Embryology.

by Gerald R. Leighton.

CHAPTER I

THE CELL AND THE INDIVIDUAL

What is Embryology, and what is its significance or interest to the ordinary educated man and woman? The answer to the question is the justification for the appearance of the following pages, and one may regard it as a somewhat striking fact, that in the production of a series of works of which this volume is one, those responsible for the subjects should have deemed it advisable to include Embryology.

Embryology may be defined as that part of the science of Biology which deals with the formation of a new individual or embryo. The definition itself ought to be sufficient to explain the significance of the subject for every one, because one can hardly conceive of any more profoundly important knowledge than that which tells of the mode of origin, manner of growth, and ultimate birth of an entirely new being. In the absence of such accurate knowledge it is quite obvious that all one's ideas concerning the manner in which the new individual is to be treated must have a more or less haphazard, or at least empirical, basis. In fact only when the science of Embryology, or the development of the individual, becomes a part of the ordinary everyday mental equipment of those who are responsible for bringing into the world new individuals, and subsequently protecting and handling them, will it be reasonable to expect that these new individuals are dealt with in the best possible manner. In a word it is evident that education, using that term in the very widest possible sense, can never be anything more than a blind groping in the dark until those into whose hands it is entrusted realise and know at least the most important fundamental facts concerning development. It is lack of this kind of knowledge which has been responsible for so much of the mistaken systems of the past in dealing with the young, and it is the spread of this knowledge which alone is the hope of better things in the future. Wherever knowledge is absent superst.i.tion is rife, and in no sphere of life is this more painfully obvious than in connection with the subject which we are about to study.

It would have been entirely impossible for many of the stupid and even cruel methods of mental and physical treatment which have been meted out to the young children in the past to have been tolerated for a moment had this knowledge been available and sufficiently widespread.

Possessing it, a flood of light is thrown upon the fascinating and otherwise obscure problems of heredity; and thus it lays open the pages of the past for those who care to read them. Possessing it also it throws upon the mental screen pictures of possibilities in the future for all those who have eyes to see. So the study of Embryology links up the past with the present and joins the present with the future. Is it not, therefore, obvious that the study of such a subject means dealing with problems the importance of which it is impossible to exaggerate; problems which the parent, the teacher, the social reformer, the politician, and the philanthropist will grapple with in vain unless they call in science to their aid? Such is the meaning and significance of the subject of our study.

In the widest sense of the word Embryology, therefore, deals with all manner of living things, be they plant or animal. But since our purpose here is to state, as far as possible in the s.p.a.ce at our disposal, the facts which are of particular importance in relation to the human subject, we shall only glance at the rest of living creatures. A brief look at them, however, is quite necessary in order to appreciate what follows. Let us be quite clear of what we are in search. We want to know as far as possible what it is that goes to the making of a man. What is the origin of the new individual? Where does the embryo come from? What elements are concerned in its formation? Where do these elements come from? How are they subsequently built up into the type of the species to which they belong? From what source do they gain their nourishment? What influences of a degenerative nature are likely to affect them? These are the questions which it is the business of the Embryologist to answer, and these are the questions the answers to which afford the explanation of man in the making. Surely they merely require to be stated that their significance may be appreciated.

We may now glance very briefly at the simplest facts which bear upon the subject, and which must precede our detailed study. The necessity for reproduction and development is involved in the universal fact of death. In all except the very simplest forms of life--those consisting of one simple ma.s.s of protoplasm--the individual sooner or later perishes, and if it were not that there were some methods by means of which the individuals could give rise to new individuals obviously the species would come to an end. No matter to what great age an individual animal may live, and there are some such as the tortoises which do live for centuries, sooner or later death overtakes them, and in all, investigation of their structure shows that nature has made provision for the carrying on of the race by means of new individuals.

Every living creature, be that creature simple or complicated, animal or vegetable, man or a jellyfish, starts life as one single cell. The very simplest living individuals never consist of anything else but one single cell, and it is in these primitive forms of life alone that what we call death can not be said to occur. Such a simple cell, after living for a certain period, simply divides itself into two halves, each of which gradually a.s.sumes the size and shape of what we may term the parent cell. The first individual has simply become two separate individuals. These two in their turn after another period of independent existence, again each divide, thus giving rise to four, and so on. Now here, although the original parent cell no longer exists _as a cell_, the actual material of which it was composed still exists in the cells which came into existence as the result of this division. The original cell, therefore, may be literally said to have been deathless, or immortal, though not everlasting. This is a profound thought, and one which must be grasped at the very commencement of our study of development, because it is one to which we shall have to recur again and again when we come to study the cells which give rise to human beings, in whom, too, there is a deathless continuity of cell protoplasm, or germ-plasm as it is then called. It is upon this fact that the whole science of Embryology depends.

The important idea to be learned from observing this process of reproduction in the single-celled animal is this: that there is nothing here which we may term the body of an animal as opposed to any of its parts. The one cell is both body and organs, and everything else; in itself it has the capacity of performing all the functions necessary for life, including that of reproduction for the perpetuation of the species. No part of the cell is set on one side for any special purpose such as happens in the bodies of higher animals. There are no special elements which go to the producing of the next generation, none of the cells which in a mammal, for example, we call "germ-cells." The whole individual is one cell. In fact one might almost say that there is no individual, but only race, or if we regard the cell as an individual then it is all germ-plasm. That is the important fact to be learned in the reproduction of single cells.

There are some single cells, such as those of the yeast, which reproduce in a slightly different manner, namely, by budding off a portion of themselves and finally becoming separate, and this might be regarded as a slightly higher stage, in so far as the original cell from which the bud came may be still identified; but in reality the process differs very little from that first described.

Then we may note that very low in the scale of living things there is a process of reproduction known as conjugation, in which, although the cells of the species appear to be all alike, yet, nevertheless, two of them join together for purposes of reproduction. In other words we have here a process of cell-union before we have the cell-division which follows. It is important to note at this stage that the creatures which we have mentioned, and even some more highly organised, such as an am[oe]ba, which has a nucleus, go through these simple or complicated reproductive processes in the total absence of anything which could suggest a distinction of s.e.x. In these cases the individuals are obviously all of one s.e.x, and, therefore, the distinction of s.e.xes into male and female is evidently something which has been added later in the scheme of evolution, not for the purpose of reproduction itself, but for something which is to be added to that.

Then in the slightly higher animals and plants we come to those in which many cells go to the making of the individual, the multicellular individuals, and amongst these we very soon see the origin of what is termed specialisation of function. That is to say, in these higher creatures which consist of many numbers of cells arranged so as to form one individual, certain cells are set apart for one purpose and others for another. Some may be for digestion, some for purposes of movement, and others for reproduction. Here we have a new phenomenon, namely, the setting aside of certain cells in a multicellular individual which from the very beginning are capable of one function alone, namely, reproducing the species. The higher one goes in the scale of life the more striking and obvious this fact becomes, and as we shall see when we come to the vertebrate kingdom, this setting aside of the cells which are to produce the individuals of the next generation is the key to the solution of the most difficult of our problems.

In these highest forms of life, however, the cell itself is becoming a much more complicated thing than that lowly form which we first noted as dividing into two to form two new individuals. Indeed, the cells in the highest animals and plants are immensely complicated in their structures and functions, and especially in connection with the changes which take place in the nucleus of such cells. Not only the nucleus but another small object within the cell which is neither part of the nucleus nor part of the cell protoplasm, also is very important, and this structure is termed the "centrosome." In fact this little body apparently begins the whole process of cell-division by itself dividing into two parts.

Then the nucleus follows suit, and ultimately the whole cell divides.

The nucleus itself is a complicated structure, as is especially seen during the processes of division, in which it breaks itself up into a number of thread-like portions, and the number of these is always the same in any given species, a fact which is of great importance in reproduction. Why do we mention these apparently dry details? Because in these minute and complicated nuclear movements the whole problems which are at the bottom of development and heredity lie. The problems of life itself can only be solved by the study of what takes place in these minute portions of cells. It is here that the new formation of an individual begins, and although it is no part of our purpose here to detail all the complicated processes of nuclear division, it is essential, in order to grasp the meaning of our subject, that we should realize that in the changes within the cell life with its variations begins.

The study of these wonderful cell processes, a work which demands the most patient investigation and high technical skill, has reached such a stage that it is a science of its own, and is called the science of "Cytology," or the science of cells, which has been made possible only in comparatively recent years by the invention of microscopes having great powers of magnification, and by the application of elaborate methods of staining to the cells themselves.

We can say no more about these processes here, but the foregoing paragraphs may perhaps be sufficient to show us how important it is to grasp these simple facts of cell life in their bearing upon development itself.

CHAPTER II

PROBLEMS OF REPRODUCTION

We have seen that in the higher types of animals and plants the single individual is made up of not one but millions and millions of cells united together for the common purpose of the individual life, and that in such complicated individualities some cells perform one function while others perform others. A human individual from this point of view, therefore, is an organised community of cells all of which, however, sprang, in the first place, from one single cell. That original single cell is termed, in animal Embryology, the "fertilised ovum." It is popularly spoken of frequently as "the egg." All the other millions of cells are the direct descendants of this fertilised ovum, or egg, even though many of them eventually become extremely unlike the original cell. In single-celled animals the offspring of the original cell remain like the parent cell, but in the highly complicated creatures the offspring split up into a great many types of cells, owing to the very fact that all remain adherent together to form the ma.s.s of the body in order to carry out different functions. So we find cells of one type in glands, of another type in the brain, of another type in bones, of another type in blood, and so forth. Nevertheless all of them sprang from one original single cell. None of these specialised types of cells, however, are capable of performing any other function than their own. A bone-cell cannot receive an impression, nor originate an idea, any more than a brain-cell can secrete bile. Each kind of cell has its own appointed duty. The most important duty that can possibly be allotted to any cell is obviously that of reproducing the individual for the purpose of continuing the race or species. So we find in higher animals that this function, like others, is relegated to a special set of cells also derived from the original single cell, and which are called "the germ-cells."

Leaving out of consideration the question of reproduction in lower types of animals we may consider the nature and origin of these cells in highest vertebrates, such as the mammals, including man. Germ-cells, which are derived from the tissues of a female animal, are termed "ova."

Those which are derived from the tissues of a male animal are termed "sperms." Notice that it is not these germ-cells themselves to which the terms male and female, indicative of the two s.e.xes, are applied, but only to the individuals. They are male and female; the germ-cells are of neither s.e.x. True the germ-cells from the male, _i.e._ the sperms, differ in appearance when seen under the microscope from those of the female, but there is no reason to believe that there is any difference between them in their capacity, for example, of transmitting the characters of ancestors to succeeding generations.

At a certain stage in the life history of the animal individual and after undergoing certain changes which need not be considered here, these germ-cells, both sperms and ova, have reached such a stage of maturity as to be capable of carrying on their sole function, namely, that of reproducing the species. The actual age in human beings, for example, at which this maturity is reached varies very much in different races, and in different individuals of the same race.

When reproduction is about to occur a union must take place between a germ-cell from a male body with a germ-cell from a female body; that is to say, a union must take place between a sperm and an ovum. This union takes place within the body of the female individual and results in the fusion of the two cells into one single cell, which is now termed a fertilised ovum. This fertilised ovum, in virtue of this process of union, is now able under suitable conditions of nutrition and shelter, such as it obtains within the female organs of reproduction, to divide and redivide again and again, thus building up a new ma.s.s of cells as the result of its division. The millions of cells so produced include, as we have already seen, cells which have all the various functions which are necessary for the continuation of the life of a human individual; that is to say, that as the result of this division of the fertilised ovum there are produced first of all germ-cells to secure the still further continuance of the race, and then mult.i.tudes of all the other kinds of cells which gradually a.s.sume the shape of an embryo or young individual, and ultimately grow into a human being.

In all the highly complicated animals fertilisation by union of germ-cells from male and female must precede reproduction. The result eventually is this multicellular individual composed of a number of different kinds of cells each set apart for its own work. But it is well to recognise that we may regard all these cells as really of two kinds, namely, the germ-cells and the others. That is to say, two kinds of cells are produced as the result of fertilisation, namely, cells whose business it ultimately will be to again take part in a similar process of fertilisation, and so perpetuate the species, and all the other cells which go to the forming of the various body tissues of the individual itself. In this way we get a simple cla.s.sification of the cells which form, for example, a human being, namely, germ-cells and body-cells, the latter often being termed "somatic." The latter are, of course, in much greater abundance than the germ-cells. They have to form all the various elements, organs, limbs, and structures known and described by the anatomist. The germ-cells are a separate little group of themselves embedded in the male and female reproductive organs for the sake of nutrition, growth, and shelter, for many years, until they again take part in the process of fertilisation. Note carefully that no other cells in the body ever unite together to produce a new individual except germ-cells.

Somatic cells reproduce by dividing directly. Germ-cells before they can do this require to be fertilised. That is to say, the cell from the male (the sperm) must fuse with the cell in the female (the ovum). As Dr.

Archdall Reid graphically states it, "Only the germs are marriageable; and, as we have just seen, in the great majority of animals and plants they observe the degrees of consanguinity very strictly, and do not unite except with members of another cell-community, and then only to found a new colony of cells, an offspring."

There are still some further considerations in connection with the subject of germ-cells and germ-plasm which we must carefully consider before leaving this part of our subject, Embryology. Everything depends upon a perfectly clear understanding at this stage. The facts themselves that have to be adduced in this connection are comparatively few and simple. No fairly educated person should have any difficulty whatsoever in grasping them. Moreover, very fortunately they are thoroughly well established and not in dispute. But the reasoning which is based upon these few and elementary facts, reasoning which is applied to the methods of treatment of the individual which is produced, may be very complicated and very debatable. Various schools of thought and opinion exist according to the att.i.tude taken towards the facts, some of which we have mentioned and others of which we are about to detail. But the facts themselves are not debatable, and we therefore see once more that their importance at this stage cannot be exaggerated.

One or two very simple general propositions bound up with the subject of Embryology, or individual development, may be stated in order to focus attention upon the nature of the problem under investigation. Thus n.o.body will be found to question the fundamental truth that children resemble their parents. That is a commonplace of experience. Similarly no one will be found to dispute another fundamental fact, namely, that children differ from their parents. This, too, is equally a commonplace of experience. If we examine a million human beings we find that they all possess certain features in common, certain characteristics in virtue of which we recognize them to be human beings. Nevertheless it is just as true that a careful examination of the same million people reveals the true saying that no two of them are exactly alike. Here then are two propositions equally true within certain limits; namely, that all human beings resemble each other, and that all human beings differ from each other. There is resemblance; and there is variation. These two things are universal because of the existence and characteristics of germ-cells. We may look at this a little closer.

Every species of animal, in the process of reproduction brings forth offspring similar to itself. This is expressed in the familiar proverb that "like produces like." One does not expect grapes from thorns, nor is it possible to construct a silk purse out of a sow's ear. But what is the explanation of this proverbial fact? The answer is of great importance, because although the fact itself is recognized as a general principle in the reproduction of a species, it is not sufficiently recognized in the full details of the characters of that individual. Too many people are still apt to expect to be able to produce grapes when the plant is a thorn, and it is unfortunately all too common to make heroic but quite futile attempts to construct human silk purses out of human sows' ears--so to speak--simply because of the ignorance of the material which is being used. The most that can be done is to give such material as is present the very best opportunity of attaining its own utmost perfection; and this, by the way, is vastly more than has ever been done for any considerable number of the human race.

But why this continuity of species? Why should like always produce like?

The answer has been sought by biologists ever since problems of life attracted man's curiosity. All sorts of weird and fantastic theories have been put forward at different times to account for this simple fact, but it is only in comparatively recent years that the real explanation has been forthcoming. It is perfectly obvious that in order to secure this continuity of racial resemblance there must be something physical or material which is actually continuous from generation to generation to account for it. The immortal Darwin saw this very clearly, and devoted much thought in the endeavor to find some explanation of this very problem. The result was his theory of Pangenesis which, ingenious as it was, was ultimately shown to have no basis on fact. In his effort to account for the fact that children resemble their parents even in such minute details as the shape of the nose, the colour of the eyes, and so forth, he formulated the idea that the parents themselves probably contributed mult.i.tudes of minute particles from their own tissues to form the cells of their offspring. He supposed, for example, that particles or gemmules from the eyes, nose, hair, and so forth, of the parent, or parents, in some way or other were fused together and gave rise to the cells which ultimately produced an embryo. Hence he thought the explanation of the resemblance between parents and children.

This was his solution to the question of the physical continuity between successive generations. It may be remarked in pa.s.sing that it is with something of pathos that one reads in Darwin's own works his own evident opinion that this theory of Pangenesis was a great discovery. One gathers almost that he himself regarded it as of greater importance than his work on natural selection.

In the course of time, however, the real actual basis of physical continuity was shown to be something quite different, and looking back now upon the history of the discoveries in this connection during the last generation one can easily imagine what speculations there must have been in the absence of the facts which are now known to embryologists.

CHAPTER III

PROBLEMS OF REPRODUCTION (_continued_)

The one outstanding discovery which has placed the science of Embryology on an absolutely firm basis, and which has made clear so many of the facts, which were previously puzzling, is this: _that the germ-cells which give rise to new individuals are themselves produced from pre-existing germ-cells_. The entire embryo, or young infant, is derived from one single cell which we have called the fertilised ovum, and that in its turn was derived from the union of two germ-cells, one from the male parent, and one from the female. These two cells in their turn were also derived in a straight line of descent from the fertilised ovum from which each parent sprang. In other words there has never been any conjugation between one fertilised ovum and another in spite of the generations of cells which have been produced between them. Put in another way the body, or somatic cells, contribute absolutely nothing to the original material or germ-plasm of which the germ-cells are composed. They do not produce them in any sense of the word whatsoever, despite the popular opinion to the contrary. This is the great discovery of modern Embryology. Until this was known it was a.s.sumed that parents did produce the cells from which their children sprang, and hence--it was thought--the resemblance between them. The fact is quite otherwise.

No parent ever produces a germ-cell, and the reason why children resemble parents and ancestors is because the germ-cells which give rise to individuals in successive generations are produced from the germ-cells of the previous generation. The line of descent or inheritance, therefore, is from germ-cell to germ-cell, and not from parents. Unless the reader makes himself absolutely familiar with the thought expressed in these facts he will never understand the science of Embryology.

Dr. Archdall Reid expresses this truth in the following words. "The somatic cells of the parent, therefore, as far as we know, contribute no living elements to the child; they merely provide temporary shelter and nutriment. The child, therefore, does not, as is popularly supposed, resemble his parent because his several parts are derived from similar parts of the parent--his head from his parent's head, his hands from his parent's hands, and so forth; he resembles him only because the germ-plasm which directed his development was a split-off portion of the germ-plasm which directed the development of the parent. The egg produces the fowl, but the fowl as a whole does not produce the egg--only one cell from the fowl, the fertilised ovum, produces it."

[Ill.u.s.tration: Diagram to show the origin of germ-cells and the embryo.]

Unite in the process of fertilisation to form the fertilised ovum, which divides a given number of times and forms daughter-cells, which are germ-cells; one of which, and one only, goes on dividing to form the body-cells, and so produces the new individual, which as it grows includes in itself those cells (germ-cells) previously formed.

The rest are germ-cells, which subsequently form the eggs and sperms of the new individual, _i.e._ they are the germ-cells of the next generation. They cannot develop independently, but when they unite with the egg or sperm of another individual, a new fertilised ovum is formed and the cycle begins again.

This is a startling thought, but it is one which a moment's careful consideration will show is the only conceivable explanation of all the facts of physical continuity. Once it is grasped a flood of light is thrown upon the whole science of Embryology. The individual is seen to be literally a "chip of the old block," and the "old block" means the whole sequence of germ-cells which has preceded his formation. In the light of this fact it is obvious why like produces like; indeed, it is obvious that it must do so. Further, we now understand at once that since one generation of germ-cells directly produces those of the next, there is no reason in the world why an individual should not more nearly resemble a remote ancestor than his own immediate parents. As a simple matter of fact this frequently happens. He does so because the germ-cell from which he sprang is composed of protoplasm handed down in direct continuity by successive generations of germ-cells from time immemorial.

In fact the problem in the light of this evidence is not so much--as it always seems to the writer--to understand why children resemble parents and ancestors, as to understand how it is that they do not resemble them more.

There is no difficulty now in explaining the fundamental propositions with which we started, namely, that children resemble their parents.

There is no difficulty in the understanding why a child resembles not only its immediate parents, but even its ancestors. There is even no difficulty in understanding why a child should resemble its ancestors, even though it does not resemble its parents. Given the simple truth that the germ-plasm is continuous from one generation to another, all these things become as clear as daylight.

But we also start with another general proposition, namely, that children differ from their parents, and it is this question of the variation in offspring which must now claim our attention for a moment.

By this term we mean to convey the fact that although every child has a real resemblance to its parents or its ancestors, it inevitably and invariably shows differences even if these be more minute than the resemblances. In other words the offspring of a human being, though obviously and necessarily, from the continuity of germ-plasm, it must be another human being, is never exactly similar to any other. Now these variations are many of them present from the very beginning, they take their origin in the germ-plasm of the two germ-cells which form the fertilised ovum. They are, that is to say, many of them germinal in origin. These must be carefully distinguished from such characteristics as are afterwards acquired by the child as the result of its adaptation to the environment in which it pa.s.ses its existence.