Gerald R. Leighton

“ It is quite obvious that the offspring of any species of animal, if they are to live and survive in the same kind of environment as that in which their parents live, must resemble them somewhat closely. The only way in which Nature can secure such a sufficiently close resemblance of offspring to parents is by insuring that they should develop along similar lines. So it is that we find that the whole of the life history of an individual is more or less a recapitulation, with, of course, variations, of that of the parents and ancestors. ”

“ It is obvious that of our total characteristics some are acquired and some are inherited, and the question then arises, How much is inherited in an embryo or individual, and therefore unavoidable, and how much acquired? ”

“ Are those differences due to the action of environment on the embryo or do they take their origin in the germ-cells from which the individuals came? Inasmuch as a litter of puppies is subjected to precisely the same environment during the whole time of development, it is perfectly obvious that such differences as they exhibit at the time of birth must have been germinal, an identical environment could not by any stretch of the imagination be held responsible for producing variations. ”

“ 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. ”

“ 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. ”

“ A human embryo, therefore, which is going to give rise to an adult individual is a community consisting of an enormous number of cells, the whole of which have descended from one common ancestor, a single cell known as a fertilised ovum. True, these descendants break up into many types of cells in order that different functions may be performed by special tissues, but none of these special cells can do everything that is necessary for the life of the whole individual; they can only play their own special small part. They can do nothing towards continuing the species of the individual. ”

“ It is only by their study that we realise that the human embryo, which is to become the human individual, consists, to a very large extent, of characters and features which are unalterably settled for it beforehand, to which nothing can be added, and from which nothing can be taken away. In other words, the possibilities for any individual are those which are pre-existent in the germ-plasm from which he or she originates. These possibilities, however, depend upon the environment in which the embryo, infant, child, and adult is subsequently placed for their full expansion. ”

“ It is important to note at this stage that the creatures which we have mentioned, and even some more highly organised, such as an amœba, which has a nucleus, go through these simple or complicated reproductive processes in the total absence of anything which could suggest a distinction of sex. In these cases the individuals are obviously all of one sex, and, therefore, the distinction of sexes 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. ”

“ Doubtless the cells which form the organs of nutrition for the embryo have a capacity for selecting the elements required for purposes of nutrition. It is their business to look after this process. How perfectly it is performed can at once be understood when we recollect how very frequently the tissues of the mother herself are in anything but perfect health, and yet the embryo is born healthy. ”

“ 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. ”

“ 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. ”

“ Tried by this test we may ask whether the changes produced in these unicellular organisms by the action of their environment are, or are not, inherited as variations. No one doubts for a single moment that a microbe as well as a germ-cell may be changed, or injured, or improved, according to its own special environment. What is in dispute is whether that change remains fixed in the succeeding generations to which these unicellular cells give rise. It is precisely here that the bacteriologist can offer evidence of a most important character. ”

“ 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. ”

“ 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. ”

“ Now we have completed what was necessary to arrive at our conclusion of what it is that goes to the making of an embryo, and therefore of a human being—a personality. The conclusion is that every characteristic which it is possible for an individual under any circumstances whatsoever to possess is traceable ultimately to the action which takes place between his inherited tendencies and his natural environment. ”

“ 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. ”

“ Wherever knowledge is absent superstition 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 ”

“ We are in search of as clear a statement as possible of the origin of the many and varied characteristics which go to the formation of a human embryo, and hence to the making of an individual. The variation in these many characteristics accounts for the differences in individualities. No two individuals are exactly similar whatever be the standard by which we estimate them. ”

“ Thus we find that the hair, the teeth, the internal ears, and the organs of reproduction, all grow to their full development in the absence of any other factor or stimulus than that of nutrition. But, as we also know, this simple stimulus is not sufficient for most of the other body tissues to develop properly. They require the additional stimulus of exercise which, indeed, may be said to begin even in the life of the embryo. ”

“ In all successive cases of multicellular organisms, development by a process of repetition of what happened in the previous generation seems to be the rule; and it would appear that only by this means could a mass of cells which constitute an individual grow into something sufficiently like the parents as to be recognised for their offspring. Given the fact that a human individual starts from a single germ-cell, it could only be by following the same steps in development trodden by the parent that the new individual could attain a similar growth. ”

“ So far we have considered merely the mode of development of the most important organs of the body, but we have said nothing of the most important supporting structure, namely, the skeleton. The earliest appearance of anything in the shape of a skeleton is the structure known as the notochord, a structure of immense importance and interest in the embryology of all vertebrate animals, in which it is a temporary thing only. ”

“ 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. ”

“ 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. ”

“ 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.” ”

“ 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 same truth applies to the cells which make up the body of the embryo and the individual, as well as to the germ-cells. The body-cells, those which make up bone, and muscle, and gland, and so forth, are constantly exposed to all sorts of influences which must tend to damage them so far as it is possible for the cells to be damaged and still live. ”

“ All that can be done under the best conceivable [Pg 41] circumstances is so to arrange the environment and surroundings of the embryo, and the subsequent individual, so that these tendencies are acted upon in such a way that the best are developed, and the worst eliminated. It must be remembered that it is under the constant action of everything that constitutes the environment of an embryo that the mass of body-cells gradually grows into a recognisable human personality. ”