Popular Science Monthly/Volume 84/June 1914/Facts and Factors of Development I
|FACTS AND FACTORS OF DEVELOPMENT|
By Professor EDWIN GRANT CONKLIN
ONE of the greatest results of the doctrine of organic evolution has been the determination of man's place in nature. For many centuries it has been known that in bodily structures man is an animal—that he is born, nourished and developed, that he matures, reproduces and dies just as does the humblest animal or plant. For centuries it has been known that man belongs to that group of animals which have backbones, the vertebrates, to that class which have hair and suckle their young, the mammals, and to that order which have grasping hands, flat nails, and thoracic mammæ, the primates, which group includes also the monkeys and apes. But as long as it was supposed that every species was distinct in its origin from every other one, and that each arose by a special divine fiat, it was possible to maintain that man was absolutely distinct from the rest of the animal world, and that he had no kinship to the beasts, though undoubtedly he was made in their bodily image. But with the establishment of the doctrine of organic evolution this resemblance between man and the lower animals has come to have a new significance. The almost universal acceptance of this doctrine by scientific men, the many undoubted resemblances between man and the lower animals, and the discovery of the remains of lower types of man, real "missing links," has inevitably led to the conclusion that man also is a product of evolution, that he is a part of the great world of living things and not a being who stands apart in solitary grandeur in some isolated sphere.
But wholly aside from the doctrine of evolution, the fact that essential and fundamental resemblances exist among all kinds of organisms can not fail to impress thoughtful men. The great life processes are everywhere the same in principles, though varying greatly in details. All the general laws of life which apply to animals and plants apply also to man. This is no mere logical inference from the doctrine of evolution, but a fact which has been established by countless observations and experiments. The essential oneness of all life gives a direct human interest to all living things. If "the proper study of mankind is man," the proper study of man is the lower organisms in which life processes are reduced to their simplest terms, and where alone they may be subjected to conditions of rigid experimentation. Upon this fundamental likeness in the life processes of man and other animals is based the wonderful work in experimental medicine, which may be counted among the greatest of all the achievements of science.
The experimental study of heredity, development and evolution in forms of life below man must certainly increase our knowledge of and our control over these processes in the human race. If human heredity, development and evolution may be controlled to even a slight extent, we may expect that sooner or later the human race will be changed for the better. At least no other scheme of social betterment and race improvement can compare for thoroughness, permanency of effect, and certainty of results, with that which attempts to change the natures of men and to establish in the blood the qualities which are desired. We hear much nowadays about man's control over nature, though in no single instance has man ever changed any law or principle of nature. What man can do is to put himself into such relations to natural phenomena that he may profit by them, and all that can be done toward the improvement of the human race is to consciously apply to man those great principles of development and evolution which have been operating unknown to man through all the ages.
Phenomena of Development
One of the greatest and most far-reaching themes which has ever occupied the minds of men is the problem of development. Whether it be the development of an animal from an egg, of a race or species from a preexisting one, or of the body, mind and institutions of man, this problem is everywhere much the same in fundamental principles, and knowledge gained in one of these fields must be of value in each of the others. Ontogeny and phylogeny are not wholly distinct phenomena, but are only two aspects of the one general process of organic development. The evolution of races and of species is sufficiently rare and unfamiliar to attract much attention and serious thought; while the development of an individual is a phenomenon of such universal occurrence that it is taken as a matter of course by most people—something so evident that it seems to require no explanation; but familiarity with the fact of development does not remove the mystery which lies back of it, though it may make plain many of the processes concerned. The development of a human being, of a personality, from a germ cell is the climax of all wonders—greater even than that involved in the evolution of a species or in the making of a world.
The fact of development is everywhere apparent; its principal steps or stages are known for thousands of animals and plants; even the precise manner of development and its factors or causes are being successfully explored. Let us briefly review some of the principal events in the development of animals, and particularly of man, and then consider some of the chief factors and processes of development. Most of our knowledge in this field is based upon a study of the development of animals below man, but enough is now known of human development to show that in all essential respects it resembles that of other animals, and that the problems of heredity and differentiation are fundamentally the same in man as in other animals.
I. Development of the Body
The entire individual—structures and functions, body and mind—develops as a single indivisible unity, but for the sake of clarity it is desirable to deal with one aspect of the individual at a time. For this reason we shall consider first the development of the body, and then the development of the mind.
1. The Germ Cells.—In practically all animals and plants individual development begins with the fertilization of a female sex cell, or egg, by a male sex cell, or spermatozoon. The epigram of Harvey, "Omne vivum ex ovo," has found abundant confirmation in all later studies. Both egg and spermatozoon are alive and manifest all the general properties of living things. How little this fact is appreciated by the public is shown by the repeated announcements by the newspapers that "Professor So-and-so has created life because he has made an egg develop without fertilization." An egg or a spermatozoon is as much alive as is any other cell—as characteristically alive as is the adult animal into which it develops. It is difficult to define life, as it is also to define matter, energy, electricity, or any other fundamental phenomenon, but it is possible to describe in general terms what living things are and what they do. Every living thing whatever, from the smallest and simplest microorganism to the largest and most complex animal, from the microscopic egg or spermatozoon to the adult man, manifests the following distinctive properties:—
1. It contains protoplasm, "the material basis of life," which is composed of the most complex substances known to chemistry. Protoplasm is not a homogeneous substance, but it always exists in the form of cells, which are minute masses of protoplasm composed of many distinct parts, the most important of these being the nucleus and the cytoplasm (Fig. 1). Protoplasm is therefore organized, that is, composed of many parts all of which are integrated into a single system, the cell. Higher animals and plants are composed of multitudes of cells, differing more
or less from one another, which are bound together and integrated into a single organism. Living cells and organisms are not static structures which are fixed and stable in character, but they are systems which are undergoing continual change. They are like the river, or the whirlpool, or the flame, which are never at two consecutive moments composed of the same particles but which nevertheless maintain a constant general appearance; in short they are complex systems in dynamical equilibrium. The principal physiological processes by which all living things maintain this equilibrium are:
2. Metabolism, or the transformation of matter and energy within the living thing, in the course of which some substances are oxidized into waste products, with the liberation of energy, while other substances are built up into protoplasm, each part of the cell converting food substances into its own particular substance by the process of assimilation.
3. Reproduction, or the capacity of organisms to give rise to new organisms, of cells to give rise to other cells, and of parts of cells to give rise to similar parts by the process of division.
4 Irritability, or the capacity of receiving and responding to pinging energies, or stimuli, in a manner which is usually, but not invariably, adaptive or useful.
Both the egg and the sperm are living cells with typical cell structures and functions, but with none of the parts of the mature organism into which they may develop. But-although they do not contain any of the differentiated structures and functions of the developed organism, they differ from other cells in that they are capable under suitable conditions of producing these structures and functions by the process of development or differentiation, in the course of which the general structures and functions of the germ cells are converted into the specific structures and functions of the mature animal or plant.
In both plants and animals the sex cells are fundamentally alike, though they differ greatly in appearances. The female sex cells of flowering plants are called ovules, the male cells pollen. The corresponding cells of animals are known as ova and spermatozoa. Collectively all kinds of sex cells are called gametes, and the individual formed by the union of a male and female gamete is known as a zygote, while the cell formed by the union of egg and sperm is frequently called the oosperm.
The egg cell of animals is usually spherical in form and contains more or less food substance in the form of yolk; it varies greatly in size, depending chiefly upon the quantity of yolk, from the great egg of a bird, in which the yolk, or egg proper, may be hundreds of millimeters in diameter, to theeggs of oysters and worms, which may be no more than a few thousandths of a millimeter in diameter. The human ovum is microscopic in size (about 0.2 mm. in diameter) but it is not smaller than is found in many other animals. It has all the characteristic parts of any egg cell, and can not be distinguished microscopically from the eggs of several other mammals, yet there is no doubt that the ova of each species differ from those of every other species, and later we shall see reasons for concluding that the ova produced by each individual are different from those produced by any other individual.
The sperm, or male gamete, is among the smallest of all cells and is usually many thousands of times smaller than the egg. In most animals, and in all vertebrates, it is an elongated, thread-like cell with an enlarged head which contains the nucleus, a smaller middle-piece, and a . very long and slender tail or flagellum, by the lashing of which the spermatozoon swims forward in the jerking fashion characteristic of many monads or flagellated protozoa. In different species of animals the spermatozoa often differ in size and appearance, and there is every reason to believe that the spermatozoa of each species are peculiar in certain respects even though we may not be able to distinguish any structural differences under the microscope. The human spermatozoa closely resemble those of other primates but are still slightly different, and the conclusion is inevitable, as we shall see later, that the spermatozoa as well as the ova of each individual differ slightly from those of every other individual.
2. Fertilization.—If a spermatozoon in its swimming comes into contact with a ripe but unfertilized egg, the head and middle-piece of the sperm sink into the egg while the tail is usually broken off and left outside. The nucleus in the head of the sperm then begins to absorb material from the egg and to grow in size and at the same time a minute granule, the centrosome, appears, either from the middle-piece or
Fig. 2. Two Human Spermatozoa. A, showing the surface of the flattened head; B, its edge; H, head; M, middle piece; T, tail. (After G. Retzius.)
from the head of the sperm, and radiating lines run out from the centrosome into the substance of the egg. The sperm nucleus and centrosome then approach the egg nucleus and ultimately the two nuclei come to lie side by side. Usually when one spermatozoon has entered an egg all others are barred from entering, probably by some change in the chemical substances given out by the egg.
This union of a single spermatozoon with an egg is known as fertilization. Whereas egg cells are usually, but not invariably, incapable of development without fertilization, there begins, immediately after fertilization, a long series of transformations and differentiations of the fertilized egg which leads to the development of a complex animal—of a person. In the fusion of the egg and sperm cells a new individual, the oosperm, comes into being. The oosperm, formed by the union of the two sex cells, is really a double cell, since parts of the egg and sperm never lose their identity, and the individual which develops from this oosperm is a double being; even in the adult man this double nature, caused by the union of egg and sperm, is never lost.
In by far the larger number of animal species the oosperm, either just before or shortly after fertilization, is set free to begin its own individual existence, and in such cases it is perfectly clear that the fertilization of the egg marks the beginning of the new individual. But in practically every class of animals there are some species in which the fertilized egg is retained within the body of the mother for a varying period during which development is proceeding. In such cases it is not quite so evident that the new individual comes into being with the fertilization of the egg—rather the moment of birth or the separation from the mother is generally looked upon as the beginning of the individual existence. And yet in all cases the egg or embryo is always distinguishable from the body of the mother and there is no protoplasmic connection between the two. In mammals generally, including also the human species, not a strand of protoplasm, not a nerve fiber, not a blood vessel passes over from the mother to the embryo; the latter is from the moment of fertilization of the egg a distinct individual with particular individual characteristics, and this is just as true of viviparous animals
Fig. 3. Entire Spermatozoon of the Annelid Nereis, showing perforatorium (P); head (H); middle piece (M), and tail (T). (From F. R. Lillie.)
Figs. 4-5. Two Stages in the Entrance of the Spermatozoon into the Egg of Nereis. Some of the protoplasm of the egg has gathered at the point of entrance to form the entrance cone (EC) which, together with the sperm head, moves into the interior of the egg in later stages. The black spheres represent yolk. (From F. R. Lillie.)
in which the egg undergoes a part of its development within the body of the mother, as it is of oviparous forms in which the eggs are laid before development begins.
The fertilized egg of a star-fish, or frog, or man is not a different individual from the adults of these forms, rather it is a star-fish, a frog, or a human being in the one-celled stage, and thereafter this new being
Figs. 6-9. Successive Stages in the Maturation and Fertilization of the entrance cone (EC) and sperm nucleus (♁N) into the egg. Fig. 6 shows the first by this division; Fig. 8, the second maturation spindle (2d Mat. Sp.) and the in the first cleavage spindle (1st CI. Sp.). (From F. R. Lillie.)
maintains its own individuality. This fertilized egg fuses with no other cells, it takes into itself no living substance from without, but manufactures its own protoplasm from food substances; it receives food and oxygen from without and it gives out carbonic acid and other waste products; it is sensitive to certain alterations in the environment such as thermal, chemical and electrical changes—it is, in short, a distinct living thing, an individuality. Under proper environmental conditions this fertilized egg cell develops, step by step, without the addition of anything from without except food, water, oxygen, and such other raw materials as are necessary to the life of any adult animal, into the immensely complex body of a star-fish, a frog, or a man. At the same time from the relatively simple reactions and activities of the fertilized egg there develops, step by step, without the addition of anything from without except raw materials and environmental stimuli, the multifarious
Egg of Nereis, less highly magnified than Figs. 4 and 5, showing the progress of the turation spindle of the egg (1st Mat. Sp.); Fig. 7, the first polar body (1st PB) sperm nucleus and spindle (♁N); Fig. 9, the division of the male and female nuclei
activities, reactions, instincts, habits, and intelligence of the mature animal.
Is not this miracle of development more wonderful than any possible miracle of creation? And yet as one watches this marvellous process by which the fertilized egg grows into the embryo, and this into the adult, each step appears relatively simple, each perceptible change is minute; but the changes are innumerable and unceasing and in the end they accomplish this miracle of transforming the fertilized egg cell into the fish, or frog, or man—a thing which would be incredible were it not for the fact that it has been seen by hundreds of observers and can be verified at any time by those who will take the trouble to study the process for themselves.
Fig. 10. Successive Stages in the Cleavage and Gastrulation of Amphioxus. A, one cell; B, two cells; C and D, four cells; E, eight cells; F, sixteen cells; Q, blastula stage of about ninety-six cells; H, section through the same showing the cleavage cavity; I, blastula seen from the left side showing three zones of cells, viz., an upper clear zone of ectoderm, a middle (faintly shaded) zone of mesoderm and a lower (deeply shaded) zone of entoderm cells; J, section through the same showing these three types of cells; K and L, successive stages in the gastrulation; cells indicated as in the preceding figure. In all figures except D the polar body is shown at the upper pole. Figs. A-H after Hatschek; Figs. I-L after Korschelt and Heider and Cerfontain. a, anterior; p, posterior; v, ventral; d, dorsal; bc, blastoccel; gc, gastroccel.
3. Cleavage.—When the two germ nuclei (egg nucleus and sperm nucleus) have come into contact after the fertilization of the egg they divide by a complicated process known as mitosis, or indirect nuclear division (Fig. 9). The centrosome, which usually accompanies the sperm nucleus in its passage through the egg, divides and forms a spindle-shaped figure with astral radiations at its two poles (Figs. 7, 8). The chromatin, or stainable substance, of the nucleus, takes the form of threads, the chromosomes (Fig. 9), of which there is a constant number for each species of animal and plant. Each chromosome then splits lengthwise, its two halves moving to opposite ends of the spindle, in which position the daughter chromosomes fuse together to form the daughter nuclei. In this way the chromatin of the egg and sperm nuclei is exactly halved.
After the germ nuclei have divided in this manner the entire egg divides by a process of constriction into two cells (Figs, 10, 28). This is the beginning of a long series of cell divisions, each of them essentially like the first, by which the egg is subdivided successively into a constantly increasing number of cells. During the earlier divisions there
Fig. 11. A and B. Two Later Stages in the Development of Amphioxus, showing the elongation of the embryo in the antero-posterior axis (a p), and formation of the somites (som); neural groove (ng) and neural tube (nt); ect, ectoderm; ent, entoderm; mes, mesoderm; ac, alimentary canal. (After Hatschek.)
is little or no increase in the volume of the egg, consequently successive generations of cells continually grow smaller (Figs. 10, 13, 14, A). This process is known as the cleavage of the egg, and by it the egg is not only split up into a considerable number of small cells, but a much more important result is that the different kinds of protoplasm in the egg become isolated in different cleavage cells, so that these substances can no longer freely commingle. The cleavage cells, in short, come to contain different kinds of, and thus to differ from one another. The differentiations of the cleavage cells appear much earlier in some forms than in others, but in all cases such differentiations appear during cleavage.
4. Embryogeny.—From this stage onward the course of development differs in different classes of animals to such an extent that it is difficult to formulate any general description which will apply to all of them. Usually the many cleavage cells form a hollow sphere, the blastula (Fig. 10, K), and this in turn becomes a gastrula (Fig. 10, L, M), in which at first two, and later three, groups or layers of cells may be recognized; the outer layer, which is formed from cells nearest the animal pole of the egg, is the ectoderm; the inner layer, or entoderm, is formed from cells nearest the vegetative pole; a middle layer, or group of cells, the mesoderm, is formed from cleavage cells which in vertebrates lie between the animal and vegetative poles.
5. Organogeny.—By further differentiation of the cells of these layers and by dissimilar growth and folding of the layers themselves the various organs of the embryo begin to appear. From the ectoderm is formed the outer layer of the skin and the nervous system; from the entoderm arises the lining of the alimentary canal and its outgrowths; from the mesoderm comes, in whole or in part, the skeletal, muscular, vascular, excretory, and reproductive systems. In vertebrates the nervous system appears as a plate of rather large ectoderm cells (Fig. 12);
Fig. 12. Cross Section of Amphioxus Larvae in Successive Stages of Development. A, through a larva similar to 11A; B and C, of a larva similar to 11B; D, of a still older larva; ect, ectoderm; ent, entoderm; mes, mesoderm; ch, notochord; np, neural plate; gc, gastroccel; ac, alimentary canal; cæl, cœlom.
this plate rolls up at its sides to form a groove (Fig. 12) and then a tube (Fig. 12); and by enlargement of certain portions of this tube and by foldings and thickenings of its walls the brain and spinal cord are formed (Figs. 12, 15, C, D). The retina or sensory portion of the eye is formed as an outgrowth from the fore part of the brain (Fig. 15, D); the sensory portion of the ear comes from a cup-shaped depression of the superficial ectoderm which covers the hinder portion of the head (Fig. 15, E and F). The back bone begins to appear as a delicate cellular rod (Fig. 12, ch), which then in higher vertebrates becomes surrounded successively by a fibrous, a cartilaginous, and a bony sheath. And so one might go on with a description of all the organs of the body,
Fig. 13. Four Cleavage Stages of the Sheep; pb, polar bodies. (After Assheton.)
each of which begins as a relatively simple group or layer of cells, which gradually become more complicated by a process of growth and differentiation, until these various embryonic organs assume more and more the mature form.
6. Oviparity and Viviparity.—This very brief and general statement of the manner of embryonic development applies to all vertebrates, man included. There are many special features of human development which are treated at length in works on embryology, but which need not detain us here since they do not affect the general principles of development already outlined. In one regard the development of the human being or of any mammal is apparently very different from that of a bird or frog or fish, viz., in the fact that in the former the embryonic development takes place within the body of the mother whereas in the latter the eggs are laid before or soon after fertilization. In man, after the cleavage of the egg, a hollow vesicle is formed, which becomes attached to the uterine walls by means of processes or villi which grow out from it (Fig. 14, D, E, F) while only a small portion of the vesicle becomes transformed into the embryo. There is thus established a connection between the embryo and the uterine walls through which nutriment is absorbed by the embryo. And yet this difference is not a fundamental one for in different animals there are all stages of transition between these two modes of development. While in most fishes, amphibians and reptiles
Fig. 14. Diagrams Showing the Early Development of the Human Oosperm. A, cleavage stage which has just come into the uterus; B and C, blastodermic vesicles embedded in the mucous membrane of the uterus; D, E and F, longitudinal sections of later stages, the anterior and posterior poles being marked by the axis a p. In C cavities have appeared in the ectoderm, entoderm and mesoderm. D, villi forming from the trophoblast (nutritive layer, tr); black indicates ectoderm (ect); oblique lines, entoderm; fewmesoderm; V, villi; am, amnion; ys, yolk sac; n, neurenteric canal (x25). (From Keibel.)
Fig. 15. A-H, successive stages in the early development of the human embryo. A, blastodermic vesicle showing primitive axis in embryonic area; age unknown. B, blastodermic vesicle attached to uterine wall at the posterior pole, showing neural groove; age unknown. C, later stage in which the neural folds are closing and five pairs of somites have appeared; age, ten to fourteen days. D, stage of fourteen somites showing enlargements of the neural folds at the anterior end which will form the brain; age, fourteen to sixteen days. E and F later stages, the latter with twenty-three somites and three visceral clefts. The ear shows as a depression at the dorsal angle of the second cleft. G, embryo of thirty-five somites showing eye, branchial arches and limb buds. H, embryo of thirty-six somites showing nasal pit, eye, branchial arches and clefts, limb buds and heart. (After Keibel.)
in fact, except in so far as the quality of the mother's blood may be changed and may affect the child. At no time, whether before or after birth, is the mother more than nurse to the child. Hereditary influences are transmitted only through the egg cell and the sperm cell and these influences are not affected by intra-uterine development. The principles of heredity and development are the same in oviparous and in viviparous animals—in fish, frogs, birds and men.
Summary.—This is a very brief and incomplete statement of some of the important stages or phases of the development of the body of man or of any other vertebrate. In all cases development begins with the fertilized egg which contains none of the structures of the developed animal, though it may exhibit the polarity and symmetry of the adult and may also contain specific kinds of protoplasm which will give rise to specific tissues or organs of the adult. From this egg cell arise by division many cells which differ from one another more and more as development proceeds, until finally the adult animal results. A specific type of development is due to a specific organization of the germ cells with which development begins, but the earlier differentiations of the egg are relatively few and simple as compared with the bewildering complexities of the adult, and the best way of understanding adult structures is to trace them back in development to their simpler beginnings and to study them in the process of becoming.
7. Development of Functions.—The development of functions goes hand in hand with the development of structures; indeed function and structure are merely different aspects of one and the same thing, namely organization. All the general functions of living things are present in
Fig. 16. A, human embryo of forty-two somites; ages about twenty-one days. B, embryo of about four weeks. C, still older embryo showing the beginnings of the formation of digits. F, embryo of about two months. (After Keibel.)
the germ cells, viz., (1) Constructive and destructive metabolism, (2) Reproduction, as shown in the division of cells and cell constituents, (3) Irritability, or the capacity of receiving and responding to stimuli. All these general functions of living things are manifested by germ cells, but as development advances each of these functions becomes more specialized, more complicated and more perfect. A cell which at an early stage was protective, locomotor and sensory in function may give rise to daughter cells in which these functions are distributed to different cells, cells which at an early stage were sensitive to many kinds of stimuli give rise to daughter cells which are especially sensitive to one particular kind of stimulus, such as vibration, light, or chemicals.
Functions develop from a generalized to a specialized condition by the process of "physiological division of labor" which accompanies morphological division of substance. But just as in the development of structures, new parts, which were not present in the germ, appear by a process of "creative synthesis," so new functions appear in the course of development, which are not merely sorted out of the general functions present at the beginning, but which are created by the interaction and synthesis of parts and functions previously present.
Much less attention has been paid to the development of functions than to the development of structures, and consequently it is not possible to describe the former with the same degree of detail as the latter. But in spite of the lack of detailed knowledge regarding the development of particular functions the general fact of such development is well established. To what extent structures may modify functions or functions structures, in the course of development, is a problem which has been much discussed, and upon the answer to which the fate of certain important theories, for example Lamarckism, depends; but this problem can be solved only by thorough-going experimental and analytical work. In the meantime it seems safe to conclude that living structures and functions are inseparable and that anything which modifies one of these must of necessity modify the other also; they are merely different aspects of organization, and are dealt with separately by the morphologists and physiologists only as a matter of convenience. At the same time there can be no doubt that minute changes of function can frequently be detected where no corresponding change of structure can be seen, but this shows only that physiological tests may be more delicate than morphological ones. In certain lines of modern biological work, such as bacteriology, cytology, genetics, many functional distinctions are recognizable between organisms which are morphologically indistinguishable. But this does not signify that functional changes precede structural ones, but only that the latter are more difficult to see than the former. For every change of function it is probable that an "unlimited microscopist" could discover a corresponding change of structure.
(To be concluded)
- First of the Norman W. Harris Lectures for 1914 at Northwestern University on "Heredity and Environment in the Development of Men"; to be published by the Princeton University Press.