Embryology: The Beginnings of Life

by Gerald R. Leighton

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 mass 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 sexes, are applied, but only to the individuals. They are male and female; the germ-cells are of neither sex. 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 mass 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 multitudes of all the other kinds of cells which gradually assume 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 classification 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 attitude 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 nobody 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.