Popular Science Monthly/Volume 24/March 1884/From Moner to Man
|FROM MONER TO MAN.|
PROFESSOR OF PHYSIOLOGY IN THE WOMAN'S MEDICAL COLLEGE OF PENNSYLVANIA.
MAN has long been regarded not only as a compendium of the entire animal kingdom, but as an epitome of the universe—as Nature's short-hand expression of a long-continued history begun with the beginning condensation of the nebulae, and still going on to the development of higher types of humanity. Nature's language is hieroglyphic, and for the correct interpretation of her occult characters a key is necessary. It is one of the many triumphs of modern science that she has found at least a partial key to this mysterious book, and it is to the unlocking of some of its secrets that your attention is invited on this occasion.
My subject—the development of the human body from a microscopic speck of living matter—is a vast one, and the attempt to condense its consideration into the space of a single hour can result, at best, in a little more than a bare outline; but even such an exposition, however imperfect, may perhaps be deemed justifiable as a means of inciting to further study, and it is in this hope that the task is undertaken.
In the earliest perceptible stage of its existence, the human being consists of a minute apparently homogeneous mass of living matter of the kind known, since the days of Von Mohl and Remak, as protoplasm. The word means simply the first formative material, or the material in which all plants and animals have their origin. That it is a fact of natural history, and not a mere figment of the scientific imagination, that all plants and animals originate in a common substance, is no longer denied. This great principle was, indeed, recognized by Harvey, and first expressed in his famous aphorism, "Omne vivum ex ovo"—an egg, whenever it occurs, consisting essentially of a minute globule of protoplasm.
What is the origin of this universal, white-of-egg-like material? As little is known of the history of the first production of protoplasm as of that of the elements—hydrogen, oxygen, nitrogen, carbon, etc., of which it is composed. So far as yet discovered, all protoplasm, whether vegetable or animal, is derived from pre-existing protoplasm. The spontaneous production of living matter from non-living materials has never been satisfactorily proved. The particular kind of protoplasm which we are about to consider—viz., the human germ—is the combined derivative of certain glands which exist in separate adult human beings who represent the opposite sexual polarities belonging to all except the lowest vegetable and animal types. At the earliest recognizable stage of his existence man may therefore be regarded, physiologically, as a secretion. Zoölogically, to what rank is he, then, entitled? The undeveloped human ovum, immediately after its fertilization, corresponds in structure to the lowest known order of the most simple class of animals, the Protozoa, which stand at the very foot of the zoölogical scale. To this most humble of all known living creatures Professor Haeckel has given the name of Moner, a word of the same origin as monad, and expressive of ultimate simplicity and primitiveness.
More simple even than the moner, however, is the bathybius, found on the deep-sea bottom, and described by Professor Huxley as consisting of an ill-defined mass of a slime-like material possessing all the properties of living protoplasm. Even granting with skeptics on this point that the existence of bathybius is not satisfactorily proved, we may nevertheless assert with confidence that, as the natural predecessor of the moner, it ought to exist, and will some time be discovered, just as certain unobserved heavenly bodies have been partially described and located by astronomers long before the telescope had penetrated the obscurity in which they were hidden.
Through the processes of nutrition, under the combined influences of growth and development, this non-nucleated mass of living protoplasm (the human ovum) acquires a nucleus; in other words, there appears at its center a minute speck of matter slightly more opaque than the surrounding matter. Differentiation has therefore begun; that is, a difference of parts has made its appearance. How does this nucleus (to which, in cell-physiology, so much importance is attached) differ from the surrounding: matter which constitutes the bulk of the germ? Chemically, it is more active; it is believed to be the part where nutrition (the assimilation of new material) mainly takes place. Its greater chemical, and, therefore, nutritive activity, is shown by its deeper staining with coloring-matters, such as carmine and hæmatoxylin, and by the fact that, with the access of nutriment, fresh nuclei make their appearance. It undoubtedly contains a larger proportion of the nitrogenous matter which enters into the composition of all protoplasm, and, like the nuclei of other cells, a certain percentage of phosphorus. At this stage of its existence the germ (still microscopic) is represented in the zoölogical scale by the Amoeba, which it closely resembles in structure, having thus ascended to the second round of the zoölogical ladder.
The amœba has received its full share of attention from biologists. Its physiological endowments are scarcely greater than those of the non-nucleated moner. Both are capable of effecting those exchanges of matter which constitute nutrition; both are capable of reproduction (a phase of nutrition); both have the power of changing their form by thrusting out portions of their mass (the so called "false-feet"), and of thus executing slight creeping movements. These little masses of protoplasm are also capable of responding to contact of other matter, thus exhibiting the rudiments of common sensation. What is the evidence of this capacity? How does the amoeba manifest a sense of touch? When some substance, perhaps a smaller representative of its own species, floats against the surface of an amœba, the precocious bit of protoplasm responds to the salute by flowing around its victim, which is thereby inclosed within the body of its captor, and gradually appropriated as food. Probably the term "victim" is of doubtful application in this case, since the difference between eating and being eaten must be trifling. However that may be, the one improvises a stomach for the occasion, and digests the other with all the nonchalance of a Feejee-Islander. The human germ is, however, preserved from a similar indulgence in incipient cannibalism by its different environment—not the only period of its existence when it escapes evil-doing through lack of opportunity—for it receives its pabulum, ready prepared, from the blood of the mother, which is doubtless one of the conditions of its future higher development.
In this response to contact by movement on the part of the amœba, it exhibits the rudiments of both muscular and nervous action, since, under the influence of an external force or stimulus, a reflex movement is produced.
The next perceptible change in the evolution of the ovum is known as segmentation. This consists in an increase of its mass by duplication and reduplication; the single cell first acquires a second nucleus, and the surrounding protoplasm then separates into two masses, each having its own nucleus; this process is continued until the enveloping membrane contains a mass of cells, each like the original amœboid cell. From the resemblance of the ovum at this period to a mulberry, this is called the mulberry or morula stage of embryonic development. In the zoölogical scale, it corresponds to the labyrinthida, a little animal which consists of an aggregation of simple nucleated cells. From this multiplication of nuclei, which are regarded as the active centers of nutrition, there must result an increased power of development and growth.
By the absorption of fluid from the maternal tissues in which it is imbedded and the accumulation of this fluid at the center of the mass, the cells of this mulberry-like body become crowded outward to the periphery, thus forming a lining for the membranous sac—i. e., the outer covering of the ovum—which incloses them, the entire globular mass now being about one twenty-fourth of an inch in diameter, and consisting of a structureless outer membrane lined with a layer of nucleated cells (the blastoderm), and filled with clear fluid. These lining cells multiply rapidly; the inner ones become larger, darker, and softer than the outer ones, and thus differentiation has again occurred—the lining having developed into two distinct layers. This is known as the gastrula stage of embryonic development. All animals, from sponges to man, pass through this phase, becoming first two and then three layered sacs; but, from this point, the different branches or sub-kingdoms diverge; and the next recognizable phase in the development of the human embryo is confined to vertebrates, with a single exception, the ascidian. The larval ascidian swims like a tadpole by means of a caudal appendage in which may be traced a rod-like body thought to be a rudimentary chorda dorsalis, since it resembles the embryonic structure which, in the perfect vertebrate, develops into the spinal column with its contained, highly endowed spinal cord. This, however, not only fails to develop but actually disappears in adult life, leaving the ascidian a simple invertebrate animal. But, whether the ascidian be a true connecting link between invertebrates and vertebrates, or, as suggested by Balfour, a reversion from the higher form, it serves equally to indicate a close relationship between these two great subdivisions of the animal kingdom.
Between the two layers of germinal cells which belong to the gastrula stage, a third layer is developed, and from these three layers (the epiblast, the mesoblast, and the hypoblast) all the tissues and organs of the body are derived. The inner layer (hypoblast) gives origin to the epithelial lining of the alimentary canal and to the various glands derived from it. From the outer layer (epiblast) are developed the brain and spinal cord, and the epidermis with its appendages and derivatives, including the organs of the special senses. From the middle layer (mesoblast) the various intermediate structures are produced. The remaining history of development is, therefore, the history of the differentiation of these three layers of the blastoderm (which alike consist of simple nucleated cells) into the various tissues and organs of the body. Accompanying this process there is a corresponding development of functions. As absorption and assimilation, so perfectly performed by these germinal cells, are, however, the fundamental facts in the nutrition of even the highest organisms, so also reaction in response to a stimulus, of which we have found even the moner and the amœba to be capable, is the fundamental fact in the functions of the fully developed muscle, nerve, and brain of the highest organisms.
The embryon, in its condition of a three-layered sac, soon begins to show a slight bilateral symmetry, and a chorda dorsalis appears. Its rank, as a vertebrate, is thus established in the dawning of that important structure, a backbone.
Allusion has been made to the ascidian as introducing the vertebrate type. Whatever may be thought of the claims of this animal to so important a place in the genealogical tree, there can be little doubt about the position of the amphioxus with its dorsal cord distinct and persistent throughout life. Though classed, on this account, among vertebrates, it is singularly wanting in vertebrate characteristics, having neither heart nor brain in the true sense of these words. It is also destitute of limbs, even of the most rudimentary kind, such as are found in the very lowest fishes. In fact, it is distinctly neither vertebrate nor invertebrate, thus admirably filling the position of a connecting link between these two great subdivisions of the animal kingdom.
At the chordonian stage of its development, the human embryon is equally destitute of a true heart, brain, and limbs, thus corresponding to a sub-type of the vertebrates called by Haeckel, Acrania, of which the amphioxus is the best-known representative. There is, nevertheless, in this heartless, brainless, limbless, and almost shapeless mass of but slightly differentiated protoplasm, that wonderful impulse of evolution by which its destiny, as an individual of the highest organic rank, is assured.
Along the line of the chorda dorsalis, rudimentary nerve-centers and spinal vertebræ gradually appear, the embryon thus entering on a grade of development comparable to that of the lowest fishes, in which the spinal column is cartilaginous rather than bony.
The budding limbs resemble budding fins; arches similar to those which, in water-breathing animals, support the gills are seen; and the rudimentary lungs are mere air-bladders.
Next arises the amnion stage, so named from an important though temporary nutritive organ whose development begins at this period; it is an extension of the yolk-sac, and contains a highly nutritious fluid.
The gill-arches gradually disappear, developing into more advanced structures; the heart becomes subdivided into four chambers; the air-bladders give place to true lungs; and, with the complete formation of a placenta, the mammalian stage of development is fully established. The embryon is henceforth recognizable as belonging to the class mammalia, the highest of the vertebrates.
As the growing organism becomes more and more complex, its progress is more and more gradual. We have seen how the germ passes, almost at a single step, from the gastrula to the rudimentary vertebrate stage; but, after the mammalian stage is reached, it moves with deliberation through various lower embryonic forms of the class mammalia, till the human type is fully developed. At birth even, differentiation is far from being complete; not only do the several human races differ materially in shape and size of skull and in weight of brain, but there are also wide possibilities of difference among individuals of the same race and even between members of the same family. Exceptional characters are not recognized in their cradles; on the contrary, growth and differentiation continue till full maturity is reached, lifting the inventor, the philosopher, and the creative genius as far above the average human being as the average human being is above the chimpanzee.
In order to illustrate the relations to each other of the different grades of animal life, Haeckel employs the figure of a tree, which is intended to exhibit the probable lines of evolution of the entire series of animal forms continued through vast geological periods; and it is a fact of the utmost significance that this tree serves equally well as an illustration of the plan and progress of human embryonic development, thus indicating that the life-history of every human embryo is a recapitulation, in brief, of the history of the development of the whole animal kingdom. The base of the trunk of this tree represents the lowest, i. e., the most simple of animal forms—those which the human germ so closely resembles after fertilization, before development has begun.
The roots of this tree have not been represented by Professor Haeckel; but the supposition that, like the roots of other trees, they are concealed in the inorganic crust of the earth, is necessary to the completeness not only of the figure, but of the theory which it is intended to illustrate; I have therefore ventured to make this addition in the copy of Haeckel's figure which is before you.
Ascending by a single step, the lowest branches represent those organisms in which the first developmental change has occurred, the amœba, it will be remembered, showing its superiority to the moner in the possession of a nucleus.
From this point the trunk is carried upward through the various stages, giving off large branches which thereafter pursue separate paths of development in different directions. These groups, in their turn, subdivide; and while at each step the divergence is a gentle one, it nevertheless leads farther and farther away from the common type with which the process of differencing began; like the terminal twigs of any widely-branching tree which, though closely surrounded by other twigs, are far removed from the common trunk, and still more widely separated from those branches which have developed on the opposite side.
This tree is one which bears all manner of fruit; but, as all the branches of a tree receive the life-supporting sap from a common trunk, so all living forms have a common origin in protoplasm with which the evolution of their life begins; the entire growth and development of the body consisting in the growth and differentiation of the protoplasm of which its tissues and organs are composed.
Observe how admirably the figure of a tree exhibits the supposed relationship between the various types of animals both extinct and living; indicating, not that each type has been derived directly from one immediately preceding it, either in time or in structural rank, but that various types have had a common ancestor from which, by development in different directions, all have more or less diverged; so that the relationship between man and the existing anthropoid apes, for example, is that of remote cousinship rather than of direct descent. The common stock is represented by the trunk of the tree; from this trunk, which rises higher and higher with each diverging offshoot, has sprung an immense variety of branches; and, at the very pinnacle of this magnificent structure, man appears—the crowning efflorescence of organic evolution.
The permanent types which represent these various phases of embryonic development show a progressively increasing differentiation from their environment. The moner and the amœba are almost as structureless as the water in which they are found, consisting of little more than water with a trace of albumen; in specific gravity, in temperature, in color, etc., the difference between these low organisms and their environment is slight. Compared with the differences—chemical, physical, and structural—between man and the invisible atmosphere in which he is submerged, the contrast in this particular is a striking one. This leads us to other considerations of still greater significance.
The true environment of any organism consists in as much of the external universe as that organism is capable of holding communication with; so that, as the life becomes higher, the environment also becomes more complex.
At the deep-sea bottom, where life is exhibited in its most simple grades, the temperature is unvarying; no light penetrates to those depths; a uniformity of conditions is thus preserved almost unbroken, and the adjustments necessary to the continuance of life under such circumstances are as trifling as the grade of life is simple.
By the greater complexity of the human organism as compared with other animals, man is brought into communication with and under the influence of a vastly increased variety of external conditions, mainly through the organs of the special senses and their intimate relations with a highly developed nervous system.
That without the eye and its connections with the brain we could have no consciousness of light is the merest commonplace of physiology; yet, could we realize the full meaning of this and other similar facts, we should be near to an understanding of the difference between a high and a low organism; the life is high when there is a high degree of correspondence with a highly complex environment.
Poets have understood this principle better, perhaps, than physiologists.
says Wordsworth; and Coleridge—
Emerson also embodies this whole philosophy in a single illustration: "The sea drowns both ship and sailor, like a grain of dust, and we call it fate; but let him learn to swim, let him trim his bark, and the water which drowned it will be cloven by it and will carry it like its own foam—a plume and a power."
When we remember that our environment consists, not only of the natural elements of earth and sky, reaching to the most distant star which communicates its vibrations to our atmosphere, but that it also includes other human beings with the influences which such an environment involves, we realize that, while physiology undoubtedly rests on chemistry and physics, it also includes psychology and reaches far toward sociology—sciences which involve the highest problems of our existence; and, though we find it impossible to sink our plummet to the depths of this ocean, or to send an arrow to the stars which gem the arching dome above, we may at least hope to gather a few shells on the shore of the one, and to intercept some gleams of light from those distant suns which fascinate by their very distance, and make glorious the night of our intellectual darkness even.
How, we next inquire, does the human embryo differ, at the progressive stages of its evolution, from the embryos of the various lower types which it successively resembles? Whence the impulse of development by which it rises from these lower levels to the human plane? In reply to these questions we can only refer to the principle of heredity which, though it imprints upon the germ no trace discoverable by any known test, unfailingly molds the plastic protoplasm into certain prescribed and prearranged forms, with their accompanying capacities and powers. The inherent forces by which one germ develops into an oak and another into a trailing vine, one into a mollusk and another into a man, are handed down from generation to generation, so that each plant and animal reproduces its own kind and not some other kind. This can not be regarded, however, as an exceptional fact; the production of the germ with all its hidden possibilities, like every other differentiation of matter, depends upon the general principle known as the persistence of force; and to deny that the power of development of any grade of life is inheritable is to deny the persistence of force—a doctrine which lies at the very foundation of the stately edifice of modern science.
What is there in the whole stupendous drama of evolution, as conceived by the most enthusiastic supporters of the hypothesis, more wonderful or more difficult of comprehension and acceptance than these facts of embryonic development at which we have briefly glanced?
By the simultaneous processes of growth and differentiation, by a gradual increase of complexity and heterogeneousness continued through a considerable period of time, a microscopic speck of apparently structureless protoplasm, undistinguishable by any known test from the germ of any other animal, develops into the most highly endowed organism of which we have any knowledge.
And through what agencies are these remarkable results accomplished? Besides the inherited impulse of growth and development already referred to, there is furnished to this germ a due supply of ready-prepared food; a certain uniform temperature is also secured to it until the time of birth. After that period, its environment becomes gradually more complex; but embryonic development does not differ essentially from the continued development of infancy, childhood, and youth, by which the adult state is reached. The minute speck of simple protoplasm which constitutes the human organism at the beginning of its career is as truly an independent individual as it ever becomes. At this, as at every subsequent stage of its existence, its life and growth and progress depend on the activities of its own tissues, brought into play by the influence of external forces. Then, as always, it receives food from its environment; while the appropriation and assimilation of this food, as well as the elimination of the products of disintegration and waste, are accomplished by means of the same processes of absorption, chemical combination and decomposition, which constitute nutrition at all periods of existence. The embryon lives its own life—a work which can not be delegated to another.
Our next inquiry is in regard to the forces manifested by living bodies. What are the relations between the highly developed varieties of protoplasm which constitute their different tissues and organs and the remarkable functions—muscular action, emotion, volition, etc.—peculiar to animal organisms?
This question will be best answered by means of a familiar illustration. By an appropriate combination of valves and pistons, of wheels and levers, and numerous other contrivances put together in strict conformity with the principles of mechanics, in which the most delicate allowances are made for unavoidable friction, and the attraction of gravitation is either annihilated by counterbalancing weights or turned to account as a source of power, a machine is constructed which strikingly illustrates the importance, not only of the particular character of the different parts of which it is composed, but of the relations of these parts to each other. The force operating such a machine may be derived from a simple fall of water, or from the oxidation of burning anthracite; but, although this may be the sole source of the actual energy expended, it is far from being the only factor concerned in the production of the special kind of work accomplished. The results are due to the transformations of this initial force into force of other kinds, the character of the work done depending on the peculiar construction of the machine—in other words, on the relations of its parts. Thus the expansive power of steam may be expended in the idle clapping of the lid of a tea-kettle, or in the driving of the piston in the engine of an ocean-steamer, according to the relations into which the steam is brought. Keeping this illustration in mind, we may perhaps attain to some conception of the meaning of a living organism, and wherein consist the differences in different organisms.
The life-processes are concerned in the building up of the tissues—that is, in the construction and constant repair of the mechanism out of materials supplied by food; coincident with this assimilation of new material, there is a corresponding accumulation of energy or force. The energies liberated, on the other hand, in the activities of muscle, nerve, brain, etc., come from the oxidation—the so-called waste—of these tissues; and (as in the machine) the results produced are due to the transformations of this initial force, derived from oxidation of the tissues, into other kinds of force, viz., those manifested by living animal organisms, the character of the work done depending (as in the illustration) on the particular construction of the mechanism concerned. In the operations of living organisms, not less than in those mechanisms whose motive power is derived from steam, not a known law of matter is violated, but all are wrought into a harmony so complete that the entire complex and heterogeneous structure acts as a unit.
Glancing in thought over the vast expanse of matter of which the universe consists, what has been the direction of the progress witnessed through the long ages since the beginning condensation of the nebulous masses in which our solar system is believed to have originated? The immense globes which whirl in repeated circles through the heavenly spaces, though bound together by the strongest and most subtile bonds, roll blindly on, forever unconscious of themselves and of one another. The lily of the field even, clothed in beauty though it be, and surrounded by the greater glories of earth and sky—the warm sunshine and green fields—has no conscious enjoyment of itself or of them; but as elements identical with those which compose these unconscious forms have combined and recombined in compounds of increasing complexity, as molecules have condensed and differentiated in the development of a higher kind of living matter, consciousness has dawned, and (mainly through the avenues of the special senses) mind has developed. Each generation, heir to the endowments of all preceding ones, has added its increment of gain, and later generations—those which belong to the historic period—have begun their lives with a vast amount of inherited intelligence. There is sound philosophy in the statement once jocosely made, that the natives of a certain part of the country, remarkable for their intellectual activity, are born with a good common-school education. By far the greater part of our education is indeed born with us.
Increased refinements of emotion, clearer subtilties of thought—these are the directions which further development of the race must take; and the individual who experiences a hitherto unrealized emotion, or who grasps a new thought which corresponds with some never before observed fact or relation in the external world, is the seat and center of progress. In such minds, nature is undergoing a still higher evolution, and the colors of humanity are thus successively planted on hitherto unsealed summits.
- Address delivered at the opening of the Twenty-ninth Annual Session.
- The lecture was illustrated by drawings.
- See "Principles of Biology," Herbert Spencer, vol. ii.