This passage from the simple cave to the many-chambered lodge is an Evolution, and a similar development may be traced in the domestic architecture of all civilized societies. The labourer's cottage of modern England and the shieling of the Highland crofter are the survivals of the one-roomed hut of Primitive Man, scarcely changed in any essential with the lapse of years. In the squire's mansion also, and the nobleman's castle, we have the representatives, but now in an immensely developed form, of the many-roomed home of the chief. The steps by which the cottage became the castle are the same as those by which the cave in the rocks became the lodge of the chief. Both processes wear the hall-mark of all true development -- they arise in response to growing necessities, and they are carried out by the most simple and natural steps.
In this evolution of a human habitation we have an almost perfect type of the evolution of that more august habitation, the complex tenement of clay in which Man's mysterious being has its home. The Body of Man is a structure of a million, or a million million cells. And the history of the unborn babe is, in the first instance, a history of additions, of room being added to room, of organ to organ, of faculty to faculty. The general process, also, by which this takes place is almost as clear to modern science as in the case of material buildings. A special class of observers has carefully watched these secret and amazing metamorphoses, and so wonderful has been their success with mind and microscope that they can almost claim to have seen Man's Body made. The Science of Embryology undertakes to trace the development of Man from a stage in which he lived in a one-roomed house -- a physiological cell. Whatever the multitude of rooms, the millions and millions of cells, in which to-day each adult carries on the varied work of life, it is certain that when he first began to be he was the simple tenant of a single cell. Observe, it is not some animal-ancestor or some human progenitor of Man that lived in this single cell -- that may or may not have been -- but the individual Man, the present occupant himself. We are dealing now not with phylogeny -- the history of the race -- but with ontogeny -- the problem of Man's Ascent from his own earlier self. And the point at the moment is not that the race ascends; it is that each individual man has once, in his own life-time, occupied a single cell, and starting from that humble cradle, has passed through stage after stage of differentiation, increase, and development, until the myriad-roomed adult-form was attained. Whence that first cradle came is at present no matter. Whether its remote progenitor rocked among the waves of primeval seas or swung from the boughs of forests long since metamorphosed into coal does not affect the question of the individual ascent of Man. The answers to these questions are hypotheses. The fact that now arrests our wonder is that when the earliest trace of an infant's organization meets the eye of science it is nothing but a one-celled animal. And so closely does its development from that distant point follow the lines of the evolution just described in the case of the primitive savage hut, that we have but to make a few changes in phraseology to make the one process describe the other. Instead of rooms and chambers we shall now read cells and tissues; instead of the builder's device of adding room to room, we shall use the physiologist's term segmentation; the employments carried on in the various rooms will become the functions discharged by the organs of the human frame, and line for line the history of the evolution will be found to be the same.
The embryo of the future man begins life. Like the primitive savage, in a one-roomed hut, a single simple cell. This cell is round and almost microscopic in size. When fully formed it measures only one-tenth of a line in diameter, and with the naked eye can be barely discerned as a very fine point. An outer covering, transparent as glass, surrounds this little sphere, and in the interior, embedded in protoplasm, lies a bright globular spot. In form, in size, in composition there is no apparent difference between this human cell and that of any other mammal. The dog, the elephant, the lion, the ape, and a thousand others begin their widely different lives in a house the same as Man's. At an earlier stage indeed, before it has taken on its pellucid covering, this cell has affinities still more astonishing. For at that remoter period the earlier forms of all living things, both plant and animal, are one. It is one of the most astounding facts of modern science that the first embryonic abodes of moss and fern and pine, of shark and crab and coral polyp, of lizard, leopard, monkey, and Man are so exactly similar that the highest powers of mind and microscope fail to trace the smallest distinction between them.
But let us watch the development of this one-celled human embryo. Increase of rooms in architecture can be effected in either of two ways by building entirely new rooms, or by partitioning old ones. Both of these methods are employed in Nature. The first, gemmation, or budding, is common among the lower forms of life. The second, differentiation by partition, or segmentation, is the approved method among higher animals, and is that adopted in the case of Man. It proceeds, after the fertilized ovum has completed the complex preliminaries of karyokinesis, by the division of the interior-contents into two equal parts, so that the original cell is now occupied by two nucleated cells with the old cell-wall surrounding them outside. The two-roomed house is, in the next development, and by a similar process of segmentation, developed into a structure of four rooms, and this into one of eight, and so on.  In a short time the number of chambers is so great that count is lost, and the activity becomes so vigorous in every direction that one ceases to notice individual cells at all. The tenement in fact consists now of innumerable groups of cells congregated together, suites of apartments as it were, which have quickly arranged themselves in symmetrical, definite, and withal different forms. Were these forms not different as well as definite we should hardly call it an evolution, nor should we characterize the resulting aggregation as a higher organism. A hundred cottages placed in a row would never form a castle. What makes the castle superior to the hundred cottages is not the number of its rooms, for they are possibly fewer; nor their difference in shape, for that is immaterial. It lies in the number and nature and variety of useful purposes to which the rooms are put, the perfection with which each is adapted to its end, and the harmonious co-operation among them with reference to some common work. This also is the distinction between a higher animal and a humble organism such as the centipede or the worm. These creatures are a monotony of similar rings, like a string of beads. Each bead is the counterpart of the other; and with such an organization any high or varied life becomes an impossibility. The fact that any growing embryo is passing through a real development is decided by the new complexity of structure, by the more perfect division of labour, and of better kinds of labour, and by the increase in range and efficiency of the correlated functions discharged by the whole. In the development of the human embryo the differentiating and integrating forces are steadily acting and co-operating from the first, so that the result is not a mere aggregation of similar cells, but an organism with different parts and many varied functions. When all is complete we find that one suite of cells has been specially set apart to provide the commissariat, others have devoted themselves exclusively to assimilation. The ventilation of the house -- respiration -- has been attended to by others, and a central force-pump has been set up, and pipes and ducts for many purposes installed throughout the system. Telegraph wires have next been stretched in every direction to keep up connection between the endless parts; and other cells developed into bony pillars for support. Finally, the whole delicate structure has been shielded by a variety of protective coverings, and after months and years of further elaboration and adjustment the elaborate fabric is complete. Now all these complicated contrivances -- bones, muscles, nerves, heart, brain, lungs -- are made out of cells; they are themselves, and in their furthest development, simply masses or suites of cells modified in various ways for the special department of household work they are meant to serve. No new thing, except building material, has entered into the embryo since its first appearing. It seized whatever matter lay to hand, incorporated it with its own quickening substance, and built it in to its appropriate place. So the structure rose in size and symmetry, till the whole had climbed, a miracle of unfolding, to the stature of a Man.
But the beauty of this development is not the significant thing to the student of Evolution; nor is it the occultness of the process nor the perfection of the result that fill him with awe as he surveys the finished work. It is the immense distance Man has come. Between the early cell and the infant's formed body, the ordinary observer sees the uneventful passage of a few brief months. But the evolutionist sees concentrated into these few months the labour and the progress of incalculable ages. Here before him is the whole stretch of time since life first dawned upon the earth; and as he watches the nascent organism climbing to its maturity he witnesses a spectacle which for strangeness and majesty stands alone in the field of biological research. What he sees is not the mere shaping or sculpturing of a Man. The human form does not begin as a human form. It begins as an animal; and at first, and for a long time to come, there is nothing wearing the remotest semblance of humanity. What meets the eye is a vast procession of lower forms of life, a succession of strange inhuman creatures emerging from a crowd of still stranger and still more inhuman creatures; and it is only after a prolonged and unrecognizable series of metamorphoses that they culminate in some faint likeness to the image of him who is one of the newest yet the oldest of created things. Hitherto we have been taught to look among the fossiliferous formations of Geology for the buried lives of the earth's past. But Embryology has startled the world by declaring that the ancient life of the earth is not dead. It is risen. It exists to-day in the embryos of still-living things, and some of the most archaic types find again a resurrection and a life in the frame of man himself.
It is an amazing and almost incredible story. The proposition is not only that Man begins his earthly existence in the guise of a lower animal embryo, but that in the successive transformations of the human embryo there is reproduced before our eyes a visible, actual, physical representation of part of the life-history of the world. Human Embryology is a condensed account, a recapitulation or epitome of some of the main chapters in the Natural History of the world. The same processes of development which once took thousands of years for their consummation are here condensed, foreshortened, concentrated into the space of weeks. Each platform reached by the human embryo in its upward course represents the embryo of some lower animal which in some mysterious way has played a part in the pedigree of the human race, which may itself have disappeared long since from the earth, but is now and for ever built into the inmost being of Man. These lower animals, each at its successive stage, have stopped short in their development; Man has gone on. At each fresh advance his embryo is found again abreast of some other animal-embryo a little higher in organization than that just passed. Continuing his ascent that also is overtaken, the now very complex embryo making up to one animal-embryo after another until it has distanced all in its series, and stands alone. As the modern stem-winding watch contains the old clepsydra and all the most useful features in all the timekeepers that were ever made; as the Walter printing-press contains the rude hand-machine of Gutenberg, and all the best in all the machines that followed it; as the modern locomotive of to-day contains the engine of Watt, the locomotive of Hedley, and most of the improvements of succeeding years, so Man contains the embryonic bodies of earlier and humbler and clumsier forms of life. Yet in making the Walter press in a modern workshop, the artificer does not begin by building again the press of Gutenberg, nor in constructing the locomotive does the engineer first make a Watt's machine and then incorporate the Hedley, and then the Stephenson, and so on through all the improving types of engines that have led up to this. But the astonishing thing is that, in making a Man, Nature does introduce the framework of these earlier types, displaying each crude pattern by itself before incorporating it in the finished work. The human embryo, to change the figure, is a subtle phantasmagoria, a living theatre in which a weird transformation scene is being enacted, and in which countless strange and uncouth characters take part. Some of these characters are well known to science, some are strangers. As the embryo unfolds, one by one these animal actors come upon the stage, file past in phantom-like procession, throw off their drapery, and dissolve away into something else. Yet, as they vanish, each leaves behind a vital portion of itself, some original and characteristic memorial, something itself has made or won, that perhaps it alone could make or win -- a bone, a muscle, a ganglion, or a tooth -- to be the inheritance of the race. And it is only after nearly all have played their part and dedicated their gift, that a human form, mysteriously compounded of all that has gone before, begins to be discerned in their midst.
The duration of this process, the profound antiquity of the last survivor, the tremendous height he has scaled, are inconceivable by the faculties of Man. But measure the very lowest of the successive platforms passed in the ascent, and see how very great a thing it is even to rise at all. The single cell, the first definite stage which the human embryo attains, is still the adult form of countless millions both of animals and plants. Just as in modern England the millionaire's mansion -- the evolved form -- is surrounded by labourers' cottages -- the simple form -- so in Nature, living side by side with the many-celled higher animals, is an immense democracy of unicellular artizans. These simple cells are perfect living things. The earth, the water, and the air teem with them everywhere. They move, they eat, they reproduce their like. But one thing they do not do -- they do not rise. These organisms have, as it were, stopped short in the ascent of life. And long as Evolution has worked upon the earth, the vast numerical majority of plants and animals are still at this low stage of being. So minute are some of these forms that if their one-roomed huts were arranged in a row it would take twelve thousand to form a street a single inch in length. In their watery cities -- for most of them are Lake-Dwellers -- a population of eight hundred thousand million could be accommodated within a cubic inch. Yet, as there was a period in human history when none but cave-dwellers lived in Europe, so was there a time when the highest forms of life upon the globe were these microscopic things. See, therefore, the meaning of Evolution from the want of it. In a single hour or second the human embryo attains the platform which represents the whole life-achievement of myriads of generations of created things, and the next day or hour is immeasurable centuries beyond them.
Through all what zoological regions the embryo passes in its great ascent from the one-celled forms, one can never completely tell. The changes succeed one another with such rapidity that it is impossible at each separate stage to catch the actual likeness to other embryos. Sometimes a familiar feature suddenly recalls a form well-known to science, but the likeness fades, and the developing embryo seems to wander among the ghosts of departed types. Long ago these crude ancestral forms were again the highest animals upon the earth. For a few thousand years they reigned supreme, furthered the universal evolution by a hair-breadth, and passed away. The material dust of their bodies is laid long since in the Palaeozoic rocks, but their life and labour are not forgotten. For their gains were handed on to a succeeding race. Transmitted thence through an endless series of descendants, sifted, enriched, accentuated, still dimly recognizable, they re-appeared at last in the physical frame of Man. After the early stages of human development are passed, the transformations become so definite that the features of the contributary animals are almost recognizable. Here, for example, is a stage at which the embryo in its anatomical characteristics resembles that of the Vermes or Worms. As yet there is no head, nor neck, nor backbone, nor waist, nor limbs. A roughly cylindrical headless trunk -- that is all that stands for the future man. One by one the higher Invertebrates are left behind, and then occurs the most remarkable change in the whole life-history. This is the laying down of the line to be occupied by the spinal chord, the presence of which henceforth will determine the place of Man in the Vertebrate sub-kingdom. At this crisis, the eye which sweeps the field of lower Nature for an analogue will readily find it. It is a circumstance of extraordinary interest that there should be living upon the globe at this moment an animal representing the actual transition from Invertebrate to Vertebrate life. The acquisition of a vertebral column is one of the great marks of height which Nature has bestowed upon her creatures; and in the shallow waters of the Mediterranean she has preserved for us a creature which, whether degenerate or not, can only be likened to one of her first rude experiments in this direction. This animal is the Lancelet, or Amphioxus, and so rudimentary is the backbone that it does not contain any bone at all, but only a shadow or prophecy of it in cartilage. The cartilaginous notochord of the Amphioxus nevertheless is the progenitor of all vertebral columns, and in the first instance this structure appears in the human embryo exactly as it now exists in the Lancelet. But this is only a single example. In living Nature there are a hundred other animal characteristics which at one stage or another the biologist may discern in the ever-changing kaleidoscope of the human embryo.
Even with this addition, nevertheless, the human infant is but a first rough draft, an almost formless lump of clay. As yet there is no distinct head, no brain, no jaws, no limbs; the heart is imperfect, the higher visceral organs are feebly developed, everything is elementary. But gradually new organs loom in sight, old ones increase in complexity. By a magic which has never yet been fathomed the hidden Potter shapes and re-shapes the clay. The whole grows in size and symmetry. Resemblances, this time, to the embryos of the lower vertebrate series, flash out as each new step is attained -- first the semblance of the Fish, then of the Amphibian, then of the Reptile, last of the Mammal. Of these great groups the leading embryonic characters appear as in a moving panorama, some of them pronounced and unmistakable, others mere sketches, suggestions, likenesses of infinite subtlety. At last the true Mammalian form emerges from the crowd. Far ahead of all at this stage stand out three species -- the Tailed Catarrhine Ape, the Tailless Catarrhine, and last, differing physically from these mainly by an enlargement of the brain and a development of the larynx, Man.
Whatever views be held of the doctrine of Evolution, whatever theories of its cause, these facts of Embryology are proved. They have taken their place in science wholly apart from the discussion of theories of Evolution, and as the result of laboratory investigation, made for quite other ends. What is true for Man, moreover, is true of all other animals. Every creature that lives climbs up its own genealogical tree before it reaches its mature condition. "All animals living, or that ever have lived, are united together by blood relationship of varying nearness or remoteness, and every animal now in existence has a pedigree stretching back, not merely for ten or a hundred generations, but through all geologic time since life first commenced on the earth. The study of development has revealed to us that each animal bears the mark of its ancestry, and is compelled to discover its parentage in its own development; the phases through which an animal passes in its progress from the egg to the adult are no accidental freaks, no mere matters of developmental convenience, but represent more or less closely, in more or less modified manner, the successive ancestral stages through which the present condition has been acquired."  Almost foreseen by Agassiz, suggested by Von Baer, and finally applied by Fritz Muller, this singular law is the key-note of modern Embryology. In no case, it is true, is the recapitulation of the past complete. Ancestral stages are constantly omitted, others are over-accentuated, condensed, distorted, or confused; while new and undecipherable characters occasionally appear. But it is a general scientific fact, that over the graves of a myriad aspirants the bodies of Man and of all higher Animals have risen. No one knows why this should be so. Science, at present, has no rationale of the process adequate to explain it. It was formerly held that the entire animal creation had contributed something to the anatomy of Man; or that as Serres expressed it, "Human Organogenesis is a transitory Comparative Anatomy." But though Man has not such a monopoly of the past as is here inferred -- other types having here and there diverged and developed along lines of their own -- it is certain that the materials for his body have been brought together from an unknown multitude of lowlier forms of life.
Those who know the Cathedral of St. Mark's will remember how this noblest of the Stones of Venice owes its greatness to the patient hands of centuries and centuries of workers, how every quarter of the globe has been spoiled of its treasures to dignify this single shrine. But he who ponders over the more ancient temple of the Human Body will find imagination fail him as he tries to think from what remote and mingled sources, from what lands, seas, climates, atmospheres, its various parts have been called together, and by what innumerable contributory creatures, swimming, creeping, flying, climbing, each of its several members was wrought and perfected. What ancient chisel first sculptured the rounded columns of the limbs? What dead hands built the cupola of the brain, and from what older ruins were the scattered pieces of its mosaic-work brought? Who fixed the windows in its upper walls? What winds and weathers wrought strength into its buttresses? What ocean-beds and forest glades worked up its colourings? What Love and Terror and Night called forth the Music? And what Life and Death and Pain and Struggle put all together in the noiseless workshop of the past, and removed each worker silently when its task was done? How these things came to be, Biology is one long record. The architects and builders of this mighty temple are not anonymous. Their names, and the work they did, are graven forever on the walls and arches of the Human Embryo. For this is a volume of that Book in which Man's members were written, which in continuance were fashioned, when as yet there was none of them.
The Descent of Man from the Animal Kingdom is sometimes spoken of as a degradation. It is an unspeakable exaltation. Recall the vast antiquity of that primal cell from which the human embryo first sets forth. Compass the nature of the potentialities stored up in its plastic substance. Watch all the busy processes, the multiplying energies, the mystifying transitions, the inexplicable chemistry of this living laboratory. Observe the variety and intricacy of its metamorphoses, the exquisite gradation of its ascent, the unerring aim with which the one type unfolds -- never pausing, never uncertain of its direction, refusing arrest at intermediate forms, passing on to its flawless maturity without waste or effort or fatigue. See the sense of motion at every turn, of purpose and of aspiration. Discover how, with identity of process and loyalty to the type, a hair-breadth of deviation is yet secured to each so that no two forms come out the same, but each arises an original creation, with features, characteristics, and individualities of its own. Remember, finally, that even to make the first cell possible, stellar space required to be swept of matter, suns must needs be broken up, and planets cool, the agents of geology labour millennium after millennium at the unfinished earth to prepare a material resting-place for the coming guest. Consider all this, and judge if Creation could have a sublimer meaning, or the Human Race possess a more splendid genesis.
From the lips of the Prophet another version, an old and beautiful story, was told to the childhood of the earth, of how God made Man; how with His own hands He gathered the Bactrian dust, modelled it, breathed upon it, and it became a living soul. Later, the insight of the Hebrew Poet taught Man a deeper lesson. He saw that there was more in Creation than mechanical production. He saw that the Creator had different kinds of Hands and different ways of modelling. How it was done he knew not, but it was not the surface thing his forefathers taught him. The higher divinity and mystery of the process broke upon him. Man was a fearful and wonderful thing. He was modelled in secret. He was curiously wrought in the lowest parts of the earth. When Science came, it was not to contradict the older versions. It but gave them content and a still richer meaning. What the Prophet said, and the Poet saw, and Science proved, all and equally will abide forever. For all alike are voices of the Unseen, commissioned to different peoples and for different ends to declare the mystery of the Ascent of Man.
 When the multicellular globe, made up of countless offshoots or divisions of the original pair, has reached a certain size, its centre becomes filled with a tiny lakelet of watery fluid. This fluid gradually increases in quantity, and, pushing the cells outward, packs them into a single layer, circumscribing it on every side as with an elastic wall. At one part a dimple soon appears, which slowly deepens, until a complete hollow is formed. So far does this invagination of the sphere go on that the cells at the bottom of the hollow touch those at the opposite side. The ovum has now become an open bag or cup, such as one might make by doubling in an india-rubber ball, and thus is formed the gastrula of biology. The evolutional interest of this process lies in the fact that probably all animals above the Protozoa pass through this gastrula stage. That some of the lower Metazoa, indeed, never develop much beyond it, a glance at the structure of the humbler Coelenterates will show--the simplest of all illustrations of the fact that embryonic forms of higher animals are often permanently represented by the adult forms of lower. The chief thing however to mark here is the doubling-in of the ovum to gain a double instead of a single wall of cells. For these two different layers, the ectoderm and the endoderm, or the animal layer and the vegetal layer, play a unique part in the after-history. All the organs of movement and sensation spring from the one, all the organs of nutrition and reproduction develop from the other.  Marshall, Vertebrate Embryology. p. 26.
 Marshall, Vertebrate Embryology. p. 26.