HOW ANIMALS DEVELOP Marine Biological Laboratory Library Woods Hole, Mass. Presented by the estate of Dr. Herbert W. Rand Jan. 1964 ;4ft^^>^^^^i^*^ CO ru =o r^ D a a m CD HOW ANIMALS DEVELOP Frontispiece. — Operating on a newt's embryo. The embryo lies in a glass dish lined with wax. The operator has a glass needle in the left hand and in the right hand a hair-loop on a glass-holder; the loop can just be seen crossing the embryo -'-•. ■■/ / I M'^rit-., HOW AMIMALS DEVELOP By Q. H. WADDINGTON Laboratory of Experimental Zoology Cambridge University W. W. NORTON & COMPANY NEW YORK Published, 1936, by W. W. Norton if Company, Inc. 70 Fifth Avenue, New York All Rights Reserved First Edition p R NTED IN THE UNITED STATES OF AMERICA ACKNOWLEDGMENTS For permission to reproduce illustrations acknow- ledgment is made to the Cambridge University Press (Engelbach, Endocrinology ; Behrens and Barr, Endo- crinology; Huxley and de Beer, Experimental Embryo- logy), Messrs. Longmans, Green & Co. (Quain, Anatomy), Messrs. Macmillan & Co. (MacBride, Invertebrate Embryology), the Railway Gazette (photo- graph of Whitemoor Marshalling Yard, L.N.E.R.), and to my own publishers (Dtirken, Experimental Analysis of Development; Stockard, Physical Basis of Personality) . FOREWORD In this book, I have tried to write an account of embryology suitable for the intelligent layman and the elementary student. I have been conscious of two main difficulties in this task. Firstly, em- bryos are complicated and unfamiliar things, so that one has to describe the problems they present. I have attempted to avoid too much description by concentrating on the early stages of develop- ment, which are as a matter of fact the most im- portant from a general theoretical point of view, and can be described fairly shortly, since the embryos have not yet had time to develop any great complexity of form. The second difficulty arises because embryology is so interesting. The development of the structures by which living things carry out the activities of life must clearly raise many of the most fundamental problems about the nature of life itself. But most of the answers to these problems are still obscure. In order to show the directions in which people's thoughts are being led by the recent progress of embryology, I have put forward some of my own FOREWORD views, perhaps without sufficient warning that they represent probabilities rather than certainties. If I had attempted to give all the possible interpre- tations of the facts the book would have become unwieldy and confusing, but it was impossible to shirk a discussion of the problems. I believe the ideas which I have put forward are those most generally held by people who are working at em- bryology at the present day, but tomorrow we may discover some new fact which will force us to modify them. When one is brought face to face with the most fundamental questions about living things, one cannot expect to obtain complete an- swers in the comparatively short time during which biology has been actively studied. C. H. W. CONTENTS FOREWORD I. INTRODUCTION 13 The development of animal organization- similarity of all young embr)os— The three fundamental layers II. THE BEGINNING OF DEVELOPMENT 26 Egg and sperm— Fertilization, heredity, vir- gin birth— Development begins III. MOVEMENTS AND FOLDINGS 40 Formation of the three layers in sea- urchins — Frogs and newts — Lampreys — Birds— Mammals IV. THE "organization CENTER" 61 The technique of micro-surgery — The focus around which the embno is inte- grated — Birds — Mammals — Sea-urchins —Insects V. THE ADDITION OF DETAILS 78 Secondary organization centers — Special factories making special parts— Mosaic eggs -Different ways of building up similar structures— Larvae 82513 CONTENTS VI. THE DEVELOPMENT OF PATTERN 93 How the organizer works — Particular causes and the general pattern— Regenera- tion VII. THE FINAL ADJUSTMENTS 102 How embryos are fed— Growth— The in- fluence of function— The nervous system and development — Hormones — Sex — The part played by the genes in develop- ment SUMMING UP 126 INDEX 128 LIST OF ILLUSTRATIONS OPERATING ON A NEWT S EMBRYO Frontispiece FIG. PAGE 1. EGG-CELL AND SPERM-CELL 26 2. ORDINARY CELL-DIVISION 20 3. cell-division in the formation of the germ-cells 29 4. formation of germ-cells 3i 5. inheritance of short fingers 33 6. the effect of yolk on cleavage 36 7. spemann's tying-up experiment 38 8. BLASTULA AND GASTRULA OF SEA- URCHIN 42 9. DEVELOPMENT OF THE NEWT ' S EGG facing page 48 10. vogt's dye experiment on the newt's embryo 45 ii. maps of the newt and lamprey EGGS 46 12. CULTURE VESSEL FOR EMBRYOS 5I 13. THE ORIGIN OF THE MESODERM IN THE CHICK 52 14. MAP OF THE CHICK EMBRYO 54 15. DEVELOPMENT OF THE RABBIT 57 16. spemann's exchange EXPERIMENT IN THE NEWT 65 HOW ANIMALS DEVELOP FIG. PAGE 17. AN ORGANIZER GRAFT IN THE NEWT 68 18. AN ORGANIZER GRAFT IN THE CHICK facing page 64 19. A GRAFTING EXPERIMENT IN THE SEA-URCHIN 74 20. DEVELOPMENT OF THE DRAGON-FLY 76 21. DEVELOPMENT OF THE EYE IN VERTEBRATES 79 22. SELF-DIFFERENTIATION OF THE LEG BONES OF A CHICK facing page ^o 23. DEVELOPMENT OF THE EYE IN THE OCTOPUS 89 24. CATERPILLAR 9I 25. WHITEMOOR MARSHALLING YARD, L.N.E.R. facing page f^^ 26. DIFFERENT TYPES OF PLACENTA IO7 27. GROWTH OF MAN AND GORILLA IIO 28. VEGETARIAN AND CARNIVOROUS TAD- POLES 112 29. EFFECTS OF THE PITUITARY GLAND facing page 112 30. FACES OF MEN AND DOGS focing page 1 16 HOW ANIMALS DEVELOP CHAPTER INTRODUCTION The Development of Animal Organization Living animals are constantly on the move. It is one of the most characteristic things about them. Often we can see them running about, breathing, catching food and eating it, and so on. If we look closer we find that an animal is made up of different organs, and in all of them there is something going on all the time. On an even smaller scale, the organs are built out of cells, little lumps of living matter, each containing a special kernel or nucleus. And each cell is always full of activity. In plants the living jelly streams slowly about from one side of the cell to the other: in animal cells we cannot usually see any movement, but nevertheless there are incessant chemical actions and reactions. The cell absorbs oxygen and other substances from outside, performs many complicated chemical operations with them, and pours out again into its surroundings the by- products for which it has no use. In a living organism these changes are not isolated but are adjusted to one another so that the right operations are carried out to produce the right quantities of the various products. It is because 14 HOW ANIMALS DEVELOP we are so impressed at the way in which all the separate processes work together harmoniously that we call animals "organisms." The processes which keep an animal alive have to be quite as highly organized as the operations in the most complicated mass-production factory. If there is a "secret of life," it is here we must look for it, among the causes which bring about the arrangement of innumerable separate processes into a single harmonious living organism. When a numerous and varied set of processes is to be organized it is obviously convenient, and often absolutely necessary, to separate the different jobs among different pieces of apparatus, each of which specializes in carrying out one particular function. Thus a motor-car has a separate apparatus — the carburettor — ^to vaporize the fuel, another apparatus — the dynamo — ^to provide electric power, still another — ^the sparking plug — to make a spark, and so on. We find the same sort of plan adopted in all animals which attain more than a very minute size. For instance, every living creature has to arrange to absorb oxygen from its surroundings and to transport it in the right quantities to the cells in the body which need it. We find* that there are special organs for absorbing it, lungs in animals which breathe air, gills in animals which absorb the oxygen dissolved in water; special organs, the blood-vessels, for transporting the oxygen all through the body after it has been absorbed and dissolved in the blood ; a heart to pump the blood along ; and INTRODUCTION 1 5 many other organs to regulate the speed at which the lungs work and the blood flows. Without this rather complicated machinery, the organization of the oxygen supply would be inconc-eivable. The de- velopment of a set of specialized structures is the first step in the business of building up a living organism. To say that an animal is an organism means in fact two things : firstly, that it is a system made up of separate parts, and secondly, that in order to describe fully how any one part works one has to refer either to the whole system or to the other parts. Thus it is impossible to describe fully a thighbone without referring to the fact that it is part of a leg, and that one end fits on to a pelvis and the other on to a shinbone. The relation with the other parts of the organism is indeed so close that if an anatomist finds a new fossil bone he can often reconstruct, in general outline, the whole unknown animal to which it belongs. There are two possible ways of investigating the organization of an animal. Firstly, we can study in the adult how the organism works as a going concern: we can find out what functions are per- formed by each separate organ; we can discover how the communications between the organs are maintained by the blood and nerves; and we can study the results of removing one or more organs. But all the processes which can be investigated in this way will be proceeding within the framework provided by the fundamental spatial pattern in which the parts of the animal are arranged, since in the adult this pattern is more or less fixed. We can 1 6 HOW ANIMALS DEVELOP move large lumps of the pattern about, but we cannot discover what caused the pattern in the first place. But there is a second line of attack. We can actually watch how the parts of a living organism come into being and fit together. Nearly all organisms start life as fertihzed eggs, though a few grow out as buds from other organisms. Fertilized eggs are very simple-looking, often apparently quite homo- geneous lumps of living matter. They consist of a watery jelly, the protoplasm, which contains a variable amount of food-matter or yolk, and which also encloses a little bag of special material which is the kernel or nucleus. As we shall see, the jelly-like protoplasm is not really as simple as it looks. But it is at any rate much simpler than the adult animal, which consists of very large numbers of cells, of several different kinds, arranged in various ways to build up the different organs. During the increase in complexity as the egg develops into the adult the spatial pattern of the animal arises. In the early stages it is fluid and unfixed; we can describe its gradual unfolding, make experiments which alter it, study its genesis and causation. The study of development, or embryology, because it offers the possibility of finding out how the most fundamental characteristic of living things, their organization, comes into being, has always been of compelling interest to everyone who has been con- cerned with the position of living things in the general philosophical scheme. Nearly all biological philosophers, from Aristotle to the present day, have INTRODUCTION 1 7 been embryologists. Aristotle, in fact, founded the science. He opened hens' eggs after they had been incubated for xarious lengths of time, and described what he saw. For centuries, embryology remained a purely descriptive science. The changes which embryos go through as they develop are so many and complicated that it took an enormous amount of careful and painstaking work simply to describe them. Scientists have always asked why the changes occur; but only in the last fifty years or so have they been able to perform experiments to try to find out ; before that they could only guess, and, naturally enough, their guesses were usually wide of the mark. Even now we know very little about the causes which underlie embryonic development, but this is the most important and interesting part of the subject, and in this book I shall lay more emphasis on the tentative beginnings of our knowledge about the causes of development than on the description of the changes which occur. One very important fact has been discovered and will be described later on in the book. It has been found that at a rather late stage the organization of an embryo is comparatively loose and the various parts are to a large extent independent of one another and of the whole embryo as regards the way they develop. 1 At an earlier stage, on the other hand, it 1 Though not, of course, as regards the way they work : in this stage a lung can develop quite independently of the heart, but it cannot function to aerate the blood without the help of a heart. 1 8 HOW ANIMALS DEVELOP has been found that the way any part develops is controlled in such a way that all the material which is available is worked up into one whole animal; and further, it has been shown that this integrating control is exerted by one particular part of the embryo. At this early stage, then, the embryo is very highly organized, because the way any part behaves in development cannot be described without referring to this special controlling part. The con- trolling part is therefore called the Organizer. I shall devote quite a considerable amount of space to a consideration of the organizers which have already been discovered, and the way in which they throw light on the other facts which have emerged in the study of development. The main interest of embryology at present is theoretical, in the way discussed above. But there are a very large number of important practical questions which we may hope to be able to tackle later, when the science has been worked out more fully. For instance, why do most of the higher animals, including man, lose the power of regenera- tion so early in life, long before they are born? It would be very convenient if we could regenerate an amputated leg. Again, why do some cells start to form an unorganized cancerous growth which the animal cannot control, escaping from the agents which keep the parts of an organism together as one whole? How can we affect the production of twins from one ^gg^ The answers to these questions are not, I think, right over the horizon of our present INTRODUCTION ig view in embryology, but are quite near in front of us. The Similarity of all young Embryos The study of embryology was given a great fillip by the publication, and general acceptance among scientists, of Darwin's theory of evolution. It had already been found that the most general features of an animal's organization, those by which it was classified as a vertebrate, say, were formed early in its development, and only later there arose the more specialized characteristics by which it could be classified as a bird or a mammal, while still later it would develop the particular features of a fowl or a duck. This means that in the very earhest stages in development all embryos only show those charac- ters which are common to all animals. They must therefore look more or less alike. We have only to describe the early stages and on the basis of their common pattern we can make a general scheme into which all embryos fit, and can classify the ways in which they gradually diverge from each other. Soon after the publication of Darwin's Origin of Species, Haeckel put forward a general theory about these early similarities. He supposed that each animal, as it develops from the tgg to the adult, passes through a series of stages, each of which is similar to one of its ancestors in the course of the evolutionary history of the species to which it belongs. The series is more or less in the right order, so that the stage representing the first generalized ancestral 20 HOW ANIMALS DEVELOP vertebrate occurs before the stages representing the various groups which gradually evolved out of the original vertebrate stock. This hypothesis brings under one head a large number of very odd facts. For instance, a young mammalian embryo, such as a young human embryo about four or five weeks old, is provided with gill slits and blood-vessels which flow along them. These are like the organs found in fish, where the blood flows through the gills and absorbs oxygen from the water, but they can be of no possible use to a mammalian embryo, which at this stage is deriving its oxygen from the blood-stream of its mother. Haeckel's theory, that such organs are * 'hang-overs" from the time when the ancestors of mammals were fish, still provides the most con- venient way of describing this whole class of phenomena. But we have slightly modified the expression of Haeckel's theory. Many details of embryonic development are better described as reflections not of adult ancestors, as Haeckel thought, but rather of the embryonic stages of those ancestors. The mammalian embryo has gill slits, not like the gills its ancestors had when they were adult, but like the gill slits they had when they were embryos. With this modification, Haeckel's hypothesis, 'the so-called "biogenetic law" or recapitulation hypo- thesis, is still one of the foundations of our system of descriptive embryology. But even so, there are very many features of development to which the law does not apply. Many embryonic characteristics do not represent any INTRODUCTION 2 1 ancestral conditions, and not by any means all its ancestors are recapitulated in an animal's develop- ment. Embryos which live in special situations, like the bird embryo developing inside its shell, or the mammal in the womb of its mother, form peculiar organs suited to their particular conditions, and these often have little to do with any ancestral forms. We shall meet other examples of Haeckel's law later on, when discussing different types of larvae. Haeckel's law is not strictly an explanation of anything. When a human embryo is developing, its remote fish-like ancestors are long dead and rotted to mud on the sea floor, and cannot possibly be the effective agents which cause the human to form embryonic gill slits. In order to give a satis- factory account of the direct causes of development, one must be able to show how the development is dependent on factors which are actually present in the fertilized tgg or its immediate surroundings. What Haeckel did was to find a good way of describing the plethora of odd facts which had been accumulated. We ought to be able to find a reason why so many facts fit into Haeckel's generalization. Actually we still have not found any satisfactory reason, although we can suggest various processes which may be involved. Thus if there is to be an evolutionary change from fish into men, it is obviously easier, or so a man would think, to stick to the old plan of development until it is no longer a help and simply must be altered. The same thing 22 HOW ANIMALS DEVELOP happens in human inventions. When motor-cars were first made, the engineers did not think the whole problem out from the beginning and produce a stream-lined model with the engine over the back wheels where the power is required: they seem to have been exhausted by thinking out the engine, and simply attached it to the current form of horse- carriage in front where the horse had been. We can call this a sort of "habit reason" ; men inventing, and embryos developing, tend to do what their fathers did if they can, because it is easier. There may be other and more important reasons for imitating an old pattern. It may act as a guide. For instance, if one is going to make a cast-iron pot, one first models it in clay, as though iron had not yet been discovered; from the clay pot a mould is made into which the molten iron can be poured. Here the "ancestral" clay pot provides what may be called a formative stimulus for the "more highly evolved" iron pot. Perhaps this sort of explanation applies to ancestral characters which appear in embryos only for a short time, eventually dis- appearing entirely. The Three Fundamental Layers It follows from Haeckel's biogenetic law that young embryos must look much alike, since they should show only the characters which are common to all types of animals. This deduction from the law is actually true. Even before Haeckel definitely formu- lated his law quite a large number of different kinds INTRODUCTION 23 of animals had been investigated, and it had been found that in their early development they all passed through the same two stages. These stages are called the hlastula and gastrula, and presumably represent the ancestors from which all animals have been derived. The original form of Haeckel's law suggests that these ancestors looked like blastulae or gastrulae when they were adult, but no animals of this kind have survived till the present day. In fact, if we adopt the modification of Haeckel's law which was advanced above, there is no need to suppose that adult blastulae and gastrulae have ever existed ; we need only assume that the original ancestral organ- ism.s from which all animals have been evolved passed through these two stages in their development. There is quite a large amount of variation in the shapes assumed by the blastulae and gastrulae of different animals, but we can imagine ideal forms from which all the others can be derived by minor modifications. The ideal blastula consists of a hollow ball of cells, the walls of which are only cell-thick. The hollow inside is called the blastula cavity, or blastocoel. The ideal gastrula is also a hollow ball, but differs from the blastula in two ways; the ball is punctured, and the walls are thicker and consist first of two layers of cells and later of three. The hollow inside, together with the innermost layer which lines it, is the primitive gut, and communicates with the outside through the hole which punctures the ball. This hole is called the blastopore, because when it becomes visible as a little pore on the blastula 2 4 HOW ANIMALS DEVELOP surface it often provides the first visible indication that the blastula is changing into a gastrula. The three layers out of which the gastrula is made are named from the Greek words for skin, and for outside, inside, and middle; thus the outer layer is the ectoderm^ the innermost layer the endoderm, and the layer between them mesoderm. The ectoderm, endoderm, and mesoderm are the three fundamental parts out of which an animal is built. We might almost say that they correspond to the three major parts of a motor-car. The ectoderm develops into the skin, the sense organs, and the brain and nervous system; analogous to the body- work, the lamps, and the controls. The mesoderm forms the skeleton, muscles, and heart, or the chassis and engine. Finally the endoderm corresponds to the fuel system, and develops into the stomach and intestines and all the apparatus for absorbing food (i.e. fuel) ; this has to be much more complicated in an animal than the fuel system is in a car, because animals cannot get their nourishment poured into them in a form in which it can be used at once, as petrol is poured into the tank ; it is as if a car had to carry round with it a whole refinery for turning crude oil into motor spirit. These three layers are called the Germ-Layers, and, when it was first pro- pounded, the idea that they could be found in some form or other in all animals stimulated scientists to investigate as many different kinds of embryos as possible to see if the hypothesis was true. Most of this work was completed by the beginning of this INTRODUCTION 25 century, and it provided a broad basis of information on which all our present-day knowledge of em- bryology has been built up. On the whole, it was found that the three germ-layers could be fairly easily recognized in most animals, but there are a few difficult cases, and in some very primitive animals only the ectoderm and endoderm are present and there is no mesoderm. The particular impor- tance of the idea of the germ-layers from our point of view is that it is the beginning of an analysis of the pattern in which the embryo is organized. The formation of the three germ-layers is usually the first structural change which the embryo achieves, and almost immediately after this the main organs are formed. The process by which the blastula turns into the gastrula is known as gastrulation, and a great deal of the discussion later on in the book as to how development is brought about will be concerned with this period of gastrulation when the main structure is blocked out. CHAPTER II THE BEGINNING OF DEVELOPMENT Egg and Sperm The history of a developing animal really begins when the egg-cell becomes fertilized by uniting with a sperm-cell, but before this can happen there must occur a very important series of processes by which NUCLEUS ... HEAD O" \ Y- CENTREPIECE - TAIL a Fig. I. — Diagram of an Egg-cell {a) and a Sperm-cell {b). The sperm is more highly magnified; in Man the c^g% is o-2 mm. in diameter and the sperm about 0-05 mm, or i /500th inch long, including the tail these cells are elaborated. Egg- and sperm-cells are known collectively as germ-cells^ and like other cells they consist of a mass of living matter or protoplasm, containing a nucleus. But in the details of their structure (Fig. i) they are very speciahzed and unlike normal cells, as might be expected from the extraordinary things they have to do. The egg-cell is usually rather large as cells go, since it has to THE BEGINNING OF DEVELOPMENT 27 contain food material for the embryo to use before it develops a digestive apparatus. This food material is stored as grains of yolk, which are scattered through the cytoplasm, that is to say, all the proto- plasm outside the nucleus. As it is fairly heavy, the yolk collects at the bottom of the ^gg, which is therefore stratified, with a yolk-laden vegetative pole below and a non-yolky animal pole at the top. The nucleus usually lies near the top, in the clear protoplasm of the animal pole. Non-yolky eggs are often not very much bigger than other cells; the human tgg, for instance, is about o • 2 mm. or a hundredth of an inch in diameter. But when there is much yolk, the egg-cell may be swollen to an enormous size. The "yolks" of birds' eggs are single cells, the biggest known, with only a tiny little patch of cytoplasm nearly hidden in the huge mass of yolk. The sperm-cell is still more highly speciahzed. It consists of three parts : the head which contains the nucleus, the centre-piece, and a long tail which beats to and fro and drives the sperm actively about through the fluid in which it exists. Sperm-cells are very small, containing no yolk and hardly any cyto- plasm, and their light construction enables them to move about comparatively rapidly. A human sperm can travel at the rateof about an inch in three minutes. Eggs, on the contrary, are rarely able to move. The most important part in the elaboration of the germ-cells is the preparation of the nucleus, and this process is essentially the same both in the eggs and in the sperm. For most of the time the nucleus of 28 HOW ANIMALS DEVELOP an ordinary cell consists of a bag made of the nuclear membrane filled with rather liquid protoplasm. When the cell is about to divide into two the nuclear membrane disappears, and out of the liquid contents there are built up a number of little solid lumps, which if the cell is killed can be stained very deeply with many dyes, and are therefore called chromosomes, from the Greek words for "colour" and "body." Different chromosomes are often different in shape, so that they can be recognized, and it is very important to notice that they always occur in pairs, so that each cell has two of each kind. The number and shape of the chromosomes in the cell is fixed for any particular species, but is different in different species ; some have as few as four, others up to one or two hundred. But as the chromosomes are always in pairs of similar ones the number must always be even. When the chromosomes become visible at the begin- ning of an ordinary cell-division, each one is already split longitudinally into two half-chromosomes lying side by side. As the cell divides, these two halves separate from each other, and one half goes into each of the two cells which are formed. When the division is over they count as whole chromosomes, and gradually disappear into a normal fluid nucleus (Fig. 2). The cell-division which results in the formation of the germ-cells seems superficially very different from the ordinary divisions, but it has recently been realized that the whole difference follows from one single slight alteration in the way the division begins. The difference is this: that when the chromosomes THE BEGINNING OF DEVELOPMENT 29 Fig. 2. Fig. 3. Fig. 2. — Diagram of ordinary cell-division, (a) The chromo- somes appear, already double, in the nucleus, (b) The cell divides and one-half of each double chromosome goes to each daughter cell Fig. 3. — Diagram of cell-division during the formation of the germ-cells, {a) The chromosomes appear single in the nucleus, (a') The two chromosomes of each kind lie side by side, {b) The cell divides and one chromosome from each pair goes into each daughter cell 30 HOW ANIMALS DEVELOP appear before the germ-cell division they are not split longitudinally (Fig. 3). They do not seem to be able to proceed with the division until they have arranged themselves into double bodies, to corre- spond with the two half-chromosomes lying side by side which are found in ordinary division. They get into a doubled condition in the only way which is open to them ; that is, by the two whole chromosomes belonging to a pair joining up with each other and lying side by side. If there are six chromosomes, two A's, two B's, and two C's, for example, the two A's always join up, and so do the two B's and the two C's. The chromosomes are now a series of paired bodies, which are just like the split chromo- somes of an ordinary cell-division to look at, except that there are only half as many of them. They go on behaving just like the split chromosomes described above ; that is to say, the two partners in each paired body, which have only just come together, now proceed to separate, one partner going into each of the two daughter-cells. This means that the two daughter-cells have only got half the normal number of chromosomes and are an exception to the general rule in that they have only one chromosome of each kind. That is one of the most important character- istics of the germ-cells. The ordinary body-cells, which all have two of each kind of chromosome, are said to have the diploid number, and the germ-cells, which have only one of each kind, are said to have the haploid number. The process which has just been described is spoken of as the reduction division of the THE BEGINNING OF DEVELOPMENT 31 chromosomes because it involves the reduction of their number to half. The daughter-cells of the reduction division are not the actual germ-cells, but each one has to go O MATURATION PERIOD REDUCTION DIVISION / o REDUCTION DIVISION o b O'o 6 O PERIOD OF DIFFERENTIATION Fig. 4. — Diagram of the formation of the germ-cells, {a) The tg%. {b) The sperm. The egg mother-cell is built up and furnished with yolk during the maturation period, and then undergoes two divisions, giving four cells of which three are very small and die. The sperm mother-cell divides twice and all four resulting cells are transformed into sperms during the period of differentiation. through one ordinary division before it is ready, giving a total of four germ-cells from each cell which started the reduction process. Actually, in the forma- tion of the eggs three of these four degenerate and never function as eggs, while the remaining one has to undergo a period of ripening when it is supplied with the yolk which it will require. This ripening usually happens in the middle of the reduction division, which therefore takes a very long time (Fig. 4). 32 HOW ANIMALS DEVELOP Fertilization, Heredity, Virgin Birth At the end of all this preparation the germ-cells are ready to carry out the complicated process of developing into an adult, and are finally ready for fertilization. Fertilization really consists of two processes, the activation of the tgg by the sperm and the union of the tgg and sperm nuclei. It is easy to see the importance of the second process ; it restores the diploid number of the chromosomes by adding the haploid number in the sperm to the haploid number in the ^gg. A properly balanced set of chromosomes is essential for the development of the animal since they contain the hereditary factors. An example will show how the influence of the hereditary factors can be detected. Men are some- times born with short fingers, each with only two joints instead of three, because of some abnormality in the development of the fingers. The character is hereditary. For instance, we find short-fingered men who have married normal wives and all of whose children have short fingers. If two children born of such parents then marry, on an average one-quarter of their children will be normal and three-quarters short-fingered. These facts are due to the presence of a hereditary factor or gene for short fingers lying in a chromosome. The short-finger gene is an abnormal form of the gene which causes the fingers to develop in the ordinary way, and is derived from it by a sudden and as yet inexplicable change called a mutation. The original short-fingered fathers have THE BEGINNING OF DEVELOPMENT 33 two similar chromosomes, each with a gene for short fingers, and each of their germ-cells contains one chromosome with the short-fingered gene. When such a sperm fertihzes a normal egg containing a gene for ordinary fingers, the children have one of each kind of gene. In this particular case it is the short-fingered gene which affects the development: it is therefore said to be dominant over the ordinary PARENTS Ss Ss GERM-CELLS CHILDREN ISS ISs ISs Fig. 5. — Diagram of the inheritance of short fingers. S is the factor for short fingers and s that for ordinary fingers. The lines show the ways in which the factors may come together in fertilization. gene, which is recessive to it. When the germ-cells are formed in the children of such a marriage, the two genes, lying in the two similar chromosomes, are separated at the reduction division, and the germ-cells have half of them one normal gene and half of them one short-finger gene. If two such children marry (Fig. 5), it is pure chance which genes come together in the fertilized eggs, so that in half the eggs a normal gene will meet a short- finger, giving short-fingered adults, in a quarter of the eggs there will be two short-finger genes, giving more short-fingered adults, and in the last quarter 34 HOW ANIMALS DEVELOP there will be two normal genes giving normal adults. Hereditary factors of this kind were discovered by Mendel in the middle of the last century, and he also gave rules for the way in which they are inherited. Chromosomes had not been described at that time, and it is only about thirty years since it was reaUzed that the hereditary factors actually lie on the chromosomes and that Mendel's laws are perfectly well explained by the behaviour of the chromosomes which we have described above. The theories propounded by Mendel are collectively known as '' MendelisnC' and are part of the science of genetics, or the study of heredity. It is clear, then, that the chromosomes, or the genes within them, play a leading role in develop- ment, and we shall have to discuss later (see Chapter vii) how they do it. But we can say now that an tgg can develop without a full double set of chromosomes ; it can develop quite well, so long as it has got a half or haploid set. Anything less than this is fatal, and so usually is anything between a half and a whole set because of its lack of balance. Professor Dalcq in Brussels is investigating the development of frogs' eggs with less than the haploid number of chromosomes, and is finding out just when and how the embryos fail. The other process in fertilization is the activation of the ^gg. We know very little about how this happens. What it does is to cause the tgg to start dividing and developing. Now the same change can be brought about by other things which are not the THE BEGINNING OF DEVELOPMENT 35 sperm, and we then get a "virgin birth," or partheno- genesis as it is called in science. The most various and unexpected agents may be effective. It is some- times only necessary to prick the t^g with a sharp needle, or to put it into very weak acid ; some marine eggs may be caused to begin developing if the salt-concentration of the water is altered. In all cases the procedures give rather variable results, and we have very little idea why they give any results at all. But the eggs treated in this way, since they have a haploid set of chromosomes, can go on developing quite normally. The adult which arises is smaller than normal, and its cells are smaller than normal, since they adjust their volume to that of the half-sized nuclei. Some eggs normally develop without being fertilized by sperm, i.e. parthenogenetically. This happens, for instance, to many of the eggs of bees, and these parthenogenetic eggs give rise to drones or males, which have only the haploid number of chromosomes. In some species such animals can produce sperm without performing another reduc- tion division, but usually they are sterile. In other cases the tgg starts developing parthenogenetically, and then succeeds in doubling its chromosome number, so that the diploid condition is restored. Development Begins The first steps in the development of the tgg are always the same; the tgg divides up into smaller and smaller cells without growing at all, till there is a mass of little cells in place of the large single 36 HOW ANIMALS DEVELOP egg-cell. This process is known as the cleavage of the tgg. The details vary according to the amount of yolk which the tgg contains. Eggs with very little yolk cleave into equal parts ; those with rather more yolk SOME YOLK MUCH YOLK EGG CLEAVAGE STAGE BLASTULA Fig. 6. — Diagrammatic sections of eggs, cleavage stages and blastulae with various amounts of yolk cleave into small cells at the top or animal pole and larger cells at the bottom where the yolk is collected. Eggs with a great deal of yolk, such as birds' eggs, do not cleave throughout their whole mass : only the patch of non-yolky cytoplasm cleaves, forming a plate of little cells swimming on the surface of the main m