es joann = hei ES ones state eres coca sea aceaan Sse HG ae : ‘e New York State Qallege of Agriculture At Cornell University Ithaca, N.Y. Library MlateniaTy Course of practical zoolog NOAA 3 1924 003 399 689 mann Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003392689 AN ELEMENTARY COURSE Or PRACTICAL ZOOLOGY AN ELEMENTARY COURSE OF PRACTICAL ZOOLOGY aw BY THE LATE JEFFERY PARKER, D.Sc., F.R.S. PROFESSUk OF BIOLOGY IN THE UNIVERSITY OF OTAGO DUNEDIN, NEW ZEALAND AND W. N. PARKER, Pu.D. PROFESSJRK OF ZOOLOGY AT THE UNIVERSITY COLLEGE OF SOUTH WALES AND MONMOUTHSHIRE, IN THE UNIVERSITY OF WALES Vith One Hundred and Fifty-six Illustrations London MACMILLAN AND CO., Limirep NEW YORK: THE MACMILLAN COMPANY 1goo All rights reserved PREFACE In the early part of 1897, my brother and IJ had arranged to collaborate in writing a practical text-book of Elementary Zoology, adapted more particularly to the requirements of Students pursuing courses in the subject as laid down by various examining bodies. We had, however, only reached the stage of deciding on a general plan at the time of my brother’s death in November of the same year. The following are the chief points on which we had agreed :— 1. To adopt the method pursued in Huxley and Martin’s Llementary Biology of giving a connected account of each example. 2. ‘To give brief practical directions which should serve mainly as a guide, the student being able to refer, in case of difficulty, to the descriptive accounts preceding them. 3. In the larger animals, to arrange for as much work as possible to be done on one specimen: there is much to be said in favour of this plan apart from the fact that the average student cannot give sufficient time to the subject to dissect a fresh specimen for each system of organs. 4. To begin the course of instruction by an introductory study of one of the higher animals ; to include in this in- troduction the elements of Histology and Physiology ; and vi PREFACE to select the Frog for the purpose: after trying various methods, I have found this plan to be the most satisfactory in practice. 5. To give drawings and diagrams of difficult dissections, and of details which the beginner cannot as a rule make out satisfactorily for himself; but otherwise to limit the number of illustrations so as not to tempt the student to neglect observing the things themselves. 6. To include a short account of methods and technique, limited to the barest essential outlines, sufficient for a student working by himself to make out the things described, but not going into such details as would naturally be learnt in a properly organised laboratory. In the meantime, my brother had in preparation a Bzology for Beginners, in which he intended to carry out the plan, suggested in the preface to his Elementary Biology, of giving a simple account, with practical directions, of one of the higher animals and one of the higher plants, as an introduction to the study of Biology. The animal he selected was the Frog, and the manuscript of the greater part of this section of the book was already finished and the rest in rough draft. He had previously suggested that some of this work might be utilised for our proposed Prac- tical Zoology ; and I found that, with certain additions and with modifications in the arrangement, the whole of it was exactly the kind of introduction I had in view for the first part of our book. Some of the illustrations that my brother had intended to insert in the Bvoloey for Beginners I have found it advisable to omit, and even now the figures in the introductory part are purposely nearly as numerous as those inthe rest of the book. But apart from these various minor PREFACE vil modifications, Chapters I--XII in Part I are almost en- tirely taken from my brother’s manuscript. We felt that there could be no object in entirely re- writing the descriptions of several familiar animals already given in my brother’s published works; and in Part II. I have intentionally made use of these descriptions, borrowing very freely from the Avementary Biology, as well as (with Professor Haswell’s permission) from the Zextbook of Zoology ; and, to a less extent, from the Zoofomy. The practical directions are mainly based on a series of Laboratory-instructions I drew up some years ago for the use of my junior classes, which consist principally of students preparing for the Intermediate Science examin- ation of the University of Wales, the Preliminary Scientific examination of the London University, and the first ex- amination of the Conjoint Board of the Royal Colleges of Surgeons and Physicians. The time such students can devote to an elementary course in the subject is limited ; and throughout the book I have borne in mind that the main object of teaching Zoology ‘tas a part of a liberal education is to familiarise the student not so much with the facts as with the ideas of the science,” but at the same time that he should be provided with a sound basis of facts so arranged, selected, and compared as to carry out this principle. Our original intention was to include one or more examples of each of the larger phyla, and also to add a practical exercise after each type, giving general directions for the examination of an allied form for comparison. But I found that this would be impossible within the space of a vill PREFACE single volume, and it was therefore necessary to limit the descriptions mainly to those animals to which the students for whom the book is chiefly intended have to give special attention. ‘This has resulted in rather a heavy balance on the side of Vertebrates; but on the whole, I think that if sufficient work is done on the lower animals to illustrate certain main facts and generalisations, a comparative study of several Vertebrates forms as good a training as any for beginners—more especially in the case of medical students. I am indebted to Mr. H. Spencer Harrison, B.Sc., Demonstrator of Biology in this College, for much assistance in testing and improving the practical instructions, as well as for various suggestions and for help while the work was passing through the press. The new figures were re-drawn from the originals by Mr. M. P. Parker. W. N. PARKER. UNIVERSITY COLLEGE, CARDIFF, Nowenher, 1899. CONTENTS PREFACE .. Part I CHAPTER. I SCOPE OF THE SCIENCE OF BIOLOGY—THE FROG: PRELIMINARY SKETCH OF ITS STRUCTURE, LIFE-HISTORY, FUNCTIONS . 1. 2. : HINTS ON DISSECTION . CHAPTER II AND VITAL THE FROG (continued) : GENERAL INTERNAL STRUCTURE PRACTICAL DIRECTIONS CHAPTER II THE FROG (cont?nued): THE SKELETON . PRACTICAL DIRECTIONS ane CHAPTER IV THE FROG (cortinued): THE JOINTS AND MUSCLES PRACTICAL DIRECTIONS . 2... go oa CHAPTER V THE FROG (continued): WASTE AND REPAIR OF —THE DIGESTIVE ORGANS—NUTRITION . . PRACTICAL DIRECTIONS . seca ate eels PAGE 16: 31 66 76 \ CONTENTS CHAPTER VI THE FROG (continued): THE VASCULAR SYSTEM-—THE CIRCU- LATION OF THE BLOOD . . +. ho, at PRACTICAL DIRECTIONS * CHAPTER VII THE FROG (continued): ‘THE MICROSCOPICAL EXAMINATION OF THE SIMPLE TISSUES . PRACTICAL DIRECTIONS . CHAPTER VIII YHE FROG (contizued): THE MICROSCOPICAL EXAMINATION OF THE COMPOUND TISSUES—GLANDS—SECRETION ANI) AB- SORPTION. : i PRACTICAL DIRECTIONS CHAPTER IX THE FROG (con/inued): RESPIRATION AND EXCRETION . PRACTICAL DIRECTIONS . . ee eee CHAPTER X YHE FROG (contdiued): THE NERVOUS SYSTEM . PRACTICAL DIRECTIONS : CHAPTER XI THE FROG (conténued): THE ORGANS OF SPECIAL SENSE PRACTICAL DIRECTIONS . , i . CHAPTER NII THE FROG (coudénucd): REPRODUCTION AND DEVELOPMENT. PRACTICAL DIRECTIONS ... . » CHAPTER NII THE FROG (covdénued) + MEANING OF THE TERM SPECIES—THE PRINCIPLES OF CLASSIFICATION—EVOLUTION-—ONTOGENY AND PHYLOGENY—HEREDITY AND VARIATION—STRUGGLE FOR EXISTENCE—SELECTION—ORIGIN OF SPECIES 104 119 126 35 Iq 152 154 175 179 191 193 210 CONTENTS Part II CHAPTER I AM(EBA—UNICELLULAR ANI) MULTICELLULAR ANIMALS PRACTICAL DIRECTIONS CHAPTER II H-EMATOCOCCUS AND EUGLENA—MONADS AND BACTERIA—DIF- FERENCES BETWEEN ANIMALS AND PLAN IS—SAPROPHYTES PRACTICAL DIRECTIONS CHAPTER III PARAMECIUM, OPALINA, VORTICELLA AND ITS ALLIES—IARA- SITIC AND COLONIAL ORGANISMS—BIOGENESIS AND ABIO- GENESIS—CLASSIFICATION OF THE UNICELLULAR ORGAN- ISMS EXAMINED . Fi PRACTICAL DIRECTIONS . c . CHAPTER IV MWYDRA: BOUGAINVILLEA—ALTERNATION OF GENERATIONS— CHARACTERS OF THE PHYLUM C@LENTERATA. , PRACTICAL DIRECTIONS 3 CHAPTER V !HE EARTHWORM—CHARACTERS OF THE PHYLUM ANNULATA PRACTICAL DIRECTIONS CHAPTER VI THE CRAYFISH—CHARACTERS OF THE PHYLUM ARTHROPODA . PRACTICAL DIRECTIONS CHAPTER VII THE FRESH-WATER MUSSEL—-CHARACTERS OF THE PHYLUM MOLLUSCA—-ENUMERATION OF THE CHIEF PHYLA OF THE ANIMAL KINGDOM iH < Cee 5 . s PRACTICAL DIRECTIONS .. . PAGE 220 238 240 259 372 381 397 A CIN TERLS CHAPTER VII PAGE CHARACTERS OF THE PHYLUM VERTEBRATA—AMIPIHIONUS 403 PRACTICAL DIRECTIONS 410 CHAPTER IX CHARACTERS OF TILE CLASS PISCES—THE DOUPISE 414 PRACTICAL DIRECTIONS , . 456 CHAPTER X CHARACTERS OF THE CLASS MAMMMALIA—THE RABBIT . 466 PRACTICAL DIRECTIONS 525 CHAITER XI THE MINUTE STRUCTURE OF CELLS—CELL DIVISION—OOGENE- SIS AND SPERMATOGEN MATURATION AND FERTILI- ZAIION OF THE OVUM—EFFECT OF FOOD-YOLK ON DE- VELOPMENT—FORMATION OF THE CHIEF ORGANS OF THE ADULT IN VERTEBRATES, AND OF THE AMNION, ALLANTOIS AND PLACENTA . ee . . 541 PRACTICAL DIRECTIONS 585 INDEX . 550 AN ELEMENTARY COURSE OF PRACTICAL ZOOLOGY raed of CHAPTER I SCOPE OF THE SCIENCE OF BIOLOGY—THE FROG: PRELIMIN- ARY SKETCH OF ITS STRUCTURE, LIFE-HISTORY, AND VITAL FUNCTIONS—HINTS ON DISSECTION 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 Pract. Zoov. B 8 4 SCOPE OF THE SCIENCE OF BIOLOGY 6 TENE, devotes so much time; so are botany and zoology, in the ordinary sense of the words, re. 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 too, 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 begun the study of biology in one of its most important branches. The in- telligent gardener who observes the habits of plants, their individual tastes as to soil, moisture, sunshine and the lke, 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 things he collects, and does not consider them merely as a hoard or as objects for barter. Indeed, 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 classroom nature cannot be studied under her broader aspects ; indeed, much out-door natural history cannot be /avg/¢ 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 con- fine ourselves to some department of biology small enough to be manageable: we can take, for instance, one or more familiar animals and plants, and, by studying them in some 1 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 atits exact meaning. 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 mathe- matics 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 educational benefit of the study of science is that it sends the student direct to nature, and teaches him that a statement 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 CHAN. 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 ¢runk, 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 aava/ 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 szouf, round the whole edge of which extends the huge slit-like mouth. On the top of the fore-end of the I EXTERNAL CHARACTERS 5 snout are the two small wzosfri/s, one on 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 7r/s, 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 being folded over the eye, but produced into a thin transparent skin, the sctitating membrane, which can be drawn upwards over the eye. The entire absence of eye- brows and eyelashes is a point worthy of notice. Extending backwards from the eye is a large black 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 trace in the frog. Attached to the trunk are two pairs of offshoots or apperd- ages, the arms and legs, or fore- and Adnud-/imbs, 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 /ugers, which are slender and tapering and have no nails. The legs, on the other hand, are very long: each consists of a stout ¢Aigh, a long shana, with a well-marked “calf,” and a very curious foot. The anhle-region is long—almost like a second shank—and has no heel: it is followed by five toes, the first or innermost short, the second of moderate length, the third longer, the fourth longer still, and the fifth 6 THE FROG CHAP. of about the same length as the third. All the toes are joined together by thin transparent zeés, 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 vet or anus. In the squatting posture the body is raised upon the arms, which are kept slightly bent at the clbows, 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, corresponds 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 proxéma/ 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, irre- gularly spotted with brown or black on the upper or dorse/ surface, and whitish on the under or vev/ra/7 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 alike in this respect The I MOVEMENTS AND GROWTH v4 large black 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 ; there is no trace of it in the females. Actions performed by the Living Frog.—kKept under suitable conditions a frog very soon shows evrdences of Uife. 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, hke 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 gvozw 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 8 THE FROG CHAP. a frog feed. Tt lives upon insects, worms, slugs, and the like. Opening its mouth it suddenly darts out a tolerably long, nearly colourless, and very sticky /ongwe ; 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 inconceivable rapidity. Like other animals the frog discharges zeasfe 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 clear fluid, the we, which is perfectly clear and colourless, and contains little beyond 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 immersion in water. Hibernation.—In winter frogs bury themselves in damp places and become sluggish, all manifestations of life becom- ing 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 Arbernation, or the winter sleep. Reproduction and Development.—If you examine a number of frogs towards the end of winter—about February in England—you will find that the full-grown females are distinguished from the males, not only by the absence of I EGGS AND TADPOLES 9 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 vent by hundreds ; each is a little globular body about yisth 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 fluid, the ml¢ or spermatic @urd, which gets access to the eggs and ‘mpregnates or Jertilises them. Without impregnation 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. 1, 1), with a large head, a short tail, and no limbs; which, after wriggling about for a time, escapes from the jelly and fixes itself, by means of a sucker on the under side of its head, to a water-weed. Great numbers of these ¢adpo/es, as the free- living immature young or /avve 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 LO THE FROG CHAP. 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 g7//s, which serve as Fic. 1.—Stages in the life-history of the Common Frog, from the newly-hatched tadpoles (7) to the young frog (8); 2a is a magnified view of 2. (After Mivart)..c respiratory organs (2, 2“), the tadpole, like a fish, breath- ing air which is dissolved in the water. After a while the gills shrivel up and the tadpole then comes periodically to the surface to breathe, lungs having in the meantime made I SUMMARY OF CHAPTER II their appearance. A little pair of hind-limbs appears at the root of the tail, and a pair of forelimbs 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 little animal now comes on land and hops about as a small, tailed frog (7). As growth goes on the tail further diminishes and finally disappears altogether, the transforma- tion 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 beating, 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 (1) that a frog has certain definite far¢s 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 so/d 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 a transformation or metamorphosis, the egg giving rise toa /arva, 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 Instruments and other Requisites for Dissection.’—In order to carry out the dissection of the frog and other animals success- fully it is necessary to be provided with proper tools, The most important are— 1. Three or fou sharp dissecting knives, or scadfels, of different sizes. 2. A large anda 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 sezssors; 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, “e., a blunt needle mounted in a handle. 5. Three or four proées: a seeker ot 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, « piece of glass-tubing, 6 or 8 inches long, with one end drawn out in the flame until it is not more than {5th to s'gth of an inch in diameter. 7. An ordinary ** medérne-dropper,” or “feeder” of a self-feeding pen (see Fig. 25), made of a piece of glass-tubing about 3 inches long, drawn out in the fame 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 white pie-dish, about 6 or 8 inches long, with rather low sides. Cut out a piece of self-coloured (brown) cork-carpet or thick linvleum 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 Peopper wire or strong thread. Place this in the a A suitable hax © or dissecting instruments can be bought from most scientific instrument makers for about £1. (For further apparatus re- quired in connection with injection and microscopical work, see Pp. 99, 11g, and 135.) I HINTS ON DISSECTION 13 dish with the lead (which is simply to keep the cork-board from float- ing in water) downwards. Or, place a few strips of sheet lead in the bottom of the dish, and then pour. Zag. right lung, laid open in A to show its internal surface ; ccd. vocal cord. (After Howes). being kept shut, air is drawn in through the nostrils. The floor of the mouth is then raised (4), by muscles attached to the hyoid. .At the same time the anterior end of the lower jaw presses upon the moveable premanillee (fwx), the upward processes of which (p. 42, Figs. 8 and 9, PALY) act upon certain cartilages in connection with the external nostrils in such a way as to produce closure of these apertures (Fig. 45, 2). The gullet (gv/) is so contracted, except during the act of swallowing, as to be practically 1X RESPIRATORY MOVEMENTS 143 closed. Thus when the floor of the mouth is raised the air contained in it can escape in one way only, viz., through the glottis into the lungs. Thus zvspiration, 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. LZ.xpiration, or Ge Lng Fic. 45.—Diagrams illustrating the respiratory movements of the frog. 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.2. external nostril; g?. glottis; gud. gullet; . internal nostril ; Zag. lung ; olf, s. olfactory sac; px. premaxilla ; zag. tongue. breathing-out, is due to a contraction of the clastic lungs, accompanied by a slight lowering of the tip of the lower jaw the latter movement releases the premanille and thus opens the external nostrils. 144 THE FROG CHAP. Respiration.—By these alternate movements of inspira- 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 Jungs by the pulmonary artery is, as we have seen (p. 94), non-aérated, 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 contained in those organs only by the extremely thin walls of the capillaries themselves and the equally delicate pavement epithelium lining the lungs (Fig. 23, p. 142, 2p. dug, Cp. dng). Under these circumstances an interchange of gases takes place between the air and the blood: the hemoglobin of the red corpuscles absorbs oxygen, and the carbon dioxide in the blood, derived from the waste of the tissues, is given off into the cavities of the lungs. The blood in the pulmonary capillaries thus become 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 chambcr are supported by cartilages. The largest of these are a pair of semilunar arrfenoid cartilages (Mig. 44, a7), 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 awcul cords (z.cd). By means of muscles these folds can be stretched and relaxed, and can be brought IX KIDNEYS 145 into either a 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 characteristic 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,and 7, 4d) 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 is covered “rte tA mn) gt g aad Fic. 46.—Transverse section of frog's kidney. gf. glomerulus ; 7. cf. Malpighian capsule; ws¢. nephrostome; /e7, peritoneum covering ventral face of kidney ; fer’. fold of peritoneum supporting its outer border ; fer’. fold supporting its inner border; UA. ureter; a7. ¢u. urinary tubules. (After Marshall and Bles.) by peritoneum (Fig. 46, fer), continued on the one hand into the parietal layer (Fig. 5, 4. per), of that membrane, on the other into the mesentery (mes.) ; its dorsal face is bathed by the lymph of the subvertebral sinus (sz. Zv. s.) From the posterior end of its outer edge a delicate tube, the ureter (Figs. 3, 4, and 7, #7), passes backwards and opens into the dorsal wall of the cloaca. On its ventral face is a singular yellowish-white stripe, the advena/ or supra-renal body, an organ of unknown function (Fig. 7, between the lines from kd and ¢s). PRACT, ZOOL, L 146 THE FROG CHAP. A thin section shows the whole kidney to be made up of a tangled mass of microscopic tubes (ig. 46, v7 ti), SO twisted together that any section cuts them in various planes, some transversely, when they appear as circles, others longitudinally or ob- liquely. Amongst these urinary tubules or ne- paridia, as they are called, are seen globu- lar sacs, the Malpighian capsules (m. cp), each having in its interior a little irregular bunch, known as the glomerulus (g2). Very accurate ex- amination of numerous sections, as well as of teased-out specimens, shows that each Mal- Fic. 47.—Diagram of a single urinary tubule pighian capsule (Fig. 47; with its blood-vessels, to illustrate the 1c ee structure of the frog's kidney. Le Pp), is connected with a/.7, afferent vessel of glomerulus ; cf. capil- gq urinary e (1 Ul Jary network of kidney; ef7. efferent y tubule ( rt ), vessel of glomerulus; gé glomerulus; to which it forms a m.¢p. Malpighian capsule, showing epi- thelium ; zs¢. nephrostome showing cilia; blind bulb-like — end. ya. renal artery; 7 ff. 7 renal portal | , y ; vein; 77. renal vein; v7. ureter; w7.¢~., The tubule itself winds ur. tu ur. tu.” ur. tu’, different portions of urinary tubule, showing epithelium and through the substance of cilia. : metros the kidney, is joined by other tubules, and finally discharges into the ureter (v7). The tubules are lined with somewhat cubical cells of glandular epithehum, which, in some parts (a7. fv, v7. fi") IX EXCRETION 147 are ciliated. ‘The Malpighian capsules are lined with flattened cells of pavement epithelium. The arrangement of the bloodvessels is peculiar. Like other organs, the kidney is permeated by a network of capillaries (cf) 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 network is supplied partly by the renal arteries (7. a), partly by the renal portal vein (7.f7. v), and is drained by the renal veins (%v). It is in the behaviour of the renal arteries that the 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 capillaries (g/), the glomerulus. 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 (*. 2). Renal Excretion.— While circulating through the glomer- ulus, 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 nitrogenous waste matter wea, 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 in the glomerulus. In this way the wz7ze is formed. Accumulating in the tubules it makes its way into the ureter and thence drop by drop into the cloaca, whence it is either expelled at once, or stored for a time in the bladder. 14s THE FROG CHAP. Note that the formation of urine is a process of secretion of asimilar 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 asa virulent poison if retained in the system. It is therefore got rid of as soon as possible. Secretions of this kind, consisting 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 hemoglobin, 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, be- sides being organs of respiration, take their share in excretion, since they get rid of the important waste product, 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 take place between the outer air and the blood in the capillaries of the derm: the carbon dioxide of the non-aérated blood brought to the skin by the cutaneous artery (p. 93 and Fig. 23) is exchanged for oxygen, and the blood, in the the aérated 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 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 1X NUTRITION 149 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 bloodvessels, 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 substance which ultimately takes 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 tissues withdraw nutrient materials from the blood, whereby the waste of substance is made good, and the cells and other elements adequately nourished (constructive metabolism). 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 the tissues is associated. By the withdrawal of its oxygen the hemoglobin 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 capil- laries of an organ as bright red, aerated blood, and leaves it as purple, non-aérated 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 which nutrient matters and oxygen are absorbed 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, Zv/. C), the 150 THE FROG CHAP. products of digestion pass into the blood (Cp. Ent, C), or in the case of fats, first into the lacteals and ultimately 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 (C/. 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. Ka), 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 appearance 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 (Zp. Lut) pass in digested food, those of the skin (Z/. S4) and glomeruli (gdm) withdraw water, those of the urinary tubules (Zp. Ur. T), urea, and so on. Similarly, the various gland- cells, such as those of the liver (Zz. C), pancreas (Px. C), gastric, and cutaneous (Cv. G/) glands withdraw specific substances, or everetions, which arc discharged on the free surface of the epithchum and serve various purposes. We sce that the blood loses—(1), nutrient matters and oxygen all over the body; (2), water in the skin, lungs, and 1X NUTRITION 151 kidneys ; (3), carbon dioxide in the lungs and skin ; (4) urea, principally in the kidneys ; and (5), various substances in the glands. It gains (1), waste products all over the body ; (2), nutrient matters in the enteric canal ; (3), liver-sugar 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, Ze, in the pulmonary aud musculo- cutaneous veins; richest in nutriment as it leaves the enteric canal, 7.e., in the portal vein; poorest in urea as it leaves the kidneys, ¢.e., in the renal veins ; poorest in water as it leaves the skin and kidneys, #¢., 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. 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 152 THE FROG CHAP. temperature of the body more than very slightly above that of the surrounding medium. In warm-blooded animals, such as ourselves, the temperature 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. 11), may be looked upon as an ex- cessive 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 IL, 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. 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.—lin out a frog in the usual way (pp. 31 and 32), remove the heart, and make out the precise relations 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. Tlarden thoroughly in spiritand note (lig. 44) the Zayitgo-tracheal chamber, which communi- cates with the pharynx through the glotlis on the one hand, and on the other with both lungs. Observe also the posterior horns of the hyotd which embrace the glottis, and then separate them fron the larynge- PRACTICAL DIRECTIONS 153 3 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 /aryngeal muscles in connection with the laryngo-tracheal chamber: remove these, and pin the respiratory organs down under water, ventral surface 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 farasztes in the lungs—small worms called Ascaris nigrovenosa, belonging to the group of Nematode worms.) Note :— 1. The two arylenotd cartilages, and a ring-shaped cartilage sur- rounding the base of the lings. 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.—<. Examine these organs zw seéu (Figs. 3, 4, and 7) and note :— 1. Their form and position, and the relations of the peritoneum, which covers them on the ventral side only. (See Fig. 5). 2. The ureters (their openings into the cloaca may be scen at a later stage). 3. The yellowish adrenals. Sketch. 4. Examine under the microscope a transverse section of the kidney, prepared as directed on p. 136, and make out (Fig. 46) :-— 1. The wrznary tubules, cut through in various planes. 2. The Malpighian capsules and their glomerult. 3. Blood-capillaries and vessels. 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 NX. THE FROG (continued): THE NERVOUS SYSTEM. In a machine of human construction, such as the engine of a steamer, the proper working of the whole depends, pro- vided 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 vigorous 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 xercieus eapulses travelling from the brain or spinal cord, along the nerves, to to the muscles. It may therefore be inferred that the con- CH. X SPINAL CORD 155 trolling apparatus, by which the functions of the body are regulated, is lodged in either the brain or the spinal cord, or both. Divisions of the Nervous System.—The nervous system is divisible into (1) the central nervous system, consisting of the brain and spinal cord (Figs. 6 and 7), and (2) the fer?- pheral 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 (1) cerebral nerves (Fig. 53), arising from the brain, and (2) spinal nerves (Fig. 51) arising from the spinal cord. The Spinal Cord.—In form the spinal cord (Figs. 6 and 7, 5p. cd) is irregularly cylindrical. It is continuous in front with the brain, and tapers off posteriorily into a fine thread- like portion, the fidum terminale (ft), while opposite the fore-limbs, and just anteriorly to the filum terminale, it pre- sents a couple of enlargements, known respectively as the brachial and scfatic swellings. Along its dorsal surface runs a delicate longitudinal line, the dorsal fissure (Fig. 48, &/), and a distinct groove, the ventral fissure (vf), extends along its lower surface. The cord is covered with a delicate pigmented mem- brane known as the péa maver (p.w) and the neural canal in which it les is lined with a stout, tough membrane, the dura mater (Fig. 52, dm). Between the two is a cavity 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 extension inwards of the pia mater. The ventral fissure is a distinct cleft. “Thus the two fissures divide the cord into paired half cylinders, right and left, joined in the middle by a narrow bridge. This 156 THE FROG CHAP. bridge is traversed from end to end by a very narrow longi- tudinal canal, the central canal (c.c), 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 homogeneous, but is composed of two different substances. Its outer part is pure white and shining in the fresh cord, and is hence pil “fr Fic. 48.—Transverse section of spinal cord of frog. ce. c. central canal ; ¢_/ dorsal fissure ; @. 4. dorsal horn of grey matter’ d. 7. fibres of dorsal root of spinal nerve ; xv, c. nerve cells of dorsal horn; #zv.c’. nerve cells of ventral horn ; 4. w. pia mater ; v. / ventral fissure ; v. 4. ventral horn of grey matter ; v.”, fibres of ventral root of spinal nerve; zw. #. white matter. (After Howes.) called the wife matter (w.m). Its internal substance has a pinkish colour when fresh, and is called the grey matter (ah, v.A). The grey matter has a squarish outline in trans- verse section. It surrounds the central canal, and is con- tinued upwards and downwards, forming what are called the dorsal (d.h) and ventral (v.) horns of the grey matter. The Brain.—In front the spinal cord passes insensibly into the brain (Fig. 49), which is of somewhat greater diameter than the cord, and is made up of several very dis- x BRAIN 157 tinct parts or divisions. The hindermost division is called the éulb, or medulla oblongata (Med. obl) ; this appears to be simply a widening of the spinal cord (Sf. cd), except that on its dorsal surface is a triangular body (D, ca. pix?) of a reddish colour in the fresh condition, and called the jos- terior 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, v*) 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, v*, cc), and the 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, Cd), 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 /obes (Figs. 49 and 50, Oft./). Each contains a cavity, the optic ventricle (Fig. 50, Of¢. v), communicating with a narrow median passage, the 7rer (Figs. 49 D and 50, 7), 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 structure, the diencephalon or "tween brain (Di). On its upper surface is a small rounded vascular body, the antertor choroid plexus (Fig. 49 A, D, ch. p/x}), formed, like the posterior choroid PtH” py | Cer ia ope HOES Ch Ch ple.2 ~ Fic. 49.—Brain of frog. A, from above ; B, from below; C, from the side; D, in longitudinal vertical section. Co. cerebellum; Ce. //. cerebral hemispheres ; ch. f/x. anterior and ch. plx.2 posterior choroid plexus ; com. transverse bands of nerve fibres or commissures CH: XS connecting the left and right halves of the brain; C7. BRAIN 159 C. crura cerebri; D7, diencephalon ; for. A/. foramen of Monro ; ; 2%, iter, or aqueduct or Sylvius } inf. infundibulum ; Jed. o0d/. optic chiasma; Of7."2. optic lobe ; body , ventricle ; medulla oblongata ; SP., first ON 2. olfactory lobe; off. ch. opt. 7. optic ventricle ; pi. stalk of pineal pit. pituitary hody ; SA. cd. spinal cord 3 I—X, cerebral nerves; 7 SA. 2 . third ventricle ; 4. fourth and second spinal nerves 5 (A—C, after Gaupp; D, from Wiedersheim: after Osborn). plexus, of a thickening of pia mater, containing numerous blood-vessels. cavity, the ‘third ventricle (Figs. 49 D and 50, v), the sides of which are formed by thickenings of nervous matter, the optic thalami (Di). On the ventral surface of the brain the diencephalon is continued into a funnel-like extension, the zafundibulum (Fig. 49,7), to which is attached a rounded structure, the piturtary body ( pit). On the dorsal surface, just behind the choroid plexus, is the delicate stalk (p77) of the pineal body—the vestige of a sensory apparatus (Fig. 148 C), part of which in some lizards, for example, has the structure of an eye, and which was probably functional in the ancestors of the frog. We shall meet with other examples of such vestigial organs in the course of our studies. Vic. ¥¢ It helps to roof over a narrow’ slit-like ! Ned ob! Sp.ca 50.—Diagrammatic horizontal section of frog's brain. c. central canal ; Ce7. /7. cerebral hemisphere ; D7. diencephalon ; for. M. foramen of Monro ; 2. iter ; Lat.v. \ateral ventricle ; Med. ob2. medulla oblongata; V7. s, olfactory nerve ; O// L. olfactory lobe ; Of 7. olfactory ventricle ; Opt, 2 optic lobe; Opt. v. optic ventricle ; Sf. cd. spinal cord ; a. 3, third ventricle; 7. 4, fourth ventricle. (After Ecker and Wie- dersheim.) 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 160 THE FROG CHAP. lateral ventricle (Fig. 50, Lat. 7), which communicates with the third ventricle by a small aperture, the foramen of Monro (for. AL). Lastly, each cerebral hemisphere is continued forwards by a rounded offactory lobe (O7f. 1), which is fused with its fellow of the opposite side, the single mass lying in the posterior compartment of the girdle bone. The lateral ventricles are continued forwards into the olfactory lobes, forming the small olfactory ventricles (Fig. 50, O/f. v). The brain, like the spinal cord, is formed of grey and white matter, but their relations are different. In the olfac- tory 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, and pass out at the neural canal through the intervertebral foramina (p. 38). There are altogether ten pairs of spinal nerves in the adult frog (Fig. 51, Z—X), each of which on leaving the neural canal divides into a smaller dorsal and a larger ventral branch (Fig. 53, 1 sf—-5 sf). The first pair leave the cord through the intervertebral foramina between the first and second vertebra. They pass at first directly outwards, the large ventral branch of each, known as the Avpoglossal, turning forwards, and going to the muscles of the tongue (Fig. 51, Z, Fig. 53, 1 Sp). The second pair (Fig. 51, 77) are very large ; they emerge x SPINAL NERVES 161 between the second and third vertebre, and each is soon joined by the small third nerve (Z/Z7) which emerges between the third and fourth vertebre, as well as by a small branch or two from the first, thus forming a simple network or plexus —the brachial plexus (br. p/), 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 (ZV) emerges between the fourth and fifth vertebra, the fifth (V) between the fifth and sixth, and the sixth (V7) between the sixth and seventh. They all pass obliquely backwards, and supply the walls of the body, being distributed to both skin and muscles. The seventh, eighth, and ninth nerves supply the muscles and skin of the hind limbs. The seventh (VIZ) leaves the neural canal between the seventh PRACT. ZOOL, a ia cm. “fy \ SOP... ' DAee ie ii Al yc OPT AM PRAT! g ys ae See Fic. 51.—The ventral branches of the spinal nerves and the sympathetic of the frog, from below: shown on the right side only. J—X, spinal nerves ; 10. systemic arch ; br. pl. brachial plexus ; C. calcareous bodies which surround the spinal ganglia; D. Ao. dorsal aorta; fem. femoral nerve ;-/¢. A. iliac artery 3 scz. sciatic nerve; scz. ZZ. sciatic plexus ; S&, skull; Sf. A. splanchnic artery ; Sy. sympathetic cord ; Sy. c. commun.- cating branches between the sympa- thetic and spinal nerves; Sy. g. sympathetic ganglia; Us¢. urostyle ; V-—V®. centra of vertebra ; Vg. vagus nerve, with its ganglion. Gaupp. slightly modified.) M (After 162 THE FROG CHAP. and eighth vertebra, the eighth (77/77) between the eighth and ninth, and the ninth (ZX) between the ninth vertebra and the urostyle. They all pass almost directly backwards, and are united with one another by oblique cross branches so as to form the sciatic plexus (set. pl), from which are given off, amongst others, two nerves to the leg, the largest of which, the sefatic nerve (Sc’) being that already mentioned in the chapter on the muscular system. The tenth (X) is a very small nerve. It emerges through a small aperture inthe side of the urostyle, and supplies the cloaca, urinary bladder, and adjacent parts. It is con- nected by cross branches with the ninth. It will be noticed that while the large ventral part 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 mo/for 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 swell- lings called gaxg/ia, each of which is connected with a spinal nerve by acommunicating branch (Figs. 51 and 53 Sy, Sy. g, Sy.c). In front of the point where the dorsal aorta (D. 4o) is formed by the union of the two systemic trunks (4o), these two sympathetic nerves, as they are called, are con- tinued 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. 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, liver, kidneys, reproductive organs, rectum, and urinary bladder. x SPINAL AND CEREBRAL NERVES 163 Origin of the Spinal Nerves.—The mode of origin of the nerves from the spinal cord is peculiar and characteristic. Traced towards the cord, each nerve is found, on reaching the intervertebral foramen from which it 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, a7, vr). The dorsal root is distinguished from the ventral by being dilated into a ganglion (gv). In Fig. 51 these ganglia lie hidden within certain calcareous bodies (C) in this region. Fic. 52.—Transverse section through the vertebral column and spinal cord, to show the mode of origin of the spinal nerves. c.c. central canal ; cz. centrum ; @./ dorsal fissure ; @. .'dura mater ; d. r. dorsal Toot ; g 12, grey matter ; g7. ‘ganglion of dorsal root 3 7. a. neural arch ; 7. sp. neural spine ; 2. 172, pia mater (the reference line should stop at the margin of the cord); #. nerve trunk; 7». fv. transverse process; 7. ventral fissure ; v.r. ventral root ; w. 72. white matter. (After Howes.) Cerebral Nerves.—There are ten pairs of cerebral nerves some of which are purely sensory, some purely motor, some mixed. The frst or olfactory nerves (Fig. 49, 7) arise from the olfactory lobes, and pass through the holes in the trans- 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 (Fig. 49 Z/) is a large nerve which springs from the ventral surface of the ’tween-brain. At M 2 164 THE FROG CHAD. 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 eyeball, and is, as we shall see, the actual organ of sight. This nerve also is purely sensory. The ¢hird or oculomotor (Fig. 49, 2/7) is a small nerve arising from the crura cerebri beneath the optic lobes. It passes through a small hole in the cartilaginous side of the skull and supplies four out of the six muscles by which the eyeball is moved, and is purely motor. The fourth or pathetic (£V) 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 just above the optic 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, /) is a large nerve arising from the side of the medulla oblongata. Its root dilates to form a large 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, ’'), going to the skin of the snout, another, the »acxr/ary (V7?) to the upper lip and lower eyelid, and the third or mandibular (V%) to the muscles and skin of the lower jaw. ‘The trigeminal is a mixed nerve. The sexth or abducent (Fig. 49, PZ) is a very small motor nerve arising from the ventral aspect of the bulb and supplying onc of the muscles of the eyeball called the posterior rectus. x CEREBRAL NERVES 165 The seventh or facial nerve (Figs. 49 and 53, VZ/) arises just behind the fifth and soon joins the Gasserian ganglion. Both it and the sixth leave the skull by the same aperture v WB x 1p oe 2D ssn SP S$ . Se) Fic. 53.—Dissection of the head and anterior part 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. Ao, systemic arch ; 67. f/. brachial plexus; D. to. dorsal aorta; dv. duodenum ; Hf, heart ; Hy. body of hyoid ; 4y1. anterior, and //y2. posterior horns of hyoid ; L. lung; M. olfactory sac ; Ox. orbit ; Pd. pulmonary artery ; Sf. A. splanchnic artery ; Sf. stomach ; Sy. sympathetic ; //. cut end of optic nerve ; //!. ophthal- mic, 2, maxillary, and V3, mandibular branch of trigeminal; V//1. palatine, and V//2. hyomandibular_ branch of facial; /Y. glossopharyngeal ; A’. vagus ; Xed. cardiac, Xgas. gastric, X/ar. laryngeal, and AZw/. pulmonary branch of vagus ; / sp. first spinal nerve (hypoglossal); 2s5f.—5 sf. second to fifth spinal nerves. (After Howes, slightly modified). as the fifth. It divides into two branches, one of which, the palatine (Fig. 53, VZZ!) supplies the mucous membrane of the roof of the mouth, and the other, or hyomandibular (V7Z*) sends a branch to the skin and muscles of the 166 THE FROG CHAP. lower jaw and to the muscles of the hyoid. It is a mixed nerve. The efghth or auditory nerve (Figs. 10 and 49, VZZ7) arises from the medulla just behind the seventh, passes through an aperture in the inner wall of the auditory cap- sule, and is distributed to the auditory organ or membranous labyrinth (see Chapter XI). It is the nerve of hearing, and is purely sensory. The xinth or glossopharyngeal (Figs. 49 and 53, 7X ) 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 fenth 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 (W/ar), the heart (Xed), the lungs (X fu/), and the stomach (Vyas), and is therefore often known as the prewsmo- gastric. 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. Microscopic Structure of Nervous Tissue.— Examination of a picce of nerve under the microscope shows it to be od NERVE-FIBRES AND CELLS 167 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, A) is a cylindrical cord in which three parts can be distinguished. Running along the axis of the fibre is a delicate protoplasmic strand, the xeuraxis or axtsfibre (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, structure- less membrane, the xeurz/emma (ne). At intervals the medullary sheath is absent, and a ode is produced, where the fibre consists simply of the neuraxis covered by the neuri- lemma. Directly beneath the neurilemma nuclei are found at intervals. In the ganglia are found, not only nerve-fibres, but nerve-celis (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 may be united at their base. One of these processes is continuous with the neuraxis of a nerve-fibre ; the other 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 white matter of the brain and spinal cord consists of nerve-fibres, those in the cord having a longitudinal direction ; the grey matter contains numerous much-branched (“ multi- } The medullary sheath may be absent in certain nerve-fibres (e.g., in the sympathetic and olfactory nerves.) 168 THE FROG CHAP. polar”) nerve-cells (Figs. 48 and 54, A), each continuous with a neuraxis and enclosed in a tissue formed partly Tn. Miu. 54.—.\, nerve-cell from the grey matter of the spinal cord of a frog, and the nerve-fibre arising from it; Bb, cell from the ganglion of a dorsal root. ue, neurilemma; v7. nucleus; 7. neuraxis; 97. s. medullary sheath. (After Howes.) of the axis-fibres of nerves which enter the grey from the white matter, losing their medullary sheath as they do so, X REFLEX ACTION 169 and partly of a delicate fibro-cellular tissue called nxewrogdia, 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 first chloroformed, and then either decapitated or pithed, z.e., the medulla oblongata is severed and the brain destroyed (p. 103): there can thus be 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, 7.e., contain both sensory and motor fibres. 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 passing into the cord by the dorsal root, the motor by the ventral root. As a consequence of 170 THE FROG CHAP, this, if the dorsal root be cut and its proximal or central end—z.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 peripheral end—be stimulated, there is no result. On the other hand, if the ventral root be cut and its distal end stimulated, the Fic. 55.—Diagram illustrating the paths taken by the nervous impulses. c.c. central canal; cod. collaterals ; c. covt. cell in rind or cortex of the cerebral hemisphere ; c. g. smaller cerebral cell; @.c. cells in dorsal horn of grey matter ; d.>. dorsal root ”. ganglion of dorsal root ; g.¢. cell in ganglion of dorsal root ; gum. srey matter; M. muscle; 7. ¢. cell in medulla oblongata ; 7. motor fibre; S skin; sensory fibre ; sfc. spinal cord ; 7. ¢. cells in ventral horn of grey matter; 7. ». ventral root ; ww. 2. white matter. The arrows indicate the direction of the impulses. muscles supplied by it will contract, while stimulation of the proximal end produces no result. Very accurate observations have shown that the connec- tion between the motor and sensory fibres is as follows (Fig. 55). .\ motor fibre (77,/) is traceable from the nerve-trunk through the ventral root (v.7) into the white matter; and then, its medullary sheath being lost, passes into the ventral horn of the grey matter, its neuraxis being directly con- x REFLEX ACTION 171 tinuous 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 (4.7); 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, enter the dorsal horn of the grey matter and branch out into a complex series of fine fibres which interlace with the similar arborisations of the nerve-cells (p. 167). The path of a nervous impulse will now be obvious. The stimulus applied to the skin (Fig. 55, S) is conducted by a sensory fibre to the nerve-trunk and by the dorsal root to the spinal cord; it then passes 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 after passing through the cells of the dorsal horn: from one of the cells of the ventral horn it is con- ducted by an axis-fibre process continuous with the neur- axis of a nerve-fibre, which, leaving the cord by a ventral root, passes 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 the skin may result in the contraction of numerous muscles—as, é.g., when the application of a drop of acid to the toe causes the lifting of the leg, and that the movements are of such a nature as to withdraw the part stimulated from the irritating substance. Moreover, as shown by the experiment of apply- ing acid to various parts of the body, the movements are 172 THE FROG CHAP, vatied according to circumstances ; if one leg is prevented from rubbing off the irritating substance, the other imme- diately 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 nerve 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, how- ever, the spinal cord of a brainless frog is quite inactive, and the body of the animal will remain without movement until it dries up or decomposes. In the uninjured frog, #.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 in- evitable movements and no others; but a single stimulus may produce varied movements, the nature and direction of which cannot be predicted ; the animal will probably give a series of leaps, but the number and extent of these varies according to circumstances. 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 communication— 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, 7.c, ¢.g, ¢cort). In certain of these brain-cells (¢.cov/), voluntary 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 pre- dicted. xX REFLEX AND VOLUNTARY ACTION 174 Moreover, it can be shown by experiment that the process of originating voluntary impulses is not performed 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, ex- hibiting 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 sits up if placed on its back, and swims if placed in water. If left alone, however, it remains in the sitting posture until it dies. Hence the bulb and cerebellum are evidently concerned with the co-ordination of muscular movements, but have no power of originating impulses. If the optic lobes as well as the medulla oblongata and cere- bellum are left, the animal is affected by light, is able to per- form complex balancing movements, 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 an 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 levers, valves, &c., acting automatically ; but it is only by his controlling intel- ligence that the whole mechanism is adapted to the circum- stances of the moment. 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. 174 THE FROG CHAP. 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 when removed from the body. This is due to the fact that the muscles of the heart, like the cilia of ciliated epithelium, have the power of contracting rhythmically quite independently of the nervous system, although the 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, however, under the control of the central nervous system. We have seen that it is supplied by a branch of the vagus; when this is stimulated, the heart stops in the dilated state and begin to beat again only after a certain interval. A feebler 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 accom- panied by a branch of the sympathetic which has an exactly opposite effect ; z.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 muscular 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 a result, the arteries are ordinarily in a state 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 js distributed will x AFFERENT AND EFFERENT NERVES 175 be temporarily increased. For instance, the presence of food in the stomach acts, through the central nervous system, upon the cceliac branch of the splanchnic artery, causing a dilatation of all its capillaries and promoting an increased secretion of gastric juice. The secretion of other glands is regulated in the same way. In some cases, how- ever, 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 impulses to the central nervous system which have nothing to do with sensation, and fibres carrying impulses 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 to 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. PRACTICAL DIRECTIONS. I. The central nervous system (Fig. 6). Lay bare the brain and spinal cord as directed on p. 33, noting the dura mater and pia mater: the latter is densely pigmented over parts of the brain. The specimen in which this operation has already been performed will do, if the dissection has been done carefully. Observe the origins of the cerebral and spinal nerves, noting the long dorsal and ventral roots of the latter (compare 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 ata 176 THE FROG CHAP. 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. Noteits cylindrical form, the érachtal and sezat7c swellings, the hilum 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 whzte 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 2/6 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 ventiicle. 2. The small ledge-like cerebellum. 3. The two rounded oftzc Jodes, and the crura cerebri beneath them. 4. The dencephalon, formed of a right and left optic thalamus. On its dorsal side is the atertor chorotd plexus, roofing in the third ventricle; and on its ventral side the zxfundrbulum, to which the pituitary body is attached ; and, more anteriorly, the optic chiasma. 5. The cerebral hemtspheres, continuous in front, with 6. The olfactory lobes, which are fused together 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 /ateral ventricle and the optte ventrécle (Fig. 50). Then with a sharp scalpel divide the whole brain into two by a longitudinal vertical cut very slightly to the left of the middle line, so as to reduce it toa longitudinal section (Fig. 49, D). Examine the cut surface of the right side under water, and make out as much as possible of the relations of the ventricles of the brain :—*¥%z., the fourth ventricle, the ter and oplte ventricle, and the third ventricle, which communicates with the daleral ventricle through the foramen of Monroe, Sketch. x PRACTICAL DIRECTIONS 177 MI. 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 the systemic trunk and 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 sp7a/ nerves passing outwards from the vertebral column on either side, and the calcareous boddes close to their points of exil, 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 p. 446. 6. 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 sympathetzc nerie- cord, covered by pigmented connective-tissue. Carefully dissect the cord away from the aorta, and note the gasglza and the branches (rami communicanies) connecting them with the spinal nerves. Sketch the spinal nerves and sympathetic. c. The cerebral nerves (Fig. 53.) The dissection of these in the frog is not an easy task for a beginner, and directions for their examination in larger animals will be given in subsequent chapters. The origin of some of them 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 (Fig. 48) under the high power of the microscope, and observe— 1. The nerve-cells, present in the grey matter only (compare 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 trans- versely as well as in other directions, and cach showing a deeply- stained central vewrax7s. Sketch. Pracr. Zoot. N 178 THE FROG CH. X b, Tease up a fresh spinal (Fig. §4, B) or sympathetic ganglion in salt- solution, and stain with methyl-green. Compare the form of the nerve- cells with those in the spinal cord, Sketch. «. Cut off a very small piece of any fresh nerve (e.g., sciatic), and tease it out carefully, 2 @ longitudinal directton, in salt solution, Note that it is made up of cylindrical, unbranched zerve-fibres, bound together by connective tissue. Examine a single fibre under the high power (Fig. 54, A), and make out the rezrelemma, the medullary sheath, and the nodes: at the nodes, the zewrax7zs can also be seen. Sketch. Tease out another piece of fresh nerve in chloroform, so as to partially dissolve the medullary sheath, and note the central zewvaxzs. 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 neurilemma, with its underlying selec, 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 con- nection 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 possessor, 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 unsuitable food, darkness from light, heat from cold. The avenues of communication between the animal and its surroundings are, as in ourselves, the sexses of touch, taste, smell, sight, and hearing. The sense of touch, including that of ‘emperature, 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 ¢actile-cedls—large flattened cells arranged in groups just below the epiderm and around which the ultimate fibres of the sensory nerves are distributed. Stimuli applied to the skin, either by direct touch or by the heat of the sun, N 2 180 THE FROG CHAP. 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 in the tongue and in the neighbour- hood of the vomerine teeth, but extending also as far back as the gullet. Certain of 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 papille of the mucous membrane. In this case the stimulus is supplied, not by direct touch or by alteration of tempera- ture, 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 mem- brane of the mouth is injured, or if the glossopharyngeal and palatine nerves are cut, or if the cerebral hemispheres are destroyed. The sense of smell is lodged in the xasa/ or olfactory sacs, which are enclosed in the olfactory capsules of the skull and separated from one another by a partition, the zasa/ septum. Each sac has two apertures, the externad nostril, opening on the surface of the snout, and the internal nostril, opening into the mouth (p. 17). The sacs are lined by a XI NOSE AND EYE 181 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 nerve are connected, and they are distinguished as olfactory cells (Fig. 56). As the epithelial cells of the skin are af- fected 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 nerve to the brain, gives rise to the sense of smell. This sense can be destroyed, as in the case of oe feeling and taste—either by destruction of the olfactory mucous membrane, or by cutting the olfactory nerves, or by de- stroying 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 /'%,5°s¢Epithelial text-books of Physiology, and it will only = jyomgpossmum be necessary to give a brief outline here. Phin. Each eye (compare Fig. 57) isa nearly — cells; & olfactory interstitial cells. (From Wie- globular organ, and when removed from _ getsheim’s Verte- : : : rata.) the orbit and cleaned by dissecting away the attached muscles, &c., 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, some- what flattened portion which is directed outwards and freely exposed between the eyelids in the living frog. The outer 182 THE FROG CHAP, coat of the concealed portion of the eyeball is the sclerotic (Sc/), and is formed of cartilage in the frog ; its 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 here- after. Entering the sclerotic on its inner side, z.¢., the side next the braincase, will be seen the cut end of the optic Fic. 57.—Diagrammatic horizontal section of the human eye. c. cornea ; CA. choroid (ately C.P. radiating folds of the choroid, known as the ciliary processes ; ¢.¢, e. cf, conjunctiva; /. iris; LZ. lens; O.M. optic nerve 3 os. ora serrata, a wavy line forming the boundary of the visual portion of the retina; f.c. R. anterior non-visual portion of retina; PF. pigmented epithe- lium (black); #. retina; Scé. sclerotic ; SA/. suspensory ligament of lens; V. H. vitreous chamber. (From Foster and Shore's Physiology.) nerve (O.V). The transparent, exposed portion of the eyeball is the cornea (c), a superficial thin layer of which, or conjunctiva (e.c., €. ¢.), 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 évis (Z), with a black spot—really a hole —in its centre, the pupil. XI EYE 183 The interior of the globe (/ #) is filled with a colour- less, transparent jelly, the wtreous 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 ve/va (Rk). Between the retina and the sclerotic is a membrane called the choroid (Ch), the inner face of which, z.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 is the inner- most layer of 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 (d//vd spot) it be- comes 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 blood- vessels as well as pigment-cells. Lying just internal to the pupil is a nearly globular body, perfectly transparent when fresh, the crystalline lens (L) ; 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 aguweous chamber of the eye, which contains a watery fluid, the agueous humour. The main cavity of the eyeball, containing the vitreous humour, is called the vitreous chamber (V. 1). The actual relations of these parts in the entire eye are best grasped in a vertical section, such as is represented in Fig. 57. The main part of the eyeball forms a chamber, enclosed by the sclerotic, darkened internally by the choroid, 184 THE FROG CHAP. 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 in front, and the retina behind and at the sides, 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 lens can be made more or less convex ; in this way the focus of the entire apparatus can be altered according to whether the object viewed is nearer or farther from the eye. This arrangement for accommodation 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 (Fig. 57). 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, “e., on the retina. A vertical section of the retina (Fig. 58) reveals a very complex structure. On its imner face, ze, the surface in contact with the vitreous humour, is a layer of “nerve-fibres (7.f.), formed by the ramifications of the optic nerve, which, XI RETINA 185 passing through the sclerotic and choroid, perforate the retina, and spreads out over its inner surface. Next comes a layer of nerve-cells (g), and then several layers of fibres and nuclei (gv, zc) ; and finally, forming the outer surface of the retina proper, is a layer of delicate, transparent bodies called, from their form, the vods (rv) 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 contact 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 rh often counted as part of the choroid.| an te In spite of its complex structure the retina is not much more than 1th mm. (qigth inch) thick, and is perfectly yg. 5.—Vertical section 3 a f frog's retina. transparent. Heénce, when an image on eerie layer ae nerve-cells; g7, g?". outer and inner granu- lar layers; 2c, 2c.’outer and inner nuclear ee eencae oe ee eee is formed on it, the rays of light easily penetrate its whole thickness until they are stopped by the opaque layer ee een of pigment. The rays are thus en- pete Hwee rou abled to stimulate the rods and cones, and the stimulus is transmitted through the layers of nuclei and nerve-cells to the fibres of the optic nerve, along which it is conveyed 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. 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 cutting 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 sight 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. Asa matter of fact, the animal can direct its gaze through a very wide range by means of eight muscles attached to 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 refractor bulbr, withdraws it, causing it to bulge into the mouth. Four others (compare p. 164 and Fig. 117), the superior, inferior, anterior and posterior vec/’, rotate it respectively upwards, downwards, forwards, and backwards. And finally, two oddigue muscles, the superior and inferior, produce a rotation along an axis joining the optic nerve with the middle of the cornea. The conjunctiva, which covers the outer side of the eye and lines the eyelids, is kept moist by the secretion of a lachrymal gland, known as the Harderian gland, situated between the eyeball and the orbit in the antero-ventral aspect. The excess of this secretion is carried away into the olfactory chamber by means of a tube, the saso- lachrymal duct. Each organ of hearing, like that of sight, consists of an essential portion and an accessory apparatus. The essential organ of hearing is a structure called the membranous laby- vinth, contained within the auditory capsule of the skull (Hig. 10), and consisting of a kind of bag of very peculiar XI EAR 187 and complicated form (Fig. 59). It is made up, in the first place, of two somewhat ovoid sacs separated by a constriction : the dorsal one is called the wfriculus (w), the ventral the saccu/us (s), and from the latter a small process, the cochlea (/), projects backwards, and a narrow canal, the endolymphatic duct (d.c), upwards. With the utriculus are Fic. 59.—External view of left organ of hearing of a Vertebrate (semi-diagrammatic). aa, ampulla of anterior canal, ae. of external canal, and af. of posterior canal ; ass. apex of utriculus ; ca. anterior semicircular canal; ce. horizontal canal ; cp. posterior canal ; czs. constriction between sacculus and utriculus ; dv, se. endo- lymphatic duct; 2 cochlea; vec, sf, ss, uv. utriculus; s. sacculus. (From Wiedersheim's Vertebrata.) connected three tubes, called, from their form, the sevzz- circular canals, each of which opens into the utriculus at either end. One of them, the anterior canal (ca), is directed forwards ; another, the posterior canal (cp), back- wards ; both these are vertical in position and are united to one another at their adjacent ends. ‘The third, the 188 THE FROG CHAP. external canal (ce), is directed outwards and has a hori- zontal position. Each canal has one end dilated into a bulb-like swelling or ampulla (aa, ap, ae); those of the anterior and external canals are anterior in position, while that of the posterior canal is posterior. 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 tissue and Fig. 60.—Longitudinal section through an ampulla. a. e. auditory epithelium; @. 4. auditory hairs ; ¢c. part of semicircular canal; 7. acoustic spot and ridge; cf. connective tissue; ¢. epithelium; 2. nerve; w, junction with utriculus. (From Foster and Shore’s Physiology.) 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 sacculus. On these acoustic spots the epithelial cells are greatly elongated and produced at the surface into delicate processes called auditory hairs : to these cells 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 contained ; but a space is left between it and the surrounding bone and cartilage (Fig. 10). This space is filled by 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, fez. ov), in which is fixed the sages (stp), a small nodule of cartilage connected with a bony rod or columella (Figs. 9 and 10, col), the cartilaginous hammer-shaped outer end of which, or extra-columella, is fixed to the inner side of the zympanic membrane (Fig. 9, tymp. mem’). 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 connective tissue ; while below it communicates with the mouth by the Eustachian tube (eus. ¢). When sound-waves impinge on the tympanic membrane, the vibrations to which they give rise are transmitted 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 certain that the semicircular canals are really organs for 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. Notice that the general plan of all the sensory organs, 190 THE FROG CHAP. those of the skin, eye, and ear, is the same. They consist of certain peculiarly modified epithelial cells, specially sensitive to impulses of particular kinds, and in communication, by means of an afferent ncrve, 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 (compare 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 controlled, 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 o/factory sacs. Note the pigmented olfactory epithelium lining these, and make out the o/factory nerves and nasal septum. Sketch. Il. Aye. a. Notice again the eye/éds, 277s and pupz7. Then remove the skin covering the head so as to expose the nearly globular eyedal/s, lying in the ordzts. In the antero-ventral angle of the orbit make out the Harderian gland, and the eye-muscles passing from the walls of the orbit to the eye-ball. The four vrecte and two oblique muscles can be more easily seen on a larger animal, and directions will be given for their examination in Part JI.; but if you make a dissection of them in the frog, you should note at the same time the /evatfor and rectractor bulbz, the latter underlying the eyeball, and the former situated internally to the recti muscles. = 6, Remove the eyeball from a freshly-killed specimen, noticing as you do so the oftze nerve, which is surrounded by the rectiand retractor bulbi muscles: dissect away these ‘muscles and note the cartilaginous sclerotic, the cornea, tris, pupil, and the cut end of the optic nerve. c. Divide the eyeball into an inner and an outer hemisphere by a rapid cut with scalpel or scissors taken vertically, midway between the cornea and the optic nerve, through the wztreous 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 vétreous humour, retina, pigmented chorord, and blind spot ox entrance of the optic nerve ; and in the outer hemisphere, the crystalline Jens and the margin of the retina, or ora serrata. Sketch. Remove the lens, and notice the z7zs, continuous with the choroid, the fupz/, and the agueous chamber. d. Examine sections through the wall of the inner hemisphere of the eyeball, prepared as directed on p. 136, first under the low, and then under the high-power of the microscope. Note :— 1. The cartilaginous selerotzc. 192 THE FROG CHAP. XI 2. The choroid, enclosing pigment-cells and blood-vessels. 3. The rveténa (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 IT. Ill. Auditory organ. Notice again the ¢ympante membrane and tympani 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 essentzal part of the auditory organ (membranous labyrinth) is very small in the frog, and can be more satisfactorily studied in a good- sized fish (e.g. Dogfish or Cod). Directions for the preparation of the membranous labyrinth of the Dogfish will be given at a later stage, but if you have sufficient time and patience to dissect it out in the frog, proceed as follows :— Place the head ofa 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 exposed. 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 wfrécules, sacculus, ololeths, and the three semzezrcular canals with their ampulle. 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 individual. 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 gozads, called in the male spermaries or testes, and in the female ovaries. Reproductive Organs of the Male.—The spermaries (Fig. 3, 7. spy, Fig. 5, spy, and Fig. 7, 4s) are a pair of ovoid bodies, each attached bya fold of the peritoneum to the cor- responding kidney, and having connected 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, @) which run in the fold of the peritoneum to the kidney. Entering this organ near its inner edge, they open into a longitudinal tube (Z) from which transverse tubes pass hori- zontally across the kidney to open into the ureter (U7). The milt, or spermatic fluid (p. 9) is thus carried off by the same duct as the urine ; the ureter is therefore often called the Pract. Zoou. fa) 194 THE FROG CHAP. urinogenital duct, On the outer side of the ureter, and com- municating with it by numerous short ducts, is a glandular body, the semna/ vesicle (Figs. 3 and 7, vs. sa), which serves to store up the spermatic fluid. The spermary itself contains a narrow, irregular, central cavity, from which the efferent ducts proceed and _ into which open a number of short tubes or cr1p/s (Fig. 62, A). g¢ showing the relations of the cfferent ducts (semidiagramma tic). . transverse tubes in kidney ; //o. spermary ; Z. longitudinal tube ; A. kidney ; a me efferent ducts of spermary; (7. urcter (urinegenital duct). (From Wieder- sheim’s sInatoniy.) Pic. 61.—Spermary and kidney of fre These are lined with epithelium (4 ¢), the cells of which divide and subdivide, forming groups of smaller cells. Each of the latter undergoes a remarkable change, becoming con- verted 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 spermatozoa (Tig. 62, 7A, sf, and B); in. spite of XII REPRODUCTIVE ORGANS 19§ 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. Fic. 62.—A, transverse section of a crypt of the spermary. 5, stages in the development of the sperms Sp. bundles of sperms; 4e. germinal epithelium. (A, after Blomfield; B, after Howes). Reproductive Organs of the Female.—Each ovary (Fig. 4, 2 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. 23), its surface is studded all over with little rounded projections, each of which is an vvsac, and contains anegg. The egg or ovum (Fig. 63) isa large globular cell with a clear nucleus (7) containing numerous nucleoli (x), and having its protoplasm (fr) full of vo/k-granules— grains of proteid material which serve as nutriment for the growing 0 2 196 THE FROG CHAP. embryo. It is covered with a delicate membrane, the w7/e/- Line 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 (v) enlarge, and give rise to the 6 y . Ths a SS AG Fic. 63 —Transverse section of frog's ovary. .v, blood vessels ; ¢. 4. connective tissue ; ef. outer layer, and ¢f', inner layer of epithelium ; ¢”. outer layer of ovisac, continuous with ef’; ef". germinal epi- thelium, derived from ¢/; ef". follicular epithelium, derived from ef 3 m0. nucleus of ovum; wv’, nucleoli; e. young ovum; #7. protoplasm of ovum containing yolk-granules. (After Marshall.) > ova, while others form an investment or ‘%/licle (ep"”) for each ovum. The oviduct (Fig. 4, 7 ovd), as you have seen, is a long and greatly convoluted tube lying above or dorsal to the ovary. Its anterior end narrows considerably, runs parallel with the gullet, passes to the outside of the root of the lung, and then opens into the ceelome by a small aper- ture (x. ovd!). ‘The greater part of the oviduct is about as wide as the small intestine, and is thick-walled and lined XII FERTILIZATION 197 with gland-cells, which secrete the jelly (p. 9) surround- ing the eggs when laid. Posteriorly it suddenly dilates into a wide, thin-walled chamber (7. 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 ccelome, 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 its little sphere of jelly secreted by the gland-cells. Passing down the oviducts the eggs accumulate in the dilated extremities, which they distend enormously, so that just before laying, the abdomen of a female frog is nearly filled with 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. Fertilization —The eggs are now laid, and immediately the spawn is passed from the oviducts of the female into the water, the male sheds over them a quantity of spermatic fluid (p. 9). The sperms, swimming actively through the water, enter the spheres of jelly and come into contact with the eggs. A single sperm then penetrates the vitelline 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 zygole. This process is known as fertdisativon or impregnation. 198 THE FROG CHAP. XII Without it, as we have seen, the egg is incapable of develop- ment; after it has taken place, the egg-—or more strictly, the vesperm, since it is now an ovum flus 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 fertilized 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 by the naked eye. The yolk is not equally distributed : on one hemisphere it is less abundant than elsewhere, and it is this more protoplasmic hemisphere which is super- ficially 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 fertilized, the eye undergoes a remarkable change. .\ furrow appears all around it, as if made with a blunt instrument, and deepening gradually, at last divides the oosperm into two hemispheres in contact with one another by their flat faces (Fig. 64, .\). ‘he examination of sections shows that this process is preceded by the division of the nucleus into two; its final result is the separation of the originally one-celled cee into two cells, Now if you refer to Chapter VITI, you will be reminded of the fact that the epithelial cells of the skin multiply by a similar process of s¢mple fission, or division into two; the nucleus in cach case dividing first and afterwards the protoplasm. Fic. 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. él. cal. segmentation cavity ; 42, 42%’. blastopore ; 471, 47%. gills ; 7. c7. branchial arches ; ¢. eye; ect. ectoderm ; exd. endoderm ; ext. enteron ; 4 47. fore-brain ; h. br. hhind-brain ; 72.6 . mid-brain ; wd. medullary fold ; sd. 7. medullary groove; wes. mesoderm; mg. large lower cells; mz. small upper cells; uch, notochord ; 2. ¢. c. neurenteric canal; pcdi. proctodeum ; Aty. invagina- tion of ectoderm which will form the pituitary body ; 7c¢. commencement of rectum ; sk. sucker; sf. cd. spinal cord; std, stomodeum; ¢. tail; yA. yolk-cells ; y&. AZ. yolk-plug which fills the blastopore. (A—D, F—H, and J rom Ziegler’s models; EK, 1, K, and L after Marshall.) 200 THE FROG CHar. 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 the first, being like it, meridional, 7., passing through the poles (Fig. 64, B). It divides what we must now call the emdryo into four cells, each half black and half white. . 475) in one piece, lay open the external auditory passage, and very carefully and gradually cut away the outer wall of both tubular and bulbous portions of the tympanic with the bone- forceps. Note :-— 1. The Qrmpante membrane and the handle of the ma/leus ; and after cutting away the former so as not to injure the latter, and removing rather more of the bulla, make out— 2. The dvapanic cavety: the malleus, Mucus, and slafes ; the fensor tympant and stapedtus muscles; and the aperture of the Eustachian fube. Remove the audilory ossicles, noting as you do so the fenestra oralts and fenestra rotunda, and examine them under the low power of the microscope: by cutting through the periotic with the bone forceps, the position of the membranous labyrinth wih the cochlea may be made out. IIT. Dissect off the museles covering the mandible on the side you are working ; detach its ascending portion from the muscles inserted on to its inner surface, remove it entire, cutting through the symphysis, and clear away the underlying muscles. Pass a probe from the cut end of the gullet forwards into the mouth: lay open the gullet along this, and pull the tongue downwards, so as to get a view into the interior of the mouth (Fig. 125). Note :— 1. a, The palate and velum palati; b, the tongue and its paprlle (p. 492); «, the Apharyny: d, the glottis and efiglolt’s ; e, the con- tinuation of the pharynx above the velum palati (vaso-pharynx), with which the internal nostrils communicate ; /, the positions of the /ee// (incisors and grinders); ¢, the apertures of the waso-falatine canals (p. 490). Then remove the nasal, premaxilla, and maailla of the same side, noting the o/factory nerves and the /urbénals ; and after passing a probe backwards from the externa! nostril into the nasal chamber as far as it will go, remove the turbinals on this side, so as to expose the cartilaginous vasa/ sepia, Also cut away all the remaining bone on the same side of the middle line, slit up the naso-pharynx, and reduce your dissection to a neat longitudinal section, noting— 2. The cranial cavity, the roof of which will have been cut away if the brain has been removed. x PRACTICAL DIRECTIONS 537 3. The aperture of the Lustachian tube: pass a probe from it into the tympanic cavity. 4. The position of Jarobson’s organ, on the ventral side of the nasal septum ; and after cutting away the latter, the e/Amo-lurbinals, maxtllo- lurbénals, and naso-lurbinals of the other side. Sketch the entire longitudinal section, IV. Remove the larynx with the anterior part of the trachea. Make a longitudinal vertical section of it, keep one half entire, and from the other dissect away the muscles and mucous membrane so as to see the cartilages clearly. Examine first the cartilages and then the soft parts, noting the relations of the ¢hyrodd,-cricord, and arytenotd cartilages, the epiglott?s, and the vocal cords, Sketch. G. The Brain. I.—External characters. Note :— 1. The medulla oblongata, with its dorsal and ventral fissures and the pons Varold’ ; the convoluted cercheH/um, consisting of a central lobe and two /ateral lobes and floccul?. 2. The office Jobes, seen on slightly separating the hemispheres and cerebellum ; the crura cerebrz. 3. The olfactory lobes and cerebral hemispheres (frontal, parietal, and temporal lobes)—by gently separating them, the corpus callosum will be seen—and just behind them is the fcveal body; the drencephalon is covered above by the hemispheres, below it is visible and is continued into the 2efundibulum ; the pituitary body is nearly always left behind in the brain-case when the brain is removed, and a small aperture is seen in the centre of the infundibulum where it was attached ; the otic chiasma. 4. The origins of the nerves from the brain; these cannot, of course, be plainly made out unless the nerves have been carefully cut through when removing the brain. Compare pp. 163-166, and note in addition the spinal accessory and hypoglossal. Sketch the brain from above and from below. II. Carefully remove successive slices from one of the hemispheres nearly parallel to its upper surface down to the level of the corpus callosum, noting the fibres of the latter and the relations of the grey and white matter in the hemispheres. Cut through the corpus callosum longitudinally, near the middle line, and remove sufficient of the hemisphere already dissected to expose the /aferal ventricle 535 THE RABBIT CHAP, and its extensions into the frontal, parictal, and temporal lobes. Note :— 1. The Avppocampus, the corpus striation, the chorotd plexus, and the septum dactdum (p. 514). Then cut away the rest of the same hemisphere sv as to expose—- 2. a, The déencephalon and its two lateral halves (optic thalam?) between which the /hird ventricle is enclosed ; from its roof the stalk of the pineal body arises ; 4, the two pairs of optie dobes. Cut away the lateral half of the cerebellum of the same side, and note— 3. The vake of Teussens, the fourth ventricle, and the tree-like appearance (arbor te) of the cerebellum in section. III. Now reduce the whole brain to a longitudinal vertical section (Fig. 132), cutting it through with a sharp knife very slightly to that side of the median line which you have already dissected, so as to expose the median ventricles in section. Make out, on the uninjured half, the relations of the (Aird ventricle, ter, fourth ventricle, foramen of Monro, infundibulum, optic chiasma, corpus callosum and fornix, lamina terminalis, antertor, middle, and posterior commissure, erura cerebrt, valee of Tecussens, pons Varoli’, &c. Sketch. IV. A series of thick transverse sections should be made through another thoroughly hardened brain, and the relations of the parts already seen by dissection examined, Il. The Eye. On account of its larger size, it is better to substitute the eye of an ox or sheep—which can be obtained fresh from a butcher—for that of the rabbit, Two specimens should be obtained, and the fat, muscles, and any portions of the eyelids remaining attached cleared away. r. Afler noting the conjunetiaa, cornea, scleratic, optic nerve, trts, and pupil, insert the scissors into the margin of the cornea and cut round and remove it, so as to expose the agueous chamber, noting the agueous humour, as well as the fris, lene, and pupil (compare Fig. 57). Sketch. Insert the handle of a sealpel between the iris and sclerotic and separate off the latter from the choroid around the outer half of the eye- ball, first making four radial cuts in the sclerotic, beginning at its margin, at equal distances from one another. Turn back the four flaps thus made, and insert pins through them so as to fix {the eye firmly down under water with the iris uppermost : note the cary muscle around x PRACTICAL DIRECTIONS 539 the outer margin of the iris, the choroi?, and the celary vessels and neives. Then make two radial cuts, a short distance from one another, through the iris, and turn back the portion between the cuts, noting the ciliary processes. Sketch. Cut through the choroid and the parts enclosed by it, horizontally, around the line up to which it has been already exposed, so as to separate the eye-ball into an inner and outer hemisphere : in doing so, the gelatinous e2¢reows humour must be cut with the scissors. Examine and sketch both sections, noting in the outer hemisphere—the reé/va, stopping short at the outer margin of the ciliary processes, and the /evs with its cafsede , and in the inner hemisphere—the veé/va with its blood- vessels, and the dZezd-sfot (or point of entrance of the optic nerve). Then remove the retina and observe the choroid with its iridescent fapetun, 2. Cut your second specimen into vertical halves with the scissors, making the cut pass through the cornea, pupil, and optic nerve. As il is difficult to cut through the lens without disturbing its relations, it should be carefully separated from one half, and left entire on the other half. Examine the relations of the parts once more, compare Fig. 57. and sketch. I. Dissection of typical flexor and extensor muscles and joints of the fore-limb. 1. Expose the dcceps (chief flexor of the fore-arm). Its ovtg7i is from the anterior edge of the glenoid cavity on the pectoral arch : it arises by a single long ¢éevdon, working in the bicipital groove of the humerus. It is spindle-shaped, consisting of a single 7d, and is tnserted on to the proximal end of the radius. 2. Expose the ¢réceps (chief extensor of the fore-arm). It arises by three main heads from the pectoral arch and humerus, and is inserted on to the olecranon process of the ulna. 3. Remove the muscles from the shoulder-joint, and note the capsular ligament (p. 55). Cut through this so as to open the syxozval capsule, and note the syrozal membrane and fludd and the cartilaginous articular surfaces of the glenoid cavity and head of the humerus respectively : or, prepare as directed on p. 64 in the case of the hip- joint of the frog. 4. Asan example of a muscle with a multiple znsertion, dissect out the extensor communis digitorum (dorsal). It arises from the distal 540 THE RABBIT CUA & end of the humerus, and at the distal end of the fore-arm divides inte four tendons, which pass through the annular ligament to digits 2-5 and are inserted into the middle and distal phalanges. Note the sesamotd bores on the palmar side of the joints of the digits. J. Side dissection. Obtain another rabbit, and make a dissection from the side (compare Fig. 125) as directed in the case of the dogfish and frog (p. 464). Make out the structure and relations of all the principal organs once more, and sketch your dissection, Kk. Transverse sections. The examination of transverse sections is most easily and satisfactorily done by preparing microscopical sections of a fretus (p. 532) not more than a couple of inches in length, as directed in the case of the young dogfish (p. 464): a foetal rat or mouse will answer the purpose equally well. With a sharp knife, cut transverse sections, about th inch thick, from the following regions: @, snout; 4, cranial region ; ¢, neck ; @, thorax ; and e, abdomen. Stain and imbed, and then cut and mount one or two scetions of each, examining them with a low power and making sketches. Then put on the high spower, and make out as much as possible of the histology of the various organs, CHAPTER XI THE MINUTE STRUCTURE OF CELLS: CELL-DIVISION | STRUCTURE OF THE OVUM: SPERMATOGENESIS + MATU- RATION AND FERTILIZATION OF THE OVUM: SEGMEN- TATION OF THE OOSPERM: EFFECT OF FOOD-YOLK ON DEVELOPMENT : FORMATION OF THE CHIEF ORGANS OF THE ADULT VERTEBRATE, AND OF THE AMNION, ALLAN- TOIS, AND PLACENTA. Structure of the Cell.—We have learnt in previous chap- ters that all organisms are formed essentially of one or More cells (compare pp. 110 and 118); that the cell consists of protoplasm, and contains a xucleus consisting of chromatin and achromatin (p. 129), in which one or more xucleolt can usually be distinguished ; and that cells multiply by a process of d/narv fission (p. 236). It will now be necessary to study the structure of a typical animal cell and of its mode of division in somewhat greater detail. There seems to be a good deal of variation in the precise structure of various animal and plant-cells, but the more recent researches show that in the cell-body or protoplasm two constituents may be distinguished, a clear semi-fluid substance, traversed by a delicate sponge-work. Now under the microscope the whole cell is not seen at once but only an opéical section of it—that is, all the parts which 542 STRUCTURE OF CELLS CHAP. are in focus at one time (sec e.g. p. 315): by altering the focus we view the object at successive depths, each view being practically a slice parallel to the lenses of the instrument. ‘This being this casc, protoplasm presents the microscopic appearance of a clear or slightly granular matrix traversed by a delicate network. In the epithelial cells of animals the protoplasm is bounded externally by a cell-membrane of extreme tenuity: in amceboid cells the ectoplasm or transparent, non-granular portion of the cell consists of clear protoplasm only, the granular endoplasm alone possessing the sponge-work. It is quite possible that the reticular character of the cell may be mercly the optical expression of an extensive but minute vacuolation, or may be due to the presence of innumerable minute granules developed in the protoplasm as products of metabolism. The nucleus is spherical or oval in form, and is enclosed ina delicate nuclear membrane (Vig. 136, A, nim). Vhe achromatin is a homogeneous semi-fluid substance which forms the groundwork of the nucleus: it resembles the clear cell-protoplasm in its general characters, amongst other things in being almost unaffected by dyes. The chromatin (chr) takes the form of a network or sponge-work of very variable form, and is distinguished from all other constituents of the cell by its strong affinity for aniline and other dyes (compare Part T., Chapters VIL and VIII). Frequently, as we have seen, one or more minute globular structures, the wuchkoel (mu), occur -in’ the nucleus either connected with the network or lying freely in its meshes : they also have a strong athnity for dyes although they often differ considerably from the chromatin in their miero- chemical reactions. In the body of some cells and possibly of all there 1s x1 STRUCTURE UF NUCLEI $43 fic. 136.—Diagrams illustrating the process of indirect cell-divisiun or mitusis. A, the resting cell; B,C, D, successive phases in the formation and arrangement of the chromatin-loops and of the nuclear spindle ; E, I’, G, separation of the two sets of daughter chromosomes and their passage towards the poles of the spindle ; H, division of the cell-body and formation of the two nuclei; I, division of a plant-cell, showing the formation of the cell-wall (c. zw) by means of a cell- plate (c. Ad) centrosome; c/7. chromatin: zz’. nucleoli; v7.7. nuclear membrane; s. astrophere ; sf. spindle. (From = Parker's Avolegy, altered from Vlemming, Rahl, ete.) > founda globular body, surrounded by a radiating arrange- ment of the protoplasm and called the asfrosphere (s) : it lies close to the nucleus, and contains a minute granule S44 CELL-DIVISION CHAP. known as the cex/ral particle or centrosome (c). In many cases two astrospheres and centrosomes are found in each cell (B). Cell-division. ‘Ihe precise changes which take place during the fission of a cell are, like the structure of the cell itself, subject to considerable variation. We will consider what may probably be taken as a typical case (lig. 136). First of all, the astrosphere, with its centrosome, divides (bk), and the products of its division gradually separate from one another (c), ultimately passing to opposite poles of the nucleus (p). At the same time the network of chromatin divides into a number of separate filaments called chromo- somes (B, chr), the number of which appears to be constant in any given species of animal or plant, although it may vary in different species from two to twenty-four or more. Soon after this the nuclear membrane and the nucleoli dis- appear (B,C), and the nucleus is seen to contain a spindle- shaped body (sf) formed of excessively delicate fibres which converge at each pole to the corresponding astrosphere. The precise origin of this nuclear spindle is uncertain : It may arise either from the nuclear matrix or, more probably, from the protoplasm of the cell: it is not affected by colouring matters. At the same time cach chromosome splits along its whole length so as to form two parallel rods or loops in close contact with one another (B): in this way the number of chromosomes is doubled, each one being now represented by a pair. The divided chromosomes now pass to the cquator of the spindle (») and assume the form of more or less V-shaped loops, which arrange themselves in a radiating manner so as to present a star-like figure when the cell is er MLIUSIS 545 viewed in the direction of the long axis of the spindle. Every- thing is now ready for division, to which all the foregoing processes are preparatory. The two chromosomes of each pair now gradually pass to opposite poles of the spindle (f, F), two distinct groups being thus produced (G) and each chromosome of cach group being the twin of one in the other group. Perhaps the fibres of the spindle are the active agents in this process, the chromosomes being dragged in opposite directions by their contraction: on the other hand it is possible that the movement is due to the contractility of the chromosomes themselves. ~ After reaching the poles of the spindle the chromosomes of each group unite with one another to form a network (1) around which a nuclear membrane finally makes its “appear- ance (rt). In this way two nuclei are produced within a single cell, the chromosomes of the daughter-niucle’, as well as their attendant centrosomes, being formed by the binary fission of those of the mother-nucleus. But part passu with the process of nuclear division, fission of the cell-body is also going on. ‘This takes place by a simple process of constriction (H)—in much the same way as a lump of clay or dough would divide if a loop of string were tied round its middle and then tightened. In comparatively few cases the dividing nucleus instead of going through the complicated processes just described divides by simple constriction. We have therefore to dis- tinguish between avrect and ¢ndirect nuclear division. ‘To the latter very elaborate method the name mosis or harvokinesis is applied . direct division is then distinguished as am/foftic. In this connection the reader will not fail to note the extreme complexity of structure revealed in cells and their nuclei by the highest powers of the microscope. When the Pract. Zoou. NN 540 STRUCTURE OF OVUM CHAD. constituent cells of the higher animals and plants were dis- covered, during the early years of the present century, by Schleiden and Schwann, they were looked upon as the ultima Thile of microscopic analysis. Now the demonstra- tion of the cells themselves is an easy matter, the problem is to make out their ultimate constitution. What would be the result if we could get microscopes as superior to those of to-day as those of to-day are to the primitive instruments of eighty or ninety years ago, it is impossible even to con- jecture. But of one thing we may feel confident—of the great strides which our knowledge of the constitution of living things is destined to make during the next half century, Structure of the ovum. The striking general resemblance between the cells of the higher animals and entire unicellular organisms has been commented on as a very remarkable fact: there is another equally significant circumstance to which we must now advert. All the higher animals begin life as an egg, which is cither passed out of the body of the parent, as such, as in earth- worms, craylishes, frogs, birds, &e. (eparous forms), or undergocs development within the body of the parent, as in some dogfishes (p. 454) and nearly all) mammals (aziparous forms). The structure of an egg is, in essential respects, the same in-all animals from the highest to the lowest (compare p. 195). It consists (ig. 137) of a more or less ¢lobular mass of protoplasm, in which are deposited particles of a proteinaceous substance known as vo/k-eraniles. Within the protoplasm is a large nucleus containing chromatin as well as one or more nucleoli- which are often known as germinal spots, the entire nucleus of the ovum being called XI MODIFICATION OF OVUM 547 the germinal vesicle. An investing cuticular membrane may or may not be present. In other words the egg, as we have already seen, is a cell. j The young or immature ova of all animals present this structure, but in many cases certain modifications are undergone before the egg is fully formed. Tor instance, the protoplasm may throw out pseudopods, the egg becoming ameeboid (p. 302); or, as mentioned above and as is usually the case, the sur- face of the protoplasm may secrete a cell-wall, often of considerable thick- ness, and known as the vttelline membrane (p. 196 and Fig. 137), which may be perforated at one pole by an aperture, the mécropyle a fl eee ‘ Ape Pic. 137-—Ovum of a Sea-urchin (7 o.re- i 0). mos xtYa 7 sila coe (p 40 ) The ost extra pnenstes lividus), showing the radially- ordinary modification takes striated cell-wall (vitelline: membrane), : the protoplasm containing yolk granules place in some Vertebrata, (vitellus), the large nucleus (germinal vesicle) with its network of chromatin, such as dogfishes (p. 454) and a large nucleolus (germinal spot), (From Balfour, after Hertwig.) and birds. In a hen’s egg, for instance (Fig. 138), the yolk-granules increase immensely , swelling out the microscopic ovum until it becomes what we know as the “yolk” of the egg: around this layers of albumen or “white” are deposited by the glands of the oviduct and finally the shell-membrane and the shell. Hence we have to distinguish carefully in eggs of this character between the entire “egg ” in the ordinary accep- tation of the term, and the ovum or egg-cell. But com- plexities of this sort do not alter the fundamental fact that all the higher animals begin life as a single cell, or in other NN 2 545 SEX-CELLS CHAP. words, that multicellular animals, however large and complex they may be in their adult condition, originate as unicellular bodies of microscopic size; and the same is the case with plants. Spermatogenesis and Oogenesis. In the preceding chapters it has more than once been stated that sperms sh &L Ic. 138.—Semi-diagrammatic view of the egg of the fowl at the beginning cf incubation. a. air-space ; a/b. dense layer of albumen; a2é’, more fluid albumen ; derm; c#. chalaza, a twisted cord of the dense layer of albumen < i of the egg, formed as the latter rotates down the oviduct ; s/. shell y 9%. 9. shell- membrane ; si.m. 7, s/t. 2, its two layers separated to enclose air-cavity. (From Parker and Haswell's Zoology, after Marshall, slightly altered.) é2. blasto- ther end arise from ordinary undifferentiated cells in the spermary, and that ova are produced by the enlargement of similar cells in the ovary. Fertilization has also been described as the conjugation or fusion of ovum and sperm (compare x1 ‘SPERMATOGENESIS 549 ‘p. 197). We have now to consider in greater detail what is known as to the precise mode of development of sperms (spermatogenesis) and of ova (oogenesis), as well as the exact steps of the process by which an oosperm or unicellular embryo is formed by the union of the two sexual elements. Both ovary and spermary are at first composed of cells of the ordinary kind, the primitive sex-cells ; and it is only by the further development of these that the sex of the gonad is determined. In the spermary the sex-cells (Fig. 139, a) undergo re- peated fission, forming what are known as the sferm-mother- cells (z). These have been found in several instances to be distinguished by a peculiar condition of the nucleus. We saw (p. 544) that the number of chromosomes is constant in any given animal, though varying greatly in different species. In the formation of the sperm-mother-cells from the primitive sex-cells the humber becomes doubled : in the case of the mole-cricket, for instance, shown in Fig. 139, the ordinary cells of the body, including the primitive sex-cells, contain twelve chromosomes, while the sperm-mother-cells contain twenty-four. ‘The sperm-mother-cell now divides (c), but instead of its chromosomes splitting in the ordinary way (p. 544, Fig 136) half of their total number—in the present instance twelve— passes into each daughter cell: thus two cells are pro- duced having the normal number of chromosomes. ‘The process of division is immediately repeated in the same pecu- liar way (D), the result being that each sperm-mother-cell gives rise to a group of four cells having half the normal number of chromosomes—in the present instance six. The four cells thus produced are the immature sperms (E): in the majority of cases the protoplasm of each undergoes a great elonga- tion, being converted into a long vibratile thread, the ¢ar/ of 550 SPERMATOGENESIS CHAP. the sperm (¥, G), while the nucleus becomes its more or less spindle-shaped /ead and the centrosome takes the form of a small énfermediate piece at the junction of head and tail. ‘Thus the sperm or male gamete is a true cell, specially Fic. 139.—Spermatoyenesis in the Mole-Cricket (G7ydlotalpa). A, primitive s ell, just preparatory to division, showing twelve chromosomes (Ar); ¢. the centrosome. B, sperm-mother-cell, formed by the division of A, and containing twenty-four chromosomes ; the centrosome has divided into two. C, the sperm-mother-cell has divided into two by a reducing division, each daughte t-cell Containing twelve chromosomes, TD, each daughter-cell has dlisidedl again in the same manner, a group of four sperme-cells being produced, each with six chromosomes. Ey a single sperm-cell about to elongate to form a sperm. F, immature sperm; thes Bent aMasaines are still visible in the head. G, fully formed sperm. (Eium Parker's Bvodogy, after vom Rath.) modified in most cases for active movement. This actively motile, tailed form is, however, by no means essential : XL VUULLN LOLS 55! some animals (e.g. Crayfish, p. 368) the sperms are non- motile. The peculiar variety of mitosis described above, by which the number of chromosomes in the sperm-mother- cells is reduced by one-half, is known as a reducing diviston. As already stated, the ova arise from primitive sex-cells, precisely resembling those which give rise to sperms. These divide and give rise to the ege-mother-cells in which, as in the sperm-mother-cells, the number of chromosomes is even- tually doubled. The egg-mother-cells do not immediately undergo division, but remain passive and increase in size by the absorption of nutriment from surrounding parts: in this way each egg-mother-cell becomes an ovum. Sometimes this nutriment is simply taken in by diffusion or osmosis, in other cases the growing ovum actually ingests neighbour- ing cells after the manner of an Amceba. Thus in the developing egy the processes of constructive are vastly in excess of those of destructive metabolism. We have seen (p. 249) that the products of destructive metabolism may take the form either of waste products which are got rid of, or of plastic products which are stored up as an integral part of the organism. In the developing egg, in addition to increase in the bulk of the protoplasm itself, a formation of plastic products usually goes on to an immense extent. In plants the stored-up materials may take the form of starch, of oil, or of proteid substance: in animals it consists, as mentioned above, of rounded or angular grains of proteid material, known as yolk-granules. These being deposited, like plums in a pudding, in the proto- plasm, have the effect of rendering the fully-formed egy opaque, su that its structure can often be made out only in sections. 552 MATURATION CHAP. \1 Maturation of the ovum.——Vhe fully-formed ovum as described on p. 546, is, however, incapable of being fertilized or of developing into an embryo: before it is mpe for conjugation with a sperm or able to undergo the first stages of segmentation it has to go through a process known as maturation. Maturation consists essentially in a twice-repeated process of cell-division. The nucleus (Fig. 140, A) loses its mem- brane, travels to the surface of the egg, and takes on the form of an ordinary nuclear spindle. Next the protoplasm grows out into asmall projection or bud, into which one end of the spindle projects. Nuclear division then takes place, one of the daughter nuclei remaining in the bud ( fv/), the other in the ovum itself. Nuclear division is followed as usual by division of the protoplasm, and the bud becomes separated as a small cell distinguished as the frst polar cell, Tn some cases development from an unfertilized female gamete takes place, the process—which is not uncommon among insects (¢.g. the common little green plant-louse or ffA/s) and crustaceans (e.g. water- fluas)—being distinguished as parthenogrnests. Tt has been proved in many instances and is probably generally true that in such cases the egy begins to develop after the formation of the first pelar cell. Thus in parthenogenetic ova it appears that maturation is completed by the separation of a single polar cell, afler which the oyum centains the number of chromosomes normal to the species. In the majority of cases, development takes place only after fertilization, and in these maturation is not complete until a second polar cell (u, pol) has been formed in the same manner as the first. The ovum has now lost a portion of its protoplasm together with three-fourths of its chromatin, half having passed into the first polar ccll and half of what remained into the second: the remaining one- Fig. ago, The maturation and impregnation of the animal ovum (diagrammatic?. A. the ov um, surrounded hy the vitelline membrane (ew), in the act of forming the first polar cell (foods 9 cent. centrosome. 8, both polar cells (fod) are formed, the female pronucleus (¢ pron.) lies near the centre of the ovum, and one of the several sperms is shown making its way into the ovum at the micrepyle (microp). C, the head of the sperm has “become the male pronucleus (d Aron), its intermediate piece the male centrosome (g ceu?); other structures as before. TD, the male and female pronuclei are in the act of conjugation. KH, onjugation is complete, and the segmentation nucleus (seg. vue) formed. (irom Parker and Haswell's Zve/egy'.) 554 FERTILIZATION CHAP. fourth of the chromatin becomes enclosed as a nucleus, which is distinguished as the female pronucleus (B, 2 pron). The formation of both polar cells takes place by a reducing dutston (p. §5t) ; so that, while the immature ovum contains double the number of chromosomes found in the ordinary cells of the species, the mature ovum, like the sperm, contains only one-half the normal number. In some animals the first polar body bas been found to divide after separating from the egg. Jn such cases the egg-mother-cell or imma- ture ovum gives rise toa group of four ccells—the mature ovum and three polar-cells, just. as the sperm-mother-cell gives rise to a group of four cells, all of which, however, become sperms (lig. 139). Fertilization of the ovum.—Shortly after maturation, the ovum is fertilized by the conjugation with it of a single sperm. .\s we have found repeatedly, sperms are produced in vastly greater numbers than ova, and it often happens that a single cgg is seen quite surrounded with sperms, all apparently about to conjugate with it (Fig. 140, B). It has, however, been found to be a general rile that only one of these actually conjugates: the others, like the drones in a hive, perish without fulfilling the one function they are fitted to perform. The successful sperm (Bb) takes up a position at right angles to the surface of the egg and gradually passes through the micropyle (ierep) or works its way through the vitelline membrane until its head lies within the egg- protoplasm. The tail is then lost, and the head, accom- panicd by the intermediate picee or centrosome, penctrating deeper into the protoplasm, takes on the form of a rounded body, the male pronuchus (c, & pron). The two pronuclet approach one another (pb) and. tinally unite to form what is called the seementation-nucleus (i, Seg. auc/), Ue single nucleus of what is not now the vzw but XI SEGMENTATION OF OVUM 558 the voosperm—the impregnated egg or unicellular embryo (compare pp. 197 and 198). The fertilizing process is thus seen to consist essentially in the union of two nuclear bodies, one contributed by the male gamete or sperm, the other by the female gamete or ovum. It follows from this that the essential nuclear matter or chromatin of the oosperm—often spoken of as the germ- flasm—is derived in equal proportions from each of the two parents. Moreover, as both male and female pronuclei con- tain only half the number of chromosomes found in the ordi- nary cells of the species, the union of the pronuclei results in the restoration of the normal number to the oosperm. Fertilization being thus effected, the process of segmenta- tion or division of the oosperm takes place as described in previous chapters. Different types of ova and their segmentation. —Before passing on to consider further details in the process of development of the oosperm, we must briefly refer to some differences already noted in the ova of different animals. We have seen that in all cases the immature egg is a simple, minute cell, but that owing to the deposition of yolk-granules in its protoplasm, it may reach a compara- tively large size (e.g. Crayfish, Dogfish, Bird). he pres- ence of a greater or less amount of yolk in the ovum results, as we know, in very considerable differences as regards its mode of segmentation, as well as in its subsequent development. The minute eggs of Amphioxus and the Rabbit, for instance, which are cach only '5 mm. (about zs0 inch) in diameter, contain so comparatively small an amount of food-yolk as not to interfere materially with the process of segmentation: such ova are called alectithal. When the quantity of food yolk is relatively greater, it may 550 SEGMENTATION OF OVUM CHAD, become accumulated towards the centre of the egg, even- tually leaving a layer of protoplasm comparatively free from yolk round the periphery (centrolecithal ova, e.g. Crayfish, Fig. 91); or, as in the case of éelolecithal ova (Figs. 64, 11g, and 138), the yolk-granules may become aggregated more at the lower than at the upper pole (Frog), until in the most extreme cases there is only a layer of yolkless protoplasm—the germinal dise (Dogfish, Bird)—lying at the upper pole of the egg. As yolk is an inert substance, the more of it an egg contains the less actively can the latter divide, and the quantity may be relatively so great in some parts as to pre- vent segmentation in these parts altogether. We can there- fore distinguish between Aoloblastic oosperms, which_undergo entire segmentation (e.g., Hydra, Earthworm, Mussel, .\m- phioxus, Frog, Rabbit), and merodlastic oosperms, in which segmentation is limited to that part of the egg in which the protoplasm is comparatively free from yolk (e.g. Crayfish, Dogfhsh, Bird), this portion, after segmentation, being known as the ddastuderm. In the centrolecithal ovum it is evident that the segmentation must be svperfictald or pertpheral (p. 309), and in the meroblastic telolecitha. ovum adscurd or restricted to a small germinal disc at its upper pole (Figs. 119 and 138). In the case of holoblastic ova the segmenting cells or déasfomeres may be equal, or nearly equal, in size (eg., Amphioxus, Rabbit); or if the yolk is. pre- sent in greater quantities towards the lower pole, unequal, (e.g. Warthworm, Tro). The influence of the food-yolk in modifying the carly processes Of development is thus evidently very great, and my order to understand these processes in their simplest form Ios necessary to select for our study an alecithal holo- blastic egy, such as that of the lancelet. XI DEVELOPMENT OF AMPHIONUS 557 Development of Amphioxus. The oosperm of Am- phioxus (Fig. 141, A) undergoes binary fission (B), each B Fic. 141.—Stages in the segmentation of th. oosperm of Amphioxus. D, represents the four-celled stage (C) from above ;YH, vertical section of G; k, vertical section of the blastula stage (I). (From Korschelt and Heider, after Hatschek.) __ 555 GASTRULA CHAP, of the two resulting cells dividing again into two (C, D). This process is continued until a globular mass of cells or blastomeres is produced by the repeated division of the one cell which forms the starting point of the series, Owing to there being rather more yolk at the lower than at the upper pole, the lower cells are slightly larger than the upper, so that the segmentation is not quite equal (E—k). The embryo has now arrived at the polyplast or morula stage, and sections show that it is hollow, the blastomeres being arranged in a single layer around a central segmenta ton-cavity (p. 200): such a hollow polyplast is often known as a Alastula (K). The lower side of the blastula then becomes tucked in, or /vzagrnafed, the result being that the single-layered sphere is converted into a double-layered cup (Mig. t42). This process can be sufficiently well imitated by pushing in one pole of a hollow india-rubber ball with the finger. ‘he resulting embryonic stage is known as the gastrula (Fig. 142, C): its cavity is the primitive enteron or archenteron (p. 201), and is bounded by the invaginated cells which now constitute the evdoderm, the remaining cells forming the outer wall of the gastrula being the ectoderm (p. 202). ‘The two layers are continuous at the aperture of the cup, the gastrila-mouth or blastopore. Between the ectoderm and endoderm is at first a space, the greatly dimi- nished segmentation-cavity, which gradually becomes entirely obliterated, so that the ectoderm and endoderm are in con- tact (A.B). The general resemblance of the gastrula to a simplified Hydra,! devoid of tentacles, will at once be ap- parent, and the stage in the development of the frog’s egg re- ‘It must, however, be remembered (pp. 303 and 313) that the ectoderm and endoderm of Tydroids are differentiated before the mouth is formed, so that the mouth does not correspond to the blasto- pere of the gastrula, XI NERVOUS SYSTEM 559 presented in rig. 64, I, though much modified by the quantity of food-yolk, will be seen to correspond to the gastrula-stage. As in the frog, the blastopore soon closes, the mouth and anus being subsequently formed from the stomodzum and the proctodzeum respectively (p. 204). The gastrula becomes clonyated, flattened on onc. side, Fic. 142.—Three stages in the formation of the gastrula of Anmphioxrus, In A, the nuclei of the endoderm have been omitted. (From Korschelt and Heider, after Hatschek.) and convex on the other. 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 plate and groove (Fig. 143, mp) are then formed, the central nervous system being developed in a manner essentially similar to that already described in the case of the tadpole (p. 202), except that the central canal of the medullary cord () is formed after the 560 NOTOCHORD CHAP. plate has become separated from the outer ectoderm. line a thickening of the endoderm form the xoefechard In the mid-dorsal (ch) soon becomes constricted off to maya Vie. ry3. -Four stages in the development of the notochord, nervous system, and mesoderm of slaephiovius, ak, ectoderm; ch notochord ; di. cavity of archenteron ; 74. ridge of ectoderm growing over medullary plate > 74. endoderm ; 24, caclome 3 y7é. cazlomic pouch ¢ mk\, parietal layer of mesoderm y ov". visceral layer; f. medullary plate 5 7. medullary cord; 2s. protovertebra, (From Korschelt and Heider, after Hatschek.) (pp. 203, 404 and 425), and on either side of this a series of hollow endodermic pouches arise, arranged metamerically (Pigs. 143, mA, and 144, us, mh’). The x1 MESODERM AND CCELOME 561 cavities of these, which subsequently give rise to the ccelome (//, ws), are thus at first in free communication with the archenteron and are known as exéeroceles ; from their walls the wesoderm is derived. Subsequently the communi- cations between the enteric and enteroccelic cavities become Fic. 144.—Embryo of .ditphiorus. A, in vertical section, slightly to the left of the middle line; L, in horizontal section, as, ectoderm; cz. neurenteric canal; dA, wd. archenteron ; é4. endo- derm; w7&. mesodermal folds; 1”. medullary canal; ws. first coelomic pouch ; ush. coelomic cavity; V. anterior; //. posterior end. (From Korschelt and Heider, after Hatchek.) closed, and the paired pouches gradually extend between the ectoderm and endoderm, both dorsally and ventrally (Fig. 143, C, D), their outer walls (parietal or somatic layer of the mesoderm, #4!) 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, Pracr. Zoou. (ome) 562 SEGMENTATION IN VARIOUS TYPES CHAP, mk) in contact with the endoderm and with it forming the splanchnopleure or wall of the enteric tube (compare p. 203). Thus the body-wall and the enteric canal are separated by a cavity, the ca'dome (D, Zh), which, much as in the adult Earthworm, is divided into a series of metamerically ar- ranged portions: later on, however, the adjacent walls of these celomic’ sacs disappear, and the cecelome becomes a continuous éavity. The embryo Amphioxus is hatched soon after reaching the yastrula-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 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 gencral rule, the greater the relative quantity of yolk present in the ovum of an animal, the less clearly can a gastrula-stage be recognised, Tn the earthworm and mussel the segmentation is entire, but unequal, and the larger, lower cells become invaginated to form the endoderm and archenteron while the smaller upper cells give rise to the ectoderm. In the earthworm the blastopore does not become closed, but gives hse to the mouth. In the frog (p. 201) the archenteron arises by a split appearing amongst the yolk-cells, beginning at the edge of the blastopore and gradually extending forwards : the process is probably supplemented by a limited amount of invagination of the ectoderm. The archenteron is at first a very narrow cleft, but soon widens considerably (Fig. 64, Fy ent): for some ume it does not actually communicate with the cx- terion, the blastopore (6/44) being filled up by aw yolkeplug (re. 2). As the archenteron extends forwards, and the relatively small segment XI GASTRULA-STAGE 563 ation-cavity (44. ca) gradually disappears, the edges of the lowermargin of the blastopore approach one another, and uniting in the median plane, give rise to a vertical groove, the Arémétive grove, as it is called. In the centrolecithal egg of the crayfish (Fig. 91) a gastrula-stage is formed by invagination, but as the centre of the vosperm is filled with solid yolk in the place of a segmentation-cavity containing fluid, the invagination only extends a short distance inwards, the archenteron Itc. 145.—Two stages in the development of the blastoderm of the chick, at about the twentieth and tw enty-fourth hour of incubation respectively ; diagrammatic. ar. op. area opaca; ar. Al. area pellucida; Ad. head 3 wed. gv. medullary ETOOVE 5 mics. mesoderm, indicated by dotted outline and deeper shade ; fr. ame. pro- amnion; A”. sé. primitive streak and groove: Av. 7. mesodermal segments or protovertebree. — (I'rom Marshall’s £7é7-yology, in part after Duval.) being relatively very small and the ectoderm separated from the endoderm by the yolk. The gastrula-stage is much less clearly distinguishable in’ the segmenting eggs of the dogfish and bird (pp. 454 and 547), in which the relatively enormous mass of unsegmented yolk is, as in the craylish, sufficient to nourish the embryo until it has’ reached a stage closely resembling the adult in almost every essential respect except size. A blaslopore can sometimes be recognised in such cases, but in the embryo of the common fowl it is only represented by a primidive greene (see above and Fig. 145 fv. st). The blastoderm soon becomes difler- 002 564 SEGMENTATION IN RABBIT CHAP. cnUated inty an outer ectoderm and an inner, lower layer of cells (compare Fig. 119), between which and the yolk the enteric cavity is formed: a segmentation-cavity is hardly recognisible. As the embryv develops, it becomes folded off from the yolk, which forms a yo/k-sac on its ventral side (Figs. 120 and 154). The minute egg of the rabbit and of most other Mammals, although alceithal and undergoing a holoblastic segmentation, has presumably Fic. 146.~ Oosperin of rabbit 70-90 hours after impregnation. dc. cavity of blastodermic vesicle (yolk-sac) ; cf. outer layer of cells (trophoblast) ; Ay. inner mass of cells of the embryonic area ; 7A. albuminous envelope. (From Balfour, after E. van Beneden.) been derived from a mervblastic type with abundant yolk like that of the bird, and some Mammals living in Australia at the present day still possess egys of this type. In the higher Mammalia the yolk has disappeared, as iis no longer needed, the embryo, as we have seen, being nourished by means of a placenta, whieh will be described pre- senly. The carly processes of development are therefore somewhat peculiar, and though the segmentation is holoblastic, the subsequent development is essentially similar to that of the bird, the embryo beginning te appear in a imass of cells (Mig 146, Ay) attached to XI MESODERM AND CCELOME 565 the upper pole of a large bastodermic vesicle (dz), representing the yolk- sac of a hird but containing a fluid instead of yolk and being surrounded by a layer of cells known as the Lrophoblast (ep). In Amphioxus alone amongst the triploblastic animals described in this book, does the mesoderm arise as a serics of enteroccelic pouches : it is usually at first solid, and may be budded off from the endoderm, from the lip of the blastopore or primitive groove at the junction of the ecto- derm and endoderm, or both endoderm and mesoderm may be differentiated at the same time from the lower layer- cells or yolk-cells (e.g. frog); or, finally, it may arise in all these ways (e.g. fowl, rabbit). The ceelome is formed by a split taking place in the mesoderm on either side (Figs. 65, mes, and Fig. .147, wsd, som, sp/), the split gradually extending with the extension of the mesoderm between the ectoderm and endoderm. Thus the ecclome is formed, not as an enteroccele, but as a schisocele. In Vertebrates each mesoderm-band becomes differ- entiated into a dorsal portion, the vertebral plate, which soon loses its coelomic space, and a ventral portion, the lateral plate, which is divided into parietal and visceral layers by the ccelome (Figs. 143 D and 147). The vertebral plate undergoes metameric segmentation, becoming divided into a row of squarish masses, the mesodermal segments or protovertebre (pr.v), from the dorsal portions of which the muscular segments or myomeres are formed (p. 203), and from their ventral portions the vertebral column, the segmentation of which alternates with that of the myo- meres. 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 566 NERVOUS SYSTEM Ms. Sh seen, In all cases formed from the cetoderm (pp. 202, 209, and Figs. 04, 65, and 143), and in crantate Vertebrates the anterior end of the hollow medullary tube becomes dilated, forming three bulb-like swellings—the fore-brain (Mig. 148, .\, £4), mid-brain (wm. 4), and hind-brain (4. 4). Soon a hollow outpushing grows forwards from the first vesicle hig. 147-— ‘Transverse section of embryo of frog. cal coslome y cad. prolongation of ceclome into protovertebra (the reference line should end at the space) + ev, mesenteron (archenteron) { 2s@. mesoderm; xch. notochord ; pre. protovertebra ; sgva. pronephric duct; sov7. somatic layer of mesoderm; spice. spinal cord; sf/. splanchnic layer of mesoderm; 94. yolk-cells, (Prom Parker and Haswell’s Zoedogy, after Marshall.) (b, frs.ex), and the third gives off a similar hollow out- vrowth (cd) from its dorsal surface. The brain now con- sists of five divisions: the Arosencephalon (prs. cr) and dtencephalon (den) derived from the fore-brain, with the pincal apparatus (pw. 4, pu. ec) and the infundibulum and pituitary body (¢af. piv): the mid-brain or mesen- cephaton (m. 6) which gives rise to the opie dobes and criura ceredyr; and the epencephalon or cerebellum (cblm) and (807007 $ JPNSUAY puv iaxyarg woy paonpay) ‘pos [eurds +2 -gs { sayouquaa A101 eyo 10 sajooutys “29 “ye £ Apo Aseyngid “47g £ uopeydaouasoid “zag + apenoosoid *29 “sa/ $ (681 -d areduioa) ada [eaurd ‘9 ‘zg $ Apoq [eautd “9 -xg $ uo[eydaduasoid jo Joor 10 ‘untyped rg {sapoizjuaa [esr] 10 sajwovied -y2 “vg sturery ondo “yz °e $saqo, ondo +7 *7g0 :saywaojdo *wa-zZo {saqo] A10x9e}O +7 “fo Saptujuaa yyinoy 410 APIDIVIBUL HI “Yue FS LJeSuoqGoO vI[Npeur 790 “pom Sasoo0sau +9 “4 S UleIq-plut °9 "74 CSI[eUlWIa? wUIWE, *7Z 7 dan ‘7 Swmynqrpunyur sf $ ureiq-puly “9 “Y SouOyY Jo UaurvIoy “we “ff uTeIq-a105 “9 *f § ajaaida -w2 “ga Sucjeydaouatp swap Sapimnuaa PAI) JO 9[MovIP “I yp seyeus viodi0d “s +2 { sasaydstmay peaqazad *y “9 $14q31a9 wD "942 “49 [euLD yeUad +2 °2 f wINI[aqasao 2799 “apts 7y5t ay} UO pasodxa SAUIATS ayy fADIA [USIOp “T faules ayy Jo uotdas [eISes ‘Mf sesaydsruray pesqaia> yitm uresq Jo Mata apis “J ! uopeqdaouaiaw yo ‘Fy uopeydaouasaur Jo ‘5 suojeydasuatp yo ‘y { uofeydaouasoid jo UoNdas asieasueyy “WS uOTDas [eIWSes ur aures ayy “Cy | uopeydaouasoid PePIATPUN YIM UleIq pawsof-A[INy JO Mata apts ‘D {s8uis puoosas ‘ey ¢ aury Pevnop v Aq payeorpur Ayaeo ayy “mata apis ‘astys sry “ye “uleaq aivlursd ary jo stuvaSeiqg—"gtr ‘org aor yu ! VOD ode) 202 Loe 207; 7p Ad. es: wWzIQP? Qu gud oud ods 192 PIL 220 pow (ole 22702 gud. x uct 508 NERVOUS SYSTEM CHAP, metencéphalon or medulla oblongata (med. obl) derived from the hind-brain. The original cavity of the brain becomes correspondingly divided into a series of chambers or ventricles (compare Figs. 148 and 50), all communicating with one another, and called respectively the fore-ventricle or prosocele, third ventricle or adiacwle, mid-ventricle or mesocele (tter and optic ventricles or optocales), cerebellar ventricle or epicaele, and fourth ventricle or melacwle. In some fishes (e.g. dogfish, Fig. 115) the brain consists throughout life of these five divisions, but in most cases (Figs. 49 and 131), the prosencephalon grows out into paired lobes, the right and left cerebral hemispheres or parencephala (Figs. 148, LL, ¢ 4), each containing a cavity, the /ateral ventricle or paracale (pa. coe) which communi- cates with the diaccele (a. caw) by a narrow passage, the foramen of Monro (f. m). Vrom the prosencephalon or the hemispheres are given off a pair of anterior prolonga- tions, the olfactory lobes or rhinencephala (olf. 1), each containing an olfactory ventricle or rhinocale (rh. ca), In the preceding description the brain has been described as if its parts were in one horizontal plane; but, asa matter of fact, at a very carly 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 fore-brain being strongly inclined to that of the hind-brain. The bend is known as the cerebsval flexure (Fig. 1§§): 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 hecomes obscure and is not noticeable in the adult. The ganglia of the dorsal roots of the spénal neries ave developed from a paired vervral ridge formed close to the junction of the medullary plate and outer ectoderm, and the dorsal roots themselves appear as outgrowths from their ganglia (see Fig. 147, above sf. ¢): the ventral roots arise as direct outgrowths from the medullary cord. Certain of the cerebral nerves ave developed in an essentially similar manner to XI NOSE AND EYE 569 the dorsal roots of the spinal nerves, while others arise as direct ventral outgrowths from the brain, like the ventral roots. The olfactory organs arise as sac-like invaginations of the ectoderm, one on either side of the snout, and become enclosed by the cartilaginous olfactory capsules, developed, with the rest of the skeleton, from the mesoderm. The aperture of invagination gives rise to the external nostril, opt.c’ tnve opt.st Fic. 149.—Early (A) and later (B) stages in the development of the eye. dien. diencephalon ; 7x. 2. invagination of ectoderm to form lens; Z. lens ; 0ft.c outer layer of optic cup; opt. c’. inner layer; oft. st. optic stalk ; eft. 2. optic vesicle ; 24. pharynx ; f7y. pituitary body. (From Parker and Haswell’s Zoology altered from Marshall.) the internal nostrils (in air-breathing forms) being developed subsequently. The mode of development of the paired ere of vertebrates is peculiar and characteristic. At an early stage of development a hollow outgrowth—the optic vesicle (Fig. 149, A, oft. 7)—is given off from each side of the fore-brain and extends towards the side of the head, where it meets with an in-pushing of the ectoderm (/v7z. /) which becomes thickened, and finally, separating from the ectoderm, forms a closed, spherical sac (B, 7) with a very 570 EYE CHAP, small cavity and thick walls (compare Vig. 64, I, e). This body is the rudiment of the lens: as it enlarges it pushes against the optic vesicle and causes it to become in- vaginated (4), the single-laycred optic vesicle thus be- comes converted into a two-layered optic cup (opt. ¢, opt. c), 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 gradually extends towards the stalk of the cup, and thus gives rise to a slit in the wall of the latter: this = choroid fissure (Vig. 150, aus), h ephoe ib as it is called, soon becomes yl : ~ os S closed by the union of its edges. ‘The outer layer of l the optic cup becomes the pigment-layer of the retina (p. 183) : from its inner layer aus Fig.f150.—Plastie representation of the the rest of th at membrane— optic cup and Tens. including the ce and cones ab outer wall ef optic cup ; aus. choroid fissure; g@. cayity of optic —is formed. ‘The stalk of the cup 3 4. space between the two walls, which afterwards disappears ; optic cup occupies in the 76. inner wall of optic cup 32. lens ; e) Sv. stalk of optic cup (rudiment of smbryonic eye, the place of optic nerve), (After Hertwig.) e ) Puce ; P 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, 7.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 imme- XI EAR S71 diately surrounding the optic cup is differentiated to form the choroid, the iris, and the sclerotic. Thus the eye of Vertebrates has a threefold origin: the sclerotic, choroid, iris, vilrcous 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 from7the inner layer of the optic vesicle, 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 lig. 149). 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 (Migs. 64 L and 155, av.s) which deepens and becomes flask-shaped ; and finally, as a rule, loses its connection with the external ectoderm, becoming 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 compart- ments, from the former of which arise the utriculus and semicircular canals, and from the latter the sacculus and cochlea. The mode of development of the enterie canal has already been dealt with (pp. 204—210). ‘The first traces of the “ver and pancreas are seen as simple offshoots of the mesenteron (archenteron), which gradually become branched in a complicated manner, the numerous lobules being more or less closely connected together by mesoderm. The gillpouches arise as paired outgrowths of the endoderm lining the) pharynx which come into contact with the $72. RESPIRATORY AND CIRCULATORY ORGANS — cHap, ectoderm, the latter becoming perforated to form the external branchial apertures. Gill-clefts appear in the cm- bryo of reptiles, birds, and mammals—animals in which gills are never developed (Mig. 155); but they early disappear with the exception of the first cleft, corresponding with the spiracle of the dogfish, which gives rise in all Vertebrates above fishes to the tympano-eustachian passage (p. 433): the branchial skeleton, as we have seen, undergoes a cor- responding reduction or modification (pp. 422 and 479). In air-breathing Vertebrates the /vngs arise as a ventral outgrowth of the pharynx. The circulatory organs are developed from the meso- derm, the Aearé arising in the visceral layer on the ventral side of the pharynx. It has at first the form of a straight tube, which soon becomes twisted into an S-shape and in which transverse constrictions are formed dividing it into the different chambers. ‘The auricular and ventricular portions are cach at first single, but from the Amphibia onwards the former subsequently becomes divided into two by a septum and the ventricle is similarly subdivided in birds and mam- mals. The modification of the arterial arches in the ex- amples studied has already been described (pp. 435 and 506). In the meroblastic eggs of the dogfish and bird the dorsal aorta, in addition to its other branches, gives rise to paired evfe/line arteries: these vessels branch up over the extra- embryonic part of the blastoderm (p. 578), which spreads over the yolk, and take an important share in the absorp- tion of the latter by the embryo. rom this avea vasculosa (Fig. 154), the blood is returned by wveld‘ne veins into vesscls which eventually give rise to the hepatic portal veins. The other chief veins in all embryonic Craniates are, as in the dogfish, the jugulars and the cardinals. In all Verte- rates above the fishes, the cardinals became subsequently xI URINOGENITAL ORGANS 573 more or Icss entirely replaced functionally by the develop- ment of a postcaval (compare p. 440): the anterior part of one or both cardinals may, however, persist as the azygos vein or veins (e.g. Rabbit, p. 509). Urinogenital organs. The exerefory organ, speaking of craniate Vertebrates as a whole, consists of three parts, all paired and situated along the dorsal wall of the ccelome: the fore-kidney or pronephros (Fig. 151, A, p. uph), the mid kidney ox mesonephros (ms. uph) and the Aivnd-kidnev or meta- nephros (mt. nph). Tach of these is provided with a duct, the pro- (s¢.d), meso- (msn.d), and metanephric (int. n. d) ducts, which open into the cloaca. The govads (gov) lie in the ccelome suspended to its dorsal wall by a fold of peritoneum : they are developed as ridges covered by ccelomic epithelium (compare pp. 194—196 and 336). The pronephros is nearly always functionless in the adult and often even in the embryo, and usually disappears alto- gether: in the young tadpole it acts as the sole excretory organ for some time. The mesonephros is usually the functional kidney in the lower Craniata, in which as a rule no metanephros is developed (see p. 576), and the meso- nephric duct acts as a ureter, often in addition carrying off the seminal fluid of the male (e.g. frog). In the higher forms the mesonephros is replaced in its excretory function by the metanephros, the metanephric duct being the ureter (e.g. Rabbit). The development of the kidney reveals a resemblance to the nephridia ef worms which would hardly be suspected from its adult structure. The provephros (Vig. 151 A, /. uph) originates as two or three coiled tubes formed from thesoderm in the body-wall at the anterior end of the celome; they are arranged metamerically and cach opens 574 URINOGENITAL ORGANS CH. XI into the ccelome by a ciliated funnel (xs). Obviously such tubes are xephridia (compare p. 331); their chief peculiarity is that their outer ends do not open directly on to the exterior, but into a longitudinal tube, the pronephric duct (sg), which passes backwards and discharges into the cloaca. It seems probable that this arrangement is to be explained by supposing that the nephridia originally opened externally into a longitudinal groove, which, by the apposition of its edges, was converted into a tube. All the nepbridia of the pronephros open, by their ciliated funnels, into the narrow anterior end of the ccglome, into which projects a branch of the aorta ending in a single large glomerulus (p. 146). The proncphros soon degenerates, its nephridia losing their connection with the duct (2), but in the mean- time fresh nephridia appear in the segments posterior to the proncphros and together constitute the mesonephros ov TVolfian body (ms. nph), from which the permanent kidney is formed in most of the lower Craniata (eg. frog). ‘The mesonephric nephridia open at one end into the duct (sg. d), at the other, by ciliated funnels (vs¢), into the cwlome ; a short distance from the funnel each gives off a blind pouch which dilates at the end and forms a Malpighian capsule (wc), and a branch from. the aorta centering it gives rise to a glomerulus. In some forms (e.g. Dogfish, p. 450) the pronephric duct now becomes divided by a longitudinal partition into two tubes: one retains its connection with the mesonephros and is now known as the mesonephric or TModfian duct (Cy ms. ad): the other, or ALadierian duct (po ned), has no conncetion with the nephridia, but opens into the ecelome in the region of the vanishing pronephros, and assumes the functions of an oviduct in the female. — In some Craniata the DDO Dumsnph Cee _ Far.b6b the development of cthe, urine nital orgstfot Craniata. thlatrophy of proverlnos, de clopment 3, pronephros andZits ducts nephros 3 C, differentiation of Wolffian and Miillerian cucts ; D, develcpment of metanephros, male type; E, female type. al, bl. aMantoic bladder: az. anus: cf. cloa iat gow. gonad; (uf, s testine + 77. ¢ Malpighian capsule + ws. 1 ad. mesune} pric (Wolffian) duct 4 Sasi ugh. mesor Fic, 151.—-Diagranis illustratin of meso- 576 MUSCLES AND SKELETON CHAP. nephros ; m¢. 2. @. metanephric duct ; #/, 7f4. mctanephros ; #s¢. nephrostomes ; ow. ovary; 4.2.a. Miillerian duct; A. 7p. pronephros ; sg.d@. pronephric duct; ¢. spermary 3 vc. efferent ducts. (from Parker and Haswell’s Zoology). Miillerian duct appears quite independently of the Wolffian duct; the latter is then simply the pronephric duct after the union with it of the mesonephric tubules. In the higher Vertebrata (Reptiles, Birds, and Mammals) a diverticulum (D, E, #4 2. 2) is given off from the pos- terior end of the Wolfhan duct, which grows forwards and becomes connected with a series of posterior nephridia. In this way is formed a mefanephros (mt. nph), which gives rise to the permanent kidney, and a metanephric duct (m/. 7. @) which becomes the ureter. The Wolffian body ceases to discharge a renal function, becomes in the female a purely vestigial organ, and in the male gives rise to the epididymis (pp. 4514 and 521), which receives the efferent ducts from the spermary and from which the Wolffian duct (spermiduct or vas deferens) arises. The homology of the hinder part of the kidney in the dogfish (p. 451) is differenUy interpreted by zoologists. It is usually considered as corresponding to part of the mesonephros, but in its differentiation from the anterior part of this organ and in the development of special ureters it resembles the metanephros of higher Vertebrates. The majority of the muscles are developed, as we have secn (pp. 203 and 565) from the mesodermal segments others arising from the parictal and visceral layers of the mesoderm. The first part of the endoskeleton to arise is the endo- dermic notochord (pp. 203 and 560), in the mesoderm surrounding which cartilage appears and undergoes seg- mentation, giving rise to the vertebra, the notochord be- coming constricted by the ingrowing cartilage, and eventually disappearing more or less completely (compare pp. 425 and 565): it at first extends into the head as far as the XI SKULL 577 pituitary body (Fig. 152 C). The cranial cartilage does not become segmented, but gives rise to a pair of horizontal bars, the parachordals (PE): these are continued forwards, diverging around the pituitary body, as the trabeculae cranti (Zr), and thus a support is formed for the developing brain. The two parachordals and trabecule then unite Fic. 152.—A and B, two stages in the development of the chondrocranium. A. eye; AF. antorbital process; B&. basal plate, formed from the parachordals ; C. notochord; Cz#, anterior process of trabecula; iV, V&A. nose; O. ear; Ol. position of foramina for olfactory nerves; PZ. parachordal cartilage; P/. post- orbital process; PA. pituitary space; S. nasal septum; 7». trabecula. (From Wiedersheim’s .4 xatomy.) respectively with one another, and so form a firm floor (B) for the future brain-case, which is gradually developed by the floor 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 form the fontanelles (pp. 43 and 420). In the meantime Pract. Zoov. PP an EMBRYONIC MEMBRANES CHAP: * ur the cartilaginous sense-capsules are developed, the olfactory and auditory capsules uniting with the brain-case in front and behind respectively. The visceral skeleton is formed asa series of cartilaginous bars within the visceral arches, the first of which forms the mandibular arch, the second the hyoid, and the others the branchial arches. The Zémés appear as small buds (Fig. 155) composed of ectoderm with a core of mesoderm, and their skeleton arises by the formation of cartilage at their bases, which extends 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 more or less entirely cartilaginous in the adult (e.g. Dogfish), but in higher forms extensive processes of ossification set in, certain bones (cartilage-bones) replacing this 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 the higher Craniata (Reptiles, Birds, and Mammals), and known as embryonic membranes. Taking the chick as a convenient example, these are formed as follows. The blastoderm 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 (Fig. 154). The extension of the ectoderm and endoderm takes places regularly and symmetrically, but the meso- derm, while extending equally in the lateral and posterior XI AMNION AND ALLANTOIS 579 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 pvo-amnion (Fig. 145 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 amniotic fold (Fig. 153, A,am. f) arises in front of the head-end of the embryo from 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 (B) and round the tail (A), but in these regions (am. 7’) it consists from the first of ectoderm plus the parietal layer of mesoderm, #.e., it is a fold of what may be called the embryonic body-wall or somatopleure (p. 561). Its cavity is a prolongation of the space between the parietal and visceral layers of mesoderm, 7.e., is an extension of the extra-embryonic ccelome. The entire amniotic fold gradually closes in above (C), forming a double-layered dome over the embryo. Its inner layer, formed of ectoderm internally and mesoderm exter- nally, is the amnion (am), the cavity of which becomes filled witha watery ammiotic fluid, serving as a protective water- cushion to the enclosed embryo. Its outer layer, formed of ectoderm externally and mesoderm internally, is the serous membrane (sr. m): this comes to lie just beneath the vitelline membrane, 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 (archenteron) at its posterior end (C, a//), and consists, therefore, of a layer of visceral mesoderm lined by endoderm. It has at first the form of a small, ovoid sac PP 2 Mic. 153.—Diagrams illustratin & the development of the fastal membranes of a bird: A, early stage in the format ton of the amnion, longitudinal vertical section; B+ CH. XI ALLANTOIS 581 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 ; ad’. the same growing round the embryo and yolk-sac ; az. amnion ; am.f, amniotic fold; az. anus; dx. brain; cal. coelome ; c@/’. extra-embryonic coelome 5 At. heart; ms. ent. mesenteron 3 mth. mouth; zch. notochord 3 sf. cd. spinal cord; s7. 7. serous membrane 3 276. d. umbilical duct ; vé. mz. vitelline membrane ; y#. yolk-sac. (Reduced from Parker and Haswell’s Zoology.) having the precise anatomical relations of the urinary bladder of the Frog. It increases rapidly in size (Figs. 155 and 154, all), and makes its way, backwards and to the right, into the extra-embryonic ccelome, between the amnion and the serous Fic. 134.—Egg of fowl, at the sixth day of incubation, with embryo and futa appendages. a, air-space ; add, allantois ; az. amnion ; a7. vasc. area_vasculosa ; cd. embryo ; yk. yolk-sac. (From Parker and Haswell’s Zoology, after Duval.) membrane (Fig. 153, C, D). Arteries pass to it from the dorsal aorta, and its veins, joining with those from the yolk-sac, take the blood through the liver to the heart. Next, the distal end of the sac spreads itself out and extends all round the embryo and yolk-sac (D, a//), fusing, as it does so, with the serous and vitelline membranes, and so coming to lie immediately beneath the shell-membrane. It finally en- closes the whole embryo and yolk-sac, together with the re- 582 ALLANTOIS CHAP. mains of the albumen, which has, by this time, been largely absorbed. The allantois serves as the embryonic respiratory organ, gaseous exchange readily taking place through the Fic. 155.—Chick at the fifth day of incubation. all, allantois ; am. cut edge of amnion; av. s. auditory sac ;_/ 47. fore-brain ; f 2. fore-limb ; 4. dx. hind-brain; /. 2. hind-limb; 42. heart; 4. hyoid arch ; a. b7. mid-brain ; #2, mandibular arch ; ¢, tail, (From Parker and Haswell’s Zoology, after Duval.) 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 cartnc/e 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 ait PLACENTA 583 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 mode of development of the amnion and allantois in the Rabbit is similar to that 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 blastodermic vesicle (p. 565) becomes adherent. The foetal part of the placenta is formed from the outer layer of the amnion (serous membrane, Fig. 156, sz) in a limited disc-shaped area where the distal portion of the allantois coalesces with it. ‘The membrane thus formed (chorion) develops vascular processes—-the chorionic villi ( pl)—which are received into depressions— the uéerine crypts—in the mucous membrane of the dorsal wall of the uterus which constitutes the maternal porticn of the placenta. The completed placenta with its villi is sup- plied with blood by the allantoic vessels, and the blood supply of the uterus is at the same time greatly increased : the fcetal and maternal capillaries and sinuses are thus brought into intimate relation with one another in the placenta, and diffusion can take place between them, nutrient matter and oxygen diffusing from the blood of the mother into that of the foetus, while excretory sub- stances pass from the blood of the foetus into that of the mother. The discoidal placenta of the Rabbit is of the type termed deciduate, the villi of the placenta being so intimately con- nected with the uterine mucous membrane that a part of the latter comes away with it at birth in the decédua, or atter 54 PLACENTA CHAP. birth, which is attached to the newly-born young by the umbilical cord, consisting of the stalks of the allantois (a) and flattened yolk-sac (ed, fd) twisted together. 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 wmdbzlicus, which repre- Tr Fic. 156.—Diagrammatic longitudinal section of the foetus and embryonic mem- branes of a rabbit. «, (on right) amnion ; a. (on left) stalk of allantois ; a2. allantois with blood-vessels ; c. embryo; ds. cavity of the flattened yolk-sac (blastodermic vesicle); ed. endo- dermal layer of yolk-sac ; ed’. inner portion, and ed”. outer portion of endoderm lining the compressed cavity of the yolk-sac; 7. vascular layer of yolk-sac ; fl. placental villi; ». space filled with fluid between the amnion, the allantois, and the yolk-sac ; s#, serous membrane ; s/. margin of vascular area of yolk-sac. (From Balfour, after Bischoff.) sents the point of connection between the feetus and the placenta. The intra-abdominal portion of the allantois is represented by a cord or ligament, the wzvachus, 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. XI PRACTICAL DIRECTIONS 585 PRACTICAL DIRECTIONS. A. Special methods are required to follow out the details of the structure and division of nuclei, but if you have not the opportunity of examining preparations illustrating these, a good deal may be made out as regards the chromatin and the behaviour of the chromosomes in the process of mitosis by the following simple method. Obtain a young, gilled-larva or tadpole of the common newt (p. 218), kill, and place it in corrosive sublimate for about half an hour: wash thoroughly in water and transfer first into weak, and then into strong alcohol (p. 137). Stain entire, either with a solution of hematoxylin, which can be bought ready prepared, or alwm-carmine, which you can prepare yourself by dissolving 200 grams of ammonia-alum in water, boiling the solution and adding carmine to excess, filtering, and diluting with three or four times its bulk of water ; a drop or two of carbolic acid should be added to the solution. Strip off small pieces of the skin of the stained preparation, and after putting them through weak and strong alcohol, transfer to absolute alcohol, then to turpentine or oil of cloves, and mount in Canada balsam (p. 139). Examine, comparing Fig. 136 and pp. 541-545, and sketch as many different stages as possible. B. Observation of the details concerned in the process of maturation and fertilization of the ovum is too difficult for the beginner, but polar ced/s may easily be seen in the living, freshly-laid eggs of one of the common pond-snails (e.g. Leémneus stagnalis), in which some of the stages of segmentation can also be observed. Keep some of these snails in a glass vessel with water weeds, and notice that the eggs, when laid, are enclosed, a number together, in a common ‘gelatinous envelope. Separate the eggs from one another with needle, and examine in water under the microscope. Note that the small egg is surrounded by fluid enclosed ina relatively large egg-case, and observe the minute polar cells at the periphery of the ovum. C. A series of models of the development of Amphroxus, to be found in most museums, should be carefully examined (conpare Figs 141- 144). “D. In order to follow out the development of the chief organs in a 580 PRACTICAL DIRECTIONS CHAP, Vertebrate animal, it is necessary to make a number of serial transverse sections. For this purpose chick-embryos (see below) are, on the whole, the most convenient and satisfactory, but if you also wish to make sections of embryos of the frog, proceed as directed on p. 214. Serial sections of embryos in different stages should be mounted on the same slide, after smearing it with collodion and oil of cloves (p. 139). It is a matter of some difficulty to make satisfactory sections of the early stages : the most important stages for the present purpose subsequently to segmentation are from the time when the embryo begins to become elongated up to hatching. A number of fresh, impregnated fowl’s eggs should be obtained and placed in an incubator at a temperature of from 37 to 4o C. 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 incuba- tion. To expose the embryo, place the egg in a dish of warm water (temperature as above), in order that, after the first day, the beating of the heart and the circulation of the blood may not be stopped. With the forceps, tap the surface of the egg lying uppermost so as to break the shell into small pieces, which can then be removed: cut away sufficient of the shell-membrane with the scissors to expose the entire embryo and blastoderm. The early stages are difficult to observe, and the most important 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. Prepare as above. 1. Examine first an enéncubaled egy, as directed above, and make out ts structure (compare Fig. 138). (The blastoderm undergoes seg- mentation before the egg is laid.) 2. First day of incubation (18-20 hours). Examine with a lens and compare Fig 145 A. Then carefully cut round the blastoderm with fine scissors, float it off in the water, preserve, stain, and mount entire in Canada balsam. (The removal and preparation of the blastoderm at this stage is rather difficult, and sections of the next stage will illustrate the chicf points equally well). XI PRACTICAL DIRECTIONS 587 3. End of first day (about 24 hours). Examine as before, com- paring Fig. 145 B. Sketch, and then preserve, stain, and eventually cut sections. Note the ectoderm, endoderm, mesoderm, medullary groove and its closure to form the medullary cord ; the neural ridges (p. 568) ; the xotochord ; the barietal (somatic) and visceral (splanchnic) layers of the mesoderm, and the calome (compare Figs. 64, 65, and 147). Sketch one or two typical sections. 4. Second day. Examine, prepare, cut sections, and sketch as before, noting the fore-, méd-, and hind-brain, the optic vesicles, lens, auditory pits, heart and vitelline vessels, and the head-fold of the amnion, as well as the increase in the number of mesodermal segments (compare Figs. 148, 149, and 153). 5. Zhird day. Examine, prepare, and sketch as before, noting, in addition to the points already referred to, the further gradual folding off of the embryo from the yolk (compare p. 454) and extension of the amnion and area vasciulosa ; the cerebral flexure; the visceral arches and clef/s and the arterial arches ; the mesonephrtc duct; and the further development of the eye and ear (compare Figs. 147, 148, 149, 150, 153, and 155). 6. Fourth to sixth days. Observe the further development of the parts already seen and also the émb-deds and the allantois (compare Figs. 153-155). Sketch. 1. Examine and compare a few later stages. E. Obtain some advanced embryos of the rabbit (p. 542) or ral. and examine ¢@x sé¢z in the uterus before removing and preserving them. Note the avezon and examine the placenta and its connection with the uterine wall and with the foetus. Sketch. INDEX INDEX (The numbers in ttalics refer to practical directions.) A Aidan abdominal cavity, 20, 347, 418, 467, 489 Abdominal pore, 416 Abiogenesis, 282 Acanthias, 415, 454 Acetabulum, 50, 486 Achromatin, 129, 231, 541, 542 Acoustic spots, 188 Acrania, 406 Acromion, 483 Adaptation, 224 Adenoids, 498 Adrenal bodies, 145, 431, 521 Agamobium, 314 Alimentary canal, see Enteric canal Allantois, 579, 583, 587 ALLOLOBOPHORA, see Earthworm. Alternation of generations, 314 Alveoli of jaws, 493 Amnion, 579, 587 AMCBA: occurrence and general characters, 229; movements, 232; resting condition, 232; nutrition, 233; growth, 234; excretion, 235 ; respiration, 235 ; metabolism, 236; reproduction, 236; immortality, 236 ; conjuga- tion, 237; death, 237 ; animal or plant ? 255; practical directions, 238 Amceboid movements, 106, 231 Amphibia, 219, 403, 466 Amphiccelous, 423 AMPHIOXUS, see Lancelet Ampulla, of semicircular canals, 188 Ampulla, of sensory canals of the integument, 448 Anal segment, 319 Analogous, analogy, 217 Anatomy, 217 Animals and Plants : comparison of typical forms, 255; discussion of doubtful forms, 255, 257 ; bound- aries artificial, 258 Ankle, see Tarsus Annulata, 220, 341 ANODONTA, see Mussel Anura, 219 Antenna, 352 Antennary gland, 361 Antennule, 352 Anus, 6, 267, 276, 319, 357, 386, 416, 429, 469, 497 Aorta, aortic arches, see Arteries Aperture, or apertures (see also Foramen, and under Nephridium, Kidney, Pores, Reproductive organs, &c.) auditory, Crayfish, 367; Rabbit, 470, 472; cloacal, see anus ; exhalant and inhalant, 352 502 INDEX Aphis, 552 Appendages, lateral, and their skele- ton: Frog, 5, 28, 36, 48, 51; Crayfish, 346, 349 5 Dogfish, 417, 426; Rabbit, 484, 486 ; Develop- ment of in Vertebrates, 578, 587 Aqueous chamber and humour, 103 Arachnoid fluid, 155 Archenteron, 201, 558 Archicerebrum, 366 Area vasculosa, 572, 581, 587 Arm, see Fore-limb Arterial arches, 80, 436 Arteries: Frog, 27, 80; Crayfish, 362 ; Mussel, 392 ; Dogfish, 433- 438; Rabbit, 503-506; Vertebrate embryo, 572 Arthobranch, 361 Arthropoda, 220, 346, 370. Articular membrane, 347, 354 Articular processes, see Zygapo- physes Artificial selection, 227 Arytenoid cartilage, 144, 500 Ascaris, 153 Asexual generation, see bium Asexual reproduction, see Fission, Budding, Spore Assunilation, 149, 234, 300 ASTACUS, see Crayfish Astragalus, 51, 457 Astrosphere, 543 Atlas vertebra, 481 Atrial pore, 405 Atrium, 404, 406 Atrophy, see Vestige Auditory capsule, 39, 41, 420, 475 Auditory organ: /yog. 45, 186; Cravjish, 367 ; Mussel, 394 ; Dog- Jish, 449; Rabbit, 518 ; Develop- ment of, in Vertebrates, 571, 587 , Auditory ossicles, 475, 519 Auricle, see Heart Automatism, see Movements Aves, 219, 403 Agamo- Axial fibre : of Vorticella, 276; of Carchesium, 281 Axial parts, 4 Axis fibre, see Neuraxis Axis vertebra, 482 B Been 152; structure, 257; nutrition, 257 ; animals or plants? 257; rapid multiplication, 282; practical directions, 260 Backbone, see Vertebral column Basal cartilages of fins, or basalia, 425-427 Bell of medusze, see Umbrella Bilateral symmetry, 291 Bile, 69, 431 Bile-duct, 68, 431, 499 Bile-passages, 133 Binomial nomenclature, 215 Biogenesis, 282 Biology, 1 Bird, development of, 563, 572, 576, 578, 587 Birds, see Aves Bladder, see Gall-bladder and Uri- nary bladder Blastoccele, see Segmentation-cavity Blastoderm, 369, 454, 563, 586 3 embryonic and extra- embryonic portions, 572, 578 Blastodermic vesicle, 565, 584 Blastomere, 556 Blastopore, 201, 340, 369, 558, 562 Blastula, 558 Blind spot, 539 Blood: #veg, 20, 78, 85; Earth- worm, 329, 330; Crayfish, 365; Mussel, 392 ; Lancelet, 405 ; Dos- fish, 442; Mammalia, 467 Blood-corpuscles: colourless, see Leucocytes ; red, 105, 442 Blood-sinus, see Sinus Blood-vessels: Frog, 78, 1073 Earthworm, 329; Crayfish, 362 3 INDEX Mussel, 392; Lancelet, 408; ‘Dogfish, 433-442 5 Rabbit, 503 Body-cavity, see Ccelome Body of vertebra, see Vertebra Body-segments, see Metamere Bojanus, organ of, 390 Bone: cartilage and membrane, 43, 4725 nature of, §2; microscopi- cal structure of, 116 Bones, see Endoskeleton, Skull, Vertebra, Ribs, Sternum.: and under individual bones of limbs Botany, I BoOUGAINVILLEA : occurrence and general charactets, 304; micro- scopic structure, 304; structure of Medusa, 307 ; nervous system, 3115; organs of sight, 308, 312; reproduction and development, 312; alternation of generations, 3143 practical directions, 3777 Brachial plexus, see Nerve-plexus Brain: /rog, 28, 156, 202; Larth- worn, 333; Crayfish, 365 3 Alus- sel, 392; Lancelet, 405, 408; Dovfish, 443; Rabbit, 501; de- velopment of, in Vertebrates, 202, 506, 587 , Brain-case, see Skull Branchia, see Gill Branchial apertures, arches, clefts, and septa: Zadpole, 204, 206; Lancelet, 405, 407; Dogfish, 416, 422, 429, 432; development of, 571 Branchial rays, 422 ‘Branchial vessels: Crayfish, 360, » 363 ; Alesse, 392 ; Lancelct, 409 ; Dogfish, 433; Tadpole, 435 Breast-bone, see Sternum Bronchus, 500 Buccal cavity,:16, 327, 429 Buccal groove, 262 Bud, budding, 301, 303, 313 Bufo, Bufonidze, 218 Bulb, see Medulla oblongata Bulbus aortz, 89 Bulla, tympanic, 475 Byssus, 396 Pract. Zoot. 593 Cc (Beret 498 Calcaneum, 51, 487 Calcar, 52 Canal : central, of spinal cord, 156, 408, 443; naso-palatine, 490; neural, see Vertebral column; neurenteric, 203; radial and circu- lar of Medusa, 307 ; semicircular of ear, see Auditory organ and Mem- branous labyrinth; sensory, of Dogfish, 416, 448; sternal, of Crayfish, 349 ; vertebrarterial,481 Canaliculi, see Bone Canine-teeth, 492 Capillaries, 95, 363, 435, 438 Capitular facet and capitulum, 481 483 Carapace, 347 Carbohydrates, 72 Carbon dioxide, 66 Carchesium, 281, 288 Cardiac division, see Stomach. Carotid arch, see Arteries of Frog, Dogfish, and Rabbit Carpus, 50, 485 Cartilage, 20, 35, II5, 419, 470; calcified, 46, 48, 419 Caruncle, 582 Castings of earthworm, 318 Cell, 110, 231, 540; sce various types Cell-colony, see Colony : Cell-differentiation, see Differentia- tion, and under development of various types Cell-division, 544, 585 Cell-membrane or wall, 232, 244, 542 Cellulose, 244, 254, 405 Cement, 429, 493 Centrosome, 544 Centrum, see Vertebra Cephalothorax, 347 Cerebellum: Frog, 157; Dogfish, 443, 513 Cerebral flexure, 568 Cerebral ganglion, see Brain QQ under 504 INDEX Cerebral hemispheres : Frog, 159; Rabbit, 511 Cerebral nerves, see Nerves Cerebral vesicles, 566 Cerebro-pleural ganglion, 392 Cervical groove, 347 Cheetopoda, 341 Chalaza, 548 Change of function, 433 Chela, 352 Cheliped, 350 Chiasma, optic, 164, 443, 513 Chick, see Bird Chitin, 320, 347 Chlorophyll, 242, 251 Choroid, 183, 517 Choroid-fissure, 570 Choroid-plexus, 157, 512 Chromatin, 129, 231, 541, 542 Chromatophores, 243, 299 Chromosomes, 544 Cilia, and ciliary movement, 109, 242, 250, 265, 273, 275 Cilia, absence of in Crayfish, 355 Ciliary folds, muscles, nerves, and vessels, 184, 517 Ciliata, Ciliate Infusoria, 261, 286 Circulation of blood, 86, 89, 329, 363, 392, 409, 441, 503 Circulatory organs, see Blood- vessels and Lymphatic system Cirri, 406 Clasper, 417, 426, 453 Class, 219 Classification, 217, 220, 223, 396, 403 Clavicle, 47, 483 Clitellum, 319, 339 Clitoris, 524 Cloaca, 23, 386, 429 Cloacal aperture, see Anus Cnidoblast, 297 Cnidocil, 297 Coagulation of blood, 78, 107 Cochlea, 187, 518 Cocoon, 339 Ceelenterata, 314 Ceelome, 20, 203, 314, 320, 325, 340, 360, 368, 386, 418 ; Develop- ment of, 203, 561, 565 Coelomata, 314 Ccelomic epithelium, 322, and see Epithelium Ccelomic fluid, 328 Collaterals, 171 Colon, 497 Colony, Colonial organisms, 281 Columella, 45, 189 Commissures, see Blood-vessels and Nervous system ; of Brain, 515 Conchiolin, 384 Concrescence, 347 Condyle, occipital, see Skull Cones of retina, 185 Conjugation, 197, 237, 268, 278 Conjunctiva, 182 Connectives, 333, 365 Connective-tissue, 18, 113 Contractility, nature of, 112 Conus arteriosus, 79, 88, 433 Coracoid, 47, 427 Coracoid process, 483 Cordylophora, 317 Cornea, 182, 366 Corpus adiposum, see Fat-body ; callosum, 511; cavernosum and spongiosum, 523, 524; striatum, 514 Corpuscles, see and Leucocytes Cortex of Brain, 160, 511 Cortical layer, 264, 269, 273 Craniata, 406 Cranium, see Skull CRAYFISH : general characters, 346, 9723 limited number and con- crescence of metameres, 346; appendages, 348, 349, 3785 exo- skeleton, 347, 349, 355, 372, 3793; muscular system, 355, 374; 375,377 3 enteric canal, 356, 3753 gills, 360, 3733; kidney, 361, 378; blood-system, 361, 374, 3753 nervous system, 365, 3777; sense- organs, 366, 378; reproductive organs, 368, 375; development, 369, 563 Creation, 221 Cribriform plate, 474 Blood-corpuscles INDEX 595 Cricoid cartilage, 500 Crop, 327 Cross-fertilization, 339 Crura cerebri, 157, 443, 513, 566 Crustacea, 292, 371 Crystalline style, 386 9 lens, see Lens of Eye Ctenidia, 383 Cutaneous glands: Rabbit, 468 Cuticle: in unicellular animals, 251, 264, 2733 in multicellular animals, 306, 320, 321, 355, 383 Cyst, see Cell-wall and Encystation Cystic duct, see Bile-duct Frog, 129; D Duescte: cells and nuclei, 250, 254, 545 Death, 11, 152 Decalcifying, directions for, 7377 Decidua, 583 Decomposition, II, 151; and see Putrefaction Degeneration, 406 ; and see Vestige Dehydrating, directions for, 7.37. Dental formula, 494 Dental lamina and papilla, 429 Dentine, 417, 429, 493 Deric epithelium, see Epiderm Derm, 128, 355, 417 Dermal teeth, 418 Descent, doctrine of, see Evolution Development, meaning of the term, 9, 273, 280. For development of the various multicellular types, see under their names. Practical work, 272-214, 585 Dialyser, 73, 77 Diaphragm, 467, 489, 490 Diastema, 490 . Diastole of heart, 90 ; of contractile vacuole, 266 Diencephalon, 157, 443, 512, 566 Differentiation, 204, 206, 237, 261 ’. Diffusion, 73, 551, 583 Digestion, 68; intra- cellular, 300, 328 and extra- Digestive glands, 359, 386, and see Digestion, Enteric canal, Liver, Glands, Pancreas Digestive system, see Enteric canal Digits, 56, 485, 488 Dimorphic, Dimorphism, 250,310 Dicecious, 312, 368 Diploblastic, 293, 304, 324 Directions for dissecting, 7g; for drawing, 14; for killing, 77, 279, 259, 287, 310, 341, 372, 397, 5253; for preparing skeletons, 53) 379, 456, 5253 for injecting blood-vessels, 99, 374; 379s 458, 459, 526; for microscopic work, 2793; for histological and embryological work, 7375, 585 Disc of vorticella, 275 Dispersal, 272, 278, 396 Dissecting instruments, &c., 72 Distal, 6 Distribution of food-materials, 148 Divergence of character, 223 Division of physiological labour, 206, 237 DoGFisH : General characters, 415, 456; exoskeleton, 417, 456; endoskeleton, 419, 4563; enteric canal, 428, 457, 459; gills, 432, 461; blood-system, 433, 458, 459, 401 ; nervous system, 442, ¢62; kidneys, 450, 4593; repro- ductive organs, 451, 459, 460; sense-organs, 448, 462, 463; de- velopment, 453, 563, 572 Dorsal, 6 Drum-membrane, see membrane Duct, see under names of individual ducts and glands Ductus arteriosus, 533 Duodenum, 22, 497 Dura mater, 155 Tympanic E ar, see Auditory organ; in- ternal, middle, and external, 450, 467, 469, 518, 519 QQ 2 596 INDEX EARTHWORM: general characters, 318, 347; metameric segmenta- tion, 319; ccelome and enteric canal, 320, 325, 328, 3742, 343, 345 ; cell-layers, 321, 345; blood- system, 328, 342, 7-44) 9453 ne- phridia, 331, 342, 344 3453 nervous system, 333, 7443 differ- entiation of organs and tissues, 335 3 reproduction and reproduc- tive organs, 336, ae 3433 de- velopment, 340, 562 Ecdysis, 355 Echinodermata, 397 Ectoderm, 202, 209, 293, 304, 307, 340, 369, 454, 558 Ectoplasm, 231 Efferent duct of spermary, 193, 338, 451 Egestion, 233 Egg of fowl, 547, 555, 556 Egg-cell, see Ovum Egg-sac, 337 Elasmobranchii, 415 Embryo, 9, 200; various types Embryology, 217 Embryonic membranes, 578, 5&7 Emulsification of fats, 75 Enamel, 418, 429, 493 Enamel-organ, 429 Encystation, 232, 244, 254, 272, 279 Endoderm, 202, 210, 293, 298, 304, 307, 340, 369, 454, 558, 565 Endoderm-lamella, 309, 310 Endolymph, 188 Endolymphatic duct, 187, 420, 449 Endoparasite, see Parasite Endoplasm, 231 Endophragmal system, 349 Endopodite, 350 Endoskeleton : Frog, 16, 35, 205; Lancelet, 406; ‘Dogfish, 419 ; Rabbit, 470; development of in Vertebrates, 576 Endostyle, 408 Energy, conversion of potential into kinetic, 235 ; source of, in chloro- phyll-containing organisms, 248 and see under Enteric canal: frog, 23, 2043 Earthworm, 320, 327; Crayfish, rece Mussel, 386 ; Luncelet, aos; 3 Dosfish, 429 5 Rabbit, 490- tao development of in Verte- brates, 204-210, 571 os Enteric epithelium, 322, and see Epithelium Enteroccele, 561 Enteron or enteric cavity, 291, 304, 308, 358 Epencephalon, 566 Epiderm: Frog, 128; Rudco, 320, 321; Crayfish, 355; Alussel, 385; Lancelet, grr; Dogfish, 4173; Rabbit, 468 Epididymis, 451, 521, 576 Epiglottis, 495, 500 Epipharyngeal grocve, 408 Epiphysis of Vertebra, 480, 486 Epipodite, 352 Epistoma, 349 Epistylis, 281, 26° Epithelial cells: columnar, 107 ; ciliated, 109; glandular, 130 e¢ seq. 3 squamous, I10 ; stratified, 128 Epithelium, 109, 311 ; ceelomic, 322, and see Peritoneum ; deric—see Epiderm ; enteric, 322, and see Endoderm Equivocal generation, see Abio- genesis EUGLENA : occurrence and general characters, 251 ; movements, 251; structure, 251; nutrition, 252; resting stage, 254 reproduction, 254; animal or plant? 255; practical directions, 259 Eustachian tube or recess, 17, 189, 495, 519 Eustachian valve, 506 Evolution : organic, 221, 285; of animals and plants, 258 Excretion, 148, 150, 235 Excretory organs, see Kidney and Nephridium, and compare Con- tractile vacuole Exopodite, 350 Exoskeleton : cuticular, 306, 355, 383; dermal, 418, 429; epider- mal, 466, 468 Expiration, 143 Extra-cellular digestion, 234 Eye: Frog, 4, 181; Craysish, 366 ; Dogfish, 416, 449; Rabbit, 517 ; development in Vertebrates, 569 Eye: compound, 366 Eyelids, 5, 416, 469 Eye-muscles, see Muscles of eye Eye-spots or ocelli: Zugdena, 254; Medusa, 309, 312; Lancelet, 408 Eye-stalks, 349, 353 Base 487 Feces, 8 Fallopian tube, 523 “Family, 218 Fascia, 59 118, Fat-body, 25 Fats, 72 Femur, 51, 487 Fenestra ovalis, 46, 189, 475, '518 Fenestra rotunda, 476, 518 Ferment, fermentation : amylolytic, 743 peptonizing or proteolytic, 745 putrefactive, 256, 257 Fertilization, 197, 554, and see also Conjugation, and under develop- ment of various types Fibrin, 107 Fibula, 51, 487 Filum terminale, 155; 515 Fingers, see Digits Fin-rays, 407, 414, 425, 426, 4275 dermal, 426 Fins: Zadpole, 207 ; Lancelet, 404, 406 ; Dogyish, 417, 425 Fishes, see Pisces Fission, 106, 198, 236, 250, 268, 272, 277; multiple, 254, 272 Fissures of spinal cord, 155 Fixing, directions for, 736 Flagellata, Flagellate 261, 286 Flagellum, 242, 251, 256, 298; of antenna and antennule, 353 F Infusoria, Flocculus, 476, 513 Feetus, 524 Follicle, ovarian, see Ovisac Fontanelle, 43, 420 Foods, 67, 72 Foot, of mussel, 382 ; and see Pes Foramen : lachrymal, 477; inter- vertebral, 38, 424, 481 ; magnum, 40, 420, 470; obturator, 486 ; of Monro, 160, 514 Foramina for cerebral nerves, see Skull Fore-brain, 202, 566 Fore-gut, 358 Fore-limb or fin, see Appendages Fornix, 514 Fossa ovalis, 504 Fossils, 223 FroG: Preliminary account, 4; mouth cavity, 16; skin and muscles, 17, 77; abdomenand its contents, 20, 72; neural cavity and its contents, 27, 33; struc- ture of limbs, 28, 34; skeleton, 35, 533 joints, 55, 6¢; muscles, 57, 643 enteric canal and diges- tion, 67, 76; vascular system, 78, 98; circulation of blood, 89, 1033 lymphatic system, 97, 98; simple tissues, 104, 727; com- pound tissues and glands, 126, 739; lungs and larynx, 141, 752; kidneys, 145, 75.3; structure and functions of nervous system, 154, 7753 sense-organs, 179, ror; reproductive organs, 193; 270; fertilization of eggs, 9, 197; de- velopment, 9, 198, 272, 274, 562, 586; metamorphosis, 11, 206, 2r2; Classification, 215; side dissection, 465; summary of characters, 466 Function, see Physiology G (yeimnaaes 22, 69, 431, 499 Gallus, see Bird Gamete, 197, 268 Gamobium, 314 598 INDEX Ganglion, 163, 167 ; and see Nerve- ganglia Gastric glands, see Glands Gastric juice, 71, 74, 132 Gastric mill, 358 Gastrolith, 359 Gastrula, 558 Gastrocnemius, 59 Gemmation, see Budding Generation, asexual, see mobium Generation, sexual, see Gamobium alternation of,see Alter- nation of generations Generative organs, see Reproductive organs Genus, 215, 218 Germinal disc, 556 Germinal epithelium, 194, 196, 336 ss vesicle and spots, 546 Germplasm, 555 Gestation, 525 Giant-fibres, 334 Gills : Tadpole, 10, 204, 207; Cray- ish, 360; .Jussel, 382, 386; Doghsh, 4325 455 Gill-arches and clefts, see Branchial apertures, arches, clefts and septa Gill-cover, 347, 360 Gill-rays, see Branchial rays. Gizzard: Earthworm, 327; Cray- fish, 356, 358 Gland-cells: Hydra, 298, 299; Earthworm, 327, 339; and see Glands and Goblet-cells. Glands: Cowper’s, 523; digestive, see Enteric canal; gastric, 131, 431, 497 ; green, 361 ; Harderian, 186, 518; lachrymal, 518 ; mam- mary, 467, 468, 489 ; Meibomian, 518 ; cesophageal, 327 ; perineal, 469, 523; prostate, 523; race- mose, 135; rectal, 523; salivary, 494; and see kidney, liver, pan- creas, &c. Glenoid cavity, 47 Glochidium, 396 Glomerulus, 146, 574 Glottis, 17, 495 Glycogen, 134 Aga- ” Goblet-cells, 109, 130, 131 Gonad, 193: and see Reproductive organs Gonaduct, see Reproductive organs Grey matter of spinal cord and brain, 156, 160, 167 Growth, 236 Gryllotalpa, 550 Gullet, 253, 265, 275; and see Enteric canal . H H zemal arch and spine, 425 Heematochrome, 243 HMarococcus: general eharac- ters, 240; rate of progression, 240; ciliary movements, 242, 250; colouring matter, 242: motile and stationary phases, 244; nutrition, 245; source of energy, 248; reproduction, 250; dimorphism, 250; animal or plant? 255; practical directions, 259 : Heemocyanin, 365 Hemoglobin, 107, 330 Hairs, 466, 468 Hallux, 487 Hand, see Manus Hardening, directions for, 736 Head, 4, 347, 349,416, 468 Heart, 20, 79, 87, 362, 392, 433, 503 ; development of, 442, 572; pulsation of, 90 Heat, evolution of, 151 Hemibranch, 432 Hepatic ducts, see Bile duct Hepatic caecum, 408 Hepatic portal system, see Portal system Hepato-pancreas, 359 Heredity, 225 Hermaphrodite, see Moncecious Heterogenesis, 284 Hibernation, 8 Tligher (tripoblastic) animals, uni- formity in general structure, 324 Hind-brain, 202, 566 INDEX 599 Hind-limb or fin, see Appendages Hind-gut, 357 Hinge of lamellibranchiate shell, 393 Hip-girdle, see Pelvic arch Hippocampus, 514 Histological methods, 585 Histology, 104 Holobranch, 432 Holophytic nutrition, 247, 253 Holozoic nutrition, 247, 253 Homogenesis, 284 Homology and homologous, 217, 310 Homology, serial, 39, 52 Host, 271 Humerus, 48, 484 Hybrids, 216 Hypra:, occurrence and general characters, 289; species, 291 ; _ Movements, 291; mode of feed-. ing, 292; microscopic structure, 292; digestion, 299; asexual, artificial, and sexual reproduction, 301 ; development, 303; practi- cal directions, 774 Hydranth, 304 Hydroid polypes, 303 Hydrozoa, 314 Hyoid, 40, 44, 422, 472, 479 Hyomandibular, 422 Hypobranchial groove, see Endo- style’ Hypostome, 289, 304, 308 120, I35, I lt 22, 497 Ilium, 50, 486 Imbedding, directions for, 777 Immortality, 236 Impregnation, see Fertilization Incisors, 468, 492, 494 Income and expenditure, 148, 236, 245 Incubation, 586 Incus, see Auditory ossicles {ndividual, 220, 281 Individuation, 281, 301 Infundibulum, of brain, 159, 443; 513, of lung, 501 Infusoria, 261, 286 Ingesta and egesta, balance of, 236 Ingestion, 8, 233 Inguinal canal, 521 Injection of blood-vessels, 99, 374, 399s 458, 459, fOL, 526 Innominate bone, 51, 486 Insertion of muscle, 60 Inspiration, 143 Integument, structure of, 127 Integumentary sense-organs, 414, Intercellular substance, 115, 117 Interneural plate, 424 Interstitial cells, 293 Intervertebral discs, 480 Intervertebral foramina, see Fora- men Intervertebral substance, 424 Intracellular digestion, 234 Interrenals, 431 ‘Intestine, 22; see its various “sub- divisions and Enteric canal Invagination, 558 Iris, 5, 182, 571 Irritability, 60, 169, 232, 276 Ischium, 50, 486 J acobson’s organ, 517 Jaws, 17, 352, 415, 420, 472, 477; 479 Joints, 55, 348, 354 z K K cctroutinesis, see Mitosis Keber’s organ, see Pericardial gland Kidney: Frog, 26, 145; Crayfish, 361 ; ALussel, 390; Doyfish, 450; Rabbit, 520; development of, in Vertebrates, 573 600 INDEX L ir palp, see Palp Labrum, 356 Lachrymal ducts and glands, see Naso-lachrymal] ducts and glands Lacune, see Bone Lamelle of gills (Mussel), 388 Lamellibranchiata, 396 Lamina terminalis, 514 LANCELEr: general characters, 404; fins, 405; skeleton, 405; gill-slits and bars, 407; enteric canal, 408; blood-vessels, 408; nephridia, 408; nervous system, 408 ; gonads, 409 ; development, 409, 5573; practical directions, FlO-4l3 Larva, 9, 349, 370, 395, 396, 409 Laryngo-tracheal chamber, 141 Larynx, 489, 500 Lateral line, 416, 448 Legs, see Appendages Lens of eye, 183, 517, 569 Lepus, see Rabbit Leucocyte, 105, 328 Life, origin of, 281; and see Bio- genesis Life-history, 8, 206; and see under various types Ligaments, 55, 57 Limbs, see .Appendages Lips, 468 Liver, 20, 133, 204, 359; 408, 431, 499 Lobster, 346 Lumbricide, 341 Lumpricus, see Earthworm Lungs, 10, 22, 141, 204, 208, 501 Lymneus, 585 Lymph, 18 Lymph-hearts, 97, 9f Lymphatic glands, see Adenoids Lymphatic system, 18, 97, 442 M M alleus, see Auditory ossicles Malpighian capsule, 146, 574 Mammalia, 219, 403, 466 Mammary glands, see Glands Mandible, 16, 44, 352, 422, 479 Mantle, 381 Mantle-cavity, 385 Manubrium: of Medusa, 307; of malleus, 519; of sternum, 483 Manus, 5, 50, 468, 485 Marrow-cavity, 48 Matrix, see Intercellular substance Maturation of ovum, 552 Maxilla: Crayfish, 352 Maxilliped, 350 Meckel’s cartilage, 44, 422 ‘Mediastinum, 501 Medulla oblongata, 157, 443, 513 Medullary: cord, folds, groove, and plate, 202, 559; sheath, 167; substance of Infusoria, 264, 269, 273 Medusa, 304, 307-312 Megagamete, 279 Meganucleus, 265, 273 Megazooid, 251, 279 ; Membranous labyrinth, 186, 449, 518 Mesencephalon, see Mid-brain Mesenteron, see Archenteron Mesentery, 22, 431, 498 Mesoderm, 202, 203, 210, 340, 369, 454, 559, 565; vertebral and lateral plates of, 565 Mesodermal segments, 565 Mesoglcea, 293, 307 Mesonephros, 573, 574 Metabolism, 149, 236 Metacarpus, 50, 485 Metamere, Metameric tion, 319, 346, 354 Metamorphosis, 11, 209, 280 Metanephros, 573, 576 Metapleural fold, 406 Metatarsus, 51, 488 Metazoa, 286 Microgamete, 279 Micrometer, 727 Micromillimetre («) = yyy of a millimetre, or gs}gq Of an inch Micronucleus, 265, 275 Micropyle, 400, 547, 554 Microtome, 138 segmenta- INDEX 601 Microzooid, 251, 279 Mid-brain, 202 Mid-gut, 357 Milk-glands, see Glands Milk-teeth, 493 Milt, see Spermatic fluid Mitosis, 545, 585 Molars, 492, 494 Mollusca, 220, 396 MONADS: occurrence and general characters, 256 ;: movements, 256 ; nutrition, 256 ; animals or plants ? 2573 vapid multiplication, 282 ; practical directions, 260 Moneecious, 302 Morphology, 217 Morula, see Polyplast Mother of pearl, see Nacre Mounting sections, directions for, 139 Mouth, 4, 16, 253, 265, 275, 290, * 307, 319, 356, 386, 408, 415, 428 Mouth-cavity, see Buccal cavity Movement, spontaneous, volun- tary, and involuntary, 7, I1, 112, 172, 232, 312, 335, 346, 417; “and see under various types Mucous membrane, 17 Mule, see Hybrid Miillerian duct, 450, 574 Multinucleate, 269, 369 Muscle-fibres: striped, 112, 355; unstriped, I11f, 131, 133; and see under various types Muscle-processes, 293, 299, 304 Muscles: Frog, 18, 57, 59, 63, 205 ; Crayfish, 321, 355 3 Afussel, 385 ; Rabbit, 489; and see Myo- “mere. Muscles: of eye, 186, 449, 517; of middle ear, 519 Muscles, papillary, 504 Muscular contraction, 160, 276 Muscular impressions on shell, 384 Muscular layers of enteric canal, “70, 75 . Muscular system, see under various .. types ; development of, in Verte- brates, 203, 565, 576 Muscularis mucosee, 133 MUSSEL: general characters, 381, 997 ; mantle, shell, and foot, 381, 397; 398 ; food-current, 382, 397 ; gills, 382, 386, 398, 399, goo; muscles, 385, 798; enteric canal, 386, 399, gor; nephridia, 390, goo; hblood-system, 392, 3993 nervous system, 392, goo ; sense- organs, 394. gor; gonads, 394, go2; development and meta- morphosis, 395, 400, 562 Mustelus, 415, 454 Myocomma, 418 Myomere, 203, 355, 404, 418, 565 Myophan layer, 264 N i N acre : nacreous layer, 384 Nasal organ, see Olfactory organ Naso-lachrymal duct, 186, 477, 518 Naso-palatine canals, see Canals Naso-pharynx, 495 Natural History, 2 Natural selection, 226 Nauplius, 370 Neck, 468 Nemathelminthes, 397, 457 Nematocyst, 295 Nephridiopore, 332 Nephridium, 146, 331, 361, 368, 390, 404, 406, 408, 450, 574 Nephrostome, 145, 146, 331, 574 Nerve-cells, 167, 298 Nerve-collar, 333 Nerve-cord : ventral, 333, 365 Nerve-fibre, 167 Nerve-foramina, 478 ; and see Skull and Vertebral column Nerve-ganglia, 162, 163, 164, 167, 333, 365, 366, 392 Nerve-plexus: brachial, 161, 515; coeliac, 516; sciatic or lumbo- sacral, 162, 515 Nerve-roots, 163, 409 Nerves: afferent and efferent, 166, 175; cerebral, 163, 445, 515, 568 ; of lateral line, 447 ; motor 602 INDEX and sensory, 162, 169, 3343 sciatic, 62, 162, 515; spinal, 160, 409, 445, 515, 568 ; sym- pathetic, 162, 445, 516; vaso- motor, 174; visceral, 366 Nervous impulse, 62,154, Nervous system: Frog, 155; Hydra, 298; Bougainvillea, 311 ; Barth- worm, 333 3 Crayfish, 365 ; ALus- sel, 392; Lancelet, 404, 408 ; Dogfish, 442; Rabbit, 511; de- velopment of, in Vertebrates, 565, 587 ’ Neural arch, spine, and canal, see Vertebra Neural plate and process, 424 Neuraxis, 167 Neurenteric canal, see Canal Neurilemma, 167 Neuroceele, 443; and see Canal, central of spinal cord, and Ven- tricles of brain Neuroglia, 169 Newt, 218 Nictitating membrane, 5, 469 Nodes of nerve-fibre, 167 Nose, see Olfactory organ Nostrils, 5, 17, 180, 416, 448, 468, 470, 517 Notochord, 576, 587 Nuclear division, 544, 585; in- direct, 545 Nuclear membrane, 542; spindle, 544 Nucleolus, 109 ef seg., 541, 542 Nucleus, 106 ef seg., 129, 231, 243, 254, 265, 269, 273 Nucleus, conjugation-, 268; seg- mentation-, 554 Nutrition, 67 ; and see under various types 203, 404, 425, 560, O Chis 317 Occipital condyle, see Skull Ocellus, see Eye-spot Oesophagus, see Gullet Odontoblasts, 429 Odontoid process, 482 Olecranon, 49, 485 Olfactory capsule, 39, 42, 420, 476 4 lobe, 160, 444, 511, 514, 568 sf organs: Frog, 180 ; Cray- Jish, 367 ; Mussel, 394; Dogfish, 416, 448; Aabhit, 511, 517; de- velopment of, in Vertebrates, 569 Olfactory pit, 408 Oligocheta, 341 Ommatideum, 366 Ontogeny, 224 Oogenesis, 196, 548 Oosperm, 198; and various types Oosperm, holoblastic and mero- blastic, 556 OPALINA: occurrence and general characters, 2693; structure and division of nuclei, 269; parasitic nutrition, 2715; reproduction, 271; means of dispersal, 272; development, 254; practical di- rections, 287 Optic chiasma, see Chiasma ; cup, 570; lobe, 157, 443, 513, 566; thalamus, 159; vesicle, 569 Oral cavity, see Buccal cavity Oral hood, 406 Orbit, see Skull Order, 218 Organ, 30, 151, 238, 335 Organism, 231 Onigin of muscles, 60 »» of species, 225 Osmosis, see Diffusion Osphradium, 394 Ossicles : of gizzard (Crayfish), 358, 376; auditory (Rabbit), see Audi- tory ossicles Ossification, 44 Ostia : of heart, 362; of gills, 389 Otocyst, 394 Otoliths, 188 Ovary : Frog, 23, 193, 195; and see Reproductive organs Oviduct, 25, 196, 337, 368, 453, 523, 574 see under INDEX 603 Ovisac, 195, 523 Ovum, 195, 546, 5553 under various types. Ovum : alecithal, centrolecithal, and telolecithal, 556 Oxidation of protoplasm, 149, 248 and see P Passes 224 Palate, 490 Pallial line, 384 Pallium, see Mantle Palp: Crayfish, 353 ; Mussel, 386 _ Pancreas, 22, 70, 134, 204, 431, 499 Pancreatic juice, 70 Papill of tongue, 180, 492 Parachordal, 577 Paragnatha, 356 Paramylum, 251 PARAMECIUM: structure, 262; mode of feeding, 266; reproduc- tion, 268; conjugation, 268 ; practical directions, 286 Parasite, parasitism, 33, 153, 269, 271 Parietal layer of peritoneum, 26, 322; see also Mesoderm Parthenogenesis, 552 Patella, 487 Pectoral arch, 20, 36, 46, 426, 483 » fin or limb, see Appendages Pedal ganglion, 392 Pelecypoda, 396 Pelvic arch, 20, 36, 50, 426, 486 », fin or limb, see Appendages Pelvis of kidney, 521 Penis, 470, 523 Pepsin, peptone, 74 Pericardial gland, 390 Pericardial sinus, 362 Pericardium, 20, 386, 418, 490, 503 - Perichondrium, 55 Perilymph, 189 Perinzeum, 469 Periosteum, 55 Periostracum, 384 Peristaltic movements. 75 Peristome, 275 Peristomium, 319 Peritoneum, 22, 26, 322, 418, 490 Pes, 5, 51, 468, 488 Peyer’s patches, 498 Phalanges, 50, 51, 485, 488 Pharynx, 17, 327, 405, 408, 429, 495 Phylogeny, 223, 224 Phylum, 219 Physiology, 1, 217 Pia mater, 155 Pigment-cells, 128 Pigment-layer of retina, 185 Pigment-spot, 254 Pineal body, 159, 443, 513, 566 Pineal eye, 159, 567 Pinna, 467, 469, 519 Pisces, 219, 403; general characters of, 414 Pithing, directions for, 707 Pituitary body, 159, 443, 513, 566 Placenta, 467, 524, 583 Planula, 313 Plasma, 104 Plastic products, 249, 551 Platyhelminthes, 397 Pleopod, 350 Pleura, pleural membrane, 490 Pleurobranch, 361 Pleuron, Crayfish, 347 Podobranch, 361 Podomere, 349 Polar cells, 552, 585 Pollex, 485 Polycheta, 341 Polymorphic, polymorphism, 310 Polyplast, 200, 558; and see under various types Polystomum, 33 Pons Varolii, 513 Pores, dorsal, 325 Portal system: hepatic, 85, 440, 510; renal, 85, 438 Post-axial and pre-axial borders of limb, 484, 486 604 INDEX Poupart’s ligament, 489 Premolars, 492, 494 Prepuce, 470 Preservative fluids, 73? Primitive groove, 563 Prismatic layer, 384 Proamnion, 579 Processes of skull, 472 e¢ seg. Proccelous, 36 Proctodeum, 204, 358, 431 Pronephric duct, 573 Pronephros, 573 Pronucleus, male and female, 554 Prosencephalon, 444, 566 Prostate, see Gland Prostomium, 319 Proteids, 72 Protista, 257 Provococcus, see Hematococcus Protoplasm, 106, 541; and see Cell Protopodite, 350 Protovertebra, 565 Protozoa, 220, 286 Proximal, 6 Pseudobranch, 433 Pseudopod, 106, 231, 250, 298 Pterygiophores, 425-427 Pubis, 50, 486 Pulmonary artery and vein; sce Arteries and Veins Pupil, 5, 182 Putrefaction, 256, 257, 261, and see Decomposition Pyloric division and stomach, see Stomach Pyrenoid, 243 valve, of R eee : general characters, 467, 5253 skeleton, 470, 525 ; muscles and body wall, 488, 526, 527, 532, 535, 539 3 celome, 489, 528, 532, digestive organs, 490, 528-530 ; respiratory and vocal organs, 489, 500, 533, 534, 5373 circulatory organs, 503, 527-5353 nervous system, 511, 527; 53? 534) 537 3 sense - organs, 516, 576-538; urinogenital organs, 520, 528- 5323 development, 524, 564, 578, 583, 587 Racemose, 135 Radial canals, see Canals Radial symmetry, 291 Radiolaria, symbiotic relations with Zooxanthella, 299 Radio-ulna, 49 Radius, 50, 484 Rana esculenta, 216 RANA TEMPORARIA, see Frog Ranide, 218 Reagents, hardening, preserving, mounting, and staining, 735 Recapitulation, theory of, 224 < Rectal gland, 431 . Rectum, 23, 386, 431, 497 Reducing division, 551, 554 Reflex action, 169, 312 Regeneration, 336 Renal portal system, system Reproduction, 8, and see under various types, and also Asexual reproduction Reproductive organs, 23, 25, 193, 302, 312, 336, 368, 394, 409, 451, 521, 573 Reptilia, 219, 403 Respiration, 141, 144, 235, 330 Respiratory movements, 7, see Portal 142, 593 Retina, 183. 184, 570 Retinula, 366 Rhabdome, 366 Khinencephalon, 568 Rhizopoda, 286 Ribs, 424, 468, 483 Rocks, sedimentary and stratified, Soi Rodentia, 525 Rods and cones, 185 Rostrum, 349, 420 : Rudiment, often used for Vestige (7.2.) INDEX 605 S Scien 187 Sacculus rotundus, 497, 498, 529 Sacrum, 482 : Salamander, 218 Salivary glands, see Glands Saprophytic nutrition, 256, 257 Sarco emma, I12 Scales, 414, 418 Scapula, 46, 427, 483 Schizoceele, 565, 587 Sclerite, 376 Sclerotic, 182, 517, 571 Scrotal sac, 470, 521 ScyYLLium, see Dogfish Section-cutting, directions for, 126, 138 : Secretion, 130 Segment, see Metamere, Podomere Segmentation-cavity, 200, 558, 562 Segmentation of oosperm, 198, 200, 212, 303, 313) 340; 369, 585 Segmentation, equal and unequal, 556; discoid, 556 4 superficial, 369, 556 is metameric, see Me- tamere 5 -nucleus, 554 Selection, natural and artificial, 226, 227 Self-fertilization, 339 Seminal funnel, 338 » vesicle, 194, 338, 451 Sense-organs and cells, 179, 308, 312, 335, 414, 416, 448, 516 Septa, of Earthworm, 325 Septum: lucidum, 514; nasal, 42, 476 Serous membrane of embryo, 579, 83 ee aa bones, 485, 487, 488 Seta, 320, 348, 367 Sex-cells, primitive, 549 Sexual characters, external, 7, 368, 417, 469 ’ Sexual generation, see Gamobium Sexual organs, see Reproductive organs Sexual reproduction, see under various types Shank, 5 Shell, 381; larval, 395 Shell of egg, 453 Rat Shell-gland of Mussel-embryo. 395 ; of Dogfish, 453 Shoulder-girdle, see Pectoral arch Sinus: blood, 360, 362, 364, 438, 441; lymph, 18, 27; urinary and urinogenital 451; venosus, 80, 89 Siphon, inhalant and exhalant. 382 Skeleton, see Endo- and Exo- skeleton Skin, see Integument Skull; Frog, 16, 35, 39; Dogyish, 420; “Rabbit, 470; development of 576 Smell, organ of, see Olfactory organ Snout, 4 Somatic layer of mesoderm, 561 Somatopleure, 561 Spawn, 9 | Species, 215 e¢ seg. ; origin of, 222, 225 Sperm, or spermatozooid, 194, 368, 549 ; and see under various types Spermary, 25. 193, 194; and see Reproductive organs Spermatogenesis, 194, 548 Spermatophore, 368 Spermotheca, 338 Sperm-reservoir, 338 Sperm-sac, 338, 451 Spinal cord, 28, 155, 443, 515 Spiracle, 416, 433 Splanchnic layer of mesoderm, 561 Splanchnopleure, 562 Spleen, 23, 98, 431, 499 Spontaneous generation, see Abio- genesis Spores, 254, 279 Stalk of Vorticella, 273, 276 Stapes, 46, 189, 473, 519 Starch, 72, 243 Sterilised infusions, 283 Sternebree, 483 Sternum, 16, 48, 347, 483 Stimulus, various kinds of, 62 606 INDEX Stock, see Colony Stomach, 22, 70, 357, 386, 429, 495 Stomodxum, 204, 358, 429, 431 Struggle for existence, 225 Substitution of organs, ‘209 Sucker, 203 Supporting lamella, see Mesogloea Suprarenals, see Adrenals Suspensorum, 40, 422 Sutures, 50, 470 ° Swimmeret, see Pleopod Symbiosis, 299 Sympathetic, see Nerves Symphysis, 486 Syn-cerebrum, 366 Synovial capsule, 56 Systemic arch, Yo, 436, 506 Systole : of heart, 90 ; of contractile vacuole, 266 T fee organs, 179, 368, 394, 516 Tadpole, 9, 203 et seg. Tail, 9, 209, 406, 416, 468 Tapetum, 539 Tarsus, 51, 487 Taste-organs, 180, 492, 517 Teasing, directions for, 723 Teats, 468, 489 Teeth, 17, 358, 428, 468, 492 Teleostomi, 415 Telson, 347 Tendon, 59 Tentacles, 290, 291, 304 Tergum, 347 Testis, see Spermary Thalamencephalon, see phalon Thigh, 5 Thoracic duct, 511 Thorax, 347, 467, 489 Thread-cell, see Nematocyst Thymus, 431, 490, 503 Thyroid, 431, 503 Thyroid cartilage, 500 Tibia, 51, 487 Dience- Tibio-fibula, 51 Toad, see Bufo Toes, see Digits Tissues, enumeration of, 31 Tongue, 8, 17, 429, 492 Tongue-cartilage or bone, see Hyoid Tonsil, 490 Trachea, 489, 500 Trabecule cranii, 577 Transverse process, see Vertebra Trichocyst, 267 Triploblastic 306, 314, 324 Trochanter, 487 Trochosphere, 340 Trophoblast, 565 Trunk, 4, 416, 468 Trypsin, 74 Tubercle, and Tubercular facet, 481, 483 Tunicata, 406 Turbinals, 476, 517 Tympanic cavity, membrane, and ring, 5, 45, 189, 433, 475, 519 Typhlosole, 327, 386 U U Ina, 50, 485 Umbilical cord, 584 Umbilicus, 584 Umbo, 383 Umbrella, 307 Unicellular, 231 Unio, see Mussel Urachus, 584 Urea, 66, 147 Ureter, 26; and see under various types Urethra, see Urinogenital canal Uric acid, 66 Urinary bladder, 23, 361, 390, 521 », tubules, see Nephridium Urine, 8, 66, 147 Urinogenital aperture, 469 sa canal, 522, 523 i duct, 194 ”» organs, 193, 450, 520 INDEX 607 Urinogenital organs, development of, 573 Urodeles, 219 Uropod, 350 Urostyle, 35, 39 Uterine crypts, 583 Uterus, 523 »» Masculinus, 522 Utriculus, 187 Vv VV acuole: contractile, 232, 243, 2541 265, 2733 food-, 233, 266, 27 Vagina, 523 valve : of Vieussens, 512, 5378; spiral 429, 498 ; ileo-colic, 498 Valves: of heart, 88, 433, 504, 506; of shell, 381 ; of veins, 89 Variability, 225 Variation, individual, 216, 225 Variety, 225 Vascular system, see Blood-vessels, Arteries, Veins Vas deferens, see Wolffian duct Vasa efferentia, see efferent ducts Veins : Frog, 19, 82; Crayfish, 363; Mussel, 392; Dogfish, 438 ; Rabbit, 504, 509; embryo Verte- brate, 572 Veliger, 396 Velum : of Medusa, 309 ; of Lance- let, 408 Velum palati, 490 Vena cava, see Veins Vent, see Anus Ventral, 6 Ventricle, see Heart Ventricles of brain, 157, 159, 408, 443, 512-514, 568 Vermiform appendix, 498 Vertebrata, 219 ; general characters of, 403 Vertebra and vertebral column, 16, 35, 36, 423, 480, 576 Spermiduct, Vertebrarterial canal, see Canal Vessels, see Blood-vessels Vestibule, see Urinogenital canal Vestige, vestigial, 159, 361 Vibrisse, 469 Villi: of intestine, 498 ; of chorion, 583 : Viscera, abdominal, 20, 429, 495 Visceral arches and clefts, 419, 422, 571, and see Branchial aper- tures Visceral ganglion, 393 Visceral layer of peritoneum, 27, (323 Visceral mass, 385 Vitelline membrane, 196, 547, 579 Vitreous body of compound eye, », chamber and humour, 183 Vocal cords, 144, 500 >» sacs, 218 VORTICELLA: occurrence and general characters, 273; struc- ture, 273; reproduction, 277; conjugation, 278; means of dis- persal, 278; encystation, 279 ; spore-formation, 279; meta- morphosis, 230; practical direc- tions, 287 Vulva, 470, 523 WwW \) \) aste-products, 8, 66, 249 White matter of brain and spina cord, 156, 160 Wolffian body, see Epididymis » duct, 450, 574 Work and waste, 66, 148, 234 Worms, 340 Wrist, see Carpus X X iphisternum, 48, 483 608 Y ee cells: of Radiolaria, 299 ; of Earthworm, 323, 327, 332 Yolk, yolk-granules or spheres, 195, 546; and see under various types Yolk-cells, 200, 202 Yolk-plug, 201 Yolk-sac, 454 THE RICHLARD CLAY AND SONS, LIMITED, LONDON INDEX Z J cia, 281; and see Meenaoet and Microzooid Zoology, I te Zoophytes, see Hydroid polypes es Zooxanthella, 299 * Zygapophysis, 36, 481 Zygoma : zygomatic arch, 477, 478 Zygote, 197, 279 END. 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