial tissue with its cuticle, mus- cular tissue, and nervous tissue, as well as blood and coelomic fluid. One result of this is that, to a far greater extent than in Hydra, we can study the morphology of the earthworm, as we have done that of the frog, under two distinct heads : anatomy and histology (p. 104). Asexual reproduction does not take place normally in the earthworm, but it frequently happens by accident that a worm is cut into two or more parts. When this occurs, each end is able to reproduce the missing portion : this process is known as regeneration (compare pp. 268 and 307). The earthworm, like Hydra, is monoecious or herm- aphrodite (p. 307), and besides the essential organs of sexual reproduction — ovaries and spermaries* — which are, as in the frog, developed from certain parts of the coelomic epithelium (p. 604), it possesses various accessory organs. The whole reproductive apparatus is situated in segments 9-15. The ovaries (Figs. 82, ov, and 86, o) are a pair of minute bodies about i mm. in length, attached by a short stalk, one on either side, to the posterior face of the septum separating segments 12 and 13, not far from the nerve-cord. The proximal end of each ovary, nearest the stalk, is composed of a mass of undifferentiated cells of germinal epithelium (compare Figs. 62 and 63) : nearer its middle, certain of these are seen to increase in size so as to be recognisable as young ova : while the 346 THE EARTHWORM distal end contains the ripe ova, arranged in a single row, each enclosed in a vitelline membrane and containing a large nu- cleus and nucleolus and a number of granules of food-yolk *(p. 195). The eggs are discharged into the coelome and are received into the fe- male gonoducts or oviducts (Fig. 82, o. d, and Fig. 86) — two short tubes, each with a wide ciliated mouth, placed opposite the corresponding ovary. The oviduct perforates the next following sep- tum (i.e., that between segments thirteen and fourteen) to open by a minute aperture on the fourteenth seg- ment, near the ventral couple of setae. Con- nected with the mouth of each oviduct is j a small egg-sac (Fig. 82, r. o, Fig. 86, e. s), developed as an out- growth from the same into the cavity of seg- 14 15 FIG. 86. — Diagrammatic longitudinal section of part of a Lumbricus, showing segments 9-15 and the contained generative organs of one side : x 7. In the body-wall the cuticle is indicated by a clear space, the circular muscles by irregular dots, the longitudinal muscles by dotted longitudinal lines, and the peritoneal membrane by a thin line. e. s. egg-sac ; o. ovary ; sp. aperture of anterior spermotheca — both spermothecae are indi- cated by dotted lines ; sp. s. posterior sperm- sac, the anterior and middle sacs are not lettered ; ss. sperm -reservoir ; t. anterior spermary — the posterior is not lettered ; • ' nTiurtCk SsJoSaj^^ "*^~> |f|j ^^.s^-|| '^ ^°x S ! .§ ..s " p § ^w ^ * Jl S^fal? -^i ^^^^ SS^2rt"^^^ ti-w^? fl £ «-5 .. " with, and by more pronounced lines running at right angles to, the long axis of the organ. Moreover, each lamina is double, being formed of two similar plates, the inner and outer lamellce, united with one another along VIII GILLS 403 the anterior, ventral and posterior edges of the lamina, but free dorsally. The lamina has thus the form of a long and extremely shallow bag, open above (Figs. 102 and 103) : its cavity is subdivided by vertical plates of tissue, the inter-lamellar junctions (i. I. j), which extend between the two lamellae and divide the intervening FIG. 102. — Diagram of the structure of a piece of gill of Anodonta. The gill-lamina is made up of V-shaped gill-filaments (/) arranged in longitudinal series and bound together by horizontal inter-filamentar junctions (i.f. j) which cross them at right angles, forming a kind of basket-work with apertures, the ostia (os) leading from the outside and opening (os') into the cavity of the gill. The latter is divided by vertical partitions, the inter-lamellar junctions (i. l.j), into compartments or water-tubes (w. t), which open also into the supra- branchial chamber ; b. v. blood-vessels. (From Parker and Haswell's Zoology.) space into distinct compartments or water-tubes (Figs. 101 and 102 , w. t), closed vent rally, but freely open along the dorsal edge of the gill. The vertical striation of the laminae is due to the fact that each lamella is made up of a number of close-set gill-filaments (Fig. 102, /) : the longitudinal striation to the circumstance that these filaments are connected by horizontal bars, the inter- filamentar. junctions (i. f. j} . At the thin free or ventral edge of the lamina the filaments of the two lamellae are continuous with one another, so that each lamina has 404 THE MUSSEL CHAP. actually a single set of V-shaped filaments, the outer limbs of which go to form the outer lamella, their inner limbs the inner lamella. Between the filaments, and bounded above and below by the inter-filamentar junc- tions, are minute apertures or ostia (os), which lead from the mantle-cavity through a more or less irregular series of cavities into the interior of the water-tubes. The filaments themselves are supported by chitinous rods, and covered with ciliated epithelium, the large cilia of which produce a current running from the exterior through the ostia into the water-tubes, and finally escaping by the wide dorsal apertures of the latter. The whole organ is traversed by blood-vessels (b. v). The mode of attachment of the gills presents certain features of importance (compare Fig. 103, A, B, C). The outer lamella of the outer lamina is attached along its whole length to the mantle : the inner lamella of the outer and the outer lamella of the inner lamina are attached together to the sides of the visceral mass little below the origin of the mantle : the inner lamella of the inner lamina is also attached to the visceral mass in front, but is free further back. The gills are longer than the visceral mass, and project behind it, below the posterior adductor (Figs. 101 and 103, C), as far as the posterior edge of the mantle : in this region the inner lamellae of the right and left inner laminae are united with one another, and the dorsal edges of all four laminae constitute a horizontal partition between the pallial cavity below and the exhalant chamber or cloaca above. Owing to this arrangement it will be seen that the water- tubes all open dorsally into a supra-branchial chamber (Fig. 103, s. br. c), continuous posteriorly with the cloaca and thus opening on the exterior by the exhalant siphon. The physiological importance of the gills will now be obvious. By the action of their cilia a current is pro- duced which sets in through the inhalant siphon into VIII TRANSVERSE SECTIONS 405 ret ret FIG. 103. — Anodonta. Three transverse sections (x ij). A, through the anterior part of the visceral mass. B, through the posterior part of the visceral mass. LHttt CHAP. (Fig. 107, 7^cA) extends to the anterior end of the animal, and there are no paired limbs. A brain (br) can hardly be said to be present, and these are no paired olfactory, optic, or auditory organs. The pharynx is rela- tively very large, and is perforated by very numerous oblique gill-slits (br. cl), which do not open on the exterior directly, but are surrounded by a chamber, the atrium (Figs. 107, atr, and 109, A, b) ; this opens externally by a pore (Figs. 106 a.ndioj,atrp), and though differing in its mode of development from the branchial chamber of the tadpole (p. 207), has somewhat ix EXTERNAL CHARACTERS 421 similar relations. There is no heart (Fig. no), and the colourless blood apparently contains no corpuscles of any kind. The nephridia remain distinct, not being united into a single kidney on either side : they are situated anteriorly, in the neighbourhood of the pharynx (Figs. 107, nph, and 109, A, ri) ; the gonads (Figs. j*)6 and 107, gon, and Fig. 109, A, g) are metamerically arranged, and have no ducts. In certain of its characters . Amphioxus resembles the members of a group of animals — the Tunicata, commonly known as " sea-squirts," in which the body is enclosed in a " test " or mantle, consisting largely of cellulose (p. 244). These, like Vertebrates, possess a notochord and a dorsal, hollow nervous system in young stages, and in the adult retain numerous pharyngeal gill-slits ; they are almost certainly degenerate descendants of primitive animals from which the* Vertebrata also arose. These numerous and marked differences between the lancelet and the higher Vertebrates make it necessary to place Amphioxus in a separate division of the Verte- brata, called — from one important negative character — the Acrania, while all the other Vertebrates, which possess skulls, are included in the division Craniata. The external appearance of Amphioxus is represented in Fig. 106. In addition to the points already referred to, it will be seen that the mouth is surrounded by a fold, the oral hood (or. hd), from which a number of tentacles or cirri (cir) are given off ; and that there is a lateral or metapleural fold (mtpl) along either side of the body extending backwards as far as the at rial pore, in addition to the median fin- fold (dors.f, cd.f, vent.f) extending round the tail as a caudal fin. In the young animal the gill-slits open directly to the exterior, but a median canal is subsequently formed along the ventral side of the body, and as this extends inwards and its edges unite to form the atrium (Fig. 108, si, p), leaving 422 THE LANCELET CHAP. only the atriopore open, it gradually surrounds the pharynx at the sides, pushing the coelome before it, so that the latter becomes greatly reduced in this region (Fig. 109, A, co). The skeleton is very simple ; besides the notochord there are rod-like bars of a substance somewhat FIG. 108. — Amphioxus lanceolatus. Diagrammatic transverse sections of larvce in three different stages to show the development of the atrium. ao. aorta ; c. derm ; ch. notochord ; d. intestine ; /. fascia ; fh. cavity for dorsal Heider's Embryology, after Lankester and Willey.) resembling cartilage supporting the oral hood and cirri (Fig. 107, sk), chitinoid rods (br. r) supporting the gill- bars between the clefts, and short rods of connective- tissue, the fin-rays, in the dorsal and ventral fins (Figs. 106 and 107, dors. f. r, vent. f. r). In the course of development, each primary gill-slit becomes divided into two by the growth, from above GILL-BARS 423 downwards, of a tongue-like process, the secondary gill- bar or septum (Fig. 107, br. sep. 2),. so that in the adult the slits and intervening bars are seen to be arranged in couples, the supporting rods (br. r. 1} of the primary A ' B air FIG. 109. — Amphioxus lanceloatus. A, transverse section of the pharyngeal region ( x 15). a. dorsal aorta ; b. atrium ; c. nofochord ; co. coelome ; e. endostyle ; g. gonad (ovary) ; kb. branchial bars ; kd. pharynx ; /. " liver " ; my. myomere ; n. nephridium, shown as if opening into the coelome ; r. spinal cord ; sn, sn. dorsal and ventral spinal nerves. B, transverse section of the intestinal region, (x 20.) atr. continuation of atrium on right side of intestine ; ccel. coelome ; d. ao. dorsal aorta ; int. intestine ; myom. myomere ; nch. notochord ; neu. spinal cord ; 5. int. v. sub-intestinal vein. (From Parker and Haswell's Zoology : A, from Hertwig, after Lankester and Boveri ; B, partly after Rolph.) bars (br. sep. i) being forked below. A further compli- cation is produced by the formation of transverse connections supported by skeletal rods, between the gill-bars. The gill-slits are more numerous than the muscle-segments or myomeres (myom), and, owing to their obliquity, a large number of them always appear in a transverse section (Fig. 109, A) . 424 THE LANCELET CHAP. On the ventral wall of the pharynx is a longitudinal groove, the endostyle (Fig. 109, A, e), lined by ciliated and glandular cells ; and a somewhat similar groove, the epipharyngeal groove, extends along the dorsal aspect of the pharynx : these serve as channels along which the particles of food pass into the intestine. The minute mouth (Fig. 107, mth) leads from the cavity of the oral hood into the pharynx, and is surrounded by a mem- brane, the velum (vl), produced at its edge into a number of tentacles (vl. /). The intestine is straight (Fig. 107, int) and the hepatic ccecum, or rudimentary liver, arises from its anterior end on the ventral side, and extends forwards to the right of the pharynx (Fig. 107, Ir, and Fig. 109, A, I). Although no heart, is present, the blood-vessels, some of which are contractile, present certain undoubted homo- logies with the more complex vessels of the Craniata, and so a distinction is made between arteries and veins. The arrangement of the chief of these may be seen from the diagram (Fig. no), which should be compared with Fig. 83 and later on with Fig. 119. Blood occurs in certain spaces as well as in the vessels. The numerous nephridial tubes (Fig. 107, nph) are situated above the pharynx on either side in relation with the reduced coelome in this region. The internal end of each is provided with bunches of peculiar knob- like cells projecting from closed nephrostomes : the other end opens into the atrium (Fig. 109, A, n). The spinal cord has a dorsal fissure but no ventral fissure (p. 155). Its central canal widens out in front (Fig. 107, br) to form a small cerebral ventricle (en. coe] in the front wall of which is an unpaired pigment-spot, representing a simple kind of eye (e. sp), and at its anterior end is a ciliated depression usually known as the olfactory pit (olf. p). There is no auditory organ. ix GONADS 425 The dorsal and ventral nerves (Fig. 109, A, sn) remain separate, and do not unite to form a nerve-trunk (compare p. 163). The sexes are distinct, and the metamcrically arranged gonads (Figs. 106, 107, and 109, A, gon, g) are situated in pouches, the cavities of which represent part of the ccelome and which project into the atrium on either side of the pharynx. When ripe, the minute eggs and cf.ao \ af.br.a' v^o cifbr.a. FIG. no. — Diagram of the chief vessels of Amphioxus. The arrows show the course taken by the blood. a/, br. a. afferent branchial arteries ; br. cl. gill-slits ; cp. intestinal capillaries; d. ao. paired dorsal aorta; ; d. ao'. median dorsal aorta ; ef. br. a. efferent branchial arteries ; hep. port. v. hepatic portal vein ; hep. v. hepatic vein ; int. intestine; Ir. " liver "; ph. pharynx; 5. int. v. sub-intestinal vein ; v. ao. ventral aorta, which contracts from behind forwards. (Representatives of the precaval and cardinal veins of fishes are also said to be present.) (From Parker and Has well's Zoology.) sperms burst through into the atrium and find their way out through the atriopore. The oosperm undergoes segmentation, and the embryo is hatched at a relatively early stage as a simple larva, which gradually develops into the adult form. The embryology of Amphioxus, which is very instructive, will be referred to in greater detail in Chapter XIII. PRACT. ZOOL, E E* 426 THE LANCELET CHAP. PRACTICAL DIRECTIONS AMPHIOXUS A. External characters. Examine an entire specimen and note : — i, the form of the body ; 2, the continuous median fins — dorsal, caudal, and ventral ; 3, the paired lateral fin-fold or metapleure, ex- tending along the body anteriorly to the ventral fin ; 4, the oval hood, anterior and ventral, with its cirri ; 5, the anus, a short distance from the posterior end, just on the left side of the caudal fin ; 6, the median ventral atriopore, at the junction of the lateral and ventral fins ; 7, the myomeres. B. Anatomy. The dissection of Amphioxus is too difficult a task for the beginner, and so it is better to proceed as follows : — 1. Place a young specimen for a short time in absolute alcohol, and then transfer to oil of cloves : if the specimen is a small one, it may with advantage be slightly stained first. Then transfer to a hollow slide, or, if you use an ordinary slide, support the cover-glass on two small pieces of wood of the thickness of the specimen, and mount in thick balsam : or the preparation may be examined in oil of cloves. (Many details are better seen in very young specimens, not more than half an inch long, which may be purchased ready mounted.) Examine and note in addition to the above points : — 1. The notochord, extending from the anterior to the posterior end of the body, rather nearer the olorsal than the ventral side. 2. The neural canal enclosing the spinal cord, lying just above the notochord, and having pigment in its walls. 3. The or al skeleton, consisting of a segmented basal bar along the sides of the oral hood, with a rod passing down each cirrus. 4. The longitudinal series of connective-tissue compart- ments, which are filled with gelatinous substance, forming the fin-rays. 5. The elastic horny rods supporting the gill-bars, and the arrangement of the gill-slits in pairs : — the primary gill- bars separating the successive pairs, the rods supporting them being forked ventrally ; and the secondary gill-bars ix PRACTICAL DIRECTIONS 427 between the members of a pair, the rods of these being unsplit ventrally. Note also the horizontal bars connecting the primary and secondary gill-bars and making the walls of the pharynx appear like a mesh work. 6. The cavity of the oral hood, bounded by lateral folds, the muscular velum between it and the pharynx, and the minute mouth. 7. The intestine, running straight from the hinder end of the pharynx to the anus, and giving off near its anterior end the hepatic ccecum or " liver " extending forward on the right side of the pharynx. 8. The myomeres and the intermuscular septa between them, arranged like a series of V's, the apices of which point forwards. The muscular fibres run longitudinally from septum to septum, and are of the striped kind. 9. The gonads (ovaries or spermaries], arranged in a single row on either side of and also rather further back than the pharynx extending as far as the atriopore. Note in the female the large ova, which, when ripe, cause a great disten- sion of the body ; and in the male the minute sperm-cells, the structure of which cannot be made out in entire speci- mens. Sketch. II. Cut a specimen into short pieces, about an eighth of an inch in length, and select portions from — a, between the hinder end of the atrium and the anus (Fig. 109, B), or through the latter ; b, just in front of or through the atriopore ; c, through the anterior part of the pharynx ; and d, through the posterior part of the pharynx (Fig. 109, A). Stain and embed these, and prepare a few sections from each (see p. 136). Examine in the order given above, first with the low, and then with the high power. Sketch a section from each region. a. i. The oval form of the section, the median dorsal and ventral fins, and the integument (columnar epiderm and thin derm). 2. The myomeres cut across in various planes, and appear- ing as squarish masses separated by the septa. 3. The central notochord, oval in transverse section, with transverse wavy lines indicating the boundaries of the vacuolated notochordal cells. It is surrounded by a connective-tissue sheath, continuous with the connective- tissue investment of the neural canal above and with the intermuscular septa at the sides : the latter pass into the derm peripherally. E E 2 428 THE LANCELET CHAP. 4. The spinal cord, lying in the neural canal. It is ovoid in transverse section, and has a dorsal fissure extending downwards nearly to the central canal, which is nearer the ventral than the dorsal surface. See if your section happens to pass through a dorsal or ventral nerve. The dorsal nerves arise by large single roots from about the middle of the sides of the cord, while the ventral nerves have numerous fine roots, and arise from the ventro-lateral angles of the cord. The dorsal and ventral nerves are not in the same transverse plane (compare Fig. 52), but have an alternating arrange- ment. 5. The gelatinous connective-tissue forming the fin-rays. 6. The intestine, with its single layer of long epithelial cells. If the section passes through the anus, the latter will be seen opening on the left side of the ventral fin. 7. The coelome, surrounding the intestine, and also enclosing the subintestinal veins, which are continuous anteriorly with the hepatic portal vein (Fig. no). 8. The dorsal aorta, just below the notochord. b. Go over 2, 3, 4, 5, 6, and 8 again — all much as in a. Note— 1. The dorsal and lateral (metapleural) fin- folds, ventral body-wall, and small metapleural canals. 2. The coelome , a narrow space round the intestine, in which three or four subintestinal (portal] veins can be seen. 3. The atrial cavity just outside the coelome, and separated from it by the atrial membrane. If the section passes through the atriopore, it will be seen in the mid -ventral line, putting the atrial cavity in communication with the exterior. c. Note — i. The triangular form of the body in transverse section ; the dorsal fin, the ventral surface, with its folded integument and thin transverse ventral muscles ; the metapleural folds and their contained large canals. Then go over a. 2, 3, 4, and 5 again, which are all much as in a and b. 2. The pharynx, lying just below the notochord, and1 extending nearly to the ventral body- walls. Note — (a) the deep, dorsal epibranchial groove ; (b) the ventral endostyle, projecting into the pharyngeal cavity and enclosing a coelomic canal ; (c) the gill-bars, a great number of which will be cut through obliquely, larger primary alternating with smaller secondary ones. Note the skeletal rods near their outer surfaces, and, in the primary bars, the small PRACTICAL DIRECTIONS 429 ccelomic spaces on the outer sides of the rods ; (d) the dorsal suspensory folds of the pharynx, which enclose the two dorsal ccelomic canals. 3. The atrial cavity, surrounding the pharynx, except at the mid -dorsal line, where the pharynx is attached to the sheath of the notochord, and communicating with the cavity of the pharynx through the gill-slits. It is enclosed by the two atrial folds, which are united in the mid-ventral line : the atrial epithelium is often much folded ventrally. 4. The single ventral aorta in the endostylar ccelomic canal, and the paired dorsal aortce, on either side of the epibranchial groove. d. Note again a. 2, 3, 4, 5 ; c. i, 3, 4, and also : — 1. The pharynx, much as in c. 2, but laterally com- pressed. The endostyle is now converted into a groove. 2. The hepatic ccecum, on the right side of the pharynx, and covered by the atrial epithelium. Note the ccelomic space around it, in which are the portal veins and the hepatic veins (dorsal side). 3. The gonads (ovaries or spermaries) — large masses on either side of the pharynx, projecting into the atrial cavity and covered by the atrial folds. The spaces around them represent portions of the ccelome. Note the small sperm- cells in the male, and in the female, the large ova. (The nephridia can only be made out satisfactorily in good sections of very well preserved specimens.) E E 3 CHAPTER X CHARACTERS OF THE CLASS PISCES — THE DOGFISH THE class Pisces (see p. 418) includes a number of aquatic Vertebrates which present a considerable amount of difference in form and structure, and which are distinguished from the Amphibia, as a whole, by certain constant characteristics, of which the following are the chief. The organs of respiration and locomotion are adapted for life in the water. The former consist, as in later stages of the tadpole (p. 207), of a series of vascular internal gills, attached to the septa separating the gill- clefts, and they persist throughout life : only in a very few instances (viz., in the small sub-class of the Dipnoi or " Mud-fishes ") are lungs and internal nostrils also present. The pectoral and pelvic limbs have the form of paddle-like fins, which, like the median fins (p. 421), are supported by skeletal fin-rays : — the median fin is usually subdivided into separate dorsal, ventral, and caudal portions. In addition to the endoskeleton, there is usually an exoskeleton, developed in the derm and consisting of scales ; and peculiar integumentary sense-organs, occurring also in aquatic and larval Amphibians, and supplied by special nerves not repre- sented in terrestrial Vertebrates, are always present. The cerebellum is relatively large ; a tympanic cavity 43° CHAP, x ELASMOBRANCHS 431 and membrane are not present, and except in the Dipnoi, there is only one auricle in the heart and no postcaval vein. A urinary bladder developed as an outgrowth of the enteric canal (p. 210) is wanting. There is never such a marked metamorphosis as in the case of the frog. The two most important sub-classes of the Pisces are the Elasmobranchii and the Teleostomi. The Elasmo- branchs are all marine forms, and include the dogfishes, sharks, rays, and skates : their endoskeleton is com- posed of cartilage, like that of the tadpole. The Teleostomi, in which the skeleton is mainly or to a large extent bony, include by far the greater number of fishes — both marine and fresh-water forms — such as the Salmon, Cod, Herring, Perch, as well as the Sturgeon and its allies. The dogfishes are small sharks, of which there are a number of genera and species. They are all powerful swimmers, and feed voraciously on other fishes, crustaceans, &c. The commonest British forms are the Rough Hound (Scyllium canicula), the Lesser Spotted Dogfish (5. catulus), the Piked Dogfish (Acanthias vulgaris), and the Smooth Hound (Mustelus vulgaris). The following description, though referring mainly to Scyllium, will apply, in essential respects, to any of these. External Characters and General Structure. — The dog- fish has a spindle-shaped body, ending in front in a bluntly-pointed snout and behind tapering off into an upturned tail. On the ventral surface of the head is the large, transversely elongated mouth (Fig. 117), sup- ported by a pair of jaws which work in a vertical, and not, like those of the crayfish, in a transverse plane : they are, in fact, like those of the frog, portions of the 432 THE DOGFISH CHAP, skull, having nothing to do with limbs. They are covered with numerous rows of teeth which vary in form in the different species. In front of the mouth, on the ventral surface of the snout, are the paired nostrils, each leading into a cup-like nasal or olfactory sac, and (in Scyllium) connected with the mouth by a groove. The eyes are situated one on either side of the head, above the mouth : they are protected by thick folds of the skin forming upper and lower eyelids, which can be partially closed over the eye. Behind the mouth are five pairs of slit-like apertures arranged in a longitudinal series : these are the gill-clefts or external branchial apertures (p. 418). Just behind each eye is a small aperture, the spiracle : like the gill-clefts, it communi- cates with the pharynx, and it is found by development to be actually the functionless first gill -cleft. On the ventral surface, about half-way between the two ends of the fish, is the vent or anus, leading into the cloaca (p. 23), and on either side of it a small pouch into which usually opens a minute hole, the abdominal pore (Fig. 127), communicating with the ccelome, which is therefore not a completely closed cavity, as in the frog. The region from the end of the snout to the last gill- cleft is considered as the head of the fish ; that from the last gill-cleft to the vent as the trunk ; and ilic rest as the tail. A number of symmetrically- arranged, minute apertures on the skin of the head, particularly numerous on the snout, lead into a series of tubes known as sensory canals, which are situated beneath the skin in this region ; and a single tube, known as the lateral-line canal, the position of which is indicated by a very faint longitudinal line, extends along either side of the body and tail. The whole apparatus constitutes an important, but im- perfectly understood, integumentary sense-organ : as • x FINS AND SKIN 433 already mentioned, it is represented in the tadpole but disappears at metamorphosis. Springing from the body are a number of flattened folds, called the fins, divisible into median and paired. The median folds are not continuous, as in the tadpole and lancelet, but are subdivided into several distinct parts, viz., into two dorsal fins along the middle line of the back, a caudal fin lying along the ventral edge of the upturned or heterocercal tail, and a ventral fin behind the anus. The paired folds are the pectoral fins, situated . one on either side of the trunk just behind the last gill- cleft, and the pelvic fins, one on each side of the vent : these correspond to the pectoral and pelvic limbs of the frog. In the male there is connected with the inner border of each pelvic fin a grooved, rod-like structure known as the clasper, which serves as a copulatory organ. It is very possible that the paired fins, like the median fins, are specialised portions of a primarily continuous fin-fold which extended along either side of the body, like the lateral or metapleural fold of Amphioxus (p. 421). The fish swims by vigorous strokes of the tail : the pectoral fins are used chiefly for steering, and the dorsal and ventral fins serve, like the keel of a boat, to maintain equilibrium. The skin or external part of the body-wall consists, as usual, of two layers, an outer la}/er of deric epithelium or epiderm (Fig. in, Der. Epthm), formed, like that of the frog, of several layers of cells ; and an inner layer of connective-tissue, the derm. In the derm are in- numerable close-set, calcified bodies, each consisting of a little irregular bony plate produced into a short spine — composed, like the teeth present in the frog and most Vertebrates, of a calcified tissue harder than bone known 434 THE DOGFISH CHAP, • as dentine, capped with a still harder tissue called enamel — which projects through the epiderm and gives a rough, sandpaper-like character to the skin. These placoid scales or dermal teeth together constitute the exoskeleton of the dogfish : it is a discontinuous, mainly dermal, exoskeleton (compare p. 445), and not a continuous cuticular one like that of the crayfish (P- 369). Beneath the derm is the muscular layer, which, as in , Amphioxus and in the tail of the tadpole, is metamerically segmented. The muscles are divided into myomeres, following one another from before backwards, and having a zigzag disposition. The fibres composing them are longitudinal, and are inserted at either end into fibrous partitions or myocommas which separate the myomeres from one another.1 The muscular layer is of great thickness, especially its dorsal portion. The fibres of all the body-muscles are, as in the frog and Vertebrates generally, of the striped kind. There is a large ccelome (Figs, in and 117), which, as in other Vertebrates, is confined to the trunk. The cavity is divisible into two parts : a large abdominal cavity, containing most of the viscera, and a small anterior and ventral compartment, the pericardia! cavity (Fig. 117, pcd. cav), containing the heart and communi- cating with the abdominal cavity by a canal which opens on the ventral surface of the gullet. Both are lined by coelomic epithelium (Fig. in, Ccel. Epthm) underlain by a layer of connective-tissue, a strong lining membrane being thus produced, called, as in the frog, perito- neum in the abdominal, pericardium in the pericardial cavity. Another very characteristic Vertebrate feature is that 1 In the adult frog a segmentation can still be seen in the rectus muscle of the abdomen (Figs. 2 and 16), ENDOSKELETON 435 Aer. the dorsal body-wall is tunnelled, from end to end, by a median longitudinal neural cavity, in which the central nervous system is contained. The greater part of the cavity is narrow and cylindrical, and contains the spinal cord (Figs, in and 117, sp. cd) : its anterior or cerebral portion is dilated, and contains the brain. Skeleton. — Embedded in the body-wall and extend- ing into the fins are the various parts of the endo- skeleton. This character- istic supporting framewTork is mainly composed; as in the tadpole and in embryos of Vertebrates generally, of cartilage, which may be more or less impregnated with lime salts (p. 46), so as to have, in part, the appearance of bone but differing in structure from true bone : this calcifica- tion is well seen in the vertebral centra (p. 439). The entire skeleton con- sists of separate pieces of cartilage, calcified or not, and connected with one another by ligaments (p. 57) : as in the frog, it is divisible into skull, vertebral column, and skeleton of the paired fins, with their arches or girdles ; in connection with the skull are certain cartilaginous visceral arches, forming the upper and lower jaws and supporting the 'f.V lie. in. — Diagrammatic transverse section through the trunk of a female dog fish (compare Fig. 5). The ectoderm is dotted, the endo- derm radially striated, the meso- derm evenly shaded, and the coelo- mic epithelium represented by a beaded line, (x £.) Card. V. cardinal vein ; Coel. ccelome; Coel. Epthm. parietal, and Coel. Epthm'. visceral layer of coelomic (peritoneal) epithelium ; D. Ao. dorsal aorta ; Der. Epthm. epi- derm ; Derm, derm ; Derm. F. R. dermal fin-ray ; D. F. dorsal fin ; /. int. V. intra-intestinal vein ; Int. intestine ; K. kidney ; Lat. V. lateral vein ; M . myomeres ; N. A ; neural arch ; N. cce. central canal of spinal cord ; Nph. nephridium ; Ovd. oviduct ; Ovy. ovary ; Sp. Cd. spinal cord ; Ur. ureter ; V . Cent. vertebral centrum. (From Parker's Elementary Biology.) 436 THE DOGFISH CHAP, x gills ; and there are also skeletal parts in the median fins. The cranium or brain-case (Fig. 112, Cr) is an irregular cartilaginous box containing a spacious cavity for the brain, very similar to the chondrocranium of the frog (p. 43). It is produced into two pairs of outstanding projections : a posterior pair, called the auditory capsules (aud. cp), for the lodgment of the organs of hearing, ridge-like projections on which indicate the position of the semicircular canals (p. 187) ; and, in front of the brain-cavity, an anterior pair, the olfactory capsules (olf. cp), for the organs of smell, open below, and separated from one another by a septum. Between the olfactory and auditory capsules, on either side, the cranium is hollowed out into an orbit (or), bounded by a supra- orbital and a sub-orbital ridge, for the reception of the eye. In front the brain-case is produced into three cartilaginous rods, forming the rostrum (r) and supporting the snout. On its posterior face is the foramen magnum (p. 40), on either side of which is an oval condyle for articulation with the first vertebra. On the roof of the skull, between and behind the olfactory capsules, there is a fontanelle (p. 43), closed over by connective-tissue only ; and between the two auditory capsules is a depres- sion into which open the two endolymphatic ducts of the ears (p. 465). The nerve-apertures will be referred to at a later stage. In the human and other higher vertebrate skulls the upper jaw, as we have seen to be the case in the frog (Fig. 9), is firmly united to the cranium, and the lower alone is free. But in the dogfish both jaws (up. j, I. j) are connected with the cranium by ligaments (Ig, lg') only, and each consists of strong, paired (right and left) moieties, united with one another by fibrous tissue. The upper jaw, which corresponds to the palatoquadrate r\ ^3 2. .12 H .rf- 2. — ^ X 437 438 THE DOGFISH CHAP. cartilage of the frog (p. 44), presents at its posterior end a rounded surface against which fits a corresponding concavity on the lower jaw, so that a free articulation is produced, the lower jaw, or mandibular cartilage (p. 44), working up and down in the vertical plane. The upper and lower jaws correspond to the first of a series of seven pairs of visceral arches, and are there- fore often spoken of as the mandibular arch. The remaining six pairs have on either side the form of cartilaginous half-hoops, lying in the walls of the pharynx, and united with one another below so as to form a basket-like apparatus supporting the gills. The second of these arches is distinguished as the hyoid, and is situated immediately behind the jaws. It consists of two parts : a strong, rod-like hyomandibular (Fig. 112, hy. m), which articulates above with the auditory capsule and is connected below by fibrous tissue with the jaws, thus helping to suspend them to the cranium and serving as a suspensorimn — formed in the frog by the quadrate (pp. 40 and 44) ; and a hyoid cornu or horn, which curves forwards inside the lower jaw, and is connected with its fellow of the opposite side by a median basi-hyal plate which supports the tongue (compare Fig. 9). The remaining five arches (br. a. i-br. a. 5) are called the branchial arches. Each is formed of several separate pieces, united by fibrous tissue so as to render possible the distension of the throat during swallowing : the fifth is connected below with its fellow by a large median basi-branchial plate, which supports the roof of the pericardial cavity. Both they and the hyoid give attachment to delicate cartilaginous branchial rays (br. r, br. rf, Fig. 118, r) which support the gill-filaments. In the frog -tad pole the gills are similarly supported by cartilaginous arches, which become greatly reduced and x VERTEBRAL COLUMN 439 modified at metamorphosis : the hyoid cartilage of the adult (p. 44) represents the hyoid and first branchial arch In addition to the parts of the dogfish's skull described above there are certain small cartilages of minor importance in relation with the nostrils, spiracle, mouth, and outer sides of the branchial arches (e.g., Ib and ex. br in Fig. 112). The vertebral column has the general character of a jointed tube surrounding the spinal portion of the neural fcr fcr'i.n.p np n.sp n.s.p n.sp n.a h.a FIG. 113. — Vertebras of Scyllium canicula. (x 2.) A and B from the trunk, C and D from the middle of the tail ; A and C, two vertebrae in longitudinal section ; B and D, single vertebrae viewed from one end. b. calcined portion of centrum ; c. centrum ; for. foramen for dorsal, and for', for ventral root of spinal nerve ; h. a. haemal arch ; h. c. haemal canal ; h. sp. haamal spine ; i. n. p. interneural plate ; n. a. neural arch ; n. c. neural canal ; n. p. neural process and plate ; n. sp. neural spine ; ntc. intervertebral substance (remains of notochord) ; tr. pr. transverse process r. proximal portion of rib. canal. Lying beneath this cavity, i.e., between it and the coelome, is a longitudinal row of biconcave or amphiccelous discs, the vertebral centra (Fig. 113, c ; Fig. 117, en) : they are formed of cartilage, but have their anterior and posterior faces denselv calcified ; 440 THE DOGFISH CHAP, calcified bars also extend longitudinally through each centrum from face to face, so as to show a radial arrange- ment in transverse section. The biconcave intervals between the centra are filled with a soft intervertebral substance (Fig. 113, ntc), which is also present between the first vertebra and the skull and which represents part of the embryonic notochord (see pp. 203 and 418). The centra are united by ligament, so that the whole vertebral chain is very flexible. In the frog it will be remembered that the completely ossified centra are proccelous, and are articulated with one another (pp. 36 and 57). Connected with the dorsal aspect of the series of centra is a cartilaginous tunnel consisting of the neural arches (Figs. 113 and 117, n. a), enclosing the spinal cord : it is divided into segments, corresponding with, but usually twice as numerous as the centra, owing to the presence of intercalary pieces. Arising from each centrum on either side is a neural process which is fused with a neural plate (Fig. 113, n. p) and perforated posteriorly for the exit of the ventral root of a spinal nerve (for'} ; and fitting between two consecutive neural plates is an intercalary piece, the interneural plate (i. n. p), perforated by an aperture for the dorsal root of a spinal nerve (for). The arch is completed above by the neural spines (n. sp), which fit in between the neural and interneural plates respectively, and are thus, like the lateral elements of the arch, twice as numerous as the centra. The first vertebra has facets for articulation with the condyles of the skull. In the anterior part of the vertebral column the centra give off paired, outstanding transverse processes (Fig. 113, B, tr. pr), to the end of each of which is articulated a short, cartilaginous rod, the rib (r). Further back ribs are wanting, the transverse processes x VERTEBRAL COLUMN 441 arc directed downwards, instead of outwards, and in the whole caudal region they unite below, forming hcrmal arches and spines (Fig. 113, D, //. a, h. sp, and Fig. 117, h. a), which together constitute a kind of inverted tunnel in which lie the artery and vein of the tail. In the region of the caudal fin the haemal spines are elongated and act as supports for the fin. A centrum together with the corresponding neural arch and trans- verse processes, or haemal arch, represent a vertebra or single segment of the vertebral column. In the frog we have seen that there are no independent ribs, and that the caudal vertebrae are represented by a single bone, the urostyle (p. 39). It should be noticed that in the vertebral column we have another instance of the metameric segmentation ' of the vertebrate body. The vertebrae do not, however, correspond with the myomeres, but alternate with them. The myocommas (p. 434) are attached to the middle of the vertebrae, so that each myomere acts upon two vertebrae and thus produces the lateral flexion of the body. In the embryo dogfish, as in the tadpole, before the development of the vertebral column, an unsegmented, cellular rod with an elastic sheath, the notochord, resembling that of Amphioxus (p. 419), lies beneath the neural cavity in the position occupied in the adult by the line of centra, by the development of which it is largely replaced. Seg- mentally arranged cartilages appear above and below the notochord, which on the one hand give rise to the arches, and on the other invade the notochord and constrict it at regular intervals, so as to replace it completely in those regions which will form the middle parts of the vertebral bodies, leaving the vacuolated notochordal cells in the biconvex spaces between the centra (Fig. 113, ntc). Thus much of the notochord persists as the soft intervertebral substance. The skeleton of the median fins consists of a series of parallel cartilaginous rods, the fin-rays or pterygio- 442 THE DOGFISH CHAP. phores, the proximal ends of which are more or less fused together to form basal cartilages or basalia. The free edges of the fins are supported by a double series of delicate horn-like fibres, the dermal fin-rays. The paired fins are also supported proximally by cartilaginous pterygiophores, fused at the bases to form basal cartilages which articulate with the corresponding arch or girdle, and distally by horny, dermal fin-rays (Fig. 115, d. f. r). The pelvic arch (Fig. 114, BP) is a transverse bar of cartilage situated just in front of the vent, and representing the pubic and ischiatic portions Fo* FIG. 114. — Diagram of the Elasmobranch pelvic arch (BP) and fin. Bas. basal cartilage ; Fo'. nerve-foramen ; /. iliac process ; Pro. anterior ray articulating directly with the arch ; Rod. the remaining radial cartilages. (From Wiedersheim's Comparative Anatomy.) of the girdle in the frog (p. 50) ; an iliac region, extending dorsally, and coming into connection with the vertebral column, being hardly represented (I). On its posterior edge are articular facets for the pelvic fins, each of which has a single very large basal cartilage (Bas), but one or two of the anterior rays (Pro) may articulate separately with the arch. In the male, the skeleton of the clasper (p. 433) is connected with the distal end of the basal cartilage. The shoulder-girdle (Fig. 115) is a strong, inverted x PECTORAL FIN 443 arch of cartilage situated just behind the last branchial arch. On its outer surface it presents three 'articular facets on either side for the corresponding pectoral fin ; the presence of these allows of the division of each side of the arch into a narrow, pointed, dorsal portion corresponding to the scapular region of the frog (pet. g), d.f.rt FIG. 115. — Ventral view of pectoral arch of Scyllium canicula with right pectoral fin. The pectoral arch is divisible into dorsal or scapular (pet. g), and ventral or cora- coid (pet. g') portions, continuous at the articular facets (art. f) for the fin. The pectoral fin is formed of three basal cartilages (bs. 1-3) and numerous radials (rod) : its free edge is supported by dermal rays (d. f. r.) (After Marshall and Hurst, slightly modified. ( X f .) and a broader ventral portion, answering to the coracoid (pet. g'), united in the middle line with its fellow of the opposite side (compare p. 46) : there is no sternum. The pectoral fin is formed of pterygiophores (rad), fused proximally to form basals which are three in number (bs. 1-3), the third, like the main basal of the pelvic fin, being the largest and supporting the greater number of the cartilaginous rays, which give rise distally to a series of polygonal plates. It will be noticed that while the skeleton of the crayfish is a series of articulated tubes with the muscles inside 444 THE DOGFISH CHAP. them, that of the dogfish and of the frog is a series of articulated rods with the muscles outside. The joints, formed by two rods applied at their ends and bound together by ligament, are not all confined to movement in one plane, like the hinge- joints of the crayfish, but may be capable of more or less rotary movement. Digestive Organs. — The mouth, as we have seen, is a transverse aperture bounded by the upper and lower FIG. 116. — Diagram of the development of a tooth. Bg, Bg. mesoderm ; DS. dentine ; EM. epithelium of mouth ; Ma. epithelium of enamel-organ; O. odontoblasts ; SK. dental lamina; ZK. dental papilla. (From Wiedersheim's Comparative Anatomy.) jaws. In the mucous membrane covering the jaws are embedded large numbers of teeth — conical, calcified bodies, with enamelled tips, arranged in transverse rows. They are to be looked upon as special develop- ments of the placoid scales or dermal teeth (p. 433) enlarged for the purpose of seizing prey, and are con- tinually renewed on the inner sides of the jaws as they are worn away on the outer sides. x DIGESTIVE ORGANS 445 The teeth, in Vertebrates generally, are developed in the following manner. The ectodermal epithelium of the mouth (stomod&um, p. 204) — or in the case of the dermal teeth of the dogfish that covering the body generally — grows inwards to form a ridge or dental lamina (Fig. 116, S K) which projects into the underlying mesodermal con- nective-tissue and becomes enlarged distally to form a bell- shaped enamel-organ, into the base of which a mesodermal dental papilla (Z K) extends: the superficial part of this papilla forms a layer of cells known as odontoblasts (O). The dentine (DS) is formed, in successive layers, from the odontoblasts, and gradually accumulates between them and the epithelium lining the interior of the enamel organ (Ma), which gives rise, also in successive layers, to the enamel, the hardest, external part of the tooth. Around the base of the papilla more or less bone-like substance — the cement — is formed (compare p. 510). The apex of the tooth eventually breaks through the overlying epithelium and comes to the surface. It will thus be seen that while the teeth are mainly mesodermal structures, a part of them — the enamel — is 'ectodermal in origin. The mouth leads into an oral cavity — on the floor of which is a rudimentary tongue (Fig. 117, tug) capable of very little movement — which passes insensibly into the throat or pharynx (ph), distinguished by having its walls perforated by five pairs of slits, the internal branchial apertures (i. br. a), as well as by the inner openings of the spiracles (sp). The pharynx is continued by a short gullet (gul) into a capacious U-shaped stomach, con- sisting of a wide cardiac division (cd. st) and a narrow pyloric division (pyl. st) . The pyloric division communi- cates by a narrow valvular aperture with the intestine (int), a wide, nearly straight tube having its lining membrane produced into a spiral fold, the spiral valve (sp. vl), which practically converts the intestine into a very long, closely-coiled tube, and greatly increases the absorbent surface. Finally the intestine opens into a large qhamber, the cloaca (cl), which communicates with the exterior by the vent. From the gullet backwards the enteric canal is con- PRACT. ZOOL. F F CHAP, x DIGESTIVE ORGANS 447 tained in the coelome, to the dorsal wall of which it is suspended by a median incomplete mesentery (Figs, in and 117, mes) . The greater part of the canal is developed from the enteron of the embryo, and is consequently lined by endoderm ; the oral cavity is formed from the stomodaeum, and the cloaca from the proctodaeum (p. 204). Outside the enteric epithelium are connective- tissue and muscular layers, the latter formed of un- striped fibres : it is generally characteristic of verte- brates that the voluntary muscles are striped, the involuntary unstriped (compare pp. in and 112). The immense liver, divided into two lobes (/. Ir, r. Ir), is situated below the stomach along the whole length of the abdomen,, to the front wall of which it is attached by a fold of peritoneum. It discharges its secretion, the bile, into the anterior end of the intestine by a tube, the bile-duct, which gives off a blind offshoot terminating in a. gall-bladder (p. 69). The pancreas (pan) lies against the anterior end of the intestine, with which it communicates independently by the pancreatic duct. Opening into the hinder part of the intestine or rectum is a small finger-like rectal gland (ret. gl), the function of which is not known. In addition to these there are, as in all Vertebrates, minute tubular gastric glands sunk in the mucous membrane of the stomach (p. 131). The spleen (spl) is an irregular, dark-red, gland-like body, of considerable size, attached by peritoneum to the stomach (compare pp. 23 and 98). Other so-called " ductless glands " — which are also repre- sented in the frog, giving rise to internal secretions the functions of which are imperfectly understood — are the thyroid in the throat (Fig. 122) and the thymus in connection with the dorsal ends of the branchial arches ; there are also suprarenal and interrenal bodies in the neighbourhood of the kidneys, corresponding to the adrenals of the frog (p. 145). F F* 448 THE DOGFISH CHAP, Respiratory Organs. — The respiratory organs consist of five pairs of pouches, each opening by one of the internal branchial apertures (Fig. 117, i. br. a) into the pharynx, and by one of the external branchial apertures on the exterior. The walls of the pouches, or inter- branchial septa, which form the bars separating the clefts, are supported by the cartilaginous visceral arches and branchial rays (Figs. 112, br. r, and 118, r) and are lined with mucous membrane raised into horizontal ridges, the branchial filaments, which are abundantly supplied with blood-vessels and are the actual organs of respira- tion. As the fish swims, water enters the mouth and passes by the internal clefts into the branchial I pouches, and thence outwards by the external clefts, a constant sup- ply of oxygen being thus ensured . The gill-pouches are developed as offshoots of the pharynx, and the respiratory epithelium is therefore endodermal, not ectodermal, as in the crayfish, mussel and young tad" pole (compare pp. 204 and 207). As already mentioned, the walls of the pharynx are supported by the cartilaginous visceral arches, which surround it like a series of incomplete hoops, each half- arch being embedded in the inner or pharyngeal side of an interbranchial septum. Thus the visceral arches alternate with the gill-pouches, each being related to the posterior set of filaments of one pouch and the anterior set of the next. An entire gill or holobranch therefore consists of two half-gills or hemibranchs — the sets of branchial fila- ments belonging to the adjacent sides of two consecutive gill-pouches (Fig. 118). On the other hand, a gill-pouch FIG. 118. — Transverse section through a gill of an Elas- mobranch. a. afferent branchial artery ; b. branchial arch ; &A. anterior, and bl2. pos- terior hemibranch ; h. septum ; r. branchial ray ; v. efferent branchial arteries. (From R. Hert- wig's Zoology.) x HEART 449 encloses the posterior hemibranch of one gill and the anterior hemibranch of its immediate successor. The first pouch is situated between the hyoid and the first branchial arch, and the hyoid thus bears a hemibranch only. The first four branchial arches bear each a holo- branch, and the fifth is without gill-filaments. There is a vestigial hemibranch, or pseudobranch, on the anterior (mandibular) wall of the spiracle. Now it is known that parts which have become useless tend to disappear more or less completely (e.g., pineal body of the frog — p. 159, and certain of the gills in the crayfish — p. 375). In some cases, however, such vanishing parts take on new relations with other organs and so once more become useful in other ways, undergoing a change of function. Thus in higher Vertebrates the spiracle is utilised in connec- tion with the auditory organ, and instead of disappearing entirely, as do the other gill-clefts, it gives rise to the tympanic cavity and Eustachian tube (compare p. 45). Circulatory Organs. — The heart is situated in the peri- cardial cavity or anterior compartment of the ccelome (p. 434) and is a large muscular organ composed of four chambers. Posteriorly and dorsally is a small, thin- walled sinus venosus (Figs. 117 and 119, s. v), opening in front into a single, capacious, thin-walled auricle (au) ; two auricles are present only in those Vertebrates which possess lungs. The auricle communicates with a very thick- walled ventricle (v), from which is given off in front a tubular chamber, also with thick muscular walls, the conus arteriosus (c. art). There are valves between the sinus and the auricle, and between the auricle and ventricle, and the conus contains two transverse rows, each of three valves : all the valves are arranged so as to allow of free passage of blood from sinus to auricle, auricle to ventricle, and ventricle to conus, but to prevent any flow in the opposite direction (compare pp. 79 and 87) . The conus gives off in front a single blood-vessel (v. ad] having thick elastic walls composed, like other arteries, of connective and elastic tissue and unstriped muscle. This vessel, the ventral aorta, passes forwards beneath the F F ** CHAP, x ARTERIES 451 gills, and from it arise on either side paired lateral branches, the afferent branchial arteries (Fig. 119, a. br. a, and compare Fig. no). Each afferent artery passes to the corresponding gill, and there branches out into smaller and smaller arteries, which finally open into a network of delicate capillaries (p. 95), with which the connective-tissue of the branchial filaments is permeated. The blood in these respiratory capillaries is therefore brought into close relation with the surrounding water, and as the blood flows through them it exchanges its carbon dioxide for oxygen, obtained from the air dissolved in the surrounding water. From the respiratory capillaries the blood is collected into minute arteries which join into larger and larger trunks, and finally unite into efferent branchial arteries (e. br. a) by which the purified blood is carried from the gills. The efferent arteries of the right and left sides communicate with a median longitudinal artery, the dorsal aorta (d. ao), which passes backwards, immediately beneath the vertebral column, to the end of the tail. From the efferent branchial arteries and the dorsal aorta are given off numerous arteries supplying the various parts with blood. The most important of these are paired carotid arteries (c. a) to the head, and sub- clavians (scL a] to the pectoral fins ; unpaired splanchnic arteries (cl. a, ms. a) to the enteric canal, liver, pancreas, and spleen ; numerous paired renals (r. a) to the kidneys, spermatic (sp. a) or ovarian arteries to the gonads, and a pair of iliacs (il. a) to the pelvic fins. The posterior part of the dorsal aorta, supplying the tail, is contained in the haemal canal of the caudal verte- brae, and is often spoken of as the caudal artery (cd. a). The arrangement of the arteries in the tadpole is very similar to that described above, and the diagram (Fig. 119) would serve almost equally well for a tadpole as for a fish. F F 2 452 THE DOGFISH CHAP. In the former there are four pairs of afferent and efferent branchial arteries (corresponding to the second to the fifth in Fig. 119) in relation with the corresponding branchial arches and their gills. The afferent and efferent vessels at first communicate with one another through the respiratory capillaries, but later on each afferent becomes directly connected with the correspond- ing efferent artery ; and at metamorphosis, when the gills gradually disappear, all the blood thus passes directly from the ventral to the dorsal aorta, through the four arterial arches (Fig. 120). The first arch (i) gives rise in the adult frog to the carotid trunk (ca), and loses its connection with the second at the dorsal end ; the second (2) forms the systemic trunk ; the third (3) disappears ; and the fourth (4), losing its connection with the dorsal aorta, forms the pulmo-cutaneous trunk (/) (compare p. 80). Having now traced the main course and arrangement of the chief arteries, there are a few minor points of detail to be noticed in the dogfish (Fig. 121). The five afferent branchial arteries (af. br i-j) of either side do not arise regularly and sym- metrically from the ventral aorta, as represented in the diagrammatic Fig. 119. The anterior end of the ventral aorta divides into right and left branches, each of which again subdivides to form the first two afferent branchial arteries, which supply respectively the hemibranch of the hyoid arch and the holo- branch of the first branchial arch. The third afferent branchial artery arises from, about the middle of the ventral aorta, and supplies the holobranch of the second branchial arch ; a short distance behind it the fourth and fifth come off FIG. 120. — Diagram of the arterial arches of an Amphibian. 1-4, the four arterial arches which pass up the corre- sponding branchial arches ; ao. dorsal aorta ; c. a. carotid artery ; k. embryonic arterial arch of the mandibular arch ; and h. of the hyoid ; /. pul- monary artery ; st. ventral aorta. (From Wiedersheim's Comp. Anatomy, after Boas.) ARTERIES 453 close together, and supply the gills on the third and fourth branchial arches respectively : it will be remembered that the fifth branchial arch bears no gill-filaments. After aeration the blood from each hemibranch passes into an efferent branchial artery (ef. br), which joins with its fellow of the same cleft (except in the case of ef. br9), and thus forms a loop surrounding the cleft, the two halves of adjacent FIG. 121. — Diagram of the heart and branchial arteries (of one side only) of Scyllium ; lateral view, (x i£.) af. br. l-5, afferent branchial arteries ; au. auricle ; c. a. conus arteriosus ; d. 1-5, branchial clefts ; cor. coronary artery ; d. ao. dorsal aorta ; d. c. dorsal carotid artery ; ef. br. 1-9, efferent branchial arteries ; ep. br. !-4, epibranchial arteries ; mn. mandibular artery ; sp. spiracle ; s. d. subclavian artery ; s. v. sinus venosus ; v. ventricle ; v . ao. ventral aorta ; v. c. ventral carotid artery. loops being connected in the middle by a commissural vessel. From, the upper ends of each of the four loops arises an epibranchial artery (ep. br} which is connected with the dorsal aorta (d. ao), the blood from the last hemibranch passing into the fourth loop. From the dorsal end of the first efferent branchial, a dorsal carotid artery (d. c) is given off ; this passes forwards and inwards, gives off a branch to the upper jaw and snout, and then runs inwards in a groove on the skull-floor, which it penetrates in the middle line so as to reach the cranial cavity. A vessel arises from the middle of the first efferent branchial and supplies the pseudobranch, and is then continued into a ventral carotid artery (v. c), which passes through the orbit into the cranium, giving off branches to the brain and anastomosing with the dorsal carotid. From 454 THE DOGFISH CHAP, x the ventral end of the first efferent branchial a small mandibular artery (mn) passes to the lower jaw. The dorsal aorta is continued forwards, anteriorly to the first epi- branchial artery, as a slender vessel which soon bifurcates and anastomoses with the dorsal carotid. From the ventral ends of the efferent branchial loops small arteries are given off which supply the lower parts of the head, the branchial region, and the heart (cor). The subclavian arteries (s. cl) arise from the aorta just before it is joined by the last epibranchials. In the short-bodied frog we have seen (p. 80) that there is only a single splanchnic or coeliaco-mesenteric artery, which soon divides into a coeliac and a mesenteric. In the dogfish there are four splanchnic arteries, arising separately from, the dorsal aorta, viz., a cceliac, supplying the proximal limb of the stomach and the liver ; an anterior mesenteric, arising a short distance further back and supply- ing the intestine, &c. ; a lienogastric, coming off from the aorta close behind the anterior mesenteric and going to the spleen and part of the stomach and pancreas ; and a small posterior mesenteric supplying the rectal gland. As in all Vertebrates, the arteries branch and branch again in the various parts to which they are distributed, their ultimate ramifications opening into a capillary network with which all the tissues except the cartilages and epithelia are permeated. From these systemic capillaries the blood is collected into larger and larger afferent thin-walled trunks or veins, parts of many of which are greatly dilated to form sinuses. Most of the blood from the head is brought back by a pair of jugular sinuses (Figs. 119,7'. v> an(^ I22> JUS) • each of these enters a cardinal sinus (card] , which will be referred to again directly. The blood from the tail is returned by a caudal vein (cd. v, caud) lying immediately beneath the caudal artery in the haemal canal ; this vessel enters the ccelome and then divides into right and left branches, the renal portal veins (r.p.v, r.p), which pass to the kidneys and join with the capillaries of these organs, impure blood brought from the tail mingling with the pure blood of the renal arteries (Fig. 119, r. a). From FIG. 122. — Diagram of the chief veins of Scyllium canicula, from the ventral side (hepatic portal system not shown). The more ventrally situated vessels are shown black, the more dorsal ones dotted. ( x £.) au. auricle ; br. brachial vein ; card, cardinal vein and sinus ; caud. caudal vein ; cl. cloacal vein ; g. s2. 2nd gill cleft ; h. hyoidean sinus ; h. s. hepatic sinus ; il. iliac vein ; inf. jug. inferior jugular sinus ; jug. jugular sinus ; k. kidney ; lot. lateral abdominal vein ; 1. c. lateral cutaneous vein ; n. s. nasal sinus ; o. c. olfactory capsule ; or. orbital sinus ; p. an. pelvic anastomosis between the two lateral abdominal veins ; pe. pectoral fin ; pi. pelvic fin ; pi. c. pelvic cartilage; pr. v. precaval sinus ; r. p. renal portal vein ; r. s. right spermatic vein ; s. c. subclavian vein ; s. v. sinus venosus ; t. g. thyroid gland ; v. ventricle. (After O'Donoghue.) 455 456 THE DOGFISH CHAP. the kidneys the blood is returned into a pair of partially united cardinal veins (crd. v) which pass forwards, become swollen to form large sinuses anteriorly (Fig. 122, card), receive veins from the reproductive organs, muscles, &c., and finally each communicates with a precaval sinus (pr. cv), which passes vertically downwards and enters the sinus venosus. From the stomach, intestine, spleen, and pancreas the blood is collected by numerous veins, which all join to form a large hepatic portal vein (Fig. 119, h.p. v). This behaves in the same way as the renal portal : instead of joining a larger vein on its way to the heart, it passes to the liver and breaks up to connect with the capillaries of that organ ; its blood, deprived of oxygen but loaded with nutrient matters from the enteric canal, mingling with the oxygenated blood brought to the liver by a branch of the cceliac artery. After circulating through the capillaries of the liver the blood is taken by a pair of hepatic veins (h. v) , which enlarge to form a large sinus (h.s) , and this opens into the sinns venosus (compare p. 85). The course and arrangement of the veins, like that of the arteries, is very similar to that existing in the tadpole, in which several important changes occur at metamorphosis. With the disappearance of the tail and caudal vein, the renal portal veins receive their blood from, the hind-limbs only. The hinder parts of the two cardinal veins, situated between the kidneys, fuse into one, and their anterior parts disappear, a new vessel being developed which conducts the blood from the fused cardinals to the sinus venosus : the whole of the great vein thus formed is the postcaval (p. 82, Fig. 21), which is present in the Dipnoi (p. 430) and in all Vertebrates above the fishes. The iliac veins of the dogfish (il) pour the blood from the pelvic fins into the lateral (abdominal) veins (Figs. in, 119, and 122, lat) — paired trunks running forwards in the side walls of the body to the sinus venosus, and communicating at their anterior ends with the subclavian X CIRCULATORY ORGANS 457 veins (scl. v, sc) from the pectoral fins. In addition to the dorsally situated jugular sinuses, there are paired inferior jugulars (Fig. 112, inf. jug}y bringing back the blood from the ventral parts of the head and each opening into the corresponding precaval together with the subclavian. As we have seen, several of the veins, e.g., the pre- cavals, jugulars, cardinals, and the genital veins, are dilated into spacious cavities called sinuses (Fig. 122). These are, however, of a totally different nature from the sinuses of the crayfish, which are mere spaces among the tissues devoid of proper walls (pp. 374 and 377). In the dogfish, as in the frog and Vertebrates generally, the blood is confined throughout its course to definite vessels ; the heart, arteries, capillaries, and veins invariably forming a closed system of communicating tubes. The general course of the circulation will be seen to agree with that already described in the frog as well as in the cray- fish and mussel, i.e., the blood is driven by the contractions of the heart through the arteries to the various tissues of the body, whence it is returned to the heart by the veins or sinuses (Fig. 123). But whereas in both crayfish and mussel the respiratory organs are interposed in the returning current — both their afferent and efferent vessels being veins, in the dogfish they are interposed in the outgoing current — their afferent and efferent vessels being arteries. An artery, it must be remembered, is a vessel taking blood from the heart to the tissues of the body and having thick walls ; a vein is a thin-walled vessel bringing back the blood from the tissues to the heart. Moreover, the circulation in the dogfish is, as in the frog, complicated by the presence of the two portal systems, renal and hepatic. In both of these we have a vein, renal portal or hepatic portal, which, instead of joining with larger and larger veins and so returning its blood directly to the heart, breaks up, after the manner of an artery, in the kidney or liver, the blood finding its way into the ordinary venous channels after having traversed the capillaries of the gland in question. Thus an ordinary artery arises from the heart or from an artery of higher order and ends in capillaries ; an ordinary vein arises from a capillary network, and ends in a vein of F F*** 458 THE DOGFISH CHAP. higher order or in the heart. But the hepatic and renal portal veins end in capillaries after the manner of arteries, and the efferent branchial arteries begin in capillaries after the manner of veins. With regard to the general morphology of the blood- system, the dorsal aorta with the caudal artery may be considered as a dorsal vessel (compare Earthworm, p. 337, Crayfish, p. 379, and Amphioxus, Fig. no) ; the caudal vein, hepatic portal vein, and ventral aorta as together representing a ventral vessel ; the afferent and efferent branchial arteries as commissural vessels ; and the lateral veins as lateral vessels. It will be seen that the heart of Vertebrates is a muscular dilatation of the ventral vessel, as is also shown by a study of its development (p. 597). The blood, like that of the frog (pp. 104-106), con- sists of a colourless plasma containing red corpuscles (the (i hra. FIG. 123. — Diagram illustrating the course of the circulation in the Dogfish. Vessels containing oxygenated blood red, non oxygenated blood blue. B. capillaries of the body generally ; E. of the enteric canal ; G. of the gills ; K. of the kidneys; L. of the liver; T. of the tail. a. br. a. afferent branchial arteries ; au. auricle ; c. a. conus arteriosus ; d. ao. dorsal aorta ; e. br. a. efferent branchial arteries ; h. p. v. hepatic portal vein ; h. v. hepatic vein ; Ic. lacteals (p. 98) ; ly. other lymphatics ; pr. cv. v. precaval vein ; r. p. v. renal portal veins ; 5. v. sinus venosus ; v. ventricle ; v. ao. ventral aorta. The arrows show the direction of the current. (From Parker and Haswell's Zoology.) colour of which is due to haemoglobin) and leucocytes. It must be remembered that the ventral aorta and the afferent branchial arteries (Fig. 123), like the pulmonary artery of the frog (p. 144), contain venous blood. As in the frog, there is, in addition to the blood- vessels, a system of lymphatic vessels x BRAIN 459 Nervous System.— The nervous system is constructed on a similar plan to that of the frog (compare Part I, Chapter X) and of Vertebrates in general. The central nervous system is dorsal in position and consists of a brain, contained within the cranial cavity, and con- tinuous posteriorly with a spinal cord, enclosed in the neural canal of the vertebral column : it consists of grey and white matter, and its cavity or neuroccele, lined with epithelium, gives rise to the ventricles of the brain and to the central canal of the spinal cord. In correspondence with the form of the body, the spinal cord is relatively much longer than in the frog; it is not swollen opposite the paired appendages. In the brain (Figs. 117 and 124) the bulb or medulla oblongata (Med. obi) broadens out anteriorly to form lateral swellings, and its contained fourth ventricle (w4) is roofed over by the " pia mater/' The cerebellum (Cb), which is very small in the frog, is here relatively enormous, and its surface is marked by slight grooves : it overlaps the bulb behind and the optic lobes in front, and contains a ventricle communicating with the fourth ventricle. The oval optic lobes (Opt. I) are hollow, their cavities communicating with the median ventricle or iter, and ventrally to them are the crura cerebri. The diencephalon (Di) is relatively narrower than in the frog. From its thin roof, which covers over the third ventricle, is a delicate tube-like structure (pin), which extends upwards and forwards and ends in a small knob attached to the roof of the skull : this is the pineal body. From the ventral surface of the diencephalon arises the infundibulum, with an oval swelling on each side, and to it is attached the pituitary body (Fig. 117, pty), with a vascular sac on either side of it. In front of the infundibulum is the optic chiasma (compare Figs. 49 and 50). 460 THE DOGFISH CHAP. Apart from the large size of the cerebellum, the most marked difference between the brain of the dogfish and that of the frog is seen in its anterior portion. In the frog, the dience- phalon is continu- ous anteriorly with the paired cerebral hemispheres ; in the dogfish there is in this region a rela- tively smaller, unpaired portion of the brain, marked in front by a slight groove, and known as the prosence- phalon (Pros) , which represents the cere- bral hemispheres of the higher Verte- brates but which does not become sub- divided externally into paired lobes. Anteriorly it gives off, right and left, a large, oval olfactory lobe (olf. /), each connected with the prosencephalon by a short, stout stalk and applied distally to the corresponding olfactory cap- sule. The prosencephalon contains paired lateral ventricles, which communicate posteriorly with the third ventricle and anteriorly are continued into the olfactory lobes. FIG. 124. — Dorsal view of the brain of Sycllium canicula, (x ij.) The posterior division of the brain is the medulla oblongata (Med. obi.), enclosing the fourth ventricle («i). The large cerebellum (Cb) nearly covers the optic lobes (Opt. I). The diencephalon (Di) shows, in the middle, the third ventricle, and the place of attachment of the pineal stalk (pin). The prosencephalon (Pros) gives off the olfactory lobes (olf. I). The origins of the following nerves are shown : — optic (//), trochlear (IV), trigeminal (V), facial (VII), auditory (VIII), glosso- pharyngeal (IX), and vagus (X). (From Wiedersheim's Comp. Anatomy.) x NERVES 461 Each spinal nerve arises, as in the frog (p. 163), by two roots, which, however, are not in the same trans- verse plane, the dorsal root being slightly anterior to the corresponding ventral root, so that successive dorsal and ventral roots of either side alternate with one another. As already mentioned (p. 440), the two roots of each nerve pass out from the neural canal indepen- dently, uniting on the outside of the canal to form the spinal nerve-trunk. Plexuses in connection with the nerves supplying the paired appendages are much simpler than in the frog (pp. 161 and 162). A sympa- thetic system (p. 162) is represented. The origin and distribution of the cerebral nerves is in the main similar to that already described in the case of the frog (p. 163), the chief differences, characteristic respectively of pulmonate and of branchiate Vertebrates in general, being as follows.1 In fishes, there are certain nerves, usually counted as belonging to the facial and vagus, which supply the sensory canals of the integument (Fig. 125, VII op, b, e.m, and X /) : these organs (pp. 432 and 464) are not present in terrestrial forms, and their nerves are consequently also wanting. The vagus, moreover, gives off a series of branchial branches (br. 1-4) to the gills instead of a pulmonary branch, and the glossopharyngeal (IX) is also a branchial nerve. The olfactory nerves (Fig. 125, I) arise from the olfactory lobe, which is situated in a large aperture in the skull communicating between the cranial and olfactory cavities. The optic nerve (II) is continued outwards from the optic chiasma, and passes through a foramen in about the middle of the orbit, towards the ventral side (compare Fig. 112). The oculomotor (III), arising from the crura cerebri, makes its exit from the skull a short distance behind and slightly 1 The nature of a small paired nerve in dogfishes and some other Vertebrates which arises from the prosencephalon and passes to the nasal mucous membrane is not understood. 462 THE DOGFISH CHAP. x CEREBRAL NERVES 463 above the optic nerve. The pathetic (IV), coming off from the dorsal side of the front end of the medulla oblongata and supplying the superior oblique muscle, pierces the cranial wall almost directly above the optic foramen (compare also Fig. 126). All the other nerves arise from the ventro- lateral regions of the medulla oblongata, the abducent, supplying the external rectus muscle, coming off nearer the middle line than, and anterior to, the others. The abducent (VI) and the greater part of the trigeminal (V) and facial (VII) nerves pass out through a single foramen in the skull in the posterior and ventral part ^of the orbit, just anterior to the auditory capsule. A short distance above this foramen are two others, the ventral slightly anterior to the dorsal : these transmit the ophthalmic branches (see below) of the trigeminal and facial (V op, VII op] respec- tively, and from them grooves pass along the dorsal side of the orbit to an aperture just behind the olfactory capsule, the nerves emerging again on the dorsal side of the skull. The auditory nerve passes through a large foramen on the inner side of the auditory capsule to supply the membranous labyrinth. The glossopharyngeal (IX) emerges behind the auditory capsule at the posterior end of a horizontal groove in this region, and the vagus (X) passes out through a foramen between the glossopharyngeal and the foramen magnum. The nerves supplying the integumentary sense organs are as follows : (i) The ophthalmic branch of the facial (VII op) runs, as we have seen, dorsally to the similarly named branch of the trigeminal, close under the skin, and supplies the sensory tubes and ampullae (see p. 464) of the upper part of the snout ; those of the lower part of the snout are innervated by (2) a buccal branch (VII b), which extends along the floor of the orbit just above the maxillo-mandibular division of the trigeminal ; and those in the region of the hyomandibular by a small (3) external mandibular branch (VII e. m), arising from the large hyomandibular nerve (see below). The lateral-line canal, extending along the body and tail, is supplied by (4) the lateral branch of the vagus (X /) which runs backwards to the inner side of the rest of the nerve and dorsally to the spinal nerves, along the inner side of the body-wall, giving off branches which extend outwards between the great lateral muscles to the lateral canal. The other branches of the facial are : — a small palatine (VII p), which extends along the floor of the orbit, just behind the trigeminal, and supplies the roof of the mouth ; and a large hyomandibular (VII hy) which passes behind the spiracle — first giving off small prespiracular branches 464 THE DOGFISH CHAP. (VII p. s] to its anterior wall — and extends along the anterior border of the auditory capsule and the posterior wall of the orbit, and just beneath the skin, to the anterior side of the hyoid arch : it thus forks over the spiracular or mandibulo-hyoid cleft. The glossopharyngeal (IX) forks above the first gill-cleft, thus giving rise to two branches, one passing down the posterior side of the hyoid, and the other down the anterior side of the first branchial arch. The main part of the vagus extends backwards to the outer side of the lateral nerve and gives off four branchial nerves (X br 1-4) forking over the second to the fifth gill-clefts respectively, and is then continued into the visceral nerves (X v), which supply the stomach and heart. Sensory organs. — The dogfish possesses, as we have seen, a series of peculiar integumentary sense-organs supplied by the nerves just described, the exact function of which is not known with certainty. They are situated within a number of epithelial canals, developed from the epiderm, the openings of which on the head have already been noticed (p. 432). These tubes are of two kinds, known respectively as sensory and ampullary canals : the former, which are at present in all Vertebrates with gills (p. 430), are all con- tinuous with one another and are situated along certain definite tracts on the head and jaws, a canal extending along the body and tail as the lateral-line canal. The ampullary canals, which are peculiar to Elasmobranch fishes, and which contain a gelatinous material, are not continuous with one another, but run side by side, and form large masses in the snout and at the sides of the head ; at their blind ends they are swollen to form ampulla, to which the nerves are distributed. The sensory cells are arranged in little conical masses in the lining epithelium of the sensory canals and of the ampullae, a section of one of which nearly resembles that of an ampulla of a semicircular canal of the ear (Fig. 60). The olfactory organs are a pair of cup-like sacs in the snout, enclosed by the olfactory capsules and opening externally on the ventral side by the nostrils. Notice that there are no internal nostrils, as in the frog : these SENSORY ORGANS 465 are only present in Vertebrates which possess lungs. The sacs are lined by the olfactory epithelium, which is supplied by the olfactory nerves and is raised up into ridges so as to increase the surface. The structure of the eye, as well as of the accessory apparatus in connection with it, is in all essential respects the same as in the frog (p. 181), except for the differences in the eyelids (pp. 5 and 432), the absence of a lacrymal apparatus (p. 186), and for the fact that the four recti muscles (Fig. 126) do not en- sheath the optic nerve, which emerges into the orbit* a short * distance in front of their point of ff.r origin. The membranous labyrinth of the ear (compare Fig. 59, p. 187) is also very similar to that of the frog, but being larger, and the auditory cap- sules being composed entirely of cartilage, it can be dissected out with comparative ease by slicing away the capsule with a knife. A tube given off from the sacculus, called the endolym- phatic duct, which in the frog communicates with the lymphatic system, opens to the exterior on the top of the head in the dogfish, and thus the endolymph is in free com- munication with the surrounding sea-water. As we have seen, the membranous labyrinth is the essential part of the ear, and it, together with its enclosing FIG. 126. — Semidiagrammatic figure of the eye- muscles and their nerves of an Elasmobranch. ///. oculomotor, IV. pathetic, and VI. abducent nerve ; e. r. posterior rectus muscle ; i. o. in- ferior oblique ; in. r. inferior rectus ; t. r. anterior rectus ; or. wall of orbit ; s. o. superior oblique ; s. r. superior rectus. (From Parker and Haswell's Zoology.) 466 THE DOGFISH CHAP. capsule, is often spoken of as the internal ear. In the frog there is also an accessory apparatus — the tympanic cavity and membrane, together with the columella — which is called the middle ear (compare Fig. 10 and pp. 189 and 449). Urinogenital organs. — The kidneys of the dogfish are narrow, lobulated organs, lying close to the vertebral column on either side and covered ventrally by the thick peritoneum. They extend primarily along almost the whole length of the ccelome, but in the course of development (see p. 601) certain important modifica- tions take place in them and their ducts. It will be remembered that in the male frog the efferent ducts from each spermary enter the comparatively short kidney of the same side, and that the duct of the latter serves as a urinogenital duct (p. 193). In the dogfish, about the anterior half of the primary kidney loses its excretory function (Fig. 127, k', k") ; in the male it becomes connected with the anterior end of the spermary by means of a small number of fine efferent ducts (ef. d) and gives rise to a glandular body, the so-called epididymis (A, k1 ', k"}, through which the sperms pass in order to reach the original kidney-duct, which serves as a spermiduct only (spd) . This duct remains in close connection with the epididymis ventrally and becomes greatly convoluted. In the female the whole of the anterior part of the primary kidney and its duct are merely represented by vestiges (B, k'). The posterior half of the embryonic kidney in each sex is the only part which serves as a renal organ ; this is some- what swollen posteriorly, and in connection with it special ducts or ureters are developed to carry off the urine (ur). In the female these open separately into the persistent posterior ends of the primary kidney- ducts, which unite together at their bases to form a median urinary sinus (B, u. s], opening by a single x URINOGENITAL ORGANS 467 aperture into the cloaca ; while in the male, four of them on either side unite to form a wide main ureter before communicating with a similar median sinus, which, as it receives the products both of the spermaries and kidneys, is called the urinogenital sinus (A, ug. s). The spermaries are a pair of soft, elongated organs, suspended to the dorsal body-wall by delicate peritoneal folds and invested by lymphoid tissue (see pp. 507 and 515) which unites them together posteriorly. From the anterior end of each the delicate efferent ducts referred to above pass to the epididymis to become connected with the convoluted spermiduct. The latter dilates posteriorly, where it underlies the functional kidney, forming an elongated, spindle-shaped seminal vesicle (s.v.), which opens (s.v') into the base of a thin- walled, blind reservoir of about the same length, the sperm-sac (sp. s) ; just to the inner side of its aperture are the openings of the ureters (urr). The sperm-sac is continuous posteriorly with the urinogenital sinus, the opening of which into the cloaca is situated on a papilla. The female Scyllium has a single ovary (B, ov) sus- pended by a fold of peritoneum, its fellow having become aborted. In the adult it is studded all over with rounded ova in different stages of development, varying in diameter from 12-14 mm. downwards : in other Verte- brates which produce large eggs, a similar reduction of one ovary may take place (e.g., Birds). The paired oviducts (ovd) are not coiled as in the frog ; they extend along the whole length of the dorsal wall of the ccelome, 'below the kidneys, and anteriorly unite with one another below the gullet and just in front of the liver, where they communicate with the ccelome by a common aperture (ovd') ; posteriorly they open together by a single aperture (ovd") into the cloaca, behind the rectum (r). About the anterior third of each oviduct is Cl9 FIG. 127.— The urinogenital organs of Scyllium canicula from the ventral side, (x * ) A male, and B, female. Only the anterior end of the gonad is represented in each ngure, and except that m B both kidneys are shown, the organs of the right CHAP, x DEVELOPMENT 469 side only are drawn. Tn A the seminal vesicle and sperm-sac are dissected away from the kidneys and displaced outwards, and the ureters inwards. ab. p. depression into which the abdominal pore opens ; cl. cloaca ; els. clasper ; ef. d. efferent ducts of spermary ; k. kidney ; k' (in B) vestigial portion of the kidney ; k', k" (in A) epididymis ; Ir. anterior portion of liver ; m. d. vestigial oviduct in the male ; oes. gullet ; ov. ovary ; ovd. oviduct containing egg ; ovd' its ccelomic aperture ; ovd". the common aperture of the oviducts into the cloaca ; r. rectum ; sh. gl. shell-gland ; spd. spermiduct ; sp. s. sperm sac ; s. v. seminal vesicle ; s. v'. its aperture into the urinogenital sinus ; ts. spermary ; u. g. s. urinogenital sinus ; ur. ureters ; ur'. their apertures into the urinogenital sinus ; «. s. urinary sinus. narrow and thin-walled ; the posterior two-thirds is wide and distensible, and at the junction of the two parts is a yellowish glandular region, the shell-gland (sh. gl). A vestige of the anterior ends of the oviducts can be recognised in the male (A, m. d). Development.^ — Impregnation is internal, and is effected through the agency of the claspers of the male (p. 433). The eggs, when ripe, break loose from the surface of the ovary into the coelome, and thence pass, through the common oviducal aperture, into one or other of the oviducts, where fertilisation occurs. On reaching the dilated portion of the oviduct, the oosperm of Scyllium becomes surrounded first by a gelatinous substance, and then by a horny egg-shell or " Mermaid's purse"1 secreted by the shell-gland, and having the form of a pillow-case produced at each of its four angles into a long, tendril-like process. The eggs are laid among sea-weed, to which they become attached by their tendrils. In Acanthias and Mustelus (p. 431) a mere vestige of the egg-shell is formed, and the eggs undergo the whole of their development in the oviducts, the young being eventually born alive with the form and proportions of the adult. The great size of the egg is due to the immense quantity of yolk it contains : its protoplasm is almost entirely aggregated at one pole, where it forms a small 1 An egg is contained in the oviduct shown in Fig. 127, B. 470 THE DOGFISH CHAP. disc. When segmentation of the oosperm takes place it affects the protoplasmic part alone, the inactive yolk taking no part in the process (compare crayfish, p. 383). The polyplast stage consequently consists of a little mass of cells, the blastoderm (Fig. 128), at one pole of an undivided sphere of yolk. The cells of the blastoderm become differentiated into the three embryonic layers — ectoderm, mesoderm, and endoderm. At the same time the blastoderm extends in a peripheral direction so as gradually to cover the yolk, and its middle part becomes raised up into a ridge-like thickening, which is moulded, step by step, into the form of the embryo fish. The head, trunk, and tail acquire distinctness, and become more and more completely separated off subsequently give rise to endoderm ana mesoaerm. sg. segmentation cavity ; below the blastoderm is the unsegmented yolk containing scattered nuclei (n). (From Balfour's Embryology.) from the bulk of the egg, the latter taking the form of a yolk-sac (Fig. 129, A, yk. s) attached by a narrow stalk to the ventral surface of the embryo and supplied with blood-vessels (see p. 599). In this condition the various parts of the adult fish can be recognised, but the proportions are different and the head presents several peculiarities. The gill-fila- ments (br.f) are so long as to project through the external branchial apertures and spiracles in the form of long threads abundantly supplied with blood-vessels, and DEVELOPMENT 471 apparently serving for the absorption of nutriment — the albumen in the egg-shell in the case of Scyllium, secretions of the oviduct in the viviparous forms referred to on p. 469. Besides this mode of nutrition, the yolk- sac communicates with the intestine by a narrow duct (st), through which, as well as by means of the blood-vessels, absorption of its contents is constantly going on. By the time the young fish is ready to be hatched, the greater part of the yolk-sac has been drawn into the ccelome, a mere remnant of it still dangling from the ventral surface of the body. FIG. 129. — A, embryo of Scyllium with yolk-sac (x i£) ; B, underside of head, enlarged, br. /. branchial filaments protruding through gill-clefts ; br. f . branchial filaments protruding through spiracle ; cd. f. caudal fin ; d. f. dorsal fins ; e. eye ; ex. br. ap. external branchial apertures ; mth. mouth ; na. nostril ; pet. f. pectoral fin ; pv. f. pelvic fin ; st. stalk of yolk-sac ; v. f. ventral fin ; yk. s. yolk-sac. (From Parker's Biology, after Balfour slightly altered.) 472 THE DOGFISH PRACTICAL DIRECTIONS Dogfishes are best preserved in 5 per cent, formaline, which, unlike spirit, does not coagulate the blood, so that the vessels can be injected in preserved specimens. They can be obtained, fresh or ready preserved, from any Marine Biological Station. A. External Characters : see pp. 431-434. Sketch from the side. Examine a small piece of the skin under the low power with reflected light and note the form and arrangement of the dermal teeth. Isolate some of these by boiling a small piece of skin in caustic potash (p. 359), and make out the bone-like basal plate, and the spine composed of dentine tipped with enamel. Sketch. It. Skeleton. It is advisable to have one skeleton prepared entire, and one in which the parts have been disarticulated. Obtain a common butcher's or cook's pointed knife (a strong pocket-knife will do) for cutting through the rough skin and for the coarser work of pre- paration. Prepare as directed on p. 53, dipping into hot water occasionally, or macerating in 2 per cent, nitric acid for a day or two. When the greater part of the muscles has been removed, disarticulate the skull from the vertebral column, leaving the branchial apparatus attached to it, and also remove the paired fins and their arches. Disarticulate the hyomandibular cartilage from the cranium so as to separate the visceral arches, includ- ing the jaws (compare Fig. 112) : these should then be thoroughly cleaned without further immersion in hot water, as the cartilages of which they are composed come apart very easily. The other parts may be dipped into hot water for a few seconds from time to time, but care should be taken that the more delicate elements do not thereby become separated. It is useful to prepare a second cranium as well as a few additional trunk- and caudal vertebrae, which should be bisected vertically into right and left halves. When prepared, the skeleton should be kept in weak spirit, and ' x PRACTICAL DIRECTIONS 473 not allowed to dry, or the cartilages will of course shrink, unless the following method is resorted to : — Thoroughly clean a skeleton, or typical parts of it (e.g., skull, limb-skeleton, and a few trunk- and caudal vertebne), and then transfer from weak into strong methylated spirit for a day or so, and afterwards into absolute alcohol for a few hours. Place in a vessel filled with turpentine for another day, and then transfer into melted paraffin in the water-bath until the parts are thoroughly permeated, after which they should be suspended in the water-bath in order to drain off the superfluous paraffin, and then allowed to cool. Any superfluous paraffin still remaining may then be removed with a hot wire. «\Yith the specimens before you, work through pp. 435-443, noting first of all the relations of the parts in the entire skeleton, (viz., cranial and visceral portions of the skull, trunk- and caudal vertebrae, and the skeleton of the median and paired fins). When examining the skull, note the nerve foramina (pp. 461 and 463). Sketch — (a) the skull (including visceral arches) from the side, and the cranium in longitudinal section ; (b) trunk- and caudal vertebrae from the side and in longitudinal sec- tion, and from the anterior or posterior face ; (c) the pectoral arch, from the side, with the pectoral fin attached ; and (d) the pelvic arch and fin. C. General dissection : Enteric Canal, &c. I. — Fix the animal down on the dissecting board with the ventral surface uppermost, by means of fine nails inserted through the paired fins, and make a median longitudinal cut with a common knife through the skin and underlying muscular layer — which is closely connected with the skin, from the pectoral to the pelvic arch. At each end of this incision cut through the body-walls transversely, and reflect and pin down the two flaps. Cut through the pelvic arch slightly to one side of the median line, so as not to injure the cloaca. The abdominal cavity, lined by the peritoneum, will then be exposed. (In the course of your dissection you will probably find many parasitic thread-worms belonging to the phylum Nemathelminthes — see p. 412.) Make out (compare Fig. 117) : — i. The liver, with the gall-bladder partly embedded in it close to the junction of its two lobes ; the gullet, U-shaped stomach, and the branches of the vagus nerve on its walls ; the wide intestine, narrowing into a short rectum posteriorly ; PRACT. ZOOL. G G 474 THE DOGFISH CHAP. the cloaca ; the pancreas, spleen, and rectal gland ; and the incomplete mesentery. Pass a seeker backwards, on one side of the cloaca, through an abdominal pore. 2. In the male, the spermaries, fused posteriorly ; and in the female, the single ovary, and the oviducts and shell- glands. The peritoneum covering the kidneys is so thick that at present they can only be recognised as slightly convex ridges. II. — Remove the skin from the dorsal surface of the head between and slightly in front of and behind the eyes, and then slice away part of the roof of the skull with a knife until the brain is exposed, being careful not to injure some nerves which you will see close beneath the skin on either side of the brain-case in front. Then cut off the tail trans- versely, a short distance behind the pelvic fins, and on the cut surface note — 1. The integument, in which runs the sensory canal of the lateral line ; the lateral cutaneous vein (Fig. 122, I.e.). 2. The centrum and the neural and hcemal arches of the vertebra, and the soft intervertebral substance (remains of the notochord) ; the spinal cord ; and the caudal artery and vein. 3. The myomeres and myocommas (see p. 480) ; and, if your section passes through a dorsal fin, the cartilaginous pterygiophores and the horny fin-rays (compare Fig. in). Sketch. III. — The dorsal aorta and its branches may now be injected (see p. 99) through the cut end of the caudal artery, into which a cannula should be inserted for some distance (tying is unnecessary). Now return to the examination of the abdominal viscera, and make out : — 1. The bile-duct, opening into the intestine just behind the pylorus. The pancreatic duct runs in the wall of the intes- tine, and careful dissection is required to make out its course (see §IV, i). 2. The hepatic portal vein and its factors, entering the liver near the median plane. If the blood has escaped from it, try to blow it up with a blowpipe. 3. The position of the dorsal aorta, which will be seen better at a later stage, but the chief branches of which should now be traced to their distribution, as follows : a, the cceliac artery, extending downwards and backwards along the stomach from above the posterior end of the gullet; b, the anterior mesenteric artery, arising about x PRACTICAL DIRECTIONS 475 T I inches behind the cceliac ; c, the Uenogastric artery, arising close behind, and then crossing, the anterior mesenteric ; and d, the small posterior mesenteric artery, passing downwards to the rectal gland. 4. The large hepatic sinus, immediately in front of the liver, below the gullet : slit it open, and note the veins entering it from the liver. On either side of the gullet in this region along the dorsal surface of the coelome, a capacious cardinal sinus will be seen : make an aperture in this, and pass a seeker backwards, noting that the sinus narrows into the cardinal vein, which passes along the inner side of the corresponding kidney and parallel to the aorta (Figs. 119 and 122). The genital (spermatic or ovarian) sinus communicates with the cardinal. 5. The lateral abdominal veins (Fig. 122, lat.}, running on either side of the body just beneath the peritoneum. Cut through the body-wall on one side, a short distance behind the pectoral fin ; insert a cannula, directed forwards, into the cut end of the lateral vein (see Fig. in), and inject. The vein will then be seen running forwards as far as the pectoral arch, when it turns towards the dorsal side. 6. In the female, the united anterior ends of the oviducts, and their coelomic aperture, ventral to the gullet and just in front of the liver. IV. — Taking care not to injure the anterior ends of the oviducts and to leave part of the hepatic sinus in situ, remove the liver, together with the stomach and intestine, \\ ilhout injuring the bile-duct, cutting through the stomach at its junction with the gullet and the intestine just in front of the rectal gland. Wash out the portion of the enteri'c canal thus removed under the tap, fill it with water, and place the whole under water in a dissecting-dish. Cut away portions of the wall of both stomach and intestine, and make out — 1. The course of the bile-duct and pancreatic duct, and their apertures into the intestine. 2. The pyloric valve, and the spiral valve of the intestine, which makes about seven or eight close turns, appearing like a series of cones one within the other. 3. The characters of the mucous membrane of the stomach and intestine. Sketch your dissection. D. Urinogenital organs. I. — After noting again the gonads, and in the male the delicate efferent ducts of the spermaries (Fig. 127, A), remove G G 2 476 THE DOGFISH CHAP. in the male all but the anterior ends of the latter, and in the female the entire ovary ; then carefully dissect away the thick peritoneum covering the kindeys, noting as you do so the dorsal aorta and its various branches, and once again the cardinal veins (C, § III, 4), which may be inflated with air. The renal portal veins are not easily traced without injection : this may be done from the cut end of the caudal vein. Note— 1. The brownish kidneys, and in the male the whitish forward continuation of each into the anterior sexual part or epididymis. 2. In the male : a, the convoluted spermiduct, indistin- guishable from the epididymis anteriorly and enlarging posteriorly to form the elongated seminal vesicle ; and b, the grooved claspers, which are supported by cartilages and have each a gland at the base of the groove. II. — Cut through the skin round the vent, and dissect the entire cloaca and the kidneys (together with the epididymes in the male) away from the body, noting as you do so some small paired and median bodies in close relation to the dorsal aspect of the kidneys ; these are respectively the suprarenal and interrenal bodies (p. 447). Pin your dissection down, ventral side uppermost, under water. Clear away with great care the connective-tissue which binds the ureters and generative ducts to the kidneys posteriorly, slit open the cloaca and make out — 1 . In the male (Fig. 127, A) the aperture of the rectum, and the urinogenital papilla. Insert the small scissors into the aperture at. the apex of the latter, and slit open the urino- genital sinus, continuing the cut into the two sperm-sacs ; make out the apertures of the seminal vesicles and ureters. Pass a seeker or probe into these apertures (the main ureter may be injected), and then dissect out, on one side — a, the elongated and pointed, thin-walled sperm-sac ; and b, the delicate ureters, four of which unite to form a widish common tube, situated towards the inner border of the kidney, before opening into the urinogenital sinus. Sketch. 2. In the female (Fig. 127, B), the thin-walled anterior united ends of the oviducts, their thick-walled posterior portion, the shell- glands , the apertures of the rectum and oviducts into the cloaca, and the urinary papilla. Insert the point of the scissors into the aperture on the apex of the latter, and slit open the urinary sinus, in which several openings of the ureters will be seen on either side. Cut x PRACTICAL DIRECTIONS 477 open the oviducts, and note, if present, the eggs enclosed in horny egg-cases. Sketch. E. Circulatory1 and Respiratory organs, &c. I. — After noting the spinal nerves, exposed by the removal of the kidneys, the body may be cut through just behind the pectoral arch, and the posterior portion thrown away. Pin down the head and anterior portion of the body, ventral side uppermost, make a median longitudinal incision through the skin from the lower jaw to the pectoral arch, and dissect it away on either side as far as the gill-clefts. Then, with- out injuring the lateral vein (p. 475), remove the middle portion of the pectoral arch and expose the pericardial cavity and heart. Insert a seeker, pointing backwards, along UK- dorsal side of the heart, through the pericardia -peritoneal canal which communicates between the pericardial and abdominal cavities (p. 434) : it opens on the ventral side of the gullet by two apertures. Make out — 1. The form, and relations of the chambers of the heart (sinus venosus, auricle, ventricle, and conus arteriosus). 2. The ventral aorta, to expose which the muscles in front of the pericardium must be carefully removed ; but before doing so, it is better to inject the ventral aorta, cutting a small hole in the ventricle, and inserting and tying the cannula into the conus arteriosus : use a blue injection if you have already used red for the dorsal aorta. 3. The five afferent branchial arteries (compare Fig. 121) : trace these outwards, and note their distribution. Sketch your dissection. II. — Cut through the ventral aorta at its junction with the conus arteriosus, and through both ends of the sinus venosus, carefully separating the latter from the walls of the pericardium and noting the entrance of the hepatic sinus (p. 475). Remove the entire heart, pin it down under water, ventral side uppermost. Cut open the ventricle and conus arteriosus and note — 1 . Their cavities and walls ; the auricula-ventricular aperture and valves : the valves in the conus arteriosus, of which there are two sets, consisting of three in each set. Sketch. 2. The sinu-auricular aperture is best made out by turning the heart over with the dorsal side uppermost, before cutting open the auricle from the ventral side. Sketch. 3. Insert a seeker into one of the cut distal ends (still left in situ] of the sinus venosus, and slit it up so as to expose 1 See also C. § III. 478 THE DOGFISH CHAP. the precaval sinus of the same side ; by means of a seeker find the apertures of the following veins or sinuses (compare Fig. 122) — a, the cardinal and jugular ; b, the inferior jugular ; and c, the lateral vein. III. — Insert the scissors into the external gill-clefts of one side, one by one, and extend the clefts by cutting dorsally and ventrally, so as to expose the gill-pouches, communi- cating with the pharynx by the internal gill-clefts. Make out — 1. The branchial filaments, and observe that there are four complete gills on the first four branchial arches, and a half-gill or hemibranch on the posterior face of the hyoid arch. Note also the pseudobranch on the anterior side of the spiracle. 2. The structure of the gills. Remove two entire gills ; dissect one, and cut the other across transversely (compare Fig. 1 1 8), noting the relations of the septum., cartilaginous branchial arch and rays, branchial filaments, and single afferent and paired efferent branchial artery. Sketch. IV. — Cut through the floor of the pharynx and mouth close to the middle line, just on one side of the ventral aorta, and extend the cut through the lower jaw. On one side, turn the floor outwards, and fix it back in this position so as to expose the roof of the mouth and pharynx and the internal gill-clefts ; dissect away the mucous membrane lining the roof, and trace out on one side (Fig. 121) — 1. The epibranchial and efferent branchial arteries, and the dorsal aorta. 2. The carotid, subclavian, coronary arteries, &c. F. Nervous System and Sense-organs. I. — Remove with the knife the rest of the skull-roof and a few of the anterior neural arches so as to expose the entire brain and the anterior part of the spinal cord. In doing so, be careful not to injure the contents of the orbit, the nerves referred to on p. 474, § II., or the auditory capsule of one side. After noting the " pia mater," make out — 1. The subdivisions of the brain (olfactory lobes, prosence- phalon, diencephalon, optic lobes, cerebellum, and medulla oblongata] . Sketch. 2 . The origins of the cerebral nerves from the brain and the points at which they penetrate the walls of the skull (pp. 461-464). 3. The spinal cord, and the alternating dorsal and ventral roots of the spinal nerves. Then cut through the spinal x PRACTICAL DIRECTIONS 479 cord just behind the medulla oblongata, and through the origins of the cerebral nerves. Remove the brain and place it in formaline or spirit. II. — Carefully dissect away the skin covering the head and pharyngeal region on the undissected side, expose the orbit, and remove the delicate connective-tissue surrounding its contents. Pin down firmly, and dissect out the following from the side (compare Fig. 125 and pp. 461-464). 1. The ophthalmic division of t he facial nerve, and imme- diately below it that of the trigeminal ; trace them back- wards to their foramina in the skull-wall and forwards, through a canal between the olfactory capsule and the cranium, to their distribution. 2. The large mass of sensory (ampullary) canals on the dorsal side of the snout. 3. The four recti and the two oblique eye-muscles (Fig. 126), and the nerves (III, IV, VI) supplying them. i. The eye, and the optic nerve — anterior to the recti muscles. The eye may now be removed by cutting through the muscles and optic nerve, and dissected as directed on p. 191. 5. The large, flat, maxillo-mandibular division of the trigeminal, running forwards and outwards along the floor of the orbit, and there dividing into maxillary and mandi- bular branches. 6. The facial nerve, entering the orbit close behind the maxillo-mandibular nerve, and giving off : — behind the spiracle — a large hyomandibular branch, passing along the anterior border of the auditory capsule and posterior wall of the orbit, and down the anterior side of the hyoid arch just beneath the skin : and in front of the spiracle — a palatine and prespiracular branches. Of the branches to the sensory canals, the ophthalmic has already been seen ; the buccal and external mandibular require very careful dissection in order to make them out satisfactorily. 7. The glossopharyngeal and vagus nerves. To expose these, slice away sufficient of the auditory capsule (noting as you do so the semicircular canals and the endolymphatic duct, p. 465) to expose the foramina by which they emerge from the skull, behind the auditory capsule, and separate the mass of muscles lying alongside the vertebral column from the branchial apparatus, by dissecting away the con- nective-tissue. Trace the glossopharyngeal to its bifurca- tion over the first gill-cleft, and in the vagus follow out — a, the four branchial branches, forking over the remaining 480 THE DOGFISH CHAP. gill-clefts ; b, the visceral branch ; and .c, the lateral line branch, above and to the inner side of the branchial branches. 8. Separate some of the ampullary sensory tubes from one another, and note the ampullce and the nerves supplying them. 9. Carefully slice away the cartilage of the auditory capsule of the side you have not already dissected so as to expose the membranous labyrinth. Examine under water, and make out the vestibule (utriculus and sacculus) with its contained otolithic mass, the three semicircular canals with their ampulla, and the branches of the auditory nerve (compare Fig. 59). III. Now examine the preserved brain from above, from below, and from the side, making out, in addition to the parts already noticed (F, § I, i) — 1. The optic chiasma, infundibulum (with an oval lobe and a vascular sac on either side) , pituitary body, crura cerebri, and, as far as possible, the origins of the nerves. Sketch from above and below. 2. On one side of the brain, cut into the olfactory lobe, prosencephalon, optic lobe, and cerebellum from above, so as to expose the olfactory ventricle, lateral ventricle, optic ventricle, and cerebellar ventricle. Then bisect the entire brain into right and left halves with a sharp scalpel, and examine the uninjured half in longitudinal section, noting, in addition to the parts mentioned above, the third ventricle, foramen of Monro, iter, and fourth ventricle. Sketch. G. Transverse Sections. — Cut thick transverse sections of an entire dogfish with a knife through — a, the anterior, and 6, the posterior part of the head (pharyngeal region) ; c, about the middle of the body ; and d, the tail. Make out the relations of the various parts and organs, and sketch the lateral half of each section. A more satisfactory method than this is to obtain a very young dogfish, not more than \ inch in diameter, and after cutting it transversely into pieces about J inch in thickness in the regions named above, stain, embed, and mount a few sections from each piece (see p. 136). These can first be examined with a lens or with the low power of the micro- scope, and then, by putting on the high power, important points in the histology can be made out. In addition to the minute structure of the tissues and organs described in Part I. of this book, the structure of the notochord (p. 441), integumentary sense-organs (pp. 432 and 464), dermal teeth (pp. 433 and 444), &c., should be studied. x PRACTICAL DIRECTIONS 481 H. Side Dissection. — It is very instructive to supplement and recapitulate your work on the anatomy of the dogfish by dissecting another specimen from the side (compare Fig. 117), as in the case of the crayfish. Dissect off the skin from the left side for an inch or so and observe the myomeres and myocommas. Cut open the abdominal cavity as before, very slightly to the left side of the middle line. Continue to cut forwards through the pectoral arch and backwards through the pelvic arch ; the arteries may be injected at this stage. Then dissect off the left half of each arch with the corresponding fin, and cut away the left body- wall. Cut through the skin in the mid -dorsal line : and then, working from the left side, remove the left half of the skull and visceral arches, as well as the left side of the vertebral column, so as to expose the brain and spinal cord. Remove the left kidney (and left spermary if your specimen is a male ; left oviduct if it is a female). Pin down under water, and tidy your dissection so as to reduce it to a neat longitudinal section, in which all the unpaired soft organs are left intact. Without tearing the mesentery, pin out the liver, stomach, and intestine beyond the ventral limits of the body-wall, so that the other abdominal organs are not hidden. Follow out the relations of the various organs as before, and sketch your dissection. It is important at this stage to refresh your memory of the anatomy of the frog, and to compare it with the dogfish, by making a similar dissection of it from the side (compare Fig. 7). Sketch. CHAPTER XI CHARACTERS OF THE CLASS MAMMALIA — THE RABBIT BEFORE examining a rabbit, as an example of the highest class of Vertebrates — the Mammalia, it will be well to recapitulate some of the characters of the frog, the organisation of which is higher than that of a fish. The frog, taken as an example of the class Amphibia, differs from a fish in the following points amongst others. Its paired limbs have not the form of paddle-like fins, but the fore-limb consists of upper arm, fore-arm, wrist, and hand, and the hind-limb of thigh, shank, ankle, and foot, each with characteristic skeletal parts ; it has no median fin in the adult, and that of the tadpole is not supported by fin-rays ; there is no hard, dermal exo- skeleton ; respiration in the adult is pulmonary, and internal nostrils are present ; there are two auricles in the heart, and the cardinal veins are replaced by a postcaval ; there is a urinary bladder formed as an outgrowth of the cloaca. Moreover, the endoskeleton, unlike that of the dogfish, is .in the adult composed mainly of bone. In all these characters the frog resembles the rabbit. But the Mammal differs from the Amphibian in many important respects, some of the chief of which are : — the presence of an epidermic exoskeleton consisting of hairs ; the high temperature of the blood, which remains 482 CHAP, xi MAMMALIA 483 almost uniformly within a few degrees of 100° Fahr., and does not vary to any appreciable extent with the temperature of the air ; the absence of nuclei in the red corpuscles of the blood ; the presence of mammary glands beneath the skin in the female which secrete milk for nourishing the young ; the subdivision of the body-cavity into two portions — thorax and abdomen — by a transverse partition, the diaphragm ; the presence of two ventricles as well as of two auricles in the heart, and of a single systemic aortic arch (that of the left side) ; the higher differentiation of the brain, and also of the skeleton ; and the mode of articulation of the lower jaw. More- over, in the large majority of Mammals, the teeth are differentiated into front-teeth for biting or seizing the food and cheek-teeth or grinders, and their succession is limited to two functional sets ; an external ear or pinna is present ; ' there is no cloaca, the anus and urinogenital apertures opening separately on the exterior, while the ureters open directly into the bladder ; the ova are minute ; the young undergo their early development in the oviduct, where they are nourished by diffusion from the blood-system of the parent by means of an organ known as the placenta, and after birth they are suckled by the mother. Bearing in mind these essential characters of the higher Mammalia as compared with the Vertebrates previously studied, we can now proceed to examine the structure of the rabbit in greater detail. External Characters. — The rabbit (Lepus cuniculus) is a very abundant and widely distributed animal which in the wild state makes burrows in the earth, and the practically hairless young are born in these or in special nests. There are a number of varieties, the habits and general appearance of which have been modified by domestication (compare p. 227). 484 THE RABBIT CHAP. XI In addition to head, trunk, and short tail, the rabbit possesses a distinct neck, and the whole animal, including the limbs and even the soles of the feet, is covered with a soft fur consisting of hairs (Fig. 130). In the wild rabbit, the fur is of a brownish colour, lighter below, and white under the tail : in the many domesticated varieties the colour is very varied. The hairs correspond to modified epidermic cells which become converted into a horny material ; they are developed in tube-like involutions of the epiderm called hair-sacs, into FIG. 130. — Lateral view of skeleton of Rabbit with outline of body. ( X (From Parker and Haswell's Zoology.) the swollen base of each of which a mesodermal hair-papilla projects, the substance of the hair, with its cortex arid medulla, being formed from the epidermic cells covering the papilla (Fig. 131). Into the hair-sacs open the ducts of sebaceous glands (HBD), the secretion of which serves to lubricate the hairs, which can be erected by means of muscles (Ap). There are five digits in the hand or manus, and four in the foot or pes, each terminated by a pointed and curved horny claw, developed, like the hairs, from the FIG. 131. — Longitudinal section through a hair-sac. (Diagrammatic.) (From Wiedersheim's Comp. Anatomy.} Ap, muscles for erecting the hair ; Co. derm ; F. outer longitudinal layer, and F 1. inner transverse layer of connective-tissue fibres of hair-sac ; Ft. adipose tissue ; GH. hyaline layer, which lies between the inner gnd outer hair-sheaths, i.fr, between the root-sheath and the hair-sac ; HDD. sebaceous glands ; HP. hair- papilla, containing vessels; M. medulla; O. cuticle of shaft; R. cortex; Sc. horny layer of epiderm ; Sc h. hair-sh jf t ; SM. Malpighian layer of epiderm ; WS, WSL. external and internal root-sheath. 485 486 THE RABBIT CHAP. epiderm. Along the ventral surface of the body in the female are four or five pairs of papillae — the teats, on which open the ducts of the milk-glands, which corre- spond to modified integumentary glands. The various parts of the skeleton (Fig, 130) can be felt through the skin, and it will be noticed that the anterior part of the trunk, or thorax, is surrounded by ribs, many of which meet below with a breast-bone or sternum, and which are absent in the posterior part of the trunk, or abdomen. Beneath the anterior end of the snout is the transverse mouth, which has a narrow gape and is bounded by upper and lower lips : the upper lip is divided by a longitudinal cleft which is continuous with the oblique, slit-like external nostrils. Just inside the lips are the upper and lower front teeth or incisors, which are chisel- - shaped, and behind them the hairy integument is continued on either side into the cavity of the mouth. The eyes are protected by movable hairy upper and lower eyelids, as well as by a hairless third eyelid or nictitating membrane (compare p. 5), supported by cartilage and situated in the anterior corner of the eye, over which it can be partly drawn : it corresponds to the little red lump in the inner corner of the human eye. On the upper lip and above and below the eye are certain very long and stiff hairs— the " whiskers " or vibrissce, and behind the eyes are a pair of long and movable external ears or pinna : these are supported by cartilage and are somewhat spout-shaped, leading to the external auditory openings. Below the root of the tail is the anus, and in front of and below this the urinogenital aperture, the space between them being known as the perineum. On either side of these apertures is a hairless depression of the skin on which open the ducts of the perineal glands, the secretion of which has a strong and characteristic odour. xi SKELETON 487 In the female the slit-like urinogenital aperture is called the vulva ; in the male the aperture is smaller and is situ- ated on the conical apex of a cylindrical organ, the penis, which can be retracted within a fold of skin, the foreskin or prepuce. On either side of the penis is an oval pouch of the skin, the scrota! sac, not very apparent in young animals, in each of which a spermary or testis is contained. Skeleton. — The skeleton of the adult rabbit consists almost entirely of bone, but it must be remembered that, in addition to certain cartilages described below, all articular surfaces are covered or lined by a thin layer of cartilage, and that the various parts of the skeleton are connected together by ligaments. In the skull, both replacing and investing bones (p. 43) are much more numerous than in the frog, and the structure of the entire skull is far more complicated and highly differentiated. A posterior, relatively large cranial region, in the side walls of which the auditory capsules are embedded, can be distinguished from an anterior, somewhat conical facial region, constituting the skeleton of the snout (Fig. 132). Just behind the junction of these two regions on either side is a large orbit, separated from its fellow by a thin interorbital septum, perforated by a foramen for the optic nerve (opt.fo}. At the sides of the foramen magnum are the two rounded occipital condyles ; the auditory apertures (and. me) are situated at the sides of the posterior part of the cranium, and the external nostrils open at the anterior end of the snout. Most of the bones remain more or less distinct throughout life, and are in contact along lines or sutures, many of which are wavy or zig-zagged : others, again, become completely fused in the adult, so that their limits are no longer distinguishable. Pen FIG. 132.— Skull of Rabbit. (Nat. size.) A, latera view with lower jaw ; B, ventral view. awg. £n>c. angular process of mandible ; as. alisphenoid (external pterygoid pro- cess below) ; aud. me. external auditory aperture in tympanic bone ; b. oc. basi- occipital ; b. sph. basisphenoid ; cond. condyle of lower jaw ; car. for. foramen for internal carotid artery ; eus. Eustachian canal ; /. m. foramen magnum ; cor. coronoid process ; fr. frontal ; i. incisors ; *'. d. f. inferior dental foramen for passage of the mandibular division of the trigeminal nerve ; int. pa. inter- 488 CHAP, xi SKULL 489 parietal ; t. o. /. infraorbital foramen for passage of the maxillary division of the trigeminal nerve ; ju. jugal ; kr. lacrymal ; kr. for. lacrymal foramen ; m molars ; max. maxilla ; IMS. nasal ; oc. c. occipital condyle ; opt. fo. optic foramen ; o. sph. orbitosphenoid ; pa. parietal ; pal. palatine ; pm. premolars • pal. max. palatine process of maxilla ; par . oc. paroccipital process of exoccipital ' pal. p. max. palatine process of premaxilla ; p. max. premaxilla ; peri, pcriotic ' pit. for. pituitary foramen ; pi. pterygoid ; p. t. sq. post-tympanic process of squamosal ; s. oc. supraoccipital ; sph. f. sphenoidal fissure ; sq. squamosal ; ty. bul. tympanic bulla ; vo. vomer ; zyg. max. zygomatic process of maxilla ; V. mn. foramen for mandibular division of trigeminal nerve ; VII. for facial nerve • IX. X. XI. for glossopharyngeal, vagus, and spinal accessory; XII. for hypo- glossal. (From Parker and HaswelTs Zoology.) The upper jaw forms part of the facial region, which encloses the olfactory chambers ; and the lower jaw, con- sisting of a single bone on either side, articulates directly with the sides of the cranium without the intervention of a hyomandibular as in the dogfish (p. 438) or of a quadrate cartilage as in the frog (p. 44). The rest of the visceral portion of the skull, representing the hyoid and first branchial arch, forms the so-called hyoid bone which is embedded in the base of the tongue (Fig. 135, ty). + The bones l which form the walls of the brain-case are arranged in three rings or segments, between the middle and posterior of which are intercalated the auditory cap- sules (Figs. 133 and 132 A, peri). The posterior, or occipital segment, consists of four bones, which in the adult become completely united with one another. The lower of these is the basioccipital (b. oc), a flattened bone bounding the foramen magnum below, and forming the hinder part cf the base of the skull and the lower part of each occipital condyle (oc. c). The two exoccipitals (e. oc) bound the foramen magnum at the sides, and form the upper part of the occipital con- dyles : each is produced downwards into a paroccipital process (par. oc) which fits closely against the posterior surface of a swollen bone (ty. bul) to be described presently, which is continuous with a tube surrounding 1 In the following description, the investing bones are dis- tinguished by an asterisk from the replacing bones (see p. 43). PRACT. ZOOL. H H 490 THE RABBIT CHAPo the auditory aperture (aud. me). The occipital segment is completed above by the supraoccipital (s. oc], bounding the foramen magnum above ; it has a pitted surface and is marked externally by a shield-shaped prominence. The middle, or parietal segment, consists of five bones — a basisphenoid (b. sph) below, an alisphenoid (a. sph, as) on either side, and two parietals* (pa) above. The broad posterior end of the basisphenoid is connected FIG. 133. — Skull of Rabbit in longitudinal vertical section. (Nat. size.) The cartilaginous nasal septum is removed. a. sph. alisphenoid ; e. oc. exoccipital ; e. tb. ethmo-turbinal ; eth. ethmoid ; fl. fossa for flocculus of brain ; i. incisors ; mx. tb. maxillary turbinal ; n. tb. naso- turbinal ; pal', palatine portion of the bony palate ; p. sph. presphenoid ; s. /. sella turcica, or depression in which the pituitary body lies ; I, region at which the olfactory nerves leave the skull; II, optic foramen; V mn. foramen for mandibular division of trigeminal ; VII. for facial nerve ; VIII, for auditory nerve ; IX, X, XI, for glossopharyngeal, vagus, and spinal accessory ; XII for hypoglossal. Other letters as in Fig. 132. with the basioccipital by a thin plate of cartilage, and tapers in front to a blunt point : it is perforated at about its middle by an oval pituitary foramen, and on its upper surface is hollowed out to form a depression in which the pituitary body lies (s.t). The alisphenoids are wing-like bones, directed upwards and outwards, and firmly united with the basisphenoid : each is produced ventrally into a pterygoid process (as), consisting of two SKULL 401 laminae which converge and unite with one another anteriorly. The parietals are a pair of thin, slightly arched bones forming a considerable part of the roof of the brain-case and united with one another by suture along the middle line ; the outer edge of each gives off a thin, ventral process which is covered by the squamosal (sq), a bone which will be referred to presently (p. 495) and which, on the external face of the skull, separates the parietal from the alisphenoid. Interposed between the parietals and the supraoccipital is a small median inter parietal* (int. pa). The frontal segment also consists of five bones— a presphenoid (p. sph), two orbito sphenoids (o. sph), and two frontals* (/>).* The small presphenoid is laterally com- pressed and is connected with the basisphenoid by carti- lage, so that in the dry skull there is a considerable interval between the two bones ; it forms the inferior and anterior boundary of the optic foramen (opt. fo, II), which puts the two orbits in communication with one another and both in communication with the cranial cavity. The orbitosphenoids are two wing-like laminae directed outwards and slightly backwards, and com- pletely fused with the presphenoid ; they surround the rest of the optic foramen. The frontals form the roof and side-walls of the anterior part of the brain-case, and are united by suture with one another in the middle line and with the parietals behind ; below they meet with one another anteriorly on the floor of the brain-case and unite with the presphenoid by suture ; the outer part of each forms a prominent crescentic ridge, the supra-orbital process. The brain-case is closed in anteriorly by a bone riddled with numerous small holes for the passage of the olfactory nerves (I) : this is the cribriform plate of the ethmoid (eth) . H H 2 492 THE RABBIT CHAP. It will be remembered that in the frog the occipital region is ossified by exoccipitals only, the parietals and irontals of either side are fused, there are no ali- or orbito-sphenoids, the cartilaginous walls of the anterior part of the cranium are ossified as a sphenethmoid, and that the floor of the skull is supported by an investing bone, the parasphenoid (pp. 40-44). The auditory capsules are comparatively loosely wedged in laterally between the parietal and occipital segments ; in the embryo each is ossified from three centres, instead of one (the pro-otic) as in the frog, but these early unite to form the periotic bone (peri], as the ossified auditory capsule is called. The internal or petrous portion of this bone (Fig. 133) encloses the membranous labyrinth of the ear and is very dense and hard ; posteriorly it is produced outwards as the porous mastoid portion, which is visible on the outer side of the skull (Fig. 132, A). Closely applied to the outer surface of each periotic is a bone called the tympanic*, consisting of a tubular portion above — the edge of which surrounds the auditory opening (aud. me) to which the cartilage of the pinna is attached, and of a swollen portion, or tym- panic bulla (ty. bul), below: this encloses the tympanic cavity, and in it, at the base of the tubular portion, is an incomplete bony ring to which the tympanic mem- brane is attached (Fig. 143). The tympanic is incom- plete on its inner side, where its cavity is closed by the outer wall of the periotic, and between the two, at the antero-inferior angle of the former, is the aperture by which the Eustachian tube leaves the tympanic cavity (compare p. 45). When the tympanic is removed, two small holes are seen on the outer wall of the periotic : the anterior of them is the fenestra ovalis and is plugged by the stapes — which, together with two small bones, the malleus and incus (Fig. 143), forms the chain of auditorv ossicles to be described later in connection xi SKULL 493 with the organ of hearing ; the posterior aperture is called the fenestra rotunda. On the internal or cranial surface of the periotic is a large depression (fl) which lodges the flocculus of the cerebellum (Fig. 141). The olfactory capsules are roofed in by two long and narrow nasal bones* (nas), which meet together in the middle line and unite by suture with the frontals pos- teriorly. Their side walls are formed by the bones which bear the teeth of the upper jaw — the premaxillce* (p. max) and maxilla* (max), and in the median line below is a single long and slender bone, deeply grooved on its upper surface, and formed by the fusion of the two vomers* (00) . The two nasal chambers are separated from one another in the middle line by a median vertical plate of cartilage, the nasal septum (Fig. 135, n. s), embraced below by the vomer. This cartilage, together with the cribriform plate and a median vertical plate of bone (eth) extending forwards from the latter into the septum, constitutes the mesethmoid. Within the nasal chambers certain scroll-like folds of the mucous mem- brane (Fig. 135) are present in order to increase the surface, and in these cartilages are developed. The cartilages become ossified, and the resulting turbinal bones unite with certain of the bones enclosing the olfactory organs, and are named accordingly. The ethmoid turbinals (Fig. 133, e. tb), or true olfactory scrolls, are two complicated, folded bones united to the cribriform plate of the ethmoid, and are covered in the fresh condition by the olfactory epithelium ; the maxillo- turbinals (mx. tb) are similar but more complex bones in the antero- ventral part of the nasal cavities ; and the naso-turbinals (n. tb) are thin, folded bones, much less complex, and fused with the inferior surface of the nasals. 494 THE RABBIT CHAP. In the front wall of each orbit, fitting comparatively loosely between the frontal and maxilla, is a small bone, the Jacrymal* (Fig. 132, A, Icr), with a notch near its outer border through which the naso-lacrymal duct passes (p. 186). As in the frog, the chief bones of the upper jaw on either side are the premaxilla* (p. max] and the maxilla* (max), and nearer the middle line are the palatine* (pal) and " pterygoid "* (pt) : in the embryo the position of the two last-mentioned bones is "taken by cartilage representing the upper jaw of the dogfish (compare Figs. 112 and 9) . The premaxillae, in which the sockets for the front or incisor teeth are situated, form the anterior boundary of the snout, and articulate with one another in the median line and with the maxilla behind : each gives off a nasal process passing backwards between the nasal and maxilla to the frontal, and a -palatine process (pal. p. max) extending backwards along the palate in contact with its fellow of the opposite side. The maxillae are large and irregular bones, parts of the sides of which are f enestrated, and in which the cheek-teeth are situated. From the inner and inferior edge of each, opposite the first two cheek-teeth, a horizontal palatine process (pal. max) is given off, which, articulating with its fellow of the opposite side, forms the anterior part of the bony support of the hard palate — -this is of much less extent in the rabbit than in most mammals : from its outer side arises a zygomatic process (zyg. max), which forms the anterior part of the strong zygomatic arch extending below and externally to the orbit. The palatines are thin, nearly vertical, bony laminae, internal to the maxillae to which they are attached in front, while above they join the presphenoid and the pterygoid process of the alisphenoid. They bound the passage cf the internal nostrils, and from the inner and xi SKULL 495 anterior region of each is given off, opposite the third cheek-tooth, a horizontal, inwardly directed process (pal1}, which, articulating in the middle line with. its fellow of the opposite side and in front with the palatine process of the maxilla, forms the posterior part of the bony support of the hard palate. The bones usually known as pterygoids are small irregular plates attached to the posterior edge of the corre- sponding palatine and the pterygoid process of the alisphenoid ; each ends ventrally in a backwardly- curved process. The squamosals* (sq) are a pair of plates which over- lap and complete the side- walls of the brain-case (p. 491) in front of the periotics : they articulate with the f rentals, parietals, orbitosphenoids, and alisphenoids. From the outer face of each is given off a strong zygomatic process, which bears on its under surface the articular facet for the lower jaw, and further back a slender process (p. t. sq) arises which is applied to the outer surface of the periotic. The zygomatic processes of the squamosal and maxilla respectively are united by a flat bar of bone, the jugal* (jii), which in the adult is fused with the latter. All these three bones therefore take part in forming the zygomatic arch. Most of the apertures for the transmission of the cerebral nerves have so far not been mentioned : the branches of the olfactory nerve, as we have seen, pass out through the numerous apertures in the cribriform plate (Fig. 133, eth, I), and the optic foramen (opt. fo, II) is situated between the orbitosphenoid and presphenoid. Behind and below the optic foramen is a vertical aperture — the sphenoidal fissure (spjlm f) — between the basisphenoid and alisphenoid, which transmits the third, fourth, and sixth nerves, as well as the ophthalmic and maxillary divisions of the fifth. Between the periotic and alisphenoid is a large space (V mn), through the anterior part of which the mandibular division of the 496 THE RABBIT CHAP. trigeminal leaves the skull.1 Between the mastoid portion of the periotic and the posterior border of the tympanic, at the junction of the tubular and bulbous portions of the latter bone, is a small aperture — the stylomastoid foramen, which transmits the seventh nerve : this and the eighth (VII, VIII) enter the periotic just below the depression for the flocculus of the cerebellum (fi). A space (IX, X, XI) between the occipital condyle and tympanic bulla gives exit to the ninth and tenth, as well as to the eleventh — which is not represented as a distinct nerve in the dogfish and frog ; and the hypoglossal (p. 160), which in Mammals is counted as the twelfth cerebral nerve, passes out through two small apertures (XII) in the exoccipital, just anterior to the condyle. Various other apertures will be noticed in the skull and jaws : through some of these branches of certain of the above-mentioned nerves pass, while others transmit blood- vessels. The lower jaw or mandible (Fig. 132, A) consists of two halves or rami, each corresponding essentially to the dentary of the frog, which unite with one another in front, at the symphysis, by a rough surface, while behind they diverge like the limbs of the letter V. Each ramus is a vertical plate of bone, broad behind and tapering towards the front, where it bears the incisor teeth : further back, on its upper margin, are the sockets for the cheek-teeth, and behind them is an ascending portion which bears the condyle (cond) for articulation with the facet on the squamosal : in front of the condyle is a curved coronoid process (cor] . The postero-inf erior border, which is rounded and inflected, is known as the angular process (ang. pro). The hyoid is a small bone situated at the root of the tongue, anterior to the larynx (Fig. 135, hy). It consists of a stout body or basi-hyal, a pair of small anterior horns, representing the ventral ends of the hyoid arch 1 In many Mammals (e.g., dog, cat) the maxillary division of the trigeminal passes out through a separate foramen, behind the sphenoidal fissure ; and the anterior part of the space referred to above is separated off as a distinct foramen for the mandibular division. xi VERTEBRAL COLUMN 497 of lower Vertebrates, and a pair of longer, backwardly- projecting posterior horns or thyro-hyals, attached to the larynx and representing the lower ends of the first branchial arch (compare p. 44). The vertebral column includes about forty-five bony vertebrae, each consisting of a centrum, a neural arch, and various processes (compare pp. 36-38), but becoming simplified towards the end of the tail. The centra have flat anterior and posterior surfaces, and are not connected by synovial articulations, as in the frog, but interposed between them are elastic iniervertebral discs of fibro- cartilage. In addition to the ossification which gives ' rise to the main part of the centrum, a separate flat disc of bone (Fig. '134, ep) is formed on the anterior and posterior surface of each. These epiphyses are character- istic of the vertebrae of all or nearly all Mammals : they unite comparatively late with the centrum proper, and so in disarticulated skeletons of young animals they often come away from the main mass of the centrum and remain attached to the intervertebral discs. In correspondence with the differentiation of the parts of the body, the vertebral column is divisible into five regions (Fig. 130) : the cervical in the neck, including seven vertebrae, the first two of which — called respec- tively the atlas and axis — are peculiarly modified in order to allow the skull free movement ; the thoracic in the thorax, twelve or thirteen in number, and bearing ribs ; six or seven lumbar in the abdominal region ; three or four sacral in the sacral region ; and about fifteen or sixteen caudal in the tail. Examining one of the anterior thoracic vertebrae first (Fig. 134), we see that the centrum (c) is continuous above with the neural arch (n. a), the lower part of which, on either side, presents an anterior and a posterior notch (i. v. n), so that when the vertebra are in their natural position, an 498 THE RABBIT CHAP. intervertebral foramen is formed for the passage of a spinal nerve. The roof of the arch is continued into a long neural spine (n. sp) projecting upwards and backwards, and just above the intervertebral notches are a pair of anterior and posterior articular processes or zygapophyses (pr. z, pt. z), I which articulate synovially with the vertebrae next in front and behind respectively. The articular surface of each I pre-zygapophysis looks upwards and outwards, that of the I post- zy g apo physis down wards and inwards, j Arising laterally from either side of the arch is 1 an outstanding trans- \ verse process (t. pr}, on the under surface of which is. an articular tubercular facet (t. f), with which the upper j fork of the rib (p. 500) articulates. The lower j fork or head of the rib i articulates with a facet 1 (c. f) formed partly by ' the anterior edge of the I corresponding centrum j just at the base of the I neural arch, and partly by the posterior edge of the centrum next in front, so that each cen- ] trum bears half a capi- tular facet, as it is called, on either side, both anteriorly and posteriorly (c. f, c. f"). There are no free ribs in relation with the vertebra} of other regions, in which, however, they are represented in the embryo, but early fuse with the corresponding transverse processes. The first cervical vertebra, or atlas, is ring-shaped, and its lower portion is narrow and unlike the other centra. The neural spine is small, and the transverse processes are broad horizontal plates, each perforated at its base by a verte- brarterial canal through which the vertebral artery runs. On the anterior face of the lateral parts of the atlas are two concave articular facets for articulation with the occipital condyles of the skull, and on its posterior face are two small facets for articulation with the second vertebra. The second cervical vertebra, or axis, has its centrum produced FIG. 72 a c 134. — Fifth thoracic vertebra of the Rabbit, from the left side. ( x . c. centrum ; c.f capitular half -facet for fifth and c. f" for sixth rib ; ep. epiphysis : t. v. n. intervertebral notch ; n. a. neural arch ; n. sp. neural spine ; pr. z. pre-zyga- pophysis ; pt. z. post-zygapophysis ; t. /. tubercular facet for nfth"rib ; t. pr. trans- verse process. xi VERTEBRAL COLUMN 499 anteriorly into a conical odontoid process, which fits into the lower part of the ring of the atlas and is held in its place by a ligament extending transversely across the latter : it is ossified from a distinct centre, which really belongs to the centrum of the atlas. The neural spine of the axis is elon- gated and compressed, and its transverse processes small and perforated each by a vertebrarterial canal ; zygapo- pliyses are present only on the posterior face of the arch. In all the other cervical vertebras, the transverse processes are also perforated by vertebrarterial canals, and, except in the seventh or last, are divided into dorsal and ventral lame! Lie. The zygapophyses resemble those of the thoracic vertebra described above. The seventh cervical vertebra has a longer spine than the others, and bears a pair of half facets on the posterior surface of its centrum with which the first pair of ribs in part articulate. A typical thoracic vertebra has already been described. In the tenth, the neural spine is vertical; and in the remain- ing two or three; which are larger than the others, it slopes forwards. In the posterior three or four there are no tuber- cular facets, the ribs in this region not being forked ; the capitular facets are entire, and are situated on the corre- sponding centrum only. Additional processes are present above the pre -zygapophyses from the ninth thoracic vertebra onwards. The lumbar vertebras are relatively large, increasing in size from before backwards, and their various processes are greatly developed. The neural spines are directed upwards and forwards, the transverse processes are large and pro- ject outwards, downwards and forwards. As in the pos- terior thoracic vertebras, there are stout processes above the pre-zygapophyses (which face inwards], and there is also a pair of more slender processes below the post-zygapophyses (which face outwards], and a median ventral process pro- jecting downwards from the centrum is present on the first two. The sacral vertebras are fused to form the sacrum, which supports the pelvic arch. The first — and to a less extent the second also — has large, expanded, transverse processes which articulate with the ilia ; these are the sacral vertebras proper ; the others, which decrease in size from before back- wards, are really the anterior caudal vertebras which fuse with the true sacral vertebras to form a compound sacrum. The more anterior caudal vertebras resemble those of the sacral region, but on passing backwards all the processes are seen to diminish in size, until only the centra are left at the end of the tail. 500 THE RABBIT CHAP.] There are twelve or occasionally thirteen pairs of ribs, which have the form of curved rods, situated in the walls of the thorax, and articulating with the thoracic vertebrae above and — in the case of the first seven — with the breast-bone or sternum below : the remaining ribs do not reach the sternum (Fig. 130). Each rib consists of a bony, dorsal vertebral portion, and of a ventral, sternal portion consisting of cartilage which is calcified or only incompletely ossified. The dorsal end — the head or capitulum of the rib — articulates with the capitular facet on the centra, and the first nine have also a tubercle, a short distance from the capitulum, which articulates with the tubercular facet ; just externally to the tubercle is a short, vertical process (compare pp. 498 and 499). The sternum, which is developed in the embryo by the fusion of the ventral ends of the ribs (and therefore has a different morphological significance from the sternum of Amphibians, see p. 48), consists of six segments or sternebr&, the first of which, or manubrium, is larger than the rest, and has a ventral keel. With the last is connected a rounded, horizontal, cartilaginous plate, the xiphisternum. The ribs articulate between the successive sternebrae except in the case of the first pair, the articulations of which are on the manubrium. The chief bone of the pectoral arch is the flat, triangular scapula, the coracoid portion (compare p. 47) becoming early fused with it and forming a small, inwardly curved, coracoid process, situated anteriorly to the glenoid cavity at the lower end or apex of the scapula : the apex lies over against the first rib, and the bone inclines upwards and backwards to its dorsal base, which in the fresh condition consists of a strip of cartilage, the supra- scapula. On its outer surface is a prominent ridge or spine, the free ventral edge of which is called the acromion, from which a process, the metacromion, projects backwards. The collar-bone or xi FORE -LIMB 501 clavicle is never strongly developed in those Mammals in which the fore-limb only moves in one plane — forwards and backwards : in the rabbit it is a small, curved, rod- like bone, attached by fibrous tissue at one end to the sternum and at the other to the coracoid process of the scapula, there being small cartilages at either end of it. The relative positions of the bones of the fore-limb are at first sight somewhat difficult to understand owing to their having become altered in the course of development. In your own fore-arm the bones can be rotated on one another, so that the thumb can be made to point outwards or inwards : while in the rabbit the first digit has permanently the same position, pointing inwards. To understand this, extend your arm outwards with the thumb pointing away from the ground. The back of the hand and arm, continuous with the dorsal surface of the body, or back, is its dorsal surface ; the palm of the hand, and the surface of the arm continuous with the chest, is its ventral surface ; the border of the arm and hand continuous with the thumb is the pveaxial border ; and that continuous with the little finger the ppstaxial border. This position is called the position of supination ; if the fore-arm and hand be now rotated, so that the thumb points inwards, the position is that of pronation. While in this position, bend the elbow at right angles and bring it inwards close to the body ; the preaxial border of the hand will now be on the inner side, and an examina- tion of the bones of the fore-arm shows that they cross one another. It is in this position that the bones of the rabbit's fore-limb are permanently fixed (Fig. 130, and compare Fig. 8). The proximal extremity of the humerus bears a rounded head for articulation with the glenoid cavity, in front of which is a groove for the tendon of the biceps muscle (p. 61) ; certain iuberosities for the attachment of muscles will also be observed. Its distal extremity presents a large, pulley-like surface or trochlea for the articulation of the bones of the fore-arm, and a deep depression or fossa, perforated by a foramen, on its posterior side, for the reception of the end of the ulna. The radius is the shorter, inner (preaxial) bone of the 502 THE RABBIT CHAF. ' fore-arm, and is slightly curved. Its head presents a large double surface for articulation with the trochlea of the humerus, and its distal extremity a pair of slight con- cavities for the bones of the carpus : the shaft is flattened where it abuts against the corresponding flattened surface of the ulna. Near the proximal end of the last- mentioned bone is a cavity for the articulation of the humerus, and proximally to this, at the elbow, the ulna is produced to form a large olecranon process, which is received into the fossa on the humerus when the lirnb is extended : its small distal end articulates with the carpus. The carpus, as in the frog (p. 50), consists of a proximal and a distal row of small, nodular bones, which articulate with one another where they are in contact. The bones of the proximal row, beginning at the inner (preaxial) side, are the radiale and intermedium, articulating with the radius, and the ulnare, articulating with the ulna. In the distal row are five bones, the middle one of which is distinctly proximal to the other four, so as really to lie in the middle of the carpus : this is the centrale, the others constituting a row of distal' carpals. Of these the first three articulate with the corresponding digits, the fourth, on the outer (postaxial) side, supporting the fourth and fifth digits and really consisting of two carpals fused with one another. A small bone, the pisiform, articulating with the ulna and ulnare on the ventral side, is usually looked upon as a sesamoid bone, i.e., an ossification in the tendon of a muscle ; but it probably represents the vestige of a sixth digit. The hand or manus consists of five digits, each made up of a metacarpal and of phalanges, articulating with one another. The innermost (preaxial) digit — the thumb or pollex — is the shortest, and the third the longest : the former has two phalanges, the others three each, the distal or ungual phalanx of all the digits xr HIND-LIMB 503 hiving a conical form, its dorsal surface being grooved for the firmer attachment of the horny claw. The ends of the long bones in both limbs are separately ossified as epiphyses (compare p. 497), which eventually unite with the shaft of the bone in question. Small sesamoid bones are situated on the under or palmar side of the joints of the digits. The pelvic arch consists of two lateral halves or in- nominate bones, the long axis of which is almost parallel with that of the vertebral column (Fig. 130), and which are firmly united anteriorly and internally with the trans- verse processes of the sacral vertebrae by a rough surface, while ventrally they are connected together by cartilage at the pelvic symphysis. On the cuter surface of each innominate bone, at about the middle of its length, is a deeply concave cup, the acetabulum, for articulation with the head of the femur : in it, in young rabbits, a triradiate suture can be seen, marking the boundaries of the three bones of which the innominate is composed (p. 51). Of these, the antero-dorsal is the ilium, which is connected with the sacrum. The postero-ventral por- tion of the innominate is perforated by a large aperture — the obturator foramen, through which a nerve of that name passes (p. 532), the bone above and behind it being the ischium, and that below and in front of it the pubis. Behind the obturator foramen the ischium has a thickened posterior edge or tuberosity, and then curves round and becomes continuous with the pubis, both bones taking part in the symphysis. In young rabbits it will be noticed that the part of the pubis " which enters the acetabulum consists of a small, distinct epiphysis. The hind- limb has undergone rotation forwards (Fig. 130), so as to be brought, like the fore-limb, into a plane parallel with the median vertical plane of the body ; but the rotation being forwards, and the bones of the shank not being crossed, 504 THE RABBIT CHAP. the preaxial border is internal in the whole limb, and the original dorsal surface looks, on the whole, forwards. Close to the proximal end of the femur, on its inner (preaxial) border, is a rounded projecting head for arti- culation with the acetabulum : the actual end of the bone is formed by a strong process, the great trochanter, while just distal to the head is a lesser trochanter, and opposite this, on the outer (postaxial) side, a third trochanter. The distal end of the bone bears two large condyles, separated from one another by a notch, for articulation with the tibia : this notch is continuous with a groove extending for a short distance along the anterior (dorsal) surface of the femur in which a large sesamoid bone (p. 502), the knee-cap or patella, slides : the patella lies in the tendon of the extensor muscles of the leg, and is con- nected by ligament with the tibia. Two other sesamoid bones, the jdbellce, occur on the opposite side of the knee-joint. The tibia, or inner (preaxial) bone of the shank, is much larger than the fibula, the distal half of which in the adult becomes completely fused with it. The proximal end of the tibia bears two slightly concave articular surfaces for the condyles of the femur, and distally it articulates with the tarsus : a prominent ridge — the cnemial crest — extends along the proximal end of its anterior (dorsal) surface. The slender fibula is attached proximally to the tibia. The tarsus consists of six bones arranged in three rows. In the proximal row (compare p. 51) are two tarsals, of which the inner (preaxial) or astragalus — probably corre- sponding to two bones fused together, the tibiale and intermedium — has a large pulley-like surface for articula- tion with the tibia ; while the outer (postaxial) calcaneum orfibulare articulates with the fused end of the fibula, and is produced into a strong heel or calcaneal process. In the xi MUSCLES 505 middle row is a single bone, the centrale (navicular) of the tarsus, and the distal row is made up of three bones, the true first, together with the corresponding digit (haUux), being absent as a distinct bone. The second (apparent first) distal tarsal articulates proximally with the centrale, and distally with the innermost (preaxial) metatarsal : the third (apparent second) with the centrale and the corresponding metatarsal : the fourth (apparent third), which corresponds to the two fused outer (postaxial) tarsals, with the centrale, calcaneum, and the remaining two digits. The foot or pes consists of four metatarsals with their phalanges, of which there are three to each digit. The metatarsal of the hallux, together with the correspond- ing distal tarsal, is probably represented by a distinct ossification which in the adult becomes fused with the second (apparent first) metatarsal, and forms a process on that bone which articulates with the centrale. The phalanges are similar to those of the manus, and sesamoid bones are also present on the under surface of the foot. Muscles and Body-wall. — It will be remembered that in the lancelet and dogfish the muscles of the trunk are divided up into myomeres (pp. 419 and 434), while in the adult frog the only indication of such a segmentation of the muscles is seen in the recti of the abdomen. In the rabbit nearly all trace of a segmentation of the muscles has also disappeared, and the muscular system, although similar in its general arrangement to that of the frog (compare Fig. 16), is more highly differentiated and com- plicated. We shall have occasion to notice certain of the muscles in the course of our examination of other organs. Immediately beneath the skin, which consists of epiderm and derm (Fig. 131), the whole ventral region of the trunk and neck is covered by a thin cutaneous muscle, PRACT. ZOOL. I I 506 THE RABBIT CHAP. by means of which the rabbit is able to twitch its skin. Internally to this muscle in the female are the mammary glands (p. 483), which, when secreting, appear as whitish branched masses, the ducts of which can be traced to the teats, on the apices of which they open by numerous small apertures. A whitish band of connective-tissue passes along the mid- ventral line of the abdomen from the xiphisternum to the pubis : this separates two longitudinal bands of muscle, the recti abdominis, from one another ; and laterally to them, the abdominal wall consists of three thin layers of muscle with their fibres running in different directions — the external oblique, the internal oblique, and the transver satis, the latter being lined on its inner surface by the peritoneum. A fibrous cord, known as Poupart's ligament, beneath which the blood-vessels and nerves pass outwards to the leg, extends upwards and forwards from each pubis to the corre- sponding ilium. In the thorax the muscles of the body-wall are broken up into separate portions by the ribs, and thus form a series of intercostal muscles, which, like the oblique muscles of the abdomen, are arranged in two layers, external and internal, and are important in respiration. Extending from the thorax to the fore-limb of either side are the large pectoral muscles ; and a number of other muscles can be seen in the neck, in the ventral middle line of which, covered by the cutaneous muscle, the v/indpipe or trachea is visible (Fig. 135). The trachea is strengthened by a series of cartilaginous rings and ends in front in the larynx, situated between the two rami of the mandible ; and just in front of the larynx is the hyoid bone (p. 496), embedded in a mass of muscle. The Ccelome and its Contents. — On cutting open the body-cavity, it will be seen to be divided into two main chambers — the thoracic and abdominal cavities — by means of the diaphragm (Fig. 135, d). The relatively small thorax — which is lined by a serous membrane corre- sponding to the peritoneum of the abdomen and known as the pleura — contains the lungs, as well as the heart xi DIGESTIVE ORGANS 507. enclosed in a pericardium, on the ventral surface of which latter is an organ known as the thymus (see p. 447) : the gullet and main blood-vessels also pass through the thorax. The abdomen encloses the greater part of the enteric canal, together with the liver and pancreas, the spleen, and the urinogenital organs. The diaphragm is convex on its anterior side, towards the thorax : it consists of a central, thin, tendinous portion into which radial muscles are inserted. These arise from the vertebral column and posterior ribs, and are especially strong on the dorsal side, where they form two bands known as the pillars of the diaphragm. When the muscles contract, the diaphragm is made flatter, and thus the thoracic cavity is enlarged. Digestive Organs. — The mouth-cavity (Fig. 135) is large, and the small gape is bounded by upper and lower lips, behind which are the incisor teeth (i). On either side of the cavity are the borders of the upper and lower jaws from which the cheek-teeth project : these are separated from the incisors by a considerable interval or diastema. Close behind the upper incisors are a pair of very small openings leading into the naso- palatine canals (n. p. c), which communicate with the nasal cavities but must not be confounded with the internal nostrils. The roof of the oral cavity is formed by the palate, the anterior part of which, or hard palate (h. p}, is transversely ridged and partly supported by bone (h. p1, p. 494) ; while the posterior part, or soft palate (s. p], is smooth, its hinder, free edge forming a pendulous flap, the velum palati, on either side of which is an organ known as the tonsil, consisting of connective and lymphoid tissue and resembling the " lymphatic glands " (compare p. 515) which occur along the lymph- trunks ; it has the form of a small pit with a broad I I 2 „ £f fl 2 ti«^ Hrfl" * S 3 *-* a '" o g^ S c^ « *> « s J3 « T3 o ^ '^ <•> S 0) lii^li »aS?i^- mi liisi :S1 CHAP, xi TEETH 509 of Eustachian tube ; /. t. uterine tube ; g. b. pall-bladder ; h. p. hard palate ; //. />*. bones of hard palate ; hy. hyoid ; t. incisors ; il. ileum ; t. n. passage of internal nostrils ; _;'. Jacobson's organ ; k. left kidney ; I. au. left auricle ; Ing. left lung, seen through pleura ; Ir. liver ; /. y4. fourth lumbar vertebra ; /. vn. left ventricle ; m. tb. maxillary turbinal ; n. p. c. naso-palatine canal ; n. ph. uaso-pharynx ; »/. tb. naso-turbinal ; n. s. nasal septum (middle part cut away) ; as. gullet ; ol. 1. olfactory lobe ; ov. left ovary ; p. a. pulmonary artery ; pc. pericardium ; per. gl. perineal gland ; ph. pharynx ; phr. n. phrenic nerve ; (origin not shown) ; p. m. a. posterior mesenteric artery ; p. mx. premaxilla ; pn. pancreas ; pn. d. pancreatic duct ; pr. c. left precaval ; p. sy. pelvic sym- physis ; pt. c. postcaval ; py. st. pyloric region of stomach; r. ribs ; ret. rectum ; r. gl. rectal gland ; r. vn. right ventricle ; sk. f. floor, and sk. r. roof of skull ; si. gl. sublingual gland ; s. mx. gl. submaxillary gland ; s. p. soft palate, ending in the velum palati, on the lower side of which a tonsil is seen ; sp. c. spinal cord ; sp. n. spinal (lumbar) nerves ; s. r. sacculus rotundus of ileum ; s. v. first sacral vertebra ; sy. sympathetic (the anterior end shown on the right side, the rest on the left) ; t. tongue ; thr. thyroid ; th» v$. ninth thoracic vertebra ; thy. thymus ; tr. trachea ; u. bl. urinary bladder ; ur. ureter ; ut. uterus ; vag. vagina ; vb. vestibule ; vg. vagus (the anterior end shown on the right side, the rest on the left) ; vul. vulva. papilla on its outer margin. The tongue (t) lies on the floor of the rruouth to which it is attached below, its anterior, rounded end being free : the surface of its posterior part is elevated, and elsewhere — but more particularly on the tip — its covering of mucous mem- brane is produced into minute, finger-shaped papilla, on which some of the microscopic organs of taste are situated (compare p. 180). Taste-organs are also present on a pair of circumvallate papilla on the dorsal side of the tongue towards its posterior end, and on a pair of transversely ridged areas — the foliate papilla, situated laterally, slightly anterior to the former. The main substance of the tongue is composed of muscles, some extrinsic, or arising from other parts, and others intrinsic, or entirely confined to the organ in question. The teeth (Figs. 132 and 133), as we have seen, are not all alike, as in the dogfish and frog ; there are incisors and cheek-teeth or grinders, the latter being divisible into two series — the premolars and the molars. In most Mammals there is also a pair of canine teeth situated between the incisors and premolars, and these are especially long and pointed in such carnivorous ani- mals as the dog and cat. In the dogfish and frog, again, 510 THE RABBIT CHAP. the teeth are continually renewed as they become worn out, but in Mammals there are only two functional sets, which are known respectively as the deciduous or " milk " teeth, and the successional or permanent teeth : certain of the former may even be absorbed before birth, as is the case with the incisors of the rabbit. The incisors and pre- molars (and in Mammals in which they are present the canines also) have deciduous predecessors, the molars developing behind the premolars and having no pre- decessors. All the teeth are embedded in deep sockets or alveoli of the jaw- bones, and each contains a pulp- cavity (Fig. 136, PH) extending into it from the base and containing blood - vessels and nerves. In the case of the rabbit, the aperture of the pulp-cavity (PH1) remains wide open in each H. pulp-cavity ; PH'. open- , ,, n ,, -, , £ ,, ing of same ; ZB. dentine ; tOOth, and the SUbstanCC Of the ZC. cement ; ZS. enamel. . , . • -\-i i i i j (From Wiedersheim's Comp. tOOth IS Continually added to at its base as it wears away at the other end : in many Mammals, however (e.g., dog, cat, man), the aperture becomes narrowed and growth ceases after a time, the base of the tooth forming one or more roots or fangs. The main substance of each tooth is formed of dentine (ZB), into which the pulp-cavity extends for a considerable distance and round which the enamel (ZS) forms an external layer, which may become more or less folded inwards (as in the cheek- teeth and front upper incisors of the rabbit), the cement extending into the folds (compare pp. 445 and 511). FIG. 136. — Longitudinal section of a mammalian tooth, semi- diagrammatic. PH. I xi TEETH 5ii The number of the various teeth in the jaws is con- veniently expressed by a dental formula, in which the kind of tooth (incisor, canine, premolar, or molar) is indicated by the initial letter, i, c, pm, or m ; and the whole formula has the arrangement of four vulgar fractions, in each of which the numerator indicates the number of teeth in the upper, the denominator that of those in the lower jaw, only those of one side being indicated, since the teeth of the right and left sides are always the same. Thus the dental formula of the rabbit is if, eg, pm%, m\ = 28. The anterior incisors in the upper jaw of the rabbit are long and greatly curved. They are surrounded by enamel, which is much 'thicker on the anterior surface, where it presents a median groove ; the posterior upper incisors are much smaller and are situated behind the anterior ones. In the lower jaw the single pair of long and curved incisors have no median groove, and they bite between the anterior and posterior upper incisors : owing to the thick layer of enamel anteriorly, they, like the large upper incisors, wear away less quickly in this region, and thus remain sharp, like a chisel, at their biting edges. The premolars and molars, on the other hand, are modified for grinding the food, to do which satisfactorily it is necessary that they should have broad crowns with a surface which wears unevenly. This is effected in most of the cheek-teeth by the enamel becoming involuted along the outer side in a longitudinal direction, so as to form a groove extending into the dentine almost to the other side, the groove becoming filled up with cement. As the enamel is harder than the dentine and cement, it thus gives rise to ridges as the crown wears. The first upper premolar and the last upper and lower molars are simpler than the others, and the first lower premolar has two grooves. Connected with the mouth-cavity are several pairs of salivary glands, not present in the other Vertebrates we have examined, the secretion of which — saliva — contains a ferment called ptyalin, which is capable of converting starch into sugar (compare p. 74). The food taken into the mouth is ground up or masticated and mixed with 512 THE RABBIT CHAP, xi the saliva before passing down the gullet, and thus digestion begins in the mouth. The infra-orbital salivary gland is a large, lobulated pinkish mass situated in the an tero- ventral region of the orbit, below and in front of the eyeball : its duct passes downwards to open into the mouth nearly opposite and externally to the second premolar. The parotid gland is a soft, irregular, flattened organ, lying close beneath the skin, just below and in front of the base of the external ear ; its duct passes forward and opens close to the duct of the infra-orbital gland. The sub-maxillary gland (Fig. 135, s. mx. gl) is a reddish, ovoid, compact body situated inside the angle of the lower jaw and near the middle line, somewhat anterior to the larynx : its duct runs forward to open into the mouth a short distance behind the lower incisors. The sublingual gland (si. gl) is an elongated structure situated on the inner side of the mandible, and having several ducts opening independently into the mouth. The oral cavity is continued backwards as the pharynx (ph) : this begins at the velum palati, above which it extends forwards as the naso-pharynx (n. ph} ; the latter is continuous with the passage of the internal nostrils, and into it open the Eustachian tubes (eus, compare pp. 17 and 45). On the floor of the pharynx, behind the base of the tongue, is the glottis, which leads into the larynx and is guarded in front by an elastic, leaf-like, cartilaginous flap, the epiglottis (epg) : this projects upwards towards the velum palati and is capable of being pressed backwards over the glottis during the passage of food from the mouth to the gullet. The gullet (Figs. 135 and 137, .as) is a narrow but dilatable tube which passes backwards along the neck and through the thorax, entering the abdomen through an aperture in the diaphragm and then opening into the stomach (py. st, cd. st), a wide, curved sac, elongated transversely and greatly dilated at the cardiac end, which lies towards the left side of the body : the pyloric end, from which the duodenum arises, towards the animal's u a. I k. TCt FIG. 137. — The stomach, duodenum, posterior portion of rectum, and liver (in outline) of the Rabbit, with their arteries, veins, and ducts, (x f.) A, the cceliac artery of another specimen. The gullet is cut through and the stomach some- 514 THE RABBIT CHAP. what displaced backwards to show the ramifications of the coeliac artery (cce. a) ; the duodenum is spread out to the right of the subject to show the pancreas (pn) • the branches of the bile-duct (c. b. d), portal vein (p. v) and hepatic" artery (h, a) are supposed to be traced some distance into the various lobes of the liver. a. m. a. anterior mesenteric artery ; cau. caudate lobe of liver with its artery, vein and bile-duct ; c. b. d. common bile-duct ; cd. st. cardiac portion of stomach ; c. il. a. common iliac artery ; cce. a. coeliac artery ; cy. a. cystic artery ; cy. d, cystic duct ; d. ao. dorsal aorta ; du. proximal, and du'. distal limbs of duo- denum ; du. a. duodenal artery ; du. h. a. (in A], duodeno-hepatic artery ; g. a. gastric artery and vein ; g. b. gall-bladder ; h. a. hepatic artery ; h. d. left hepatic duct ; I. c. left central lobe of liver, with its artery, vein, and bile-duct ; 1. g. v. lieno-gastric vein ; I. I. left lateral lobe of liver, with its artery, vein, and bile- duct ; ms. branch of mesenteric artery and vein to duodenum ; ms. r. mesentery of the rectum ; m. v. chief mesenteric vein ; ces. gullet ; p. m. a. posterior mesenteric artery ; p. m. v. posterior mesenteric vein ; pn. pancreas ; pn. d. pancreatic duct ; p. v. portal vein ; py. st. pyloric portion of stomach ; ret. rectum ; r. c. right central lobe of liver, with artery, vein, and bile-duct ; spg. Spigelian lobe of liver with its artery, vein, and bile-duct ; spl. spleen ; sp. a. splenic artery. (From Parker's Zootomy.) right, is less dilated and has much thicker muscular walls. The mucous membrane of the stomach, in which the microscopic gastric glands (p. 131) are con- tained, is raised into ridges or ruga, and there is a circular pyloric valve at the entrance to the intestine. The duodenum (du) extends backwards along almost the whole length of the abdomen and then turns for- wards again, forming a slightly coiled U-shaped loop, and becoming continuous with the very long and coiled second portion of the small intestine or ileum (Fig. 135, H), which finally dilates to form a rounded sac (s. r) opening into the proximal end of the dark-coloured colon (col), or first portion of the large intestine. The colon has a much greater diameter than the small intestine, and presents a series of sacculations arranged in three rows, separated by flat regions of its wall : it passes insensibly into the second portion of the large intestine or rectum (ret), which is of about the same diameter as the small intestine, and is recognisable by its rounded swellings containing the pill-like faeces : it passes into the pelvic cavity to open by the anus (a). At the junction of the ileum and colon is a relatively enormous blind-gut or cacum (ccec) — a structure not met with in either the dogfish or the frog, and only reaching xi ENTERIC CANAL 515 such a relatively large size as in the rabbit in certain other herbivorous Mammals in which the stomach has a simple form : .in those which possess a complicated stomach (viz., Ruminants) the caecum is comparatively small. It is continuous with the proximal end of the colon, which contains an intra-colic valve and into which the round sac at the distal end of the ileum opens by a circular aperture provided with an ileo-colic valve. From this point arises the thin-walled caecum, which lies coiled on itself amongst the folds of the rest of the intestine : it is about an inch in diameter, and a spiral constriction is seen on the outside marking the attachment, on the inside, of a spiral valve — like that of the dogfish's intes- tine but narrower — which makes about twenty-four turns and ends at the base of a blind, finger-shaped process, the vermiform appendix, which forms the apex of the caecum. The whole canal is supported by a mesen- tery (p. 27), the right and left halves of which are united, and which has a very complicated arrangement in correspondence with the numerous folds of the intestine. It will be noticed that the intestine is much more differ- entiated as regards its subdivisions than in the Vertebrates previously examined, and also that it is relatively much longer, being fifteen or sixteen times as long as the body (compare p. 207). On cutting open the small intestine, its mucous membrane is seen to be raised into minute, finger-shaped elevations or villi. and here and there certain patches present a hon.ey- combed appearance ; these portions are known as Peyer's patches, and, like the tonsils, thymus, and spleen, consist of masses of lymphoid follicles composed of a connective- tissue framework in which numerous leucocytes are em- bedded. Other so-called " lymphatic glands " or adenoids are present in the mesentery and elsewhere. Peyer's patches also occur in the proximal end of the colon, close to the ileo-colic aperture ; and the round sac with which the colon communicates, as well as the vermiform appendix, is lined with similar lymphoid tissue. The mucous 516 THE RABBIT CHAP. membrane of the colon has no villi, but, like that of the spiral valve, is raised into small papillae ; while that of the rectum is smooth. The liver (Figs. 135, Ir, ancT 137) is a large organ, consisting of five lobes. Its anterior surface is convex and is applied to the diaphragm, its posterior concave surface fitting against the stomach. A median vertical fold of the peritoneum attaches it to the diaphragm, and marks the boundary between the right and left central lobes (Fig. 137, r.c, l.c). Externally to the left central lobe, between it and the cardiac end of the stomach, is the left lateral lobe (I. I), and externally to the right central lobe the caudate lobe (cau), applied to the pyloric end of the stomach and hollowed posteriorly, when it fits over the right kidney : a small Spigelian lobe (spg) fits closely against the concave anterior surface of the stomach. The bile-duct is made up of several hepatic ducts (h. d) from the various lobes of the liver, as well as of a cystic duct (cy. d) from the pear-shaped gall-bladder (g. b), which is embedded in the right central lobe of the liver : the common bile-duct (c.b. d) thus formed opens into the dorsal side of the duodenum by a prominent aperture a short distance beyond the pylorus. The pancreas (Figs. 135 and 137, pn) is a diffuse gland, consisting of a number of small lobules looking not unlike masses of fat, spread all over the mesentery which connects the two limbs of the duodenum. The small ducts from the various lobules run together to form the main pancreatic duct (pn. d) which opens, independently of the bile-duct, into the distal limb of the duodenal loop a couple of inches or so beyond the bend. The spleen (Fig. 137, spl) is a long, flat body of a dark red colour, attached to the cardiac end of the stomach by a sheet of peritoneum (compare pp. 23 and 447). xi RESPIRATORY ORGANS 517 Organs of Respiration and Voice. — Owing to the presence of a neck, the lungs are situated at some distance from the glottis (Fig. 135), and, instead of a short laryngo-tracheal chamber, as in the frog (p. 141), there is a windpipe or trachea (tr) extending along the neck just ventrally to the gullet, its anterior end forming the larynx or organ of voice (p. 506), and communi- cating with the pharynx through the glottis. The cartilaginous rings of the trachea are incomplete dor- sally, and the cartilages of the larynx are more highly differentiated than in the frog, apart from the presence of an epiglottis (epg, see p. 512). The largest and most anterior laryngeal cartilage is the thyroid, which, like the- epiglottis, is peculiar to Mammals : it has the form of a broad ring, incomplete dorsally, and is the part of the larynx which can be felt externally. The second cartilage is the cricoid, represented in the frog by a ring-shaped cartilage at the base of the lungs (p. 153) : its form is somewhat like that of a signet ring, being broad dorsally — where it lies mainly between the edges of the thyroid — and narrow ventrally. A pair of arytenoid cartilages are articulated to the dorsal and inner surface of the cricoid, and each is produced into a projecting process situated between the two edges of the thyroid cartilage. The vocal cords (p. 144) are a pair of elastic folds extending across | the cavity of the larynx from the thyroid below to the arytenoids above, each bounded in front by a depression. In the position of rest, the vocal cords lie at an acute angle to one another, as in the frog ; they can be brought into parallelism and regulated by the action of a number of intrinsic and extrinsic muscles, and are set in vibration by the respiratory current of air. After entering the thorax, the trachea divides into two bronchi (Fig. 135, br), one entering each lung and giving off branches to its different lobes : the bronchi, like the 518 THE RABBIT CHAP. trachea, are supported by incomplete cartilaginous rings at their proximal ends, but these gradually disappear after the bronchi have entered the lungs. The elastic lungs (Figs. 135 and 138, Ing) are not hollow sacs, like those of the frog, but are spongy bodies, of a light pink colour, situated on either side of and above the heart, and filling the greater part of the thoracic cavity, but collapsing as soon as the wall of the thorax is perforated. Each is subdivided into two main lobes, and the right lung has in addition two small accessory lobes, an anterior and a posterior, the latter lying in the median line, behind the heart and being closely applied to the gullet. Each pulmonary artery (Fig. 135, p. a) crosses the main bronchus anteriorly to the point at which it branches into the various lobes, except in the case of the anterior accessory lobe, the bronchus to which comes off in front of the artery and may even arise from the trachea before its bifurcation. Microscopic examination shows that the bronchi divide and subdivide to form a ramifying system of tubes, each ultimate branch of which opens into a minute chamber or infundibulum, which in structure closely resembles a frog's lung in miniature. The parietal layer of the pleura (p. 506) lines the cavity of the thorax, and is reflected over each lung at the entrance of the bronchus to form the visceral layer : in the median line it forms a vertical partition, the mediastinum, with which it is continuous on the ventral side of the vertebral column above, and beneath the pericardium below (Fig. 138). Thus each lung (/. Ing, r. Ing) has its own separate pleural cavity (I. pi, r. pi), separated from its fellow by the right and left media- stinum, the space between which is called the mediastinal space. The anterior and dorsal parts of this space are narrow, and enclose the posterior part of the trachea an< the bronchi, as well as the gullet (ces) and main blood- RESPIRATION 510 vessels (aort, az. v, pt. cav} ; its middle part is wide, and encloses the heart (/. vent, r. vent), the mediastinum here fusing with the visceral layer of the pericardium (vis. per) and thus obliterating the space ; below this it again narrows to form the ventral mediastinal space (v. med), in which the thymus (p. 507) is situated. cent FIG. 138. —Diagrammatic transverse section of the Rabbit's thorax in the region of the ventricles, to show the relations of the pleurae and mediastinum (dotted line), etc. The lungs are contracted, (x § ) aort. dorsal aorta ; az. v. azygos vein ; cent, centrum of thoracic vertebra ; /. Ing. left lung ; /. pi. left pleural cavity ; /. vent, left ventricle ; my. spinal cord ; oes. gullet ; par. per. parietal layer of pericardium ; pt. cav. postcaval, close to its entrance into right auricle ; r. Ing. right lung ; r. pi. right pleural cavity ; r. vent, right ventricle ; st. sternum ; vis. per. visceral layer of pericardium ; v. med. ventral mediastinal space. (From Parker and Haswell's Zoology.) In the entire animal, the air-tight pleural cavities are completely filled by the lungs, so that the parietal and visceral layers, of the pleurae are practically in contact, there being only a lubricating serous fluid (lymph) between them. The pressure of the air in the bronchial cavities of the lungs is therefore sufficient to keep them distended ; but as soon as the pleural cavities are 520 THE RABBIT CHAP. perforated, air passes in and equalises the pressure : the elasticity of the lungs then comes into play, causing them to collapse. When the muscles of the diaphragm contract (p. 507), air is di$wn into the lungs, and this process is aided by the external intercostal muscles (p. 506) and, in forced respiration, by other muscles of the body- wall also. The mechanism of respiration may therefore be compared with a suction-pump, while that in the frog resembles a force-pump (p. 142). On either side of the larynx is a soft, vascular, gland-like thyroid body, consisting of two lateral portions connected ventrally by a median bridge. Its function is not thoroughly understood : morphologically it represents a gland developed from the pharynx, but it loses its connection with the latter and thus has no duct. The glandular vesicles of which it is composed give rise to an albuminous substance containing iodine, which is passed into the blood and lymph ; if extir- pated in the living animal, various functional disturbances result. We are also ignorant of the function of the thymus (p. 507), which is largest in young animals, becoming reduced in size in adults (compare p. 447). Organs of Circulation. — The heart, as in all Verte- brates, is enclosed in a pericardium consisting of parietal and visceral layers (Fig. 138), between which is a serous pericardial fluid. There is a complete^ separation between the arterial and venous blood in the heart, for in addition to an auricular septum, as in the frog (p. 88), the ventricular portion is divided into right and left chambers by a partition (Fig. 138), the arterial blood from the lungs entering the left auricle and thence passing into the left ventricle to be pumped into the aorta, and the venous blood entering the right auricle and thence into the right ventricle to pass to the lungs through the pulmonary artery. A distinct conus arteriosus and sinus venosus (pp. 79 and 449) can no longer be recognised, the former having become practi- xi THE HEART 521 cally absorbed into the ventricular portion of the heart, and the latter into the right auricle ; so that the aorta (together with the carotids) and the pulmonary artery now arise directly from the left and right ventricles respectively, and the precavals and postcaval enter the right auricle directly (Figs. 135, 139, and 140). The line of separation between the two ventricles can be seen externally as an oblique depression extending from the base of the heart backwards and to the right, but not reaching the apex, which is formed by the left vent rick only. The small, irregular cavity of the latter is enclosed by very thick muscular walls, and is partly surrounded by the right ventricle, the cavity of which is crescentic in transverse section (Fig. 138), while its walls are much thinner than those of the left ventricle as it has only to pump the blood to the lungs. The auricles have thin walls : each is produced into a little flap or appendix which envelops the base of the cor- responding ventricle, and the walls of which are strengthened by a network of muscular bands. In the auricular septum is a thin, oval area, the fossa avalis (Fig. 139. /. ov), which in the embryo is perforated and so allows the blood from the body to pass directly into the left auricle without going to the lungs, which are not, of course, functional until the animal is born« The two auriculo-ventricular apertures are guarded by valves — that of the left side, or mitral valve, consist- ing of two membranous flaps, that of the right, or triciispid valve (Fig. 139, tri. v), Of three flaps : the valves are attached by their bases to the margins of the aper- tures, their apices extending into the corresponding ventricles. Attached to their edges are tendinous cords arising from conical elevations of the ventricular walls known as papillary muscles, which are much larger in the left ventricle than in the right (m. pap] : these serve to PRACT. ZOOL. K K 522 THE RABBIT CHAP. prevent the valves from being pushed into the auricles when the ventricles contract. The right ventricle narrows towards its base, on the ventral side of the heart, to form a conical prolongation from which arises the pulmonary artery (Figs. 135 and 140, p. a), its aperture being guarded by three pocket- like, semilunar valves (Fig. 139, sem. v) : the aperture sem.is FIG. 139. — Heart of the Rabbit, seen from the right side, the walls of the right auricle and right ventricle partly removed so as to expose the cavities, (x i£.) ao. aorta ; /. ov. fossa ovalis ; /. pr. c. opening of left precaval ; m. pap. papillary muscles ; pt. c. postcaval ; pt. c'. opening of postcaval, with Eustachian valve below ; r. pr. c. right precaval ; r. pul. right pulmonary artery ; sem. v. semi- lunar valves at base of pulmonary artery ; tri. v. tricuspid valve. (From Parker and HaswelTs Zoology.) of the aorta from the left ventricle is similarly provided with three semilunar valves. The two precavals (I. pr. c, r. pr. c) and the postcaval (pt. c) communicate, as we have seen, directly with the right auricle, the right pre- caval opening into it anteriorly, the left precaval posteriorly, the aperture of the postcaval being just anterior to that of the left precaval. The pulmonary veins from each lung "unite and open close together into the left auricle. i ARTERIES 523 A membranous fold, the Eustachian valve, extends into the right auiicle between the apertures of the postcaval and left precaval : in the embryo this helps to conduct the blood through the aperture in the auricular septum (p. 521). It, like the " ductus arteriosus " (p. 550, and compare Fig. 120), affords another example of a vestigial structure, representing the remains of the sinu-auricular valves (pp. 89 and 449). You will remember that in the frog (p. 80) there are two systematic trunks, representing the second arterial arch of the tadpole and fish (p. 451 and Fig. 120), and uniting above to form the dorsal aorta. In the Mam- mal, one of these — the right — disappears in the course of development and all the blood from the left ventricle passes into the .single left aortic arch (Figs. 135 and 140) from the base of which both carotid arteries arise, the aortic arch then curving over the left bronchus to pass into the dorsal aorta (d. ad). Close to the origin of the aortic arch, just beyond its semilunar valves, two small coronary arteries are given off to the walls of the heart ; and more anteriorly, at the curve of the arch, arise the vessels which supply the head and fore-limb. There is a certain amount of variation as to the origin of these, which is asymmetrical, and is usually as follows. Springing from the arch of the aorta towards the right side is an innominate artery (Fig. 140, in), which gives off close to its origin the left common carotid (I. c. c), and then, passing forwards, divides into the right common carotid (r. c. c) and the right subclavian (s. cl. a), the left subclavian (br) taking its origin independently from the left side of the arch. Each common carotid passes forwards along the neck, close to the trachea, and at about the level of the larynx divides into an internal carotid (i. c), which supplies the brain, and an external carotid (e. c), which goes to the head and face. Each subclavian forms several branches, K K 2 524 THE RABBIT CHAP. the most important of which are a brachial (br) to the fore-limb, a vertebral (or) which passes through the vertebrarterial canal of the cervical vertebrae (p. 498) and supplies the spinal cord and brain, and an anterior epigastric or internal mammary (a. epg) running along the inner side of the ventral wall of the thorax. The aorta gives off, in the thorax, a series of small paired inter- costal arteries (i. cs) to the body-walls, and then passes into the abdomen, between the pillars of the diaphragm. A short distance behind the diaphragm the cceliac artery (Figs. 137 and 140, cce) arises, and supplies the liver, stomach, and spleen ; and about half or three- quarters of an inch further back is the anterior mesenteric artery (a. m. a], the branches of which pass to the small intestine, pancreas, caecum, and colon. Close behind the anterior mesenteric is the right — and rather further back the left — renal artery (Fig. 140, r), and still more posteriorly, a posterior mesenteric (p. m) to the rectum, and a pair oi^spermatic or ovarian arteries (spm) to the spermaries or ovaries, as the case may be. A small caudal artery (m. sc), corresponding to the caudal con- tinuation of the aorta, arises from the dorsal surface of the posterior part of the latter just in front of a pair of large common iliac arteries (c. il. a), which appear like a bifurcation of the aorta. These are continued outwards and backwards towards the hind-limbs, each giving off an ilio-lumbar artery (i. I) to the dorsal body- wall and then dividing into an internal iliac (i. il. a] passing along the dorsal side of the pelvic cavity, and an external iliac (e. il. a) which gives off an artery to the bladder (s. vs), and in the female one to the part of the oviduct known as the uterus (ut) ; and then, passing beneath Poupart's ligament (p. 506) to the hind-limb, becomes the femoral artery (fm. a], from the proximal end of which a posterior epigastric (p. epg} runs forwards XI VEINS 525 in the ventral abdominal wall. Small lumbar arteries are also given off from the aorta to the walls of the abdomen. The pulmonary artery (p. a) divides soon after its origin from the right ventricle into two branches, one supply- ing each lung. Just before its bifurcation it is connected by a short cord, known as the ductus arteriosm, with the aorta : this is the solid vestige of the embryonic connec- tion between the fourth arterial arch and the aorta (compare p. 452, and Fig. 120). Each precaval (1. pr. c, r. pr. c) receives — a subclavian (s. d. v) from the fore-limb ; an external jugular (e. ju] from the head, running along the neck just beneath the skin ; a small anterior epigastric from the ventral thoracic wall, as well as small vessels from some of the anterior intercostal spaces (i. cs) and the anterior surface of the diaphragm (a. ph) ; and a small internal jugular (i. ju) from the brain, opening into the corresponding external jugular nearly opposite the subclavian. An azygos vein (az. v), representing part of the right cardinal of the embryo (compare p. 456) and receiving blood from the posterior intercostal spaces, also opens into the base of the right precaval. There is no renal portal system, as in the dogfish and frog (pp. 454 and 85). A pair of internal iliac veins (i. il. v) in the pelvic cavity unite to join a median vessel (c. il. v), the hinder end of the postcaval, which receives on either side an external iliac (e. il.. v) : this is constituted by a femoral vein (fm. v) from the hind-limb ; a posterior epigastric (p. epg), from the ventral walls of the abdomen, entering the femoral just external to Poupart's ligament ; and by small veins from the bladder as well as from the uterus in the female. Slightly in front of the external iliacs the postcaval receives a pair of large Mo-lumbar veins (i. 1) from the body-walls : the left ilio-lumbar FIG. 140.— The vascular system of the Rabbit from the ventral side. The heart is somewhat displaced towards the left of the subject ; the arteries of the right and e veins of the left side are in great measure removed, (x £.) 526 CHAP, xi VEINS 527 a. epg. anterior epigastric artery ; a. /. anterior facial vein ; a. tn. anterior mesen- teric artery ; a. ph. anterior phrenic vein ; az. v. azygos vein ; br. right brachial artery ; c. il. a. common iliac artery ; c. il. v. hinder end of post- caval ; cos. coeliac artery ; d. ao. dorsal aorta ; e. c. external carotid artery ; e. il. a. external iliac artery ; e. il. v. external iliac vein ; e. ju. external jugular vfin ;/m. a. femoral artery ; fm. v. femoral vein ; h. v. hepatic veins ; t. c. internal carotid artery ; t. cs. intercostal vessels ; t. ju. internal jugular vein ; t. /. ilio- lumbar artery and vein ; in. innominate artery ; /. au. left auricle ; I. c. c. left common carotid artery ; /. pr. c. left precaval vein ; /. v. left ventricle ; m. sc. caudal artery ; p. a. pulmonary artery ; p. ep%: posterior epigastric artery and vein ; p. f. posterior facial vein ; p. m. posterior mesenteric artery ; p. ph. posterior phrenic veins ; pic. postcaval vein ; p. v. pulmonary vein ; r . renal artery and vein ; r. au. right auricle ; r . c. c. right common carotid artery ; r. pro. right precaval vein ; r. v. right ventricle ; sc/. a. right subclavian artery ; sc/. v. right subclavian vein ; spm. spermatic artery and vein ; s. vs. vesical artery and vein ; ut. uterine artery and vein ; vr. vertebral artery. (From Parker's Zootomy.) sometimes runs forwards to open into the corresponding renal vein. Rather more anteriorly still are a pair of spermatic (spm) or ovarian veins, and a large renal vein (r) enters the postcaval from each kidney. As the postcaval passes through 'the dorsal border of the liver, it receives several large hepatic veins (Figs. 137 and 140, h. v) from the lobes of that organ. Other small veins from the body- walls and from the posterior surface of the diaphragm also open into the postcaval, which then passes through the central tendon of the diaphragm and runs forward in the mediastinal space (Fig. 138, pt. cav) to open into the right auricle. The hepatic portal vein (Fig. 137, p. v) is a large vessel situated in the mesentery, ventral to the postcaval. Anteriorly it passes into and divides up in the liver, send- ing a branch to each lobe : posteriorly it is constituted by a large anterior mesenteric vein (m. v) returning the blood from the small intestine, colon, and caecum, and by smaller veins from the stomach, spleen, and duode- num, as well as by a posterior mesenteric vein (p. m. v) from the rectum. The pulmonary veins have already been described (p. 522). In the freshly-killed animal a number of the delicate, trans- parent Ivmphatic vessels (p. 9?) can be made out, those from the intestine (lacteals] running in the mesentery. They 528 THE RABBIT CHAP. come into connection with numerous adenoids (p. 515) in the mesentery and elsewhere, and most of them communicate with a main trunk — the thoracic duct — which extends from the abdomen through the thorax on the left and upper side of the aorta. The thoracic duct also receives the lymphatics from the right side of the head and neck and the fore-limb, and opens into the veins at the junction of the left external jugular and subclavian : the lymphatics of the right side of the head and neck and right fore-limb communicate with the corresponding veins of the right side. Nervous System. — The brain (Figs. 135, 141, and 142) reaches a much higher development than in the other Vertebrates we have already studied. The prosence- phalon is subdivided into two cerebral hemispheres (c.h), of much larger relative size than those of the frog (Fig. 49) and forming about two-thirds of the whole brain. They are closely applied to one another along their flat internal surfaces, are roughly conical in form, narrower in front (frontal lobes), broadening out posteriorly (parietal lobes) — where they overlap the diencephalon and optic lobes and abut against the cerebellum, and produced downwards into the prominent temporal lobes which partly overlap the crura cerebri below. Their external layer or cortex is formed of grey matter, and their surface is convex and smooth, except for the presence of slight lateral grooves between the lobes : in many Mammals the hemispheres are highly convoluted, i.e., raised into numerous winding elevations oigyri, separated by narrow grooves or sulci. A broad transverse band of nerve-fibres forms a commissure connecting the two hemispheres known as the corpus callosum (Figs. 141 and 142, cp. cl) : this structure is confined to the Mammalia, and is even wanting in certain of the lower members of the class. The olfactory lobes (olf) are club-shaped, and extend backwards along the ventral surface of the hemispheres in the form of narrow bands as far as the temporal lobes. XI BRAIN 529 The diencephalon consists of a right and left optic thalamus (o. tli) between which is the slit-like third o.th. FIG. 141. — Two dissections of the Rabbit's brain, from above, (x ij.) In A, the left hemisphere is dissected down to the level of the corpus callosum ; on the right side the lateral ventricle is exposed. In B, the hemispheres are dissected down to a little below the level of the anterior end of the corpus callosum ; only the frontal lobe of the left hemisphere is retained, of the right a portion of the temporal lobe also remains ; the choroid plexus and pineal body are removed, as well as the greater part of the body of the fornix and the whole of its left posterior pillar ; the cerebellum is removed with the exception of a part of its right lateral lobe with the flocculus. a. co. auterior commissure ; a. fo. anterior pillar of fornix ; a. pn. anterior peduncles of cerebellum ; b. fo. body of fornix ; cb\. central lobe of cerebellum ; cb*. its lateral lobe ; c. gn. elevation on the optic thalamus ; c. ft. cerebral hemisphere ; ch. pi. part of choroid plexus ; cp. cl. corpus callosum ; cp. s. corpus striatum ; c. rs. and d. p. elevations on the bulb ; fl. flocculus ; hp. in. hippocampus ; m. co. middle commissure ; o. /l, o. 12, optic lobes ; olf. olfactory lobe ; o. tit. optic thalamus ; o. tr. optic tract (continuation of chiasma) ; p, co. posterior commissure ; p.fo. posterior pillar of fornix; pn. pineal body ; pd. pn. its peduncle ; p. pn. posterior peduncles of cerebellum ; p. va. fibres of pons Varolii forming middle peduncles of cerebellum ; sp. lu. septum lucidum ; st. I. line on corpus callosum ; /. s. band of white matter lying beneath choroid plexus ; v. vn. valve of Vieussens ; v'-\ third ventricle ; v*. fourth ventricle. (From Parker's Zoototny.) ventricle (v3) roofed over by a thin membrane continuous with a vascular choroid plexus (Fig. 142, vl. ip), and from 530 THE RABBIT CHAP. its hinder part arises a stalk bearing at the end a small, rounded pineal body (pri) . The floor of the diencephalon is produced downwards to form the infundibulum (inf), to which the pituitary body (pty) is attached. In front of the infundibulum is the optic chiasma (o. ch), and behind it a small, rounded lobe (c. ma}. Each optic lobe is divided into two by a transverse furrow, so that there are four rounded elevations in this region — an anterior, larger pair (o. I1), and a posterior, smaller pair (o. I2). Below the optic lobes are the crura cerebri (c. c) — two strong, diverging bands passing forwards and outwards from the bulb to the hemispheres. The bulb or medulla oblongata (m. o) is slightly flattened dorso-ventrally, and passes behind into the spinal cord, the dorsal and ventral fissures of which are continued into it : the fourth ventricle (t>4) which it contains is roofed over by the thin pia mater only (p. 155). Ventrally its anterior border is marked by a stout band of nerve-fibres running transversely, and known as the pons Varolii (p. va). The large cerebellum is connected with the dorsal surface of the brain by three pairs of peduncles (Fig. 141, a. pn, p. va, p. pn), and consists of a median central lobe (cjb1) and of two lateral lobes (cb2), on the outer side of each of which is a smaller floccular lobe (fl) . The grey matter is superficial, and the surface is marked by numerous folds which in section present a tree-like pattern (arbor vita], brought about by the arrangement of the grey and white matter (Fig. 142). The fourth ventricle is not prolonged into the cere- bellum to any extent : it is continued forwards as the iter, from which no optic ventricles are given off (com- pare pp. 157 and 459) and which passes into the narrow but deep third ventricle in front (Fig. 142) : this is bounded anteriorly by a thin wall, the lamina terminalis xi BRAIN 501 (/. t), and extends into the infundibulum below. At its anterior end are the foramina of Monro (/. m), leading into the middle of the lateral ventricles in the hemispheres (Fig. 141). In this region each lateral ventricle is broad from side to side, but narrow from above downwards ; FIG. 142. — Rabbit. Longitudinal vertical section of the rabbit's brain. (X ij.) L< 'tiers as in preceding figure ; in addition— cb. central lobe of cerebellum, showing arbor vitae ; c. c. crus cerebri ; c. h^. parietal, and cp. h~. temporal lobe of cerebral hemisphere ; c. ma. elevation behind the infundibulum ; /. m. foramen of Monro ; inf. infundibulum ; /. t. lamina terminals ; ly. part of hippocampus ; m. o. medulla oblongata ; o. ch. optic chiasma ; pty. pituitary body ; vl. ip. choroid plexus ; v. vn. valve of Vieussens ; //. optic nerve. (From Parker's Zootoiny.} it extends forwards into the frontal lobe, backwards into the parietal lobe, and downwards into the temporal lobe. The olfactory lobes are solid. A prominent, convex ridge of white matter — the hippo- campus (Fig. 141, hp. m) — projects into the inner side and floor of each lateral ventricle where it descends into the temporal lobe, and closely applied to it is a continuation of the choroid plexus (ch. pi), which passes from the roof of the third ventricle into the lateral ventricle through the foramen of Monro. In front of the hippocampus the outer side and floor of the anterior part of the lateral ventricle is thickened to form an eminence of grey matter, the corpus slriatum (cp. s). Just beneath the corpus callosum the internal wall of each lateral ventricle is thin, and is known as the septum lucidum (sp. lu) ; and below it and above the foramina of Monro is another commissure known as the body of the fornix (Figs. 141 and 142, b.fo] which is continuous on either 532 THE RABBIT CHAP. side with two bands — one (posterior pillar) lying along the anterior edge of the hippocampus, and the other (anterior pillar) passing backwards in the side walls of the third ventricle. Connecting the two optic thalami are three transverse bands of nerve-fibres, known respectively as the anterior (a. co), middle (m. co), and posterior (p. co) commis- sures : the middle commissure, which is much the largest, is not represented in the lower Vertebrata. The spinal cord (Fig. 135, sp. c) is similar in structure to that already described in other Vertebrates (pp. 155 and 459). It extends through the entire neural canal, ends in a filum terminate, and is swollen opposite the fore- and hind-limbs, where the nerves arise which form the limb-plexuses (pp. 161 and 162). The dorsal and ventral roots of the spinal nerves lie in the same transverse plane, as in the frog (p. 163), but are rela- tively shorter than in that animal ; after uniting to form the nerve-trunks, they pass directly outwards through the inter- vertebral foramina. The brachial plexus is formed from the four posterior cervical and the first thoracic nerves, and gives off a number of nerves to the shoulder and fore-limb. The sciatic or lumbosacral plexus is constituted by the two or three hindermost lumbar and the first two or three sacral nerves, and gives off branches to the pelvic region and hind- limb, a femoral and a peroneal going to the extensor muscles, and a large sciatic and an obturator (which passes through the obturator foramen, p. 503) supplying the flexor muscles. Arising from the fourth cervical spinal nerve of either side is a phrenic nerve (Fig. 135, phr. n\ which passes backwards, between the heart and lungs, to supply the muscles of the diaphragm ; and a large auricular nerve, arising from the third cervical nerve, supplies the external ear. In addition to the ten cerebral nerves enumerated in the frog (p. 163) and dogfish (p. 461), two others — the spinal accessory and the hypoglossat (represented in the frog by fibres in connection with the vagus and by the first spinal nerve respectively, p. 160) — emerge from the skull and are therefore counted as the eleventh and twelfth cerebral nerves. The former arises from the side of the spinal cord and bulb by numerous fibres, the xi NERVES 533 posterior of which are opposite the fifth spinal nerve, from which point it runs forwards between the dorsal and ventral roots and leaves the skull together with the glossopharyngeal and vagus (p. 496), supplying certain muscles of the neck and shoulder. The hypoglossal arises by a number of fibres from the ventral surface of the bulb, passes out through the condylar foramen, and supplied the muscles of the tongue, as well as certain muscles of the neck. Tho origin and distribution of the first ten pairs of cerebral nerves correspond in their main features with those seen in the frog (p. 163). The facial is almost entirely a motor nerve and is chiefly important in supplying the facial mus- cles, which a.re very highly developed in Mammals. An anterior and a -posterior or recurrent laryngeal nerve are given off from the vagus. The relations of the sympathetic nerves (Fig. 135, sy) are also essentially similar to those occurring in the frog (p. 162). Each passes backwards along the neck close to the vagus (vg) and alongside the carotid artery, enlarging to form an anterior and a posterior cervical ganglion, In the thorax it runs just beneath the heads of the ribs, having a ganglion in each intercostal space : it then passes into the abdomen, lying close to the centra of the vertebrae and having ganglia at intervals. From all the sympathetic ganglia branches are given off con- necting them with the spinal nerves, others going to the blood-vessels : others again, in the thorax and abdomen, are connected with plexuses from which nerves pass to the heart and abdominal viscera. In the abdomen these plexuses can be seen in the mesentery, a large cceliac or solar plexus being present close to the origin of the anterior mesenteric artery. Sensory Organs. — The sense of touch is situated in microscopic tactile organs in the skin, and groups of cells, 534 THE RABBIT CHAP. called taste-buds, are present on the papillae of the tongue (p. 509) and on the soft palate (compare pp. 179 and 180). The organs of smell are situated in the olfactory cap- sules, the form of which has already been described (p. 493). They open externally by the external nostrils, and are produced backwards above the palate into the passage of the internal nostrils, which communicate with the naso-pharynx (Fig. 135, i. n, n. ph, and see p. 512). The olfactory epithelium, supplied by the olfactory nerves, is situated on the ethmo-turbinal (e. tb) : the mucous membrane of the maxillo-turbinal (m. tb) probably serves merely to warm the inspired air. On the ventral side of the nasal septum is a pair of small, tubular structures known as the vomero-nasal or Jacobson's organs (Fig. 135, j], lined by epithelium and enclosed in cartilages situated just to the inner side of the palatine pro- cesses of the premaxillae (p. 494). Each of them opens anteriorly into the corresponding naso-palatine canal (p. 507), and receives a special branch of the olfactory nerve. The function of these organs is not understood. The structure of the eye is similar to that already described in other Vertebrates (pp. 181 and 465), except that the sclerotic is not cartilaginous, but is composed of dense fibrous tissue, and the lens is relatively smaller than in the dogfish and frog and is markedly biconvex in form, the outer surface being rather flatter than the inner : it is capable of adjustment by means of the ciliary muscles contained in the radiating ciliary processes into which the choroid is thrown just externally to the iris (compare p. 184). The eyelids have already been described (p. 486). The four recti muscles ensheath the optic nerve, as in the frog (p. 1 86, compare Fig. 126), but the superior oblique, instead of arising — like the inferior oblique — in the anterior part of the orbit, takes its origin further back, near the recti, passes forwards through a nbro-cartilaginous pulley at the anterior angle of the orbit, and then backwards and outwards to its insertion on the eyeball. xi AUDITORY ORGAN 535 Between the wall of the orbit and the eyeball are two glands, the secretion of which, passing through ducts perforating the conjunctiva lining the eyelids, serves to keep the outer surface of the eye moist, and is then conducted into the nasal chambers by means of the naso-lacrymal duct (pp. 186 and 494) . These two glands correspond to special differentiations of a primarily continuous structure : one, the Harderian gland — already met with in the frog — is situated in the antero-ventral region of the orbit : the other, or lacrymal gland proper, in its postero-dorsal region. Besides these, a series of small Meibomian glands is present on the inner side of the edges of the eyelids, and produces a fatty secretion. The essential part of the auditory organ consists, as in other Vertebrates, of the membranous labyrinth with its three semicircular canals (pp. 186 and 465) enclosed in the auditory capsule (periotic bone, p. 492), and constituting the internal ear. The small outgrowth of the sacculus seen in the dogfish and frog, and known as the cochlea (Fig. 59, cc), is represented by a relatively larger struc- ture, coiled on itself in a spiral manner, and specially important as regards the sense of hearing. The part of the periotic bone which directly surrounds the cavity in which the membranous labyrinth lies is especially hard, and when the outer portion of the bone is cut away, is seen to form a sort of cast of the enclosed organ, the form of which it repeats : this is known as the bony labyrinth (Fig. 143). Internally it is separated from the membranous labyrinth by a narrow space all round, con- taining the perilymph (p. 189) and only shut off from the tympanic cavity at the fenestra ovalis and fenestra rotunda (p. 193) by the membrane which closes each of them. The membranous cochlea does not run up the middle of the spiral of the bony cochlea, but is attached between its 536 THE RABBIT CHAP. outer wall and a spiral shelf arising from its inner wall. Thus the entire cochlea shows three cavities in transverse section — a middle, the membranous cochlea or scala media, and a scala vestibuli and a scala tympani on either side of it respec- tively, which communicate with one another at the apex of the cochlea and with the perilymphatic cavity surrounding the rest of the membranous labyrinth at its base, where the scala tympani abuts against the membrane of the fenestra rotunda, and the scala vestibuli against that of the fenestra ovalis. On the wall of the scala media which separates it from, the scala tympani is a specially modified series of auditory cells forming whabis known as the organ of Corti, which receives nerve-fibres from a branch of the auditory nerve extending along the spiral shelf of the cochlea. The middle ear (p. 466) is constituted by the tympanic cavity in the tympanic bulla (p. 492), and communicates with the pharynx by the Eustachian tube (Fig. 143, E). The tympanic membrane (M), situated obliquely at the boundary of the bulbous and the tubular portions of the tympanic bone, separates the middle ear from the external ear, consisting of the auditory passage (Ex) and the pinna (p. 486). The fenestra ovalis is plugged by a small stirrup- shaped bone, the stapes (Fig. 143, Oj), one of the three auditory ossicles (p. 492) connecting the internal ear with the tympanic membrane, and probably correspond- ing morphologically to part or the whole of the frog's columella (Fig. 10) : with it is connected a small stapedius muscle, serving to keep the membrane of the fenestra ovalis on the stretch; The middle bone of the chain is the incus (Fig. 143, O2), a short process of which is articulated to the stapes by the intermediation of a small bony nodule, while its body articulates with the outer bone of the series, the malleus (O3). Arising from the body of the malleus is a handle-like process or manubrium, which is attached to the tympanic membrane (M) : this has the form of the roof of a tent, and is kept on the stretch by a small muscle, the tensor tympani. XI URINOGENITAL ORGANS 537 arising from the wall of the tympanic cavity and inserted in to the manubrium of the malleus. The study of development indicates that the malleus corresponds'to the ossified articular part of the mandibular cartilage of lower Vertebrates, and the incus to the quadrate Ex FIG. 143. — Diagram of the mammalian body labyrinth, tympanic cavity, and external auditory passage. Cch. bony cochlea ; E. Eustachian tube ; Ex. external auditory passage ; L. bony semicircular canals ; M. tympanic membrane; N. auditory nerve; Oj stapes ; 0-2 incus ; 0$ malleus. (After Headley.) (pp. 44 and 438) : the articulation of the bony mandible with the squamosal in Mammals has rendered these parts unnecessary for their original purpose ; and they have undergone a change of function (p. 449), forming an accessory part of the auditory apparatus. Urinogenital Organs. — The kidneys (Fig. 135, k) are of a somewhat compressed, oval shape, with a notch or hilus on the inner side. They are in close contact with the dorsal wall of the abdominal cavity, the right being somewhat in advance of the left. Towards the hilus, in PRACT. ZOOL. L L 538 THE RABBIT CHAP. the tubules of which the kidney is composed (p. 146) converge to open into a wide chamber or pelvis, which forms the dilated commencement of the ureter. When the kidney is cut across, its substance is seen to be divided into a central mass or medulla and a peripheral portion or cortex. The former appears radially striated, owing to the tubules in this region being straight and converg- ing to open on the surface of a conical process or pyramid, which projects into the pelvis : the cortex contains the coiled portions of the tubules and the Malpighian bodies, and thus has a dotted appearance. The ureter (Figs. 135 and 144, ur) runs backwards along the dorsal wall of the abdomen to open into the urinary bladder (u. bl, bl), a pyriform sac with elastic walls which vary in thickness according as the organ is dilated or contracted. Near the front end of each kidney towards its inner side is a small yellowish adrenal body (Fig. 135, adr, compare P-447)- In the male rabbit the spermaries are oval bodies which in the young animal are situated close to the kidneys on the dorsal wall of the abdomen, but which pass back- wards and downwards as the animal approaches maturity until they come to lie each in a scrotal sac (p. 487), situated at the side of the urinogenital opening. The cavity of each scrotal sac is in free communication with the cavity of the abdomen by an opening — the inguinal canal. A convoluted epididymis (p. 466), closely adherent to the spermary and connected with the distal end of the scrotal sac, forms the proximal part of the spermiducl or vas deferens (Fig. 144 A, v. d), which, together with the blood-vessels and nerves of the spermary, passes out through the inguinal canal : it then loops round the corresponding ureter, and extends back between the neck of the bladder and a median sac on the dorsal side of the latter known as the uterus masculinus (u. m). The neck XI URINOGENITAL ORGANS 539 of the bladder is continued backwards, through the pelvic cavity, as the urinogenital canal or urethra, on the dorsal side of which, just in front of a rounded II 3f uya, FIG. 144. — The urinogeiiital organs of the Rabbit.— A, of male ; B, of female ; from the left side (about nat. size). The kidney and proximal end of the ureter, in A the spermary, and in B the ovary, uterine tube, and uterus are not shown. an. anus ; bl. urinary bladder ; c. c. corpus cavernosum ; c. s. corpus spongiosum ; c. gl. Cowper's gland ; g. cl. apex of clitoris ; g. p. apex of penis ; p. gl. perineal gland, and />. gl'. aperture of its duct on the perineal space ; pr. anterior ; pr'. posterior, and pr", lateral lobes of prostate ; ret. rectum ; r. gl. rectal gland ; u. g. a. urinogenital aperture ; u. m. uterus masculinus ; ur. ureter ; va. vagina ; vb. vestibule ; v. d. vas deferens. (From Parker's Zootomy.) elevation, is an aperture by means of which the uterus masculinus and vasa deferentia open into it. A prostate gland (pr. gl), consisting of several lobes, is embedded in the walls of the uterus masculinus and opens by small ducts on either side of the elevation just referred to ; and a pair of smaller, ovoid Cowper's glands communicate with the urinogenital canal further back. L L 2 540 THE RABBIT CHAP. The terminal part of the urethra traverses the copu- /atory organ or penis ; that part of its wall which in the retracted condition is dorsal is constituted by a soft vascular portion, the corpus spongiosum (c. s), while the opposite surface is strengthened by two harder bodies, the corpora cavernosa (c.c), which are closely applied together through the greater portion of their length, but diverge proximally and are attached to the ischia. In both sexes a pair of perineal glands (p. gl) open on the perineal spaces (p. 486) at the sides of the penis, and two larger rectal glands (r. gl) lie at the side of the rectum. In the female the ovanes (Figs. 135 and 145, ov) are small ovoid bodies attached by peritoneum to the dorsal wall of the abdomen behind the kidneys, the ovarian follicles or ovisacs (p. 195), which are hollow, forming, when ripe, very small, rounded projections on their outer surface.1 The oviducts, instead ot remaining separate along their whole length, are fused proximally to form a wide, median portion, the vagina (Figs. 144, B, and 145, va), opening into the urinogenital canal or vestibule (vb), with which the bladder communicates and which opens ex- ternally at the vulva (Fig. 144, B, u. g. a). Into the other or distal end of the vagina, the paired, thick-walled uteri (Fig. 145, r. ut, I. ut), or middle portions of the oviducts, open by separate thick-walled apertures. The eggs undergo development in the uteri, which vary in size according to whether or not they contain embryos, and according to the stage of development of these. Each uterus is continued forwards as a narrow, slightly coiled tube — the anterior section of the oviduct, or uterine tube (fl. t), which communicates with the ccelome by a small aperture (fl. t'} surrounded by a wide, membranous 1 After the ova are discharged, a proliferation of cells into the follicles takes place, and gives rise to the so-called corpora lutea. XI FGETUS AND PLACENTA 541 funnel with thin walls and folded margins, which is applied to the outer surface of the corresponding ovary. On the ventral wall of the hinder or proximal end of the urinogenital canal is a small, hard, rod-like body, the clitoris (Fig. 135, d), corresponding to the penis of the male, and strengthened by two small corpora cavernosa (Fig. 144, B, c. c) attached at their proximal ends to the ischia. fit *•** 1Mb' rut FIG. 145. — The anterior end of the vagina, with the right uterus, Fallopian tube, and ovary of the Rabbit ( x about 2). Part of the ventral wall of the vagina is removed, and the proximal end of the left uterus is shown in longitudinal section. fl. t. uterine tube ; fl. t' . its coelomic aperture ; /. ut. left uterus ; /. ut'. aperture of same (os uteri) into vagina ; ov. right ovary ; r. ut. right uterus ; r. ut'. aperture of right uterus into vagina ; $. vaginal septum ; va. vagina. (From Parker's Zootomy.) The rabbit is viviparous. The minute ova are dis- charged into the uterine tubes, where they become fertilised, and then pass into the uterus, in which each develops into a fcetus, as the intra-uterine embryo is termed, and is nourished by means of an organ known as the placenta, which will be described in Chapter XIII. The young animal escapes from the uterus in a condition in which all the parts have become fully formed, except 542 THE RABBIT CHAP. that it is practically hairless ; the eyelids are at first coherent. As many as eight or ten young are produced at a birth, and the period of gestation, i.e., the time elapsing between the fertilisation of the ovum and the birth of the young animal, is thirty days. Fresh broods may be born once a month throughout a considerable part of the year, and, as the young rabbit may begin breeding at the age of three months, the rate of increase is very rapid. The class Mammalia is divided into a number of orders, that to which the rabbit belongs being called the Rodentia and also including rats and mice, squirrels, beavers, porcupines, and many others. All these are vegetable feeders and are mostly of small size. They possess no canine teeth, and their incisors, which are adapted for gnawing, are never more than two in number in the lower jaw, and in most of them there are only two in the upper jaw also. PRACTICAL DIRECTIONS The specimen used should be over three months old. Place it in a sufficiently large jar or box with a close-fitting lid together with a piece of cotton-wool well soaked in chloroform, and leave it until a short time after all move- ments have ceased. A pair of bone-forceps (p. 12) will be required. A. External characters (see pp. 483-487). Microscopic preparations of the skin of a Mammal should be examined, and the hairs, hair-sacs, and sebaceous glands (Fig. 131) noted. (For mode of preparation, see p. 136.) B. Skeleton. If more convenient, the detailed examina- tion of the skeleton may be postponed until after the soft parts have been dissected. The skeleton of a young rabbit, about six weeks old, as well as that of an adult, should be obtained for examination : the former is the more important of the two for making out the individual bones, which should all be separated from one xi PRACTICAL DIRECTIONS 543 another by prolonged maceration in water, or by boiling for a short time. In the adult skeleton the bones are best kept in their natural connection. An additional skull should be prepared, and a longitudinal vertical section made of it with a fine saw ; but as it is difficult to do this very accurately . and perfectly in the rabbit, owing to the delicacy of some of the bones, the skull of a young dog or cat may be used for the purpose. Work through the account of the skeleton given on pp. 487-505, as well as of the teeth on pp. 509-511, comparing the form, number, and arrangement of the teeth in the herbivorous rabbit and the carnivorous dog or cat, noting the presence of canines in the two last mentioned ; sections of the teeth (see p. 510) should be examined. Sketch typical parts of the skeleton. C. Superficial dissection, and injection of the arteries. [For the blood-corpuscles, examine a drop of blood obtained from a freshly-killed rabbit, or by pricking the tip of your finger (compare p. 121). Note the circular and slightly biconcave form of the red corpuscles, their relatively small size (about ^o1^ in. or -008 mm. in diameter), and the absence of a nucleus.] i . Fix the animal on its back on the dissecting-board by inserting large pins or nails through the limbs. If you wish to inject the arteries, cut through the skin on the inner side of one of the thighs, reflect it, and with the seeker expose the femoral artery (p. 524) ; or if your speci- men is a small one, expose one of the carotid arteries (p. 523) instead, by carefully cutting through the skin along the middle ventral line of the neck. Pass a piece of thread round the artery thus laid bare, and:"make a small slit in it with the fine scissors distal ty to the thread. Insert and tie in a cannula, directed towards the body, and first inject a little strong formaline (i part formaline and 2 parts water), following this with the coloured starch injection -mass (p. 99), which will force the formaline into the capillaries and help to preserve the specimen : this method is of special advantage for the subsequent examination of the enteric canal, especially if it is not possible to get to the end of §VI, p. 547, on the first day: the veins need not be specially injected, as they will be naturally injected with blood. Make a median longitudinal incision through the skin in the sternal region, and continue the cut backwards to the pelvic symphysis and forwards to the mandibular sym- 544 THE RABBIT CHAP. physis ; then dissect away the skin from the underlying muscles over the whole ventral surface, being careful not to injure any of the larger blood-vessels (e.g., the jugular veins in the neck). Note : — 1. The thin cutaneous muscle, part of which you have very likely removed together with the skin ; and in the adult female, the mammary glands (p. 506) — trace the main ducts of some of these to their apertures on the teats. 2. In the neck : — a, the trachea and larynx (p. 506), to see which and the following structures clearly the cervical portion of the cutaneous muscle should be dissected away and the underlying parts carefully separated with a seeker ; b, the hyoid bone, situated in a mass of muscle just anterior to the larynx ; c, the submaxillary glands (p. 512) ; and d, the large external jugular veins (p. 525). 3. In the thorax : — a, the sternum and xiphisternum ; b, the small clavicles (p. 501) ; c, the pectoral muscles ; d, the vertebral and sternal portions of the ribs and the external intercostal muscles ; and e, the blood-vessels and nerves going to the fore-limbs. 4. In the abdomen : — a, the muscles of the abdominal wall, separated in the middle ventral line by a whitish fibrous band ; b, Poupart's ligament (p. 506) ; c, the blood- vessels and nerves passing beneath Poupart's ligament to the hind -limbs. II. On the same day on which the animal is killed, remove the skin from the dorsal surface of the head and anterior part of the neck, and also the muscles covering the anterior cervical vertebrae. A membrane between the skull and first vertebra will then be exposed : cut through this, and part of the spinal cord will be seen. With the bone-forceps cut away the arches of the first three or four vertebras (noting the roots of the spinal nerves) and the roof of the skull — taking particular care in the auditory region — so as to expose the brain : note the olfactory lobes, cerebral hemi- spheres, cerebellum, and medulla oblongata, and also the dura mater and pia mater. Then cut through the spinal cord about a quarter of an inch beyond its junction with the brain, lever up the latter with the handle of a scalpel, and carefully cut through all the nerves arising from it and separate it from the cranial walls, working from behind forwards. Dissect away the olfactory lobes from their attachments, remove the entire brain, and place it in 3 per cent, formaline or in strong spirit. The animal may be preserved from day to day by wrap- xi PRACTICAL DIRECTIONS 545 ping a cloth round it soaked in 3 per cent, formaline or strong methylated spirit and placing it in a closed vessel. D. Dissection of the abdomen. I. Once more pin the animal down, ventral side upper- most, and make an incision along the middle line of the abdominal muscles from the xiphisternum to the pelvic symphysis, so as to open up the abdominal cavity. From the anterior extremity of the incision make transverse cuts, and turn back the flaps of muscle.1 Note : — 1. a, The peritoneum ; b, the diaphragm, with its muscles and central ^ tendon ; c, the pink lungs, seen through the latter : make a small aperture in the diaphragm on one side of the median line and note the collapse of the corresponding lung. 2. a, The liver, stomach, small intestine, ccecum, colon, and rectum (pp. 512-516) ; b, the urinary bladder ; c, the scrotal sacs in the male ; and — by turning aside the intestines — d, the kidneys; in the female e, the ovaries and oviducts (varying much in size according to age). 3. a, The characters and relations of the lobes of the liver (compare Fig. 137), and the folds of peritoneum which attach the liver to the diaphragm ; b, the gall-bladder ; e, the gullet, and its entrance into — /, the stomach (note its cardiac and pyloric portions) ; g, the spleen. 4. a, The parietal and visceral layers of the peritoneum and the various subdivisions of the mesentery; b, the U-shaped duodenum, passing into the ileum, and the connection of the latter with the proximal end of — c, the ccecum, at the distal end of which is the vermiform appendix, while proximally it passes into — d, the colon, continuous with — e, the rectum, which enters the pelvic cavity to open by the anus. II. Turn over the stomach to the animal's right side and make out — i. a, The postcavalvein, passing from the pelvis forwards, near the ventral surface of the backbone, through a notch in the liver to the diaphragm ; b, the dorsal aorta, running partly above, partly alongside, the postcaval ; c, the coeliac artery, given off from the aorta about an inch posteriorly to the diaphragm — trace its main branches ; d, the anterior 1 You will very probably notice a number of transparent, rounded vesicles, about | inch in diameter, in the abdomen. These are tape- worms (phylum Platyhelminthes, p. 412) in the encysted stage, which develop into the adult form when swallowed by a dog. 546 THE RABBIT CHAP. mesenteric artery, arising about half an inch further back than the cceliac — trace its main branches (Fig. 137). Note the cceliac or solar plexus (p. 533). 2. a, The left kidney, with its artery, vein, and ureter ; b, the yellowish left adrenal, anterior to the origin of the renal artery and vein ; c} the left ovary, uterine tube, and uterus in the female. 3. a, The small posterior mesenteric artery, leaving the aorta a short distance posteriorly to the left kidney and supplying the rectum ; b, the posterior mesenteric vein, in the mesentery of the rectum. III. Turn the intestines over to the animal's left side and spread out the duodenum, putting its mesentery slightly on the stretch but taking care not to rupture it, so as to make out — 1. a, The large anterior mesenteric vein, into which the posterior mesenteric vein opens ; b, the pancreas and its duct, opening into the distal limb of the duodenal loop an inch or so beyond the bend. 2. a, The right kidney, partly overlapped by the caudate lobe of the liver, and its ureter ; b, the right adrenal. IV. Replace the intestines in their natural position ; cut through the gullet close to the diaphragm, draw the stomach backwards, turn forwards the lobes of the liver, and dissect out the following structures (Fig. 137) : — 1. The common bile-duct — made up of cystic and hepatic ducts, and its entrance into the duodenum. 2 . a, The large hepatic portal vein, running in the mesentery ventrally to the postcaval, made up by the union of the mesenteric, gastric, and splenic veins, and entering the liver, sending a branch to each lobe ; b, the transparent lymphatic vessels (lacteals] in the mesentery. V. Tie two pieces of thread, about half-an-inch apart, round the portal vein just before it enters the liver (the hepatic artery and bile-duct may be included in the liga- ture) ; cut through the rectum just anteriorly to the pelvic cavity, and through the portal vein between the ligatures, as well as through the mesenteric attachments of the stomach and intestine, and remove these from the body entire. Unravel the intestine by cutting or tearing the mesentery — except that part of it in which the pancreas lies — and spread it out on the dissecting-board or in a large dissecting-dish, arranging it so that the various subdivisions may easily be distinguished from one another, and blowing it up with xi PRACTICAL DIRECTIONS 547 a blow-pipe. In addition to the relations of these parts, note : — 1. The sacculus rotundus (Fig. 135, 5. r), and the characters of the caecum, which proximally passes insensibly into the colon. 2. The length of the intestine as a whole (about fifteen or sixteen times that of the animal) , and the relative length of its five divisions. Sketch the entire canal. VI. Remove and cut open the stomach and parts of the small intestine, colon, and rectum : wash thoroughly, and examine under water. Remove the caecum together with a small portion of the ileum and colon, wash it out by directing a stream of water through it, distend with water (it is better to harden it first with formaline) and examine in a dish of water ; with the scissors cut away a piece of the wall here and there, between the constrictions and in the appendix. Note : — 1. The mucous membrane, muscular coat, and peritoneum. 2. The longitudinal ridges or ruga in the stomach, and the pyloric valve. 3. The aperture of the bile-duct, and the villi and Peyer's patches (p. 515) in the small intestine. 4. a, The Peyer's patches and intra-colic valve in the proximal part of the colon ; b, the thick lymphoid tissue in the walls of the sacculus rotundus ; c, the ileo-colic aperture and valve. 5. The spiral valve of the caecum, and the lymphoid tissue of the vermiform appendix. 6. The characters of the mucous membrane of the caecum and large intestine. (Microscopic sections of the small intestine — injected and uninjected, should also be examined, compared with Fig. 39, and the villi and intestinal glands noted.) VII. Ligature the postcaval at the points where it enters and leaves the liver : remove the entire liver, noting as you do so the aorta, gullet, postcaval, hepatic veins, and phrenic veins. Sketch the liver from the posterior surface. Then return to the examination of the abdomen, and make out (Fig. 140)— 1. The renal and spermatic (or ovarian) arteries and veins. 2. The caudal artery, arising from the dorsal side of the aorta. 548 THE RABBIT CHAP. 3 The common iliac arteries, each of which gives off an ilio-lumbar branch and divides into an external and an internal iliac, the former becoming continuous, beyond Poupart's ligament, with the femoral. (The course of these arteries and of the corresponding veins can be more easily traced after the urinogenital organs have been removed — see § IX.) 4. The ilio-lumbar veins, and the trifurcatlon of the post- caval into the external iliacs and a median vein formed by the fusion of the two internal iliacs. Sketch all these vessels later, after the removal of the urinogenital organs. VIII. Dissect away the peritoneum and fat from the kidneys, ureters, and generative organs, and trace each ureter from the hilus of the kidney backwards to the bladder, which, if contracted, may be inflated from the urinogenital aperture. Then make out — A. In the male (Fig. 144, A) : — 1. The. penis, with the prepuce, and the aperture of the urinogenital canal or urethra, which latter is lined by the soft corpus spongiosum, and strengthened by the corpora caver - nosa, which diverge proximally and are attached to the ischia. 2. The scrotal sacs, each communicating with the abdo- minal cavity by the inguinal canal, through which the corresponding spermiduct and the spermatic artery and vein pass to the spermary, to see which and the epididymis the scrotal sac should be slit open along its ventral wall. (Sperms should be examined from a fresh spermary — compare p. 210.) 3. The course of the spermiducts. B. In the female (Figs. 144, B, and 145) : — 1. The vulva; and the clitoris, with its corpora cavernosa, on the ventral wall of the urinogenital canal. 2. The vagina, uteri, and uterine tubes with their funnels. 3. The ovaries, studded with ovisacs. IX. Dissect away the kidneys, ureters — and in the male, the scrotal sacs, in the female, the ovaries — from the sur- rounding parts : cut through the pelvic symphysis with a knife, sever the attachment of the corpora cavernosa to the ischia, and cut through the skin around the urinogenital aperture and anus. Remove the whole of the urinogenital organs, together with the posterior end of the rectum, from the body and, after noting § VII, 3 and 4, pin them out in a dissecting-dish, with the ventral side uppermost, taking xi PRACTICAL DIRECTIONS 549 care to preserve the natural relations of the parts. Dissect away all the superfluous connective-tissue and fat, separate the rectum at its cut end from the urinogenital canal, and turn it on one side, noting the rectal and perinea! glands. Make out, in addition to the parts referred to in § VIII : — A. In the male. The uterus masculinus and prostate, and the spermiducts passing between the former and "the bladder. Sketch the entire dissection. Then slit open the uterus masculinus so as to see the openings of the spermiducts, remove the rectum and rectal glands, and make out the relations of the corpus spongiosum. Make a median incision along the whole length of the penis, beginning at the apex and cutting through the fibrous septum between the two corpora cavernosa : continue the incision forwards, so as to open the bladder along its ventral wall, and note and indicate on your sketch the openings of the ureters into the bladder, and the crescentic aperture of the uterus mascu- linus into the urinogenital canal, just at the anterior edge of a cushion-like fold. B. In the female. The wide urinogenital canal with its vascular walls. Sketch the entire dissection. Make a median longitudinal incision through the urino- genital canal, and continue it forwards until the cavity of the bladder is exposed ; slit open the vagina and one of the uteri and uterine tubes in the same way. Note and indicate on your sketch the openings of the ureters into the bladder, the connection of the neck of the bladder with the urino- genital canal, the large aperture in the latter leading into the vagina, and the thick-lipped aperture (os uteri) communi- cating between the vagina and thick-walled uterus, between which and its fellow is a vestigial septum, indicating the primarily paired character of the vagina. Insert a seeker in the coelomic aperture of the uterine tube. If your speci- men is in an early stage of pregnancy, young embryo (blastocysts, p. 586) may be present in the uterine tubes, and should be carefully removed and fixed in corrosive sublimate (p. 136) : if the uteri contains later embryos they present a series of swellings : cut one of the swellings open from the ventral side, and note the foetus, enclosed by the amnion (p. 606), and attached to a discoid placenta (p. 610) ; cut out the other swellings, together with their contents, and preserve them in formaline or alcohol. Microscopic sections of the ovary should be examined, and the hollow ovisacs noted, each consisting of follicular cells enclosing a minute ovum. 550 THE RABBIT CHAP. With a scalpel or razor cut across one of the kidneys through the hilus, parallel to the dorsal and ventral faces of the organ, and note the pelvis, and its prolongations — the calices, the urinary pyramid, and the cortical and medullary portions. Sketch. Examine also microscopic preparations of injected and uninjected kidney (compare pp. 146 and 537). X. Dissect out the lumbo-sacral plexus in the pelvic cavity (p. 532), and then cut through the body just behind the diaphragm ; the abdomen will be no longer required, but you should retain one of the hind-limbs if you wish to examine any of its muscles and joints (compare p. 64). E. Dissection of the thorax and neck. I. Dissect away the pectoral muscles, cut through all the vertebral ribs of the left side except the last five, at about a quarter of an inch from their junction with the sternal ribs : from the posterior end of the incision thus made, cut down- wards (i.e., towards the sternum) for about an inch, and then forwards through the sternal ribs. Turn forward the flap thus separated and carefully dissect it away from, the under- lying tissues at the anterior end, so as to detach it altogether without injuring the jugular and brachial veins. Note in the thoracic cavity thus laid open : — 1. The pericardium and heart, and the thymus (Fig. 135). 2. The collapsed lungs : make a small aperture in the trachea and inflate. 3. The parietal and visceral layers of the pleura and the mediastinum (Fig. 138). To make out the relations of these and of the mediastinal space more accurately, cut away a flap from the right wall of the thorax, as you have already done on the left side, leaving the sternum intact. 4. The relations of the pericardium, to see which cut through the posterior end of the sternum : separate the mediastinum from the dorsal surface of the bone, turn the latter forwards and remove it : then cut the pericardium open longitudinally, noting the pericardial fluid. II. Dissect away the pericardium, the thymus, and any fat about the base of the heart which may obscure the vessels arising from it. Follow out these vessels to the head by clearing away the connective-tissue, fat, &c., by which they are surrounded, but taking care not to injure any of the nerves of the neck. Make out : — i. a, The left and right ventricles and auricles, and the ramifications of the coronary artery and vein ; b, the two bronchi. xi PRACTICAL DIRECTIONS 551 2. a, The pulmonary artery and its division into left and right trunks/ b, the pulmonary veins (the course of which will be better seen at a later stage) ; c, the two precaval veins, each formed by the union of a subdavian, an external jugular, and a smaller internal jugular ; dt the thoracic portion of the postcaval vein. 3. a, The £7i//m the crelomic pouches, 578 CHAP, xni DEVELOPMENT OF LANCELET 579 bp. blastopore ; blc. segmentation-cavity ; cp. coelomic pouch (enterocoele) ; ent. enteron ; ep. ectoderm ; hyp. endoderm ; m.s. mesodermic segments ; not. noto- chord ; n.p. neuropore, or anterior opening of medullary tube ; n.pl. medullary plate ; n.t. medullary tube ; o.c.p. openings of ccelomic pouches into enteron. (From Dendy, adapted from Ziegler's models, after Hatschek.) its cavity is the primitive enteron or archenteron (ent), and is bounded by the invaginated cells which now constitute the endoderm (hyp), the remaining cells form- ing the outer wall of the gastrula being the ectoderm (ep} (compare pp. 201 and 202). The two layers are con- tinuous at the aperture of the cup, the gastrula-mouth or blastopore (bp) . Between the ectoderm and endoderm is at first a space, the greatly diminished segmentation- cavity, which soon becomes entirely obliterated, so that the ectoderm and endoderm are in contact. The general resemblance of the gastrula to a simplified Hydra,1 devoid of tentacles, will be at once apparent, and the stage in the development of the frog's egg represented in Fig. 64, I, though much modified by the quantity of food-yolk (compare p. 574), will be seen to correspond to the gastrula-stage. As in the frog, the blastopore soon closes over, the mouth and anus being subsequently formed from the stomodaeum and the proctodaeum respectively (p. 204). The gastrula becomes elongated, flattened on one side and convex on the other (J). The flattened side corresponds to the dorsal surface of the adult, and the blastopore now comes to be situated, as in the frog- embryo (Fig. 64, H, K), at the posterior end of the dorsal surface. A medullary or neural plate and groove (K, n.pl) are then formed, the central nervous system being de- veloped in a manner essentially similar to that already described in the case of the tadpole (p. 202), except that 1 It must, however, be remembered (pp. 308 and 321) that the ectoderm and endoderm of Hydroids are differentiated before the mouth is formed, so that the mouth does not correspond to the blastopore of the gastrula. 580 DEVELOPMENT OF LANCELET CHAP. the central canal of the medullary or neural cord (L, n.t) is formed after the plate is separated from the outer ectoderm. In the mid-dorsal line a thickening of the endoderm (K-N, not) soon becomes constricted off to form the notochord (pp. 203, 419, and 441), and on either side of this a series of hollow endodermic pouches arise from the archenteron, arranged meta- merically (K-O, c.p, o.c.p, m.s). The cavities of these, which subsequently give rise to the body-cavity, are thus at first in free communication with the definitive enteron and are known as enterocceles ; from their walls the mesoderm l is derived. Subsequently the communi- cations between the enteric and enterocoelic cavities become closed, and the paired pouches gradually extend between the ectoderm and endoderm, so as eventually to meet one another both dorsally and ventrally (M), their outer walls (parietal or somatic layer of the meso- derm) being in contact with the ectoderm and forming with it the somatopleure or body-wall, and their inner walls (visceral or splanchnic layer of the mesoderm) in contact, below the notochord, with the endoderm and with it forming the splanchnopleure or wall of the enteric canal (compare p. 203). Thus the body-wall and the enteric canal are separated by a cavity, the ccelome, which, much as in the adult earthworm, is divided into a series of metamerically arranged portions : later on, however, the adjacent walls of these ccelomic sacs dis- appear, and the ccelome becomes a continuous cavity. The embryo lancelet is hatched soon after reaching the gastrula-stage, when it moves about by means of cilia developed on the ectoderm cells, and has to get its own living, having by this time used up its small 1 The terms epiblast, hypoblast, and mesoblast are often used to designate the embryonic ectoderm, endoderm, and mesoderm respectively. xm EARLY DEVELOPMENT IN VARIOUS TYPES 581 reserve of yolk. It then passes through a complicated series of larval stages, gradually leading up to the adult form. Early Development of other Types. — The presence of a greater amount of food-material in the egg renders it possible for the embryo to go on developing further than the gastrula-stage before being hatched, and, as a general rule, the greater the relative quantity of yolk present in the ovum of an animal, the less clearly can a gastrula-stage be recognised. In the earthworm and mussel, it will be remembered (pp. 349 and 410), the segmentation is entire, but unequal, and the larger lower cells become invaginated to form the endo- derm and enteron while the smaller upper cells give rise to the ectoderm. ' In the earthworm the blastopore does not become closed, but gives rise to the mouth. In the frog (p. 201) the enteron arises by a split appearing amongst the yolk-cells, beginning at the edge of the blastopore and gradually extending inwards : the process is supplemented by an invagination of the ectoderm. The enteron is at first a very narrow cleft, but soon widens considerably (Fig. 64, I, ent) I for some time it does not actually communicate with the exterior, the blastopore (Up) being filled up by a yolk- plug (yk* pi). As the enteron extends forwards, and the relatively small segmentation-cavity (U. cat) gradu- ally disappears, the edges of the lower margin of the blastopore approach one another and, uniting in the median plane, give rise to a vertical, grooved streak— the primitive streak and groove, as it is called. In the centrolecithal egg of the crayfish (Fig. 98) a gas- trula-stage is formed by invagination, but as the centre of the oosperm is filled with solid yolk in the place of a seg- mentation-cavity containing fluid, the invagination only extends a short distance inwards, the enteron being rela- 582 DEVELOPMENT OF CHICK CHAP. lively very small and the ectoderm separated from the endoderm by the yolk. The gastrula-stage is much less clearly distinguish- able in the segmenting telolecithal eggs of the dogfish and bird (pp. 470 and 583), in which the relatively enormous mass of unsegmented yolk. is, as in the crayfish, sufficient to nourish the embryo until it has reached a stage closely resembling the adult in almost every essential respect except size. A blast opore can sometimes be recognised in such cases, but in the embryo of the common fowl it is to some extent represented by a primitive streak and groove (see p. 581 and Fig. 153, E, pr. st). The blastoderm becomes differentiated into an outer ectoderm and an inner, lower layer of cells (com- pare Fig. 128), between which and the yolk the enteric cavity is formed : a segmentation-cavity is recognisable in early stages. As the embryo develops, it becomes folded off from the yolk, which is gradually reduced in amount and in later stages is contained in an extra- embryonic part of the embryo — the yolk-sac (Figs. 129 and 159). A more detailed account of the early stages of the chick is given below, as the fowl's egg is an easily procurable type for the practical study of develop- ment. The fowl's egg has already been described in detail (p. 565, Fig. 149). Its essential part, the yolk, corresponds, as we have seen, to a single cell enormously enlarged by the mass < of yolk-granules it contains, the only part in which they are absent being a small area which, owing to its lesser specific gravity, is always found at the upper pole ; this area, in which the nucleus lies, is known as the germinal disc. Fer- tilisation takes place in the anterior end of the oviduct, and then the nucleus divides and initiates the process of seg- mentation which is completed by the time the egg is laid, when the germinal disc has thus become converted into a multicellular blastoderm (p, 574). xm DEVELOPMENT OF CHICK 583 The first stage of segmentation is indicated by the forma- tion of a vertical furrow across the centre of the germinal disc (Fig. 153, A), and this is followed by another furrow at right angles to it (B). Further radial and concentric furrows then arise (C) until the blastoderm is seen to consist of a number of irregular cells (D). Horizontal furrows are in the meantime formed, so that the originally single layered blastoderm, beneath which a narrow space representing the segmentation-cavity can be recognised, comes to consist of several layers of cells, the outer of which corresponds to the ectoderm ; the inner, or lower layer cells, extend rapidly through the segmentation-cavity between the ectoderm and the yolk and give rise to the endoderm, the space remaining beneath this layer and the yolk forming the rudiment of the enteron (F). New cells are gradually formed round the edge of the blastoderm, which, when the egg is laid, forms a circular patch, about 3-5 mm. in diameter, on the surface of the yolk (Fig. 149) : its central part, or area pellucida, owing to the presence of fluid in the underlying space, is more transparent than the rim, or area opaca. When the egg is incubated, the blastoderm grows rapidly at its periphery (area opaca) , and eventually, as we shall see, encloses the whole yolk (Fig. 159). The area pellucida extends less rapidly and becomes pear-shaped in outline (Fig. 153, E), its broader end corresponding to what will become the anterior end of the embryo : in this region most of the lower layer cells are arranged so as to form a definite layer of endoderm, which can be recognised somewhat later in the area opaca also. The ectoderm and lower layer cells are at first continuous round the edge of the blastoderm, but later on this is only the case at the posterior border (F), and results in the appearance of a narrow band, the primitive streak (E, pr. st), along which a median primitive groove is formed. This, as already mentioned, represents the fused lips of the blastopore of the frog and other forms ; it extends from the centre to the posterior border of the area pellucida, and rapidly grows backwards. The primitive streak is due to the multiplication of ectoderm cells, which grow inwards and spread out right and left under the covering ectoderm, between it and the endoderm, so as to form a horizontal wing of cells on either side (G). The mesoderm is formed from these wing-like ectodermic sheets, with which the endo- derm becomes united, so that the three layers are for a time indistinguishable in this region. The mesoderm eventually forms a sheet of loosely arranged cells which spreads rapidly in all directions except anteriorly (E), in which region, known as the pro-amnion, the blastoderm consists of ectoderm and UJ CHAP, xiii DEVELOPMENT OF RABBIT 585 F, vertical section of blastoderm and adjacent part of the egg at the time of laying, before incubation : the anterior edge of the blastoderm is to the right the posterior edge to the left of the figure (x 25) ; G, transverse section o ge o e e o e gure x 25 ; , transverse section of blastoderm at about the twentieth hour of incubation, the section passing through the primitive streak and groove at about the middle of its length x 200. (x 200). mesoerm ; me. gr. meuary groove ; pg. primitive groove ; pr. am. pro- amnion ; pr. st. primitive streak ; y. yolk ; zf. formative cell ; zl. lower layer cells. (A and B, from Foster and Balfour ; C — G, from Marshall, after Duval and Coste. . and Coste.) endoderm only until the end of the second day of incubation, and by the third day the whole blastoderm has become three-layered. Before tracing the development of the various organs derived from the three germinal layers respectively, it must be remembered that the actual chick embryo is formed from the area pellucida, and gradually becomes folded off from the yolk, while the area opaca extends over the yolk until the latter is entirely covered by it (iyth day of incubation) : thus we can distinguish between an embryonic and an extra-embryonic portion of the blastoderm. In the meso- derm of the latter portion blood-vessels are formed, so that it is then known as the area vasculosa (see Fig. 159). The minute egg of the rabbit (compare p. 574) and of most other mammals, although microlecithal and undergoing a holoblastic segmentation, has presumably been derived from a meroblastic type with abundant yolk, like that of the bird, and some lower mammals (Ornithorhynchus and Echidna) living in Australia at the present day still possess eggs of this type. In the higher Mammalia the yolk has disappeared, as it is no longer needed, the embryo, as we have seen (p. 483), being nourished by means of a placenta, which will be described presently. The early processes of development are therefore somewhat peculiar, and though the segmenta- tion is holoblastic, the subsequent development is for some time essentially similar to that of the bird : it is not until later stages that the more special characteristics of the mammal on the one hand and the bird on the other become apparent. PRACT. ZOOL. O O 5S6 DEVELOPMENT OF RABBIT CHAP. Fertilisation and segmentation take place in the uterine tubes (p. 540), and each oosperm is surrounded by a mem- brane, the zona radiata, apparently formed from the follicle cells, and a layer of albumen secreted from the oviduct. The oosperm divides into two blastomeres (Fig. 154, A), and then further divisions follow and result in the formation of a mulberry-like mass (B), which, however, is not hollow, as ir Amphioxus (Fig. 152), but consists of an outer layer en- closing a solid mass of larger and more granular cells (C), and at this stage it passes into the uterus. The outer layc of cells then grows rapidly, so that a space appears and gradually increases in size, between it and the inner ma except at one point where the latter is attached to tr former (D, E). This hollow ball is known as the blast ocyst or blastodermic vesicle, and may be compared to the embryo and yolk-sac of a bird, except that the latter contains a fluid instead of yolk. The outer layer is called the trophoblastic ectoderm or trophoblast ; it probably corresponds to the extra-embryonic ectoderm of the chick (p. 585) and takes no direct part in the formation of the embryo, which arises from the inner mass (embryonic area) , in much the same way as the chick is formed from the blastoderm in the case of the fowl's egg. Thus later on an embryonic portion with a primitive streak, and an extra-embryonic portion gradually growing round beneath the trophoblast, can be recognised in the embryonic area, the body of the embryo being gradu- ally folded off from the blastocyst, which is then often known as the umbilical vesicle and has similar relations to the embryo that the yolk-sac has to the body of the chick, to which it corresponds (see p. 582 and Figs. 159 and 166). In the lancelet alone amongst the triploblastic animals described in this book does the mesoderm arise as a series of enterocoelic pouches : as we have seen, it is usually at first solid, and may be budded off from the ectoderm and endoderm, or from the ectoderm only, at the margin of the blastopore or primitive groove (chick, p. 583) ; or both endoderm and mesoderm may be differentiated at the same time from the lower-layer cells or yolk-cells (frog, p. 202) ; or finally, it may arise in all these ways. The ccelome is formed secondarily by a split taking place in the mesoderm on either side (Figs. 65 and 155), the split gradually extending with the FIG. 154. — Early stages in the development of the oosperm of the Rabbit. A, two-celled stage, from middle part of the uterine tube, about twenty-two hours after copulation ; B, polyplast or morula stage, from lower end of uterine tube, about the middle of the third day ; C, close of segmentation, from lower end of uterine tube just before entering uterus, seventy hours after copulation ; D, first stage in formation of blastocyst, from uterus, seventy-five hours after copulation ; E, section of blastocyst at the end of fourth day. (x about 160.) cb, blastomere ; cc, trophoblastic ectoderm ; cd, inner mass of cells (embryonic area) ; cv, cavity of blastocyst ; mo, sperms imbedded in the zona radiata (z) ; n, nucleus. (From Marshall ; A, B, after Bischoff ; C — E, after Van Beneden.) 587 002 588 MESODERM AND CCELOME CHAP. extension of the mesoderm between the ectoderm and endoderm. Thus the coelome is formed not as an enterocoele, but as a schizoccele. In Vertebrates each mesoderm-band becomes differ- entiated into a dorsal portion abutting against the medullary cord and notochord known as the vertebral plate — which soon loses its ccelomic space, and a ventral portion, the lateral plate, which is divided into parietal and visceral layers by the coelome (Figs. 65, 152, and 155). The vertebral plate undergoes metameric seg- mentation, becoming divided into a row of squarish masses, the mesodermal segments or " protovertebrce " (pr. v, m. s, ms. s), from the outer parts of which (muscle- plates, Fig. 164, m. p) the muscular segments or myomeres are formed (p. 203), and from the inner parts the vertebral column, the segmentation of which alternates with that of the myomeres (compare p. 441). Outline of the Development of the Chief Organs in the Craniata (compare pp. 201-210). — The nervous system, as well as the essential parts of the sensory organs, are, as we have seen, in all cases formed from the ectoderm. The development of the central nervous system takes place in essentially the same manner as in the lancelet (P- 579) and frog (p. 202). In the chick the ectoderm in front of the primitive streak • early becomes thickened along the median line to form the medullary plate (Fig. 153, E) ; this gradually increases in length at the expense of the primitive streak, from the cells of which the greater part of the embryo is eventually developed. A medullary groove, bounded by medullary folds (Fig. i55,Aand B), is formed along the median line of the medullary plate and passes into the primitive groove posteriorly ; while near their anterior ends the folds meet and unite, so as to constrict off a medullary tube from the outer ectoderm (Figs. 155 and 158, A). This closure of the tube then gradually extends both forwards and backwards, the groove remaining open longest at its posterior end. By XIII NERVOUS SYSTEM 589 the formation of a head-fold in the region of the pro-amnion (p. 583, Figs. 153, E, and 158, A) the anterior end of the embryo (head-process) becomes marked off from the blastoderm. In craniate Vertebrates the anterior end of the hollow medullary tube becomes dilated, forming three bulb-like m.f nc ms.cf m.c m-s.s ect ccel- -mes' -mes" FIG. 155. — Transverse sections across the body of a chick embryo at the twenty- fourth to thirtieth hour of incubation, to show stages in the formation of the medullary cord and coelome. (x 100.) oo. aorta ; mes. mesoderm ; mes' parietal, and mes" visceral layer of mesoderm ; m. f. medullary fold ; m. g. medullary groove ; ms. d. mesonephric duct ; ms. s. mesodermal segment ; nc. notochord. veilings— the fore-brain (Fig. 156, A, /. b), mid-brain (m. b), and hind-brain (h. b). Soon another hollow swelling grows forwards from the first vesicle (B, prs. en), and the third gives off a similar hollow outgrowth (cblm) 590 BRAIN CHAP, xin from its dorsal surface. The brain now consists of five divisions : the prosencephalon (prs. en) and diencephalon (dieri) derived from the fore-brain, with the pineal structures (pn. b, pn. e) and the infundibulum, to which the pituitary body becomes attached (inf, pty) ; the mid-brain or mesencephalon (m. b), which gives rise to the optic lobes and crura cerebri ; and the epencephalon or cerebellum (cblm) and metencephalon or medulla oblongata (med. obi) derived from the hind-brain. The original cavity of the brain becomes correspondingly divided into a series of chambers or ventricles (compare Figs. 156 and 50), all communicating with one another, and called respectively the fore-ventricle or prosoccele, third ventricle or diaccde, mid-ventricle or mesoccele (iter, and optic ven- tricles or optocceles), cerebellar ventricle or epiccde, and fourth ventricle or metaccele. In some fishes (e.g. dogfish, Fig. 124) the brain con- sists throughout life of these five divisions, but in most cases (Figs. 49 and 141) the prosencephalon grows out into paired lobes, the right and left cerebral hemispheres or parencephala (Figs. 156, I-L, c. h), each containing a cavity, the lateral ventricle or paraccde (pa. cce) which communicates with the diacoele (di. cce} by a narrow passage, the foramen of Monro (/. m) . From the pros- encephalon or the hemispheres are given off a pair of anterior prolongations, tjie olfactory lobes or rhinencephala1 (olf. I), each containing an olfactory ventricle or rhinoccele (rh. cce). In the preceding description the brain has been described as if its parts were in one horizontal plane ; but, as a matter of fact, at a very early period of development the anterior part becomes bent down over the end of the notochord so that the whole organ assumes a retort-shape, the axis of the 1 The prosencephalon together with the olfactory lobes is often described as the telencephalon, g Ms'ii " > *•"*"• * H CD /! ' !> ,H \ ji ip! i-8«| 8-s IS* 2 5«?J B.i^1 i:!i ife] JJd •g-s,..0^^ S"^1^ g <-> o _ ^- . ^"2 59" 592 NERVOUS SYSTEM CHAP. fore-brain being strongly inclined to that of the hind-brain. The bend is known as the cerebral flexure (Figs. 158 and 159) : it is really permanent, but as the hemispheres grow forwards parallel to the hind-brain, and the floor of the mid-brain and hind-brain thickens, it becomes obscure and is not noticeable in the adult. The central cavity of the embryonic spinal cord has at first the form of a narrow vertical slit (Fig. 155, c), the walls of which eventually fuse dorsally, while ventrally part of the slit forms the central canal of the adult spinal cord, in which dorsal and ventral fissures are developed. The ganglia of the dorsal roots of the spinal nerves are developed from a paired neural crest (Figs. 65 B, 161, and 164) arising close to the junction of the medullary plate and outer ectoderm : this becomes seg- mented to form the ganglia, from the cells of which the sensory fibres arise and extend into the spinal cord. The ventral roots arise as direct outgrowths from cells in the ventral region of the medullary cord. Certain of the cerebral nerves are developed in an essentially similar manner to the dorsal roots of the spinal nerves, while others *Qrise as direct ventral outgrowths from the brain, like the ventral roots. The fibres of all the typical nerves grow peripherally until they reach the parts which they ultimately supply. The olfactory organs arise as sac-like invaginations of the ectoderm, one on either side of the snout (Fig. 158), and become enclosed by the cartilaginous olfactory capsules, developed, with the rest of the skeleton, from the mesoderm. The aperture of invagina- tion gives rise to the external nostril, the internal nostrils (in air-breathing forms) being developed subse- quently. The mode of development of the paired eye of verte- brates is peculiar and characteristic. xiu EYE . 593 At an early stage of development a hollow outgrowth —the optic vesicle (Fig. 157, A, op. v)—is given off from either side of the fore-brain and extends towards the side of the head, where it meets with an in-pushing of the ectoderm (/), which becomes thickened, and finally, separating from the ectoderm, forms a closed, spherical sac (B, /) with a very small cavity and thick walls (compare Figs. 64, L, e, and 158). This body is the rudiment of the lens : as it enlarges it pushes against the optic vesicle which becomes invaginated, the single- layered optic vesicle thus being converted into a two- layered optic . cup (Fig. 157, B, oc', oc"), its cavity, originally continuous with the diaccele, becoming obliterated. Between the edge of the cup and the lens, on the ventral side, is a small space which gradu- ally extends towards the stalk of the cup, and thus gives rise to a slit in the wall of the latter : this choroid fissure (C, aus), as it is called, soon becomes closed by the union of its edges. The outer layer of the optic cup gives rise to the pigment-layer of the retina (p. 183) : from its inner layer the rest of the retina — including the rods and cones — is formed. The stalk of the optic cup occupies, in the embryonic eye, the place of the optic nerve, but the actual fibres of the nerve are formed from the nerve-cells of the retina and grow inwards to the brain. During the formation of the lens, mesoderm extends in between the ingrowth from which it arises and the external ectoderm ; from this the main substance of the cornea and its inner or posterior epithelium are formed, the adjacent ectoderm becoming the external epithelium, i.e., that of the conjunctiva (p. 182). Mesoderm also makes its way into the optic cup through the choroid fissure, and gives rise to the vitreous humour. Lastly, the mesoderm immediately surrounding the optic cup S^.soo ;;; CO |,i||.g 3«j*i 1 iiiii : i ^•53 o •o •- > S fl CX ^ G --3g j^.a -S^-.H «,a ^_.^H -O ^_> o O * 111! B^f^I-.fi Il,88sl8^« CHAP, xm EYE AND EAR 595 is differentiated to form the choroid, the iris, and the sclerotic. Thus the eye of Vertebrates has a threefold origin : the sclerotic, choroid, iris, vitreous humour, and the greater part of the cornea are mesodermal ; the lens and external epithelium of the cornea are derived from the ectoderm of the head : the retina and optic nerve are developed from a hollow pouch of the brain, and are therefore in their ultimate origin ectodermal. The sensory cells of the retina — the rods and cones — although not directly formed from the external ectoderm, as in Invertebrates, are ultimately traceable into the superficial layer of ectoderm, since they are developed from the inner layer of the optic cup, which is a prolongation of the inner layer of the brain, the latter being continuous, before the closure of the medullary groove, with the ectoderm covering the general surface of the body (compare Figs. 155 and 157). The organ of hearing, like that of smell, arises in the embryo as a paired invagination of the ectoderm in the region of the hind-brain, a shallow depression being formed (Figs. 64, L, 157, A, and 158, an. s) which deepens and becomes flask-shaped ; and finally, as a rule (com- pare p. 465), loses its connection with the external ectoderm, giving rise to a closed sac surrounded by mesoderm in which the cartilaginous auditory capsule is subsequently developed. At first simple, it soon becomes divided by a constriction into dorsal and ventral compartments, from the former of which arise the utriculus and semicircular canals, and from the latter the sacculus and cochlea. The early development of the enteric canal has already been dealt with in the case of the frog (pp. 204 and 210), and it will be remembered that the greater part of it (mesenteron) is lined by endoderm, its cavity being at first bounded by the yolk below, but gradually becom- ing closed in by endoderm cells. When a yolk-sac is formed (dogfish, bird, mammal. Figs. 129, 159, 166), it .X frC'oJ _X" IT'S ^-sit* 1 ^l-8"- 1 8-lllil -M (/) \-, — < CL| O 03 T1 . « \SS ! i:-i^: I f J^iP ^'STJ |.S^ «3 ^"I^IT.II iti^lls !fii$! •+H O fl (-, rS O ^ §|J^-§« 3 M~rt2.-45.g q'S'S^l S53stJ 2?||° J2*|sj « l^rs.- 3 ^g §^s . « •- ^ ex S CD S3 CC ^ O.S 'Q * VTtJ • • !rt ** 42 8 ^ H 2 •n|* *1 f |||1||1 'o^js >-; ' S^ » ^ c f ^' «S J5 •«! S < CHAP, xm ENTERIC CANAL 597 of course communicates with the mesenteron ; but as the embryo is gradually folded off from the yolk-sac, the mesenteron becomes tubular along its whole length, and eventually the stalk of the yolk-sac becomes solid, at hatching (or birth) its point of communication with the body being marked by the navel (umbilicus). The ectodermic stomoct&um (p. 204), which gives rise to the mouth-cavity and also to the greater part of the pituitary body, comes to communicate with the mesenteron anteriorly, and the ectodermic proctodceum opens into it posteriorly. The muscular and peritoneal layers of the canal are formed, it will be remembered, from the mesoderm. The first traces of the liver and pancreas are seen as simple endodermic offshoots of the mesenteron, which gradually become branched in a complicated manner, the numerous lobules being more or less closely connected together by mesoderm. The gill-pouches arise as paired outgrowths of the endoderm lining the pharynx, which come into contact with the ectoderm, the latter becoming perforated to form the external branchial apertures. Four gill-clefts appear in the embyro of reptiles, birds, and mammals — animals in which gills are never developed (Figs. 158 and 160) ; but they early disappear with the exception of the first cleft, corre- sponding with the spiracle of the dogfish, which gives rise in all Vertebrates above fishes to the tympano- eustachian passage (p. 449) : the branchial skeleton, as we have seen, undergoes a corresponding reduction or modification (pp. 438 and 496). In air-breathing Verte- brates the lungs arise as a ventral outgrowth of the pharynx, lined by endoderm and covered by mesoderm. The circulatory organs are developed from the mesoderm, the heart arising in the visceral layer on the m-e/n sv FIG. 159. — The yolk of a Hen's egg at, A, the thirty-sixth hour from the beginning of incubation ; B, the third day ; C, the end of the fifth day ; and D, the end of the ninth day (about nat. size). (Compare Fig. 158.) ad. area pellucida ; an. inner or " true " amnion ; ak. area opaca ; av. area vasculosa (in C its outer margin) ; az. outer or " false " amnion (serous membrane), together with the vitelline membrane ; em. embryo ; km. hyomandibular cleft ; sh. egg-shell ; sm. vitelline membrane (in C, shell membrane) ; sv. air-chamber ; ta. allantois ; wa. white or albumen ; y. ys. yolk-sac. (From Marshall.) 598 CHAP, CIRCULATORY ORGANS 599 ventral side of the pharynx. It has at first the form of a straight tube, developed in the chick and rabbit by the fusion of two longitudinal vessels (vitelline veins, see below, and Figs. 161 and 164); it soon becomes twisted into an S-shape, and transverse constrictions are formed dividing it into the different chambers. The auricular and ventricular portions are each at first single, but from the Amphibia onwards the former subsequently be- comes divided into two by a septum, and in birds and mammals the ventricle is similarly subdivided. The modification of the arterial arches in the examples studied has already been described (pp. 452 and In the dogfish, bird, and rabbit the dorsal aorta, in ad- dition to its other branches, gives rise to paired vitelline arteries : these branch over the extra-embryonic part of the blastoderm (p. 585) which spreads over the yolk- sac, and in the two former takes an important share bm- vb -£ dC- -ns FIG. 160. — A section through a Chick embryo at the end of the third day of incubation, the section being taken along a plane indicated by the two arrows and crosses in Fig. 158, B ; the right side of the section is at a level slightly dorsal to that of the left side. (X 30.) (After Marshall.) a. dorsal aorta ; a1, first aortic arch, in the mandibular arch ; a*, second aortic arch, in the hyoid arch ; a:{. third aortic arch, in the first branchial arch ; ac. carotid artery ; at. internal carotid artery ; bm. cavity of mid- brain ; 6rl. first branchial arch ; ch. notochord ; Am. hyomandibular cleft ; hy. hyoid arch ; mn. mandibular arch ; nip. muscle-plate ; ne. ganglion of spinal nerve ; ns. spinal cord ; pt. pituitary pouch ; tp. pharynx ; vb. jugular vein. types the capillary network in the absorption of the 600 CIRCULATORY ORGANS CHAP. yolk by the embryo. From this area vasculosa (Fig. 159) the blood is returned by splanchnopleuric (p. 580) vitelline veins, which join with a subintestinal vein (compare Amphioxus, Fig. no) and open into vessels which eventually give rise to the hepatic portal veins. The chief somatopleuric veins in all embryonic Craniates are, as in the dogfish, the jugulars and the cardinals (Figs. 160 and 164). In all Vertebrates above n.cr -mes" end" FIG. 161. — Transverse section across the head of a Chick embryo at about the fortieth hour of incubation, in the region oi the hind-brain and heart. ( X about go.) ao. aorta ; au. beginning of auditory pit ; end', endoderm of pharynx ; end". endoderm of yolk-sac ; h'. muscular wall of heart ; h". epithelial lining of heart (the septum, which subsequently disappears, indicates the paired origin of the heart) ; h.b. hind-brain ; mes'. parietal, and mes". visceral layer of mesoderm; nc. notochord ; n. cr. neural crest ; pc. pericardial coelome ; ph. pharynx. the fishes, the cardinals become subsequently more or less entirely replaced functionally by the postcaval (compare p. 456) : the anterior part of one or both cardinals may, however, persist as the azygos vein or veins (e.g. rabbit, p. 525). Urinogenital Organs. — The excretory organs, speak- ing of craniate Vertebrates as a whole, arise as a series of tubules having the general character of nephridia xm URINOGENITAL ORGANS 601 (pp. 146 and 340) situated along the dorsal region of the ccelome ; but owing to the fact that they do not all arise at the same time and that their development is modified by neighbouring structures, there is much variation as regards these tubules and their relations. They may in general be classified into three groups, constituting what are known respectively as the fore-kidney or pronephros (Fig. 162, A, p. nph), the mid-kidney or mesonephros (ms. nph), and the hind-kidney or metanephros (mt. nph}. The pronephros appears at an early stage of develop- ment, and very rarely remains functional throughout life, though in some cases (e.g. tadpole) it serves as an excretory organ in the larva : in many cases it is merely represented by vestiges (e.g. dogfish, bird, rabbit). The pronephric tubules open at one end into the ccelome by ciliated nephrostomes, and at the other into a longi- tudinal pronephric duct (sg. d), which communicates with the cloaca posteriorly. They are generally from two to four in number, are not strictly segmental, and are con- fined to the anterior end of the ccelome : opposite their nephrostomes a single large glomerulus (see p. 146) is present on either side of the body. The mesonephros appears rather later than the pro- nephros, and extends along the greater part of the ccelome. Its tubules are segmentally arranged, and thus furnish another example of metamerism in the Verte- brate body ; each tubule is provided with a separate glomerulus enclosed within a Malpighian capsule (me). The nephrostomes of the mesonephros may lose their connection with the tubules and come to open into certain veins (e.g. frog, Fig. 47 and p. 98), or may disappear at a later stage to a greater or less extent (dogfish) or entirely (e.g. bird, rabbit) : at their other ends the tubules communicate with the pronephric duct, which may now be spoken of as the mesonephric duct (Fig. PR ACT. ZOOL. P P an. .bl FIG. 162.— Diagrams illustrating the development of the urinogenital organs of Craniata. A pronephros and its duct ; B, atrophy of pronephros, development oi 'mesonephros ; C, differentiation of Wolffian and Mullerian ducts ; D, develop- ment of metanephros, male type ; E, female type. 602 CHAP, xm URINOGENITAL ORGANS 603 al. bl. allantoic bladder ; an. anus ; cl. cloaca : gon. gonad ; int. intestine ; me. Malpighian capsule ; msn.d. mesonephric (Wolffian) duct : ms.nph. mesonephros ; mt.n. d. metanephric duct ; mt.nph. metanephros ; nst. nephrostomes ; ov. ovary ; p. n. d. Miillerian duct ; p.nph. pronephros ; sg. d. prone phric duct t. spermary ; v.e. efferent ducts. (From Parker and Haswell's Zoology.) 164, ms. d, ms. t) . In Fishes and Amphibians the mesone- p%hros remains as the functional kidney throughout life, though the posterior part alone may retain the renal function and develop special ducts (dogfish, Fig. 127), thus ^ foreshadowing the metanephros of, higher Vertebrates. The metanephros is characteristic of Reptiles, Birds, and Mammals. Its tubules arise later than those of the mesonephros and open into a special ureter or metanephric duct, which arises as an outgrowth from the posterior end of the mesonephric duct : they resemble the mesonephric tubules except that they possess no nephrostomes and usually show no signs of a metametric arrangement. In the majority of Vertebrates the generative organs take on a close connection with certain of the urinary tubules and their ducts (pp. 193 and 466), which may thus undergo a change of function. In the dogfish the pro- nephric duct early undergoes longitudinal subdivision into two, one of which is retained as a mesonephric duct (now often spoken of as the Wolffian duct] ; while the other (Miillerian duct), retaining its connection anteriorly with the ccelome by a persistent pronephric nephro- stome, assumes the function of an oviduct in the female, and atrophies in the male, though traces of it may persist in the adult (Fig. 127, m.d). Owing to the connection which is formed between the spermary and certain of the mesonephric tubules by means of efferent ducts, the Wolffian duct may now serve as a urinogenital duct (frog, p. 193) ; or, when an epididymis is formed from the mesonephros, and special ureters arise p p 2 604 MUSCLES AND SKELETON CHAP. in connection with the functional renal tubules, the Wolffian duct serves as a spermiduct mainly if not entirely (dogfish, p. 4^6; mammal, p. 538). In most Vertebrates the oviducts arise quite inde- pendently of the mesonephric ducts, each being formed as a groove of the ccelomic epithelium which becomes closed in and grows from before backwards to the cloaca, its anterior opening into the ccelome representing a nephrostome of the pronephros. In the higher Vertebrates, in which the kidney of the adult is a metanephros (p. 537), the only parts of the mesonephros and its duct which persist as more than a vestige are the epididymis and spermiduct of the male. The gonads arise as ridges covered by ccelomic epithelium on the dorsal wall of the body-cavity close to the inner side of the developing kidneys. Their epithelium is known as germinal epithelium, and from it either ova or sperms are eventually developed (pp. 194- 196, and 566). The majority of the muscles are developed, as we have seen (pp. 203 and 588), from the mesodermal segments, others arising from the parietal and visceral layers of the mesoderm. The first part of the endoskeleton to be formed is the notochord (pp. 203 and 580), developed primarily from the endoderm, but in the chick and rabbit arising from the central part of the mass of cells in the primitive streak (Fig. 153, G), in which the layers become in- distinguishable from one another (p. 583). In the meso- derm surrounding the notochord cartilages appear and give rise to the vertebra, the notochord becoming con- stricted by the ingrowing cartilage and eventually dis- appearing more or less completely (p. 441) : it at first XIII SKULL 605 ctr •o/ extends into the head as far as the pituitary body (Fig. 163). The cranial cartilages do not become segmented, but give rise to a pair of horizontal bars, the parachordals (pa. ch) : these are continued forwards, diverging around the pituitary body, as the trabeculcz cranii (tr). The two parachordals and trabeculae then unite respectively with one another, and so form a firm floor for the fu- ture brain-case : this is gradually developed by the cartilage 'growing up on either side and eventually meeting to a greater or less extent above the brain : there is never, however, a complete cartilaginous roof to the cranium, parts of which are Only membranOUS and FIG. ^63.— Dissection of the head of an embryo form the fontanelles (pp. 43 and 436). In the meantime the cartilaginous sense- capsules are deve- loped, the olfactory and auditory capsules uniting with the brain-case in front and behind respectively. The visceral skeleton is formed as a series of cartila- ginous bars within the visceral arches, the first of which forms the mandibular arch, the second the hyoid, and the others the branchial arches. Dogfish (Scyllium canicula) from the dorsal side, to show the developing chondro- cranium (dotted), (x 8.) au. auditory capsule ; br. external gills ; c.tr. cornu of trabecula ; cfi, c/4. gill-clefts ; inf. infundibulum of brain (left in situ) ; nc. notochord ; ol. olfactory sac ; pa. ch. parachordal cartilage ; pn. prenasal cartilage ; py. pituitary body (left in situ) ; sp. spiracu- lar cleft ; ' Parker.) tr. trabecula. (After W. K. 606 AMNION CHAP. The limbs appear as small buds (Fig. 158, C) composed of ectoderm with a core of mesoderm, in which latter their skeleton arises by the formation of cartilage extending inwards to form the arches, and outwards to form the skeleton of the free portions of the limbs. As we have seen, the endoskeleton may remain practically entirely cartilaginous in the adult (e.g. dog- fish), but in higher forms extensive processes of ossifica- tion set in, certain bones replacing the cartilage to a greater or less extent, and others being formed in the surrounding connective-tissue (compare p. 43). Development of the Amnion, Allantois, and Placenta. — We must now consider some important and characteristic structures which are developed in the embryos of Reptiles, Birds, and Mammals, and known as embryonic membranes. Taking the chick as a convenient example, these are formed as follows. The blastoderm, as we have seen (p. 585 and Fig. 159), gradually extends peripherally so as to cover the yolk, and thereby becomes divisible into an embryonic portion, from which the embryo is formed, and an extra- embryonic portion, which invests the yolk-sac and takes no direct share in the formation of the embryo. The extension of the ectoderm and endoderm takes place regularly and symmetrically ; but the mesoderm, while extending equally in the lateral and posterior regions, grows forwards in the form of paired prolongations which afterwards unite, so that for a time there is an area of the blastoderm in front of the head of the embryo formed of ectoderm and endoderm only and called the pro-amnion (p. 583 and Fig. 153, pr. am). Before the embryo has begun to be folded off from the yolk the rudiment of one of the two embryonic membranes, the amnion, has appeared. A crescentic XIII AMNION 607 amniotic fold (Fig. 158, A, and 165, A, am. /') arises in front of the head end of the embryo frcm the region of the pro-amnion : it consists at first of ectoderm only, the mesoderm not having yet spread into the pro-amnion. The fold is soon continued backwards along the sides of the body (Fig. 165, B) and round the tail (A), but in these regions (am. /) it consists from the first of ectoderm plus the parietal layer of mesoderm, i.e., it is a fold of am am FIG. 164. — Transverse section across the body of a Chick embryo at about the sixtieth hour of incubation, (x about 90.) ao. aorta ; am. amniotic folds ; am', true amnion, and am", serous membrane ; cd.v. cardinal vein ; coeV. ccelome in body of embryo ; coel". extra-embryonic coelome ; ect. ectoderm ; end. endoderm ; m. c. medullary cord ; mes'. parietal, and mes". visceral layer of mesoderm ; m. p. muscle-plate ; ms. d. mesonephric duct ; ms. t. mesonephric tubule ; nc. notochord ; n. cr. neural crest ; v. v. vitelline veins. what may be called the embryonic body-wall or somato- pleure (p. 580, Fig. 164, am). Its cavity is a prolonga- tion of the space between the parietal and visceral layers of mesoderm — i.e., is an extension of the extra- embryonic coelome. The entire amniotic fold gradually closes in above (Fig. 165, C) forming a double-layered dome over the embryo. Its inner layer, formed of ectoderm internally and meso- p P* FIG. 165 —Diagrams illustrating the development of the fcetal membranes of a Bird. A, early stage in the formation of the amnion longitudinal vertical section ; B, 608 CHAP, xin ALLANTOIS 609 slightly later stage, transverse section ; C, stage with completed amnion and commencing allantois ; D, stage in which the allantois has begun to envelop the embryo and yolk-sac. The ectoderm is represented by a blue, the endoderm by a red line ; the mesoderm is grey. all. allantois ; all' the same growing round the embryo and yolk-sac ; am. amnion ; am./, amniotic fold ; an. anus ; br. brain ; ccel. coelome ; cod', extra-embryonic coelome ; ht. heart ; ms. ent. mesenteron ; mth. mouth ; nch. notochord ; sp. cd. spinal cord ; sr. m. serous membrane ; umb. d. umbilical duct ; vt. tn. vitelline membrane ; yk. yolk-sac. (Reduced from Parker and Has well's Zoology.) derm externally, is the amnion (am, and Fig. 164, am'), the cavity enclosed by which becomes filled with a watery amniotic fluid, serving as a protective water-cushion to the contained embryo. Its outer layer, formed of ectoderm externally and mesoderm internally, is the serous mem- brane, sometimes spoken of as ihe false amnion (sr. m, am"} : this comes to lie just beneath the vitelline membrane (p. 565), with which it subsequently fuses. The second of the embryonic membranes, the allantois, is developed as an outpushing of the ventral wall of the mesenteron near its posterior end (Figs. 165, C, all, and 166), and consists, therefore, of a layer of visceral meso- derm lined by endoderm (splanchnopleure, p. 580). It has at first the form of a small, ovoid sac having the precise anatomical relations of the urinary bladder of the frog. Increasing rapidly in size, it makes its way, back- wards and to the right, into the extra-embryonic coelome, between the amnion and the serous membrane (C, D). Allantoic arteries pass to it from the dorsal aorta, and its veins, joining with the vitelline veins from the yolk-sac, take the blood through the liver to the heart (p. 600). Next, the distal end of the sac spreads itself out and extends all round the embryo and yolk-sac (D, all'), fusing, as it does so, with the serous and vitelline membranes, and thus coming to lie immediately beneath the shell- membrane. It finally encloses the whole embryo and yolk-sac, together with the remains of the albumen, which has, by this time, been largely absorbed (Fig. 159, D). The allantois serves as the embryonic respira- 610 PLACENTA CHAP. tory organ, gaseous exchange readily taking place through the porous shell ; its cavity is an embryonic urinary bladder, excretory products being discharged into it from the kidneys. At the end of incubation the embryo breaks the shell by means of a little horny elevation or caruncle at the end of the beak. By this time the remainder of the yolk-sac has been drawn into the ccelome, and the ventral body-walls have closed round it. On the shell being broken the allantois gradually shrivels up, respiratory movements begin, the aperture in the shell is enlarged, and the young bird is hatched and begins a free life. In the higher Mammalia the allantois takes on a further important function. The relations of the amnion and allantois in the rabbit are essentially similar to those described above in the case of the bird. But the later history of the allantois is widely different, owing to the modifications which it undergoes in order to take part in the formation of the placenta, the structure by means of which the foetus receives its nourishment from the walls of the uterus, with which the blastocyst (p. 586) first becomes adherent over the future placental area by the proliferation of the cells of its outer layer (trophoblastic ectoderm) in this region, which forms irregular processes extending into the thickened mucous membrane on the dorsal side of the uterus (Fig. 166, e'}. As the embryo develops, it sinks down and causes the vascular " yolk-sac " or umbilical vesicle (ys) to be doubled in and take on a flattened form (Figs. 167, ds, 166 and 168, ys) : this early becomes attached to the mucous membrane of the uterus, and represents a vitel- line or yolk-sac placenta such as occurs in some viviparous dogfishes ; but its nutritive function in the mammal is of only minor importance. The fatal part of the placenta is formed from the outer layer of the amnion (serous membrane) in a PLACENTA 611 limited disc-shaped area covered by the thick . part of the trophoblastic ectoderm where the distal portion of the allantois coalesces with it (Fig. 166). The membrane thus formed (chorion) develops vascular processes or septa — the chorionic villi, which are .c ys FIG. 1 66. — A Rabbit embryo and blastodermic vesicle (" yolk-sac ") at the end of the tenth day. The embryo is represented in surface view from the right side, the course of the enteric canal being indicated by the broad dotted line ; the blastodermic vesicle is shown in median longitudinal section. The greater part of the tail has been removed, (x 7\.) an', pro-amnion ; a.x. cavity of amnion, between the inner or true amnion and the embryo ; c. extra-embryonic portion of ccelome ; e. trophoblastic ectoderm ; e' '. thickened part of e by which the blastodermic vesicle is attached to the uterus, and from which the development of the foetal part of the placenta proceeds ; ei. auditory vesicle ; ek. ectodermal villi ; gf, gt, gh. enteron ; h. endoderm ; ol. eye ; r. heart ; si. circular marginal vessel of area vasculosa ; ta. cavity of allantois ; ys. cavity of " yolk-sac." (From Marshall, in part after Van Beneden and Julin.) received into depressions — the uterine crypts — in the thickened placental folds of mucous membrane on the dorsal wall of the uterus which constitute the maternal portion of the placenta (Figs. 167 and 168). The foetal portion of the placenta with its villi is supplied with muc j-h me & FIG. 167. — Portion of the uterus of a Rabbit containing an advanced foetus, (x £.) In A, the uterus has been opened and its wall reflected, and part of the " yolk- CHAP, xm PLACENTA 613 sac " (umbilical vesicle) cut away (left of the figure) to show the underlying amnion. In B, part of the uterine wall has been removed and part of the yolk- sac and allantois cut out so as to expose the foetus and placenta. a. amnion ; all.-r. dotted line indicating margin of allantois (compare Figs. 166 and 168, to) ; all.-u. line along which the allantois is reflected over the placental region ; ch. chorion ; ds. yolk-sac ; dsn point at which the umbilical vessels branch out from the umbilical stalk ; mes. mesentery of uterus ; muc. mucous membrane pt uterus ; pi. foetal surface of placenta covered by the distal wall of the allantois ; rs. circular marginal vessel of area vasculosa of yolk-sac ; u-h. undilated narrow portion of uterus between two foetus-containing swellings ; u-l. lumen of uterus ; v. om. vitelline vessels ; v. u. allantoic vessels. (From Grosser's Ver»L Anal. »*. Entwick. geschichte der EihauU und der Placenta.) blood by the allantoic vessels, and the blood-supply of the uterus is at the same time greatly increased : the foetal and dilated maternal capillaries and sinuses are thus brought .into intimate relation with one another in the placenta, being only separated by thin layers of epithelium : diffusion can thus take place between them, nutrient matter and oxygen diffusing from the blood of the mother into that of the foetus, while excre- tory substances pass from the blood of the foetus into that of the mother. The disc-shaped or discoidal placenta of the Rabbit is of the type termed deciduate, its villi being so intimately connected with the uterine mucous mem- brane that a part of the latter comes away with it at birth in the decidua, or afterbirth, which is attached to the newly-born young by the umbilical cord, consisting of the stalks of the allantois and flattened yolk-sac twisted together (Figs. 167 and 168). The blood-vessels at the plane of separation of the placenta and uterus are very small, and the uterus contracts so rapidly when the young are expelled at parturition that very little bleeding takes place, and the mucous membrane rapidly undergoes regenera- tion, ready for the next conception. The cord is gnawed through by the parent rabbit, the blood-vessels being compressed in the process ; and it soon shrivels up and comes away at the navel or umbilicus (see p. 597) 614 PLACENTA CHAP. which represents the point of connection between the foetus and the placenta. The intra-abdominal portion mi mk va • aa ny ys FTG, 1 68. —A transverse section across the uterus and the contained embryo of a Rabbit at the end of the nineteenth day. The embryo is cut transversely about the middle of the body, the section passing through the " yolk-stalk " and alia ntoic stalk, (x af.) A, dorsal aorta ; aa. allantoic artery ; a.x. cavity of amnion, between the inner or true amnion and the embryo ; ex. space between the inner and outer layers of the amnion ; gu. uterine glands ; h. endoderm of upper or vascular wall oi " yolk-sac " (umbilical vesicle) ; mi, mk. outer or longitudinal, and inner or circular muscles of wall of uterus ; mm. mesentery of uterus ; ns. spinal cord ; ny. sympathetic nerve-cord ; ph. lobule of placenta ; po. region along which the separation of the placenta occurs at birth ; pr. inter-placental groove ; pw. sub-placental cavity ; si. circular marginal vessel of yolk-sac ; ta. cavity of the allantois (see Fig. 166) ; ts. stomach; uc. dilated uterine capillary, with thick perivascular wall ; up. uterine or maternal sinuses of placenta ; uv. blood- vessels of uterus ; va. allantoic vein ; w. liver ; yk. stalk of yolk-sac ; yl. dotted line representing the lower or non- vascular wall of the yolk-sac, now completely abeorbed ; ys. cavity of yolk-sac, continuous with the uterine cavity owing to absorption of the lower wall of the yolk-sac. (From Marshall.) xm PRACTICAL DIRECTIONS 615 of the allantois is represented by a cord or ligament, the urachus, which connects the navel with the apex of the bladder, so that only a small portion of the allantoic outgrowth and not the whole of it, as in the frog, persists in the adult. PRACTICAL DIRECTIONS A. A series of models of the development of Amphioxus, to be found in most Zoological Museums, should be examined. Note — a, the various stages of segmentation and the formation of the segmentation-cavity (Fig. 152, A-G) ; b, the oblitera- tion of the segmentation-cavity during the formation of the gastrula, with its ectoderm, endoderm, and blastopore (H, I) ; c , the elongation of the gastrula, the flattening of the surface which will become dorsal, and the position of the blastopore at the posterior end (J) ; d, the formation of the medullary plate and cord, the mesodermal segments and enterocceles, and the separation of the latter from the enteron, the walls of these cavities forming the parietal and visceral layers of the mesoderm ; and the formation of the notochord and the neurenteric canal (K — O). B. In order to follow out the development of the chief organs in the Vertebrata, it is necessary to make a number of serial sections of various embryonic stages. For this purpose chick-embryos (see below) are, on the whole, the most convenient and satisfactory ; but if you have not already followed out the instructions given on pp. 212-214, you should not only examine externally the stages in the development of the tadpole up to hatching, but also attempt to make sections of some of the early stages so as to show a, the segmentation-cavity (Fig. 64, E) ; b, the enteron, blastopore, and yolk-plug (I) ; and c, the medullary cord, notochord, mesoderm, and ccelome (Figs. 64 K, and 65, A and B). Compare with the corresponding stages in Amphioxus. C. A number of fresh, impregnated fowls' eggs should be obtained and placed in an incubator at a temperature of about 39-5° C. (103° F.), or under a "broody" hen, first marking each with the date. One or two should be examined each day or oftener for the first four or five days of incubation. Expose the embryo as directed on p. 576, using warm normal (075 per cent.) salt solution (tempera- ture as above), in order that after the first day the beating of the heart and the circulation of the blood may not be 616 PRACTICAL DIRECTIONS CHAP. stopped. Remove sufficient of the shell and shell-membrane to expose the entire blastoderm, as well as the embryo proper when developed. Examine with a hand-lens before removing, and then, after carefully cutting through the vitelline membrane with fine scissors around the margin of the blastoderm, the latter should be floated off in a watch- glass, and the covering vitelline membrane removed by the aid of a needle — an operation which requires considerable care in the early stages. Fix with corrosive sublimate, and after about half an hour wash in water and preserve in increasing strengths of alcohol up to 90 per cent, (see p. 136). Two specimens of each stage should be preserved, one for examining entire, and the other for sectioning l ; they should subsequently be stained entire with borax-carmine or haematoxylin, the one mounted in Canada balsam on a slide after treating with absolute alcohol and xylol or oil of cloves, and the other imbedded in paraffin (see p. 137). Serial sections of each embryo should be mounted in order, after smearing the slide with collodion and oil of cloves or gly- cerine and albumen (p. 139). Sketch typical preparations of the various stages, both entire embryos and sections. I. External characters. 1. Unincubated egg. Segmented blastoderm (Figs. 149, 153, D). 2. First day of incubation (18-20 hours). Note the area pellucida, area opaca, primitive streak, and medullary plate (Fig. 153, E). 3. End of first day (about 24 hours}. Note the area pellucida, area opaca, pro-amnion, head-process and medullary groove, primitive streak and groove, and the small number of mesodermal segments, which increase in number from before backwards (compare Figs. 153, E, and 158, A). 4. Second day. Note — a, that the blastoderm has further increased in size, that blood-vessels are apparent in the area opaca, and that the head end of the embryo has become raised above the yolk and is beginning to be covered by the head-fold of the amnion ; b, the medullary folds, which meet in the middle line except at the posterior end, in front of the primitive streak, while in front the medullary cord thus formed has become swollen to form the vesicles of the brain, and the optic vesicles are seen standing out right and left 1 The early stages are difficult to prepare, and the most im- portant of those referred to below are from the end of the first to the third day of incubation. As the medullary groove only closes gradually from before backwards in the body-region, sections showing different stages in the development of the central nervous system may be obtained from the same embryo at these stages. xm PRACTICAL DIRECTIONS 617 from the fore-brain ; c, the auditory pits, one on either side of the hind-brain region ; d, the increase in number of the mesodermal segments ; e, the heart and main vitelline veins (Figs. 518, A, and 159, A). 5. Third day. Note — a, the further extension of the area vasculosa and its blood-vessels and the folding off of the embryo from the rest of the blastoderm investing the yolk ; b, the amnion ; c, the head, which is proportionately very large, has become twisted over — so that its right side is now uppermost, and has undergone flexure, the prominent mid- brain being now at the anterior end ; observe the cerebral hemispheres and the cerebellum in addition to the other parts of the brain ; d, the visceral arches and clefts ; e, the olfactory pits, optic cups and lens, and closed auditory vesicles ; /, the mesodermal segments ; g, the S-shaped heart, arterial arches, dorsal aorta, and vitelline veins (Figs. 158, B, and 159, B). 6. Fourth to sixth days. Note — a, the advances in the parts mentioned above, including the further extension of the area vasculosa over the yolk, the narrowing of the yolk- stalk, the flexure of the whole embryo, and the amnion ; b, the budding fore- and hind-limbs ; c, the allantois, at first a small sac extending from the ventral side of the body and gradually increasing in size (Figs. 158, C, and 159, C). By the sixth day, the embryo can be recognised as that of a bird from the characters of the fore-limbs (wings), hind- limbs, head, etc. : feather-rudiments cannot be recognised until the ninth day. 7. A few further stages should be examined (compare Fig. 159, D), especially that just before hatching, at the end of the third week of incubation, when it will be found that the albumen has become absorbed, and the very vascular allan- tois will be seen all round the egg directly under the shell, its stalk entering the body at the umbilicus ; the remains of the yolk-sac have been withdrawn into the body at the same point. Note also the neb or caruncle on the beak, used for breaking the egg-shell at hatching. II. Transverse sections. 1. Unincubated egg.1 Note the ectoderm and the lower layer cells (Fig. 153, F). 2. First day of incubation. Less difficulty will be experi- enced if you select embryos towards the end of the first day (20-24 hours). Distinguish between the ectoderm, endoderm, and mesoderm. Note — a, the formation of the medullary groove and folds ; b, the primitive streak and groove ; c, the notochord (Figs. 153, G, and 155). 1 See note about early stages on p. 616. 618 PRACTICAL DIRECTIONS CHAP. 2. Second day. Note — a, the parts mentioned under first day, tracing the series of sections through the whole embryo ; b, the gradual meeting of the medullary folds to form the hollow medullary cord ; c, that anteriorly the head has become folded off from the yolk, so that the enteron in this region forms a closed tube, lined all round by endcderm ; d, the cerebral vesicles, and the optic vesicles arising from the fore-brain, the lens and the auditory pits from the ectoderm ; e, the mesoderm, consisting on either side of dorsal mesodermal segments and of a ventral portion, split into parietal and visceral layers, with the ccelome between them — the former layer, with the ectoderm, constituting the body- wall or somatopleure, and the latter, with the endoderm, the wall of the enteron, or splanchnopleure ; f, the heart, vitelline vessels, paired aorta;, and cardinalveins (Figs. 155 and 157, A). 3. Third day. Note — a, the parts mentioned above under the first day, and their further development, including the flexure of the brain, formation of the optic cup or secondary optic vesicle, choroid fissure, and the closure of the auditory pits ; b, the mesonephric (Wolffian) ducts on the dorsal side of the ccelome ; c, in the pharyngeal region, the relations of the visceral arches and clefts and of the arterial arches ; d, the somatopleuric amniotic folds, and the way in which they meet to form the serous membrane ("false amnion") and the true amnion and amniotic cavity. (The allantois arises from the hinder part of the splanchnopleure on the fourth or fifth day) (Figs. 157, B, C, 160, 161, and 164.) D. i. If you have been fortunate enough to obtain some blastocysts of the rabbit (see p. 549), examine them carefully with a lens, and note the outer layer of cells (trophoblast) and the smaller mass (embryonic area) attached to the former on the inner side of the vesicle at one pole (Fig. 154, E). 2. Obtain, either fresh or preserved, a gravid uterus of the rabbit or rat containing advanced embryos. Slit open each swelling of the uterus carefully on the ventral side, and note — a, the flattened umbilical vesicle (" yolk-sac ") ; b, the foetus, enclosed by the thin, transparent amnion, containing a fluid ; c, the vascular discoid placenta, closely attached to the dorsal side of the uterus, and connected with the navel or umbilicus of the foetus by a vascular umbilical stalk, consisting of the stalk of the allantois and that of the umbilical vesicle twisted around one another (Figs. 167 and 168). 3. Sections should be made through the uterine wall and placenta in order to make out the relations between the maternal and fcetal portions of the placenta (Fig. 168). INDEX (The numbers in italics refer to practical directions.} Abdomen, abdominal cavity, 20, 361, 434, 483, 506 Abdominal pore, 432 Abiogenesis, 288 Acanthias, 431, 4^9 Acetabulum, 50, 503 Aciculum, 352 Acoustic spots. 1 88 Acrania, 421 Acromion, 500 Adaptation, 225 Adenoids (lymphatic glands), 507. 515 Adrenal bodies, 145, 447, 538 Agamobium, 321 Albumen, of bird's egg, 566 Alimentary canal, see Enteric canal Allantois., 609, 610 Allolobophora, see Earth- worm Alternation of generations, 321 Alveoli of jaws, 510 Amnion, 606, 610 AMCEBA : occurrence and gene- ral characters, 229 ; move- ments, 232 ; resting condi- tion, 232 ; nutrition, 233 ; growth, 234 ; excretion, 235 ; respiration, 235 ; metabol- ism, 236 ; reproduction, 236 ; immortality, 236 ; conjuga- tion, 237 ; death, 237 ; ani- mal or plant ? 255 ; practical directions, 238 Amoeboid movements, 106, 231 Amphibia, 219, 418, 482 Amphiccelous, 439 PRACT. ZOOL. — B. D. 619 AMPHIOXUS, see Lancelet Ampulla, of semicircular canals, 188 Ampulla, of sensory canals of the integument, 464 Anal segment, 327, 352 Analogous, analogy, 217 Anatomy, 217 Angular process of mandible, 496 Animals and Plants : compari- son of typical forms, 255 ; discussion of doubtful forms, 255, 257; boundaries artificial, 258 Ankle, see Tarsus Annulata, 220, 349 Anodonta, see Mussel Anopheles, 287 Antenna, 366 Antennary gland, 375 Antennule, 366 Anura, 219 Anus, 6, 267, 272, 327, 352, 371, 401, 418, 426, 432, 486, 514 Aorta, aortic arches, see Arte- ries Aperture, or apertures (see also Foramen, and under Nephri- dium, Kidney, Pores, Repro- ductive organs, &c.), auditory : Crayfish, 381 ; Rabbit, 486, 492, 536 ; cloacal, see anus ; exhalant and inhalant, 397 Aphis, 570 Appendages, lateral, and their skeleton : Frog, 5, 29, 36, 48, 51 ; Crayfish, 360, 363 ; Dog- fish, 433, 441 ; Rabbit, 501, 503 ; development of, in Vetebrates, 606 620 INDEX Aqueous chamber and humour, 183 Arachnoid fluid, 155 Archenteron, 579 Archicerebrum, 380 Area opaca, 583 Area pellucida, 583 Area vasculosa. 585 Arm, see Fore-limb Arterial arches, 80, 452, 523, 599 Arteries : Frog, 27, 80 ; Crayfish, 376 ; Mussel, 407 ; Dogfish, 449 ; Rabbit, 522 ; Vertebrate embryo, 599, 609 Arthobranch, 375 Arthropoda, 220, 360, 386, 411 Articular membrane, 361, 368 Articular processes, see Zygapo- physes Artificial selection, 227 Arytenoid cartilage, 144, 517 Asexual generation, see Agamo- bium Asexual reproduction, see Fis- sion, Budding, Spore Assimilation, 149, 234, 305 ASTACUS, see Crayfish Aster, 562 Astragalus, 51, 504 Atlas vertebra, 498 Atrial pore, 420 Atrium, 420, 421 Atrophy, see Vestige Auditory capsule, 39, 41, 436, 492 Auditory organ (incl. statocyst) : Frog, 45, 1 86 ; Crayfish, 381 ; Mussel, 409 ; Dogfish, 465 ; Rabbit, 535 ; development of, in Vertebrates, 595 Auditory ossicles, 492, 536 Auricle, see Heart Automatism, see Movements Aves, 219, 418 Axial fibre : of Vorticella, 272 ; of Carchesium, 277 Axial parts, 4, 29 Axis fibre, see Neuraxis Axis vertebra, 499 Axon, 167 B Backbone, see Vertebral column BACTERIA, 152 ; structure, 257 ; nutrition, 257 ; animals or plants ? 257 ; rapid multipli- cation, 289 ; practical direc- tions, 260 Basal cartilages of fins, or basalia, 442-443 Bell of medusae, see Umbrella Bilateral symmetry, 296 Bile, 68, 148 Bile-duct, 68, 447, 516 Bile-passages, 134 Binomial nomenclature, 215 Biogenesis, 288 Biology, i Bird, development of, 582 et seq, Birds, see Aves Bladder, see Gall-bladder and Urinary bladder Blastocoele, see Segmentation- cavity Blastocyst, see Blastodermic vesicle Blastoderm, 384, 470, 574, 582 ,, embryonic and extra- embryonic portions, 585, 606 Blastodermic vesicle, 586, 610 Blastomere, 577 Blastopore, 201, 349, 384, 579, 581, 582 Blast ostyle, 311, 313, 314 Blastula, 577 Blind spot, 183, 556 Blood : Frog, 20, 78, 85, 105 ; Earthworm, 336, 339 ; Cray- fish, 379 ; Mussel, 408 ; Lance- let, 424 ; Dogfish, 458 ; Rabbit, 483, 543 Blood-corpuscles : colourless, see Leucocytes ; red, 105, 458, 483, 543 Blood-sinus, see Sinus Blood-vessels : Frog, 80 ; Earth- worm, 337 ; Crayfish, 376 ; Mussel, 407 ; Lancelet, 424 ; Dogfish, 449 ; Rabbit, 520 INDEX 621 Body-cavity, see Coelome Body of vertebra, see Vertebra Body-segments, see Metamere Bojanus, organ of, 406 Bone : replacing and investing, 43, 489 ; nature of, 52 ; mi- croscopical structure of, 116 Bones, see Endoskeleton, Skull, Vertebra, Ribs, Sternum, and under individual bones of limbs Botany, I Brachial plexus, see Nerve-plexus Brain : Frog, 28, 156, 202 ; Earthworm, 342 ; Crayfish, 379 ; Mussel, 408 ; Lancelet, 420, 424 ; Dogfish, 459 ; Rab- bit, 528 ; development of, in Vertebrates, 588 Brain-case, see Skull Branchia, see Gill Branchial apertures, arches, clefts, and septa : Tadpole, 204, 206 ; Lancelet, 420, 422 ; Dogfish, 432, 438, 448 ; develop- ment of, 597 Branchial rays, 438, 448 Branchial vessels : Crayfish, 377 ; Mussel, 408 ; Lancelet, 424 ; Dogfish, 449 ; Tadpole, 451 Breast -bone, see Sternum Bronchus, 517 Brood-pouch, 410 Buccal cavity, 16, 335, 445, 5°7 Buccal groove, 262 Bud, budding, 306, 309 Bufo, Bufonidae, 218 Bulb, see Medulla oblongata Bulbus aortae, 89 Bulla, tympanic, 492 Byssus, 411 Gecum, 514 Ca^nosarc, 311 Calcaneum, 51, 504 Calcar, 52 PRACT. ZOOL. Calciferous glands, 335 Canal : central, of spinal cord, 156, 424, 459 ; naso-palatine, 507 ; neural, see Vertebral column ; neurenteric, 203 ; radial and circular of Medusa, 315 ; semicircular, of ear, see Auditory organ and Mem- branous labyrinth ; sensory, of Dogfish, 432, 4646 ; sternal, of Crayfish, 363 ; vertebrar- terial, 498 Canaliculi, see Bone Canine teeth, 509 Capillaries, 95, 377, 451, 454 Capitular facet and capitulum, 498, 500 Carapace, 361 Carbohydrates, 72 Carbon dioxide, 66 Carchesium, 277 Cardiac division, see Stomach Carotid arch, see Arteries of Frog, Dogfish, and Rabbit Carpus, 50, 502 Cartilage, 20, 35, 115, 431, 435. 487 ; calcified, 46, 48, 435 Caruncle, 6icf Castings of earthworm, 326 Cell, no, 231, 558; and see under various types Cell -colony, see Colony Cell-cMerentiation, see Differen- tiation, and under develop- ment of various types Cell-division, 106, 560, 575 Cell-membrane or wall, 232, 244, 254. 255, 275, 285, 559 Cellulose, 244, 254, 421 Cement, 445, 510 Centrale, 502, 505 Centrolecithal ova, 574 Centrosome, 560, 568, 572 Centrosphere, 560 Centrum, see Vertebra Cephalothorax, 361 Cerebellum : Frog. 157 '> Dogfish. 459 ; Rabbit, 530 ; develop- ment of, 590 Cerebral flexure, 592 Q Q 622 INDEX Cerebral ganglion, see Brain Cerebral hemispheres : Frog, 160 ; Rabbit, 528 ; development of, 590 Cerebral nerves, see Nerves Cerebral vesicles, 202, 589 Cerebro-pleural ganglion, 408 Cervical groove, 361 Chaetopoda, 350 Chalaza, 576 (Fig. 149) Change of function, 449, 537 Chela, 364, 366 Cheliped, 364 Chiasma, optic, 164, 459, 530 Chick, see Bird Chitin, 232, 328, 361 Chlorophyll, 242, 251, §04 Chorion, 611 Choroid, 183, 556 Choroid fissure, 593 Choroid plexus, 157, 529 Chromatin, 129, 231, 560, 573 Chromatophores, 243, 251 Chromosomes, 560, 569, 587 Cilia, and ciliary movement, 109, 242, 250, 265, 271, 280 Cilia, absence of, in Crayfish, 386 Ciliary folds, muscles, nerves, and vessels, 184, 534 Ciliata, Ciliate Infusoria, 261, 292 Circulation of blood, 86, 90, 337, 375, 4°7. 425, 449, 520 • Circulatory organs, see Blood- vessels and Lymphatic system Cirri, 352, 421 Clasper, 433, 442, 469 Class, 219 Classification, 217, 220, 223, 411 Clavicle, 47, 501 Clitellum, 327, 348 Clitoris, 541 Cloaca, 23, 401, 445 Cloacal aperture, see .Anus Cnemial crest, 504 Cnidoblast, 302 Cnidocil, 302 Coagulation of blood, 78, 107 Cochlea, 187, 535 Cocoon, 328, 349 Ccelenterata, 322 Coelomata, 322 Coelome, 20, 203, 322, 328, 401, 422, 434, 506 ; development of, 203, 580, 586 ; extra- embryonic, 607 Crelomic epithelium, 330; and see Ephithelium and Germinal Epithelium Ccelomic fluid, 336 Collaterals, 171 Colon, 514 Colony, Colonial organisms, 277 Columella, 46, 189 Commissures, see Blood-vessels and Nervous system ; of brain, 158, 531 • Conchiolin, 399 Concrescence, 361 Condyle, occipital, see Skull Cones of retina, 185 Conjugation, 197, 237, 251, 268, 274, 283, 286, 287, 308, 566, 572 ; and see under develop- ment of various types Conjunctiva, 182 Connectives, 342, 379, 408 Connective-tissue, 18, 113 Contractility, nature of, 112 Conus arteriosus, 79, 88, 449, 520 Coracoid, 47, 443 Coracoid process, 500 Cordylophora, 325 Cornea, 182, 354, 380, 555 Coronoid process, 496 Corpus adiposum, see Fat-body ; callosum, 528 ; cavernosum and spongiosum, 540 ; stria- turn, 531 Corpuscles, see Blood-corpuscles and Leucocytes Cortex of brain, 160, 528 Cortical layer, 264, 269, 280, 286 Craniata, 421 Cranium, see Skull CRAYFISH : general characters, 360, 386 ; limited number and concrescence of metameres, 360 ; appendages, 363, 393 ; exoskeleton, 361, 363, 387, INDEX 623 394 ; muscular system, 369, 390, 392 ; enteric canal, 370, 390 ; gills, 374, 388 ; kidney, 375, 393 ; blood-system, 375, 389 ; nervous system, 379, 392 ; sense-organs, 380, 393 ; reproductive organs, 382, 3'JO ; development, 383, 581 Creation, 221 Cribriform plate, 491 Cricoid cartilage, 517 Crop, 335 Cross-fertilisation, 348 Crura cerebri, 157, 459, 530, 590 Crustacea, 297, 386 Crystalline lens, see Lens of Eye Crystalline style, 401 Ctenidia, 398 ' Cutaneous glands : Frog, 129 ; Rabbit, 484 Cuticle : in unicellular animals, 251, 264, 269, 286; in multi- cellular animals, 313, 328, 369 Cyst, see Cell-wall and Encys- tation Cystic duct, see Bile-duct Cytoplasm, 558 Daughter-cells and nuclei, 250, 254, 562 Death, n, 152, 236 Decalcifying, directions for, 137 Deeidua, 613 Decomposition, n, 152 ; and see Putrefaction Degeneration, 421 ; and see Vestige Dehydrating, directions for, 137 Dendron, 167 Dental formula, 511 Dental lamina and papilla, 445 Denticles (Nereis), 352 Dentine, 445, 510 Deric epithelium, see Epiderm Derm, 128, 369, 433, 485 Dermal teeth, 434 Descent, doctrine of, see Evolu- tion Development, meaning of the term, 9, 276, 284. For develop- ment of the various multi- cellular types, see under their names. Practical work 212-214, 615 Dialyser, 73, 77 Diaphragm, 483, 506 Diastema, 507 Diastole of heart, 90 ; of con- tractile vacuole, 265 Diencephalon, 157, 459, 529, 590 Differentiation, 204, 206, 237. 261 Diffusion, 73, 305, 336, 613 Digestion, 68, 233 ; intra- and extra-cellular, 305, 336 Digestive glands, 373, 401, and see Digestion, Enteric canal, Liver, Glands, Pancreas Digestive system, see Enteric canal Digits, 50, 502, 505 Dimorphic, Dimorphism, 250 Dioecious, 320, 382 Diploblastic 298, 311 Directions for dissecting, 14 ; for drawing, 14 ; for killing, 31, 239, 259, 278, 293, 324, 354, 386, 412, 542 ; for preparing skeletons, 53, 394, 472, 542 ; for injecting blood-vessels, 99, 389, 414, 474, 543 ; for micro- scopic work, 119', for his- tological and embryological work, 135, 575, 615 Disc of Vorticella, 271 Discoid segmentation, 575 Dispersal, 274, 283, 287 Dissecting instruments, etc., 12 Distal, 6 Distribution of food-materials, 148 Divergence of character, 223 Division of physiological labour, 206, 238 DOGFISH : General characters, 431 ; exoskeleton, 433, 472 ; QQ2 624 INDEX endoskeleton, 435, 472 ; en- teric canal, 444, 473 ; gills, 448, 478 ; blood-system, 449, 474, 477 ; nervous system, 458, 478 ; kidneys, 466, 476 ; reproductive organs, 466, 475 \ sense-organs, -464. 479 ; de- velopment, 469, 582 Dorsal, 6 Drum-membrane, see Tympanic membrane Ducts, see under names of indi- vidual ducts and glands, Ductus arteriosus, 550 Duodenum, 22 ; and see Enteric canal Dura mater, 155 Ear, see Auditory organ ; in- ternal, middle, and external, 466, 492, 535, 595 EARTHWORM : general characters, 326 ; metameric segmenta- tion, 327 ; ccelome and en- teric canal, 328, 333, 335, 355, 358 \ cell-layers, 329, 358; blood- system, 337, 355, 357, 358 nephridia, 340, 355, 357, 358 nervous system, 342, 357, 358 differentiation of organs and tissues, 344 ; reproduction and reproductive organs, 345, 355, 356 ; development, 349, 581 Ecdysis, 369 Echinodermata, 412 Ectoderm, 202, 209, 298, 311, 314, 317, 349, 384, 470, 583, 586 Ectoplasm, 231 Efferent duct of spermary, 193, 347, 466, 603 Egestion, 233 Egg, 8 Egg of fowl, 565 Egg-cell, see Ovum Egg-sac, 346 Elasmobranchii, 431 Embryo, 9, 200 ; and see under various types Embryology, 217 Embryonic area, 586 Embryonic membranes, 606 Emulsification of fats, 75 Enamel, 445, 510 Enamel-organ, 445 Encystation, 232, 244, 254, 275, 283, 285 Endoderm, 202, 210, 298, 303, 311, 314, 349, 384, 470, 583, 586 Endoderm -lamella, 315, 317 Endolymph, 188 Endolymphatic duct, 436, 465 Endoparasite, see Parasite Endoplasm, 231 Endophragmal system, 363 Endopodite, 364 Endoskeleton : Frog, 16, 35, 205 ; Lancelet, 422 ; Dogfish, 435 ; Rabbit, 487 ; develop- ment of, in Vertebrates, 604 Endostyle, 424 Energy, conversion of potential into kinetic, 235 ; source of, in chlorophyll-containing organ- isms, 248 Enteric canal : Frog, 22, 204 ; Earthworm, 328, 335 ; Cray- fish, 370 ; Mussel, 401 ; Lance- let, 424 ; Dogfish, 444 ; Rabbit, 507-516 ; development of, in Vertebrates, 204-210, 595 Enteric epithelium, see Epithe- lium Enteroccele, 580 Enteron or enteric cavity, 296, 298, 311, 314, 330, 373, 580, 595 Epencephalon, 590 Epiblast, see Ectoderm and p. 580 Epiderm : Frog, 128 ; Earth- worm, 328, 329 ; Crayfish, 369 ; Mussel, 400 ; Lancelet, 419 ; Dogfish, 433 ; Rabbit, 484 Epididymis, 466, 538, 603 Epiglottis, 512, 517 INDEX 625 Epipharyngeal groove, 424 Epiphysis, 497, 503 Epipodite, 366 Epistoma, 363 Epistylis, 277 Epithelial cells : columnar, 107 ; ciliated, 109 ; glandular, 130 et seq. ; squamous, no ; strati- fied, 128 Epithelium, 109, 319 ; ccelomic, 330 (and see Peritoneum) : deric — -see Epiderm ; enteric, 330 (and see Endoderm) Equilibration, organ of, see Auditory organ Equivocal generation, see Abio- genesis EUGLENA : occurrence and gene- ral characters, 251; movements, 251 ; structure, 251 ; nutri- tion, 254 ; resting stage, 254 ; reproduction, 254 ; animal or plant ? 255 ; practical direc- tions, 259 Eustachian tube or recess, 17, 189, 512, 536 Eustachian valve, 523 Evolution : organic, 221, 293 ; of animals and plants, 258 Excretion, 148, 150, 235 Execretory organs, see Kidney and Nephridium, and compare Contractile vacuole Exopodite, 364 Exoskeleton : cuticular, 313, 369, 398 ; dermal, 434, 444 ; epidermal, 482, 484 Expiration, 143 Extra-cellular digestion, 234, 305 Eye : Frog, 5, 181 ; Nereis, 351, 353 * Crayfish, 380 ; Lancelet, 424; Dogfish, 465 ; Rabbit 486, 534 ; development in Vertebrates, 593 Eye : compound, 380 Eyelids, 5, 432, 486 Eye-muscles, see Muscles of eye Eye-spot : Euglena, 254 ; Lance- let, 424 Eye-stalks, 363, 380 Fabellae, 504 Facet, 38 Faeces, 8 Family, 218 Fascia, 18, 59 Fat-body, 23 Fats, 72" Femur, 51, 504 Fenestra ovalis, 46, 189, 492, 536 Fenestra rotunda, 493, 536 Ferment, fermentation : amylo- lytic, 74 ; peptonising or pro- teolytic, 74 ; putrefactive, 256, 257 Fertilisation, 197, 566, 572 ; and see also Conjugation, and under development of various types Fibrin, 107 Fibula, 51, 504 Filum terminale, 155, 532 Fingers, see Digits Fin-rays, 422, 430, 441-443 ; dermal, 442 Fins : Tadpole, 207 ; Lancelet, 421 ; Dogfish, 433, 441 Fishes, see Pisces Fission, 106, 198, 236, 250, 268, 273, 283 ; multiple, 237, 254, 283, 286 Fissures of spinal cord, 155 Fixing, directions for, 136 Flagellata, Flagellate Infusoria, 261, 292 Flagellum, 242, 251, 256, 303 Flocculus, 493, 530 Foetus, 541 Follicle, ovarian, see Ovisac Fontanelle, 44, 436 Foods, 67, 72 Foot, of mussel, 397 ; and see Pes Foramen : lacrymal, 494 ; inter- vertebral, 38, 440, 498 ; mag- 626 INDEX num, 40, 436, 487 ; obtura- tor, 503 ; of Monro, 160, 531 Foramina for cerebral nerves, see Skull Fore-brain, 202, 589 Fore-gut, 371 Fore-limb or fin, see Appen- dages Fornix, 531 Fossa ovalis, 521 Fossils, 223 FROG : Preliminary account, 4 ; mouth-cavity, 16 ; skin and muscles, 17, 31 ; abdomen and its contents, 20, 32 ; neural cavity and its contents, 27, 33 ; structure of limbs, 28, 34; skeleton, 35, 53; joints, 55, 64 ; muscles, 57, 64 ; enteric canal and digestion, 67, 76 ; vascular system, 78, 98 ; circulation of blood, 89, 103 ; lymphatic system, 97, 98 ; simple tissues, 104, 121 ; compound tissues and glands, 126, 139 ; lungs and larynx, 141, 152; kidneys, 145, 153 ; structure and func- tions of nervous system, 154, 175; sense-organs, 179, 191; reproductive organs, 193, 210 ; fertilisation of eggs, 9, 197 ; development, 9, 198, 212, 214, 575. 5Sl J metamorphosis, n, 206, 212 ; classification, 215 ; side dissection, 481 ; summary of characters, 482 Function, see Physiology Gall-bladder, 20, 68, 447, 516 Callus, see Bird Gamete, 197, 250, 268, 275, 286, 287, 566, 573 Gametocyte, 286, 287, 348, 356, 566 Gametogenesis, 566 Gamobium, 321 Ganglion, 163, 167 ; and see Nerve-ganglia Gastric glands, see Gland Gastric juice, 71, 74, 132 Gastric mill, 372 Gastrocnemius, 59 Gastrolith, 373 Gastrula, 577, 582 Gemmation, see Budding Generation, alternation of, see Alternation of generations Generation, asexual, see Aga- mobium Generation, sexual, see Gamo- bium Generative organs, see Repro- ductive organs Genus, 215, 218 Germ-cells, see Sex-cells Germ-plasm, 573 Germinal disc, 575, 582 Germinal epithelium, 194, 196, 345, 383, -5.66 Germinal vesicle and spots, 565 Gestation, 542 Giant-fibres, 343 Gill-arches and clefts, see Bran- chial apertures, arches, clefts and septa Gill-cover, 361, 374 Gill-rays, see Branchial rays Gills : Tadpole, 9, 204, 207 ; Crayfish, 374 ; Mussel, 398, 401 ; Dogfish, 448, 470 Gizzard : Earthworm, 335 ; Cray- fish, 370 Gland-cells : Hydra, 303, 305 ; Earthworm, 335 ; and see Glands and Goblet-cells Glands : Cowper's, 539 ; cu- taneous, 129, 484 ; digestive, see Enteric canal ; ductless, 447; gastric, 131, 447, 514; green, 375 ; Harderian, 186, 535 ; lacrymal, 535 ; mam- mary, 483, 486, 506 ; Mei- bomian, 535 ; cesophageal, 335 ; perineal, 486, 540 ; pros- tate, 539; racemose, 133; INDEX 627 rectal, 540 ; salivary, 511 ; sebaceous, 484 ; and see kid- ney, liver, pancreas, etc. Glands, ductless, 447, 507 Glenoid cavity, 47, 500 Glochidium, 411 Glomerulus, 146, 601 Glottis, 17, 512 Glycogen, 135 Goblet-cells, 109, 130, 131 Gonad, 193 ; and see Reproduc- tive organs Gonaduct, see Reproductive organs Gonotheca, 312 Gregarines, 284 Grey matter of spinal cord and brain, 156, 160; 167 Growth, 236 Gryllotalpa, 567 Gullet, 22, 253, 265, 271 ; and see Enteric canal H Haemal arch and spine, 441 Haematochrome, 243 Haematococcus, see SPH^ERELLA Haemocoele, 374 Hsemocyanin, 379 Haemoglobin, 107, 339 Hairs, 482, 484 Hallux, 505 Hand, see Manus Hardening, directions for, 136 Harderian gland, 186, 535 Head, 4, 350, 361, 431, 484, 486 Head-fold and process, 589 Heart, 20, 79, 87, 378, 407, 449, 520 ; development of, 458, 597 ; pulsation of, 90 Heat, evolution of, 152 Hemibranch, 448 Hepatic caecum, 424 Hepatic ducts, see Bile-duct Hepatic portal system, see Por- tal system Heredity, 225 Hermaphrodite, see Monoecious Heterocercal tail, 433 Heterogenesis, 291 Hibernation, 8 Higher (triploblastic) animals, uniformity in general struc- ture, 332 Hind-brain, 202, 589 Hind-gut, 371 Hind-limb or Fin, see Appen- dages Hinge of lamellibranchiate shell, 396, 398 Hip-girdle, see Pelvic arch Hippocampus, 531 Histological methods, 120, 135, 575 Histology, 104 Holoblastic segmentation, 574 Holobranch, 448 Holophytic nutrition, 247, 255 Holozoic nutrition, 247, 255 Homogenesis, 290 Homology and homologous, 217, 3i8 Homology, serial, 39, 52, 327 Host, 282, 284, 287 Humerus, 48, 501 Hybrids, 216 HYDRA : occurrence and general characters, 294 ; species, 296 ; movements, 296 ; mode of feeding, 297 ; microscopic structure, 297 ; digestion, 304 ; asexual, artificial, and sexual reproduction, 306 ; de- velopment, 308 ; practical directions, 322 Hydranth, 309 Hvdroid polypes, 308 Hydrotheca, 312 Hydrozoa, 322 Hyoid, 40, 44, 438, 496 Hyomandibular, 438 Hypoblast, see Endoderm and p. 580 Hypobranchial groove, see Endo- style Hypostorne, 295, 309, 317 628 INDEX Ileum, 22 ; and see Enteric canal Ilium, 50, 503 Imbedding, directions for, 138 Immortality, 236 Impregnation, see Fertilisation Incisors, 509 Income and expenditure, 148, 236, 245 Incubation, 615 Incus, see Auditory ossicles Individual, 220, 277 Individuation, 277, 306 Infundibulum, of brain, 159, 459, 53°, 590 ; of lung, 518 Infusoria, 261, 292 Ingesta and egesta, balance of, 236 Ingestion, 8, 233 Inguinal canal, 538 Injection of blood-vessels, 99, 389, 414, 474, 477, 543 Innominate bone, 51, 503 Insertion of muscle, 60 Inspiration, 143 Integument, structure of, 127 ; and see under various types Integumentary sense - organs, 430, 464 Intercellular substance, 115, 118 Interneura.] plate, 440 Interrenals, 447 'Interstitial cells, 298 Intervertebral discs, 497 Intervertebral foramina, see Foramen Intervertebral substance, 440, 441 Intestine, 22 ; see its various subdivisions and Enteric canal Intracellular digestion, 234, 305 Invagination, 384, 577 Invertebrata, 412 Iris, 5, 182, 555 Irritability, 60, 169, 232, 272, 319 Ischium, 51, 503 Jacobson's organ, 534 Jaws, 17, 353, 364, 367, 436, 489, 496 Joints. 55, 362, 368 K Karyokinesis, 563 Keber's organ, see Pericardial gland Kidney : Frog, 25, 145 ; Cray- fish, 375 ; Mussel, 406 ; Dog- fish, 466; Rabbit, 537 ; develop- ment of, in Vertebrates, 600 Labial palp., see Palp Labrum, 370 Lacrymal glands, 186, 535 Lacunae, see Bone and cartilage Lamellae and laminae of gills (Mussel), 401 Lamellibranchiata, 411 Lamina terminalis, 530 LANCELET : general characters, 419 ; fins, 419 ; skeleton, 422 ; gill-slits and bars, 420, 422 ; enteric canal, 424 ; blood- vessels, 424 ; nephridia, 424 ; nervous system, 424 ; gonads, 421, 425 ; development, 425, 577 ; practical directions, 615 Larva, 9, 354, 384, 411, 425, 580 Laryngo-tracheal chamber, 141 Larynx, 144, 506, 517 Lateral line, 432, 464 Laverania, see Malaria organism Legs, see Appendages Lens of eye, 183, 353, 534, 593 LEPUS, see Rabbit Leucocyte, 105, 336, 379, 458 Life, origin of, 288 ; and see Bio- genesis Life-history, 8, 206 ; and see under various types INDEX 629 Ligaments, 55, 57 Limbs, see Appendages Limnaea, 576 Linin, 560 Lips, 486, 507 Lithite, 315, 409 Lithocyst, 315 Liver, 20, 133, 147, 204, 424, 447. 5i6, 597 Liver-fluke, 412 Lobster, 360 Lumbricidae, 350 LUMBRICUS, see Earthworm Lungs, 10, 22, 141, 204, 208, 518, 597 Lymph, 18 Lymph-hearts, 97 Lymphatic glands, see Adenoids Lymphatic system, 18, 97, 458, 527 M Malaria organism, 287 Malleus, see Auditory ossicles Malpighian capsule, 146, 538, 60 1 Mammalia, 219, 418, 482, 585, 610 Mammary glands, see Glands Mandible, 16, 44, 366, 438, 496, 537 Mantle, 396 Mantle-cavity, 400 Manubrium : of Medusa, 314 ; of malleus, 536 ; of sternum, 500 Manus, 5, 50, 484, 502 Marginal sense-organs, see Litho- cyst Marrow-cavity, 48 Matrix, see Inter-cellular sub- stance Maturation of ovum, 569 Maxilla : Crayfish, 366 Maxilliped, 364 Meckel' s cartilage, 44, 438 Mediastinum, 518 Medulla oblongata, 157, 459, 53°, 590 Medullary cord, folds, groove, and plate, 202, 588 ; sheath, 167 ; substance of Protozoa, 264, 269, 280, 286 Medusa, medusa-buds, 311, 313, , 314. 315 Megagamete, 275, 287 Megalecithal ova, 574 Meganucleus, 265, 271 Megazooid, 250, 275 Meibomian glands, 535 Membranous labyrinth, 186, 465, 535, 595 Meroblastic segmentation, 574 Mesencephalon, see Mid-brain Mesenteron, 371, 595 ; and see Enteron Mesentery, 22, 27, 447, 515 Mesoblast, see Mesoderm and P- 580 Mesoderm, 202, 203, 210, 349, 384, 470, 580, 586 ; vertebral and lateral plates of, 588 Mesodermal segments, 580 Mesogloea, 298, 311, 314, 315, 317 Mesonephros and mesonephric duct, 601 Metabolism, 149, 236 Metacarpus, 50, 502 Metacromion, 500 Metagenesis, 321 Metamere, Metameric segmenta- tion, 327, 360, 368 Metamorphosis, n, 209, 276 ; and see Larva Metanephros and metanephric duct, 603 Metapleural folds, 421 Metatarsus, 51, 505 Metazoa, 293 Metencephalon, see Medulla ob- longata Microgamete, 275, 287 Microlecithal ova, 574 Micrometer, 121 Micromillimetre (/*) = fJw °f a millimetre, or 2TJozr °* an Micronucleus, 265, 271 Micropyle, 415, 572 630 INDEX Microtome, 779 Microzooid, 250, 275 Mid-brain, 202, 589 Mid-gut, 371 Milk-glands, see Glands Milk-teeth, 510 Milt, see Spermatic substance Mitosis, 563 Molars, 509 Mollusca, 220, 411 MONADS : occurrence ana gene- ral characters, 256 ; move- ments, 256 ; nutrition, 256 ; animals or plants ? 258 ; rapid multiplication, 289 ; practical directions, 260 MONOCYSTIS : occurrence, 284 ; structure, 286 ; parasitic nu- trition, 286 ; conjugation and reproduction, 286, 287 ; en- cystation and spore-forma- tion, 285, 286 ; means of dispersal, 287 ; practical directions, 293 Monoecious, 307 Morphology, 217 Morula, see Polyplast Mosquito, 288 Mother of pearl, see Nacre Mounting sections, directions for, 139 Mouth, 4, 16, 253, 265, 271, 295, 311, 314, 370, 401, 421, 431, 486 Mouth-cavity, see Buccal cavity Movement, spontaneous, volun- tary, and involuntary, 7, n, 112, 172, 232, 319, 327, 433 ; and see under various types Mucous membrane, 1 7 Mule,, see Hybrid, 216 Mullerian duct, 603 Multinucleate, 113, 281 /Muscle-fibres : striped, 112 ; un- striped, in, 131, 132 ; and see under various types Muscle-plate, 580 Muscle-processes, 298, 304, 311 3i8 Muscles : Frog, 18, 57, 59, 63, 205; Obelia, 311; Earth- worm, 329 ; Crayfish, 369 ; Mussel, 399, 400'; Lancelot, 419 ; Doefish, 434 ; Rabbit, 505 ; and see Myomere Muscles : of eye, 186, 465, 534 ; of middle ear, 536 Muscles, papillary, 521 Muscular contraction, 60, m, 2?3 Muscular impressions on shell, 399 Muscular layers of enteric canal, 7°, 75, 131, 132 Muscular system, see under various types ; development of, in Vertebrates, 203, 588, 604 Muscularis mucosae, 133 MUSSEL : general characters, 396 ; 412 ; mantle, shell, and foot, 396, 397, 412 ; food- current, 404, 412 ; gills, 397, 401, 414 ; muscles, 399, 400, 413 ; enteric canal, 401, 416 ; nephridia, 406, 415 ; blood- system, 407, 414 ; nervous system, 408, 415 ; sense- organs, 409 ; 416 ; gonads, 409, 417 ', development and metamorphosis, 410, 415, 581 Mustelus, 431, 469 Myocomma, 434 Myomere, 203, 369, 419, 423, 434. 588 N Nacre : nacreous layer, 399 Nasal organ, see Olfactory organ Naso-lacrymal duct, 186, 535 Naso-palatine canals, see Canals Naso-pharynx, 512 Natural History, 2 Natural selection, 227 Nauplius, 384 Navel, 597, 613 Navicular bone, 505 Neck, 484 INDEX 631 Nemathelminthes, 412 Nematocyst, 300, 311, 317 Nephridiopore, 341 Nephridium, 146, 340, 406, 421, 424, 600 Nephrostome, 98, 145 (Fig. 320, 345, 382, 409, 421, 425, 467, 538, 603 Reptilia, 219, 418 Reservoir of contractile vacuole, 254 Respiration, 141, 143, 235, 339, 374, 448, 517 Respiratory movements, 7, 142, 375, 520 Retina, 183, 184, 353, 556 Retinula, 380 Rhabdome, 380 Rhabdonema, 153 Rhinencephalon, 590 Rhizopoda, 292 Ribs, 486, 498, 500 Rocks, sedimentary and strati- fied, 223 Rodentia, 542 Rods and cones, 185 Rostrum, 363, 436 Rudiment, often used for Ves- tige, (q. v.) Sacculus, 187 Sacculus rotundus, 515,3^/7 Sacrum, 38, 499 Salamander, 218 Salivary glands, see Glands Saprophytic nutrition, 256, 257 Sarcolemma, 113 Scales, 430, 433 Scapula, 47, 443, 500 Schizocoele, 588 Sclerite, 390 Sclerotic, 182, 534, 555 Scrotal sac, 487, 538, 548 Scyllium, see Dogfish Sebaceous glands, see Glands Section-cutting, directions for, 126, 138 Secretion, 130 Segment, see Metamere, Podo- mere Segmentation-cavity, 200, 577, 581, 583 Segmentation of oosperm, 198, 200, 212, 308, 320, 349, 383, 410, 470, 574, 577, 581, 582, 585 INDEX G35 Segmentation, equal and un- equal, 575 ; dis- coid, 575 ,, superficial, 383, 575 „ metameric, see Metamere ,, -nucleus, 572 Selection, natural and artificial, 226, 227 Self-fertilisation, 348 Seminal funnel, 347 vesicle, 194, 347, 467 Sense-organs and cells, 179, 315, 344> 351, 380, 409, 424, 432, 464. 533, 592 Septa, of earthworm, 333 Septum : lucidurn, 531 ; nasal, 42, 493 ; interorbital, 487 Serous membrane of embryo, 609, 610 Sesamoid bones, 502, 505 Seta, 328, 350, 351, 362 Sex-cells, primitive, 566, 568 Sexual characters, external, 7, 382, 433, 486 Sexual generation, see Gamo- bium Sexual organs, see Reproductive organs Sexual reproduction, see under various types Shank, 5 Shell, 396 ; larval, 410 Shell of egg, 470, 566 Shell-gland of mussel-embryo, 410 ; of dogfish, 469 Shoulder-girdle, see Pectoral arch Sinus : blood-, 376, 377, 408, 457; lymph-, 18, 27; urinary and urinogenital, 466, 467 : venosus, 80, 89, 449, 520 Siphon, inhalant and exhalant, 397 Skeleton, see Endo- and Exo- skeleton Skin, see Integument Skull : Frog. 16, 35, 39 ; Dogfish, 436 ; Rabbit, 487 ; develop- ment of, 604 Smell, organ of, see Olfactory organ Snout, 4 Somatic layer of mesoderm, 580 Somatopleure, 580 Spawn, 9 Species, 215 et seq. ; origin of, 222, 225 Sperm, or Spermatozooid, 194, 382, 568 ; and see under various types Spermary, 25, 193, 194 ; and see Reproductive organs Spermatic substance, 9 Spermatogenesis, 194, 566 Spermatophore, 382 Spermiduct, 193, 347, 382, 466, 538- 603 Spermotheca, 347 Sperm-reservoir, 347 Sperm-sac, 347, 467 SPH^ERELLA : general characters, 240 ; rate of progression, 240 ; ciliary movements, 242, 250 ; colouring matter, 242 ; motile and stationary phases, 244 ; nutrition, 245 ; source of energy, 248 ; dimorphism, 250 ; reproduction, 250 ; con- jugation, 251 ; animal or plant ? 255 ; practical direc- tions, 259 Sphincter, 71 Spinal cord, 27, 155, 424, 459, 532, 592 Spindle, nuclear, 560 Spiracle, 432, 449 Spiral valve, 445, 515 Spireme, 560 Splanchnic layer of mesoderm, 580 Splanchnopleure, 580 Spleen, 23, 98, 447, 516 Spontaneous generation, see Abiogenesis Spores, 254, 275, 286 Sporozoa, 284, 292 Sporozoite, 287 636 INDEX Staining, directions for, 137 Stalk of Vorticella, 269, 272 Stapes, 46, 189, 492, 536 Starch, 72, 243, 252 Starfishes, 414 Statocyst, 315, 381, 409 Sterilised infusions, 289 Sternal canal, 363 Sternebrae, 500 Sternum, 16, 47, 361, 486, 500 Stigma, 254 Stimulus, various kinds of, 62 Stock, see Colony Stomach, 22, 70, 401, 445, 512 Stomoda^um, 203, 384, 445, 447, 597 Struggle for existence, 226 Submucosa, 130 Substitution of organs, 209 Sucker, 203 Superficial or peripheral seg- mentation, 575 Supination, 501 Supporting lamella, see Meso- glcea Suprarenals, 447 ; and see Adrenals Suspensorium, 40, 438 Sutures, 50, 487 Swimmeret, see Pleopod Symbiosis, 304 Sympathetic, see Nerves Symphysis, 496, 503 Syn-cerebrum, 380 Synovial capsule, 56 Systemic arch, 80, 452, 523 Systole : of heart, 90 ; of con- tractile vacuole, 266 Tactile organs, 179, 353, 382, 533 Tadpole, 9, 203, et scq. Tail, 9, 209, 432, 483, 484 Tape-v/orms, 412, 545 Tapetum, 556 Tarsus, 51, 504 Taste-organs, 180, 509, 534 Teasing, directions for, 123 Teats, 486, 506 Teeth, 17, 434, 444, 509 Telencephalon, 590 Teleostomi, 431 Telolecithal ova, 574 Telson, 361 Tendon, 59 Tentacles, 295, 309, 314, 315, 351 Tergum, 361 Testis, see Spermary Tetrad, 568 Thalamencephalon, see Dience- phalon Thigh, 5 Thoracic duct, 528 Thorax, 361, 483, 486, 506 Thread-cell, see Nematocyst Thymus, 447, 507, 519 Thyroid, 447, 520 Thyroid, cartilage, 517 Tibia, 51, 504 Tibio-fibula, 51. Tissues, enumeration of, 31 Toad, see Bufo Toes, see Digits Tongue, 8, 17, 445, 509 Tonsil, 507 Trabeculae cranii, 605 Trachea, 506, 517 Transverse process, see Vertebra Trichocyst, 267 Trimorphic, 318 Triploblastic, 311, 322, 332 Trochanter, 504 Trochlea, 501 Trochosphere, 354 Trophoblast, 586 Trophozoite, 286 Trunk, 4, 432, 484 Trypanosoma, 284 Trypsin, 74 Tubercle, and Tubercular facet, 498, 500 Tuberosity, 501, 503 Tunicata, 421 Turbinals, 493, 534 Tympanic cavity, membrane, and ring, 5, 45, 189, 536 Typhlosole, 335, 401 INDEX CSV U "Ulna, 50, 502 Umbilical cord and umbilicus, 597, 613, 615 Umbilical vesicle, 586 Umbo, 398 Umbrella of Medusa, 314 Unicellular, 231 UNIO, see Mussel Urachus, 615 Urea, 66, 147 Ureter, 25 ; and see tinder various types Urethra, see Urinogenital canal Uric acid, 66 Urinary bladder, 23, 147, 375, 406, 538, 615 , Urinary tubules, see Nephridium Urine, 8, 66, 147 Urinogenital aperture, 486 ,, canal, 539, 540 duct, 194, 603 organs, 193, 466, 537 organs, develop- ment of, 600 Urodeles, 219 Uropod, 364 Urostyle, 35, 39 Uterine crypts, 611 Uterine tube, 540 Uterus, 540 ,, masculinus, 538 Utriculus, 187 Vacuole : contractile, 232, 243, 254, 265, 269 ; food-, 233, 266, 2-2 \ a^ina, 540 Valve : of Vieussens, 529, 531 (Figs.) ; spiral, 445, 515 ; ileo- colic, 515 Valves : of heart, 88, 376, 521, 523 ; of shell, 396; of veins, 89 Variation, 216, 225, 292, 483 Variety, 225 Vascular system, see Blood- vessels, Arteries, Veins Vas deferens, see Spermiduct, Wolffian duct Vasa efferentia, see efferent ducts Veins : Frog, 19, 82 ; Crayfish, 377 ; Mussel, 408 ; Dogfish, 454 ; Rabbit, 525 ; embryo Vertebrate, 599, 600 Veliger, 411 Velum: of Medusa, 315; of Lancelet, 424 Velum palati, 507 Vena cava, see Veins Vent, see Anus Ventral, 6 Ventricle, see Heart Ventricles of brain, 157, 159, 424 459, 529, 53i. 590 Vermiform appendix, 515 Vertebra and vertebral column, J6, 35, 36, 439, 497, 604 Vertebrarterial canal, see Canal Vertebrata, 220 ; general char- acters of, 418 Vessels, see Blood-vessels Vestibule, see Urinogenital canal Vestige, vestigial, 159, 364, 521, 550 Vibrissze, 486 Villi : of intestine, 515 ; of chorion, 611 Viscera, abdominal, 20, 444, 512 Visceral arches and clefts, 438, 448 ; and see Branchial aper- tures Visceral ganglion, 408 Visceral layer of peritoneum, 27, 331 Visceral mass, 399 Vitelline membrane, 565, 609 Vitelline vessels, 3-1.) Vitreous body of compound rye, 380 Vitreous chamber and humour, 183 Vocal cords, 144, 517 Vocal sacs, 218 Vomero-nasal organ, 534 638 INDEX VORTICELLA : occurrence and general characters, 269 ; struc- ture, 269 ; reproduction, 273 ; conjugation, 274 ; means of dispersal, 274 ; encystation, 275 ; spore-formation, 276 ; metamorphosis, 276 ; prac- tical directions, 277 Vulva, 487, 540 W Waste-products, 8, 66. 249 White matter of brain and spinal cord, 156, 160, 167 Wolffian body, see Epididymis duct, 603 Work and waste, 66, 148, 234 Worms, 349 Wrist, see Carpus X Xiphisternum, 48, 500 Y Yellow cells of Earthworm, 331, 335, 34i Yolk, yolk-granules or spheres, I95> 564, 569 ; and see under various types Yolk-cells, 200, 202 Yolk-plug, 20 1, 581 Yolk-sac, 470, 582, 586 Zona radiata, 586 Zoochlorella, 304 Zooid, 277, 309 ; and see Mega- zooid and Microzooid Zoology, i Zoophytes, see Hydroid polypes Zygapbphysis, 36, 498 Zygoma, zygomatic arch, 494, 495 Zygote, 197, 251, 275, 284 PRINTED IN GREAT BRITAIN* nv RICHARD CI,AY & SONS, LIMITED, lit'NGAV, SCFFOI.li. UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY Return to desk from which borrowed. This book is DUE on the last date stamped below. JAN uu,cci teca»»5 immediateij ., LD 21-100m-ll,'49(B7146sl6)476 Bid UNIVERSITY OF CALIFORNIA LIBRARY