Popular Science Monthly/Volume 67/June 1905/Von Baer and the Rise of Embryology
|VON BAER AND THE RISE OF EMBRYOLOGY.|
THE process of building an animal's body is one of the most wonderful in all nature. After three weeks' incubation of the hen's egg, for illustration, the young bird steps into the world with heart, lungs, brain, eyes and other organs completely formed, which straightway adjust themselves to the new conditions of life. At the beginning of incubation, this living, breathing organism was merely a single element of structure—a cell, or at most a group of a few cells surrounded by a quantity of nourishment in the form of yolk—potentially an animal, in reality simply an egg.
To arrive at the period of hatching, a succession of changes has taken place whereby the food material has been transformed into the living matter of organic units, and these have become aggregated into the tissues of the body. That such a sweeping change has been wrought in such a short time is a marvel of organic architecture involving much more than mere rearrangement of material.
The history of the development of a single individual becomes endowed with greater interest, when observation teaches us that all animals, in the process of becoming, pass through a similar series of steps. In whatever group of living forms the penetrating insight of the scientific observer has been turned, fortified by the microscope, there is the same remarkable story—complex living forms arising from relative simplicity to great complexity in a short time. Every organism, above the very lowest, no matter how complex, starts its existence in the condition of a single microscopic cell, and between that simple state and the fully formed condition every gradation of structure is exhibited. Each time an animal is developed these constructive changes are repeated in orderly sequence.
But, strangely enough, the course of development in any higher organism is not straightforward, but devious. The bird, as well as all higher animals, acquires gill-clefts and other rudimentary structures not adapted to its condition of life. Most of the rudimentary organs are transitory and bear testimony, as hereditary survivals, to the line of ancestry. They are clues by means of which phases in the evolution of animal life may be deciphered.
Bearing in mind these shifting changes, one begins to see why the adult structures of animals are so difficult to understand. They are not only complex they are also greatly modified. The adult condition of any organ or tissue represents the last step in a series of gradually acquired, modifications, and is, therefore, the farthest departure from that which is ancestral and archetypal. But in the process of formation all the simpler conditions are exhibited. If, therefore, we wish to understand an organ or an animal we must follow its development, and see it in simpler conditions, before the great modifications have been added.
The tracing of the stages whereby cells merge into tissues, tissues into organs, and how the organs by combinations build up the body, is embryology. It has become one of the richest and most suggestive of the biological sciences in furnishing clues to the past history of animals and throwing light on their relationships.
It is the purpose of this paper to trace in a summary way the rise of that interesting division of biological science, pointing out its epochs, and telling something about the men who laid its foundations, the discussion being limited to the animal side, with no attempt to represent the rise of plant embryology. That we can 'read all history in the lives of a few great men' is essentially true in reference to the progress of embryology. There are many individual workers, each contributing his share, but the ideas of the science are molded into effective form only in the minds of the leaders. In this group of 'the leaders,' Von Baer stands as a monumental figure, at the parting of the ways between the new and the old—the sane thinker, the great observer.
The story of the rise of embryology can, for convenience, be divided into five periods each marked by an advance in general knowledge. These are: (1) the period of Harvey and Malpighi; (2) the period of Wolff; (3) the period of Von Baer; (4) the period from Von Baer to Balfour; and (5) the period of Balfour with an indication of present tendencies.
The Period of Harvey and Malpighi.
In General.—The conventional way of looking at the rise of embryology has been derived mainly through the channels of German scholarship. But there is reason to depart from the traditional aspect of the subject, in which Wolff is heralded as its founder, and the one central figure prior to Pander and Von Baer.
The embryological work of Wolff's great predecessors Harvey and Malpighi has been passed over too lightly. Although these men have received ample recognition in closely related fields of investigation, their insight into those mysterious events which culminate in the formation of a new animal has been rarely appreciated. Now and then a few writers, as Brooks and Whitman, have pointed out the great worth of Harvey's work in embryology, but fewer have spoken for Malpighi in this connection. Koelliker, it is true, in his address at the unveiling of the statue of Malpighi, in his native town of Crevalcuore, in 1894, gives him well-merited recognition as the founder of embryology, and Sir Michael Foster has written in a similar vein in his delightful 'Lectures on the History of Physiology.'
However great was Harvey's work in embryology, I venture to say that Malpighi's was greater when considered as a piece of observation. Harvey's work is more philosophical; he discusses the nature of development and shows unusual powers as an accurate reasoner. But that part of his treatise devoted to observation is far less extensive and exact than Malpighi's, and throughout his lengthy discussions he has the flavor of the ancients.
Malpighi's work, on the other hand, flavors more of the moderns. In terse descriptions, and with many sketches, he shows the changes in the hen's egg from the close of the first day of development onwards.
It is a noteworthy fact that, at the period in which he lived, Malpighi could so successfully curb the tendency to indulge in wordy disquisitions, and that he was satisfied to observe carefully, and tell his story in a simple way. This quality of mind can not be too much admired. As Emerson has said: "I am impressed with the fact that the greatest thing a human soul ever does in this world is to see something and tell what it saw in a plain way. Hundreds of people can talk for one who can think, but thousands can think for one who can see. To see clearly is poetry, philosophy and religion all in one." But 'to see' here means to interpret as well as to observe. Harvey was also an original observer, but, in embryology, not in so eminent a degree as Malpighi. Could, we have had the insight of Harvey united to the observing powers of Malpighi, we should have had an almost perfect combination.
Although there were observers in the field of embryology before Harvey, little of substantial value had been produced. The earliest attempts were vague and uncritical, and, naturally, embraced only fragmentary views of the more obvious features of body formation. Nor, indeed, are we to look for much advance in the field of embryology even in Harvey's time. The reason for this will be obvious when we remember that the renewal of independent observation had just been brought about in the preceding century, when, in 1543, the science of anatomy had been reformed by Vesalius.
Harvey himself was one of the pioneers in the intellectual awakening. By his immortal discovery of circulation of the blood (1619-1628) he had established a new physiology. Now, studies on the development of the body are more special, they involve observations on minute structures and recondite processes, and must, therefore, depend upon considerable advances in anatomy and physiology. Naturally the science of embryology came later.
Harvey.—Harvey's was the first attempt to make a critical analysis of the process of development, and that he did not attain more was not due to limitations of his powers of discernment, but to the necessity of building on the general level of the science of his time, and, further, to his lack of instruments of observation and technique. Nevertheless, Harvey may be considered as having made the first independent advance in embryology.
By clearly teaching, on the basis of his own observations, the gradual formation of the body by aggregation of its parts, he anticipated Wolff. This doctrine came to be known under the title of 'epigenesis,' but Harvey's epigenesis was not, as Wolff's was, directed against a theory of predelineation of the parts of the embryo, but against the ideas of the medical men of the time regarding the metamorphosis of germinal elements. It lacked, therefore, the dramatic setting which surrounded the work of Wolff in the next century. Had the doctrine of preformation been current in Harvey's time, we are quite justified in assuming that he would have assailed it as vigorously as Wolff did.
Harvey's embryological work was published in 1651 under the title 'Exercitationes de Generatione Animalium.' It embraces not only observations on the development of the chick, but also on the deer and some other mammals. He being the court physician of Charles I., that sovereign had many deer killed in the park, at intervals, in order to give Harvey the opportunity to study their development.
As fruits of his observation on the chick, he showed the position in which the embryo arises within the egg. viz., in the white opaque spot or cicatricula. He also corrected Aristotle, Fabricius and his other predecessors in many particulars.
Harvey's greatest predecessor in this field, Fabricius, was also his teacher. When, in search of the best training in medicine, Harvey wended his way from England to Italy, in Padua, he came under Fabricius as one of his teachers. In 1600, Fabricius published the earliest illustrations on the development of animals, and, again, in 1625, six years after his death, appeared his illustrated treatise on the development of the chick Altogether his figures show developmental stages of the cow, sheep, pig, galeus, serpent, rat and chick. The value of his work may be easily overestimated by a casual examination of the plates.
Harvey's own treatise was not illustrated. With that singular independence of mind which he showed in all his work, the vision of the pupil was not hampered by the authority of his teacher, and, trusting only to his own sure observation and reason, he described the stages of development as he saw them in the egg, and placed his own construction on the facts.
One of the earliest things to arrest his attention in the chick was a pulsating point, the heart, and, from this observation, he supposed that the heart and blood were the first formations. He says: "But as soon as the egg, under the influence of the gentle warmth of the incubating hen, or of warmth derived from another source, begins to pullulate, this spot forthwith dilates, and expands like the pupil of the eye, and from thence, as the grand center of the egg, the latent plastic force breaks forth and germinates. This first commencement of the chick, however, so far as I am aware, has not yet been observed by any one."
It is to be understood, however, that his descriptive part is relatively brief (about 40 pages out of 350 in Willis's translation), and that the bulk of the 106 'exercises' into which his work is divided is devoted to comments on the older writers and discussions of the nature of the process of development.
Portraits of Harvey are by no means uncommon. The one in the National Portrait Gallery, in London, is represented in Fig. 1. This is usually regarded as the second-best portrait of Harvey, since the one painted by Jansen, now in possession of the Royal College of Physicians, is believed to be the best one extant. Permission to reproduce the latter is not given.
The picture in the National Gallery shows a countenance of composed intellectual strength, with a suggestion, in the forehead and outline of the face, of some of the portraits of Shakespeare.
The aphorism 'Omne vivum ex ovo' though not invented by Harvey, was brought into general use through his works. It is not to be taken in its full modern significance. With Harvey it meant simply that the embryos of all animals, the viviparous as well as the oviparous, originate in eggs, and it was directed against certain contrary medical theories of the time.
The first edition of his 'Generatione Animalium' London, 1651, is provided with an allegorical title-page embodying this idea. As
shown in Fig. 2, it represents Jove on a pedestal, uncovering a round box—or ovum—bearing the inscription 'ex ovo omnia' and from the box issue all forms of living creatures including also man.
Malpighi.—The observer in embryology who looms into prominence between Harvey and Wolff, is Malpighi. He supplied what was greatly needed at the time—an illustrated account of the actual stages in development of the chick from the end of the first day to hatching, shorn of verbose references and speculations.His observations on development are in two separate memoirs, both
sent to the Royal Society in 1672, and published by the society in Latin, under the titles 'De Formatione Pulli in Ovo' and 'De Ovo Incubato.' The two taken together, are illustrated by twelve plates, containing eighty-six figures, and the twenty—two quarto pages of text are nearly all devoted to descriptions.
His pictures, although not correct in all particulars, represent what he was able to see, and are very remarkable for the age in which they were made, and considering the instruments of observation at his command. They show successive stages from the time the embryo is first outlined, and, taken in their entirety, they cover a wide range of stages.
His observations on the development of the heart, comprising twenty figures, are the most complete. He clearly illustrates the aortic arches,—those transitory structures of such great interest as showing a phase in ancestral history.
He was also the first to show by pictures the formation of the head-fold and neural groove as well as the brain vesicles and eye
pockets. His delineation of heart, brain and eye vesicles are far ahead of those illustrating Wolff's 'Theoria Generationis' made nearly a hundred years later. But Wolff rose to a higher level in his later work on the development of the intestine, and produced some figures better than any of Malpighi's.
The original drawings for 'De Ovo Incubato,' still in possession of the Royal Society, are made in pencil and red chalk. They far surpass the reproductions in finish and accuracy. In looking them over in 1902, I noticed four aortic arches represented in one figure where the engraver has shown only three.The portrait of Malpighi shown in Fig. 3 is taken from his Life
by Atti. From descriptions of his personal appearance, it is probably a better likeness than the handsome idealized portrait painted by Tabor, and presented by Malpighi to the Royal Society of London.
Fig. 4 shows a few selected figures from the various plates of his embryological treatises, to compare with those of Wolff.
While Harvey taught the gradual formation of parts, Malpighi, from his own observations, supposed the rudiments of the embryo to preexist in the egg. He thought that, possibly, the blood vessels were in the form of tubes, closely wrapped together, which by becoming filled with blood were distended. Nevertheless, in the treatises mentioned above he is very temperate in his expressions on the whole matter, and evidently believed in the new formation of many parts. In the work published after his death he appears to have been less circumspect.
Malpighi's work, with that of some of his contemporaries, marks the beginning of the theory of preformation.
On the whole, Malpighi should rank above Harvey as an embryologist, on account of his discoveries and fuller representation, by drawings and descriptions, of the process of development. As Sir Michael Foster has said: "The first adequate description of the long series of changes, by which, as they melt the one into the other, like dissolving views, the little white opaque spot in the egg is transformed into the feathered, living, active bird, was given by Malpighi. And where he left it, so for the most part the matter remained until even the present century. For this reason we may speak of him as the founder of embryology."
The Period of Wolff.
Between Harvey and Wolff, embryology had become dominated by the theory that the embryo exists already preformed within the egg, and, as a result of the rise of this new doctrine, the publications of Wolff had a different setting from that of any of his predecessors. It is only fair to say that to this circumstance is owing, in large part, the prominence of his name in connection with the theory of epigenesis. As we have already seen, Harvey, more than a century before the publications of Wolff, had clearly taught that development was a process of gradual becoming. Nevertheless, Wolff's work as opposed to the new theory was very important.
While the facts fail to support the contention that he was the founder of epigenesis, it is to be remembered that he has claims in other directions to rank as the foremost student of embryology prior to Von Baer.
As a preliminary to discussing Wolff's position we should bring under consideration the doctrine of preformation and encasement.
Rise of the Theory of Predelineation.—The idea of preformation in its first form is easily set forth. Just as when we examine a seed, we find within an embryo plantlet, so it was supposed that the various forms of animal life existed in miniature within the egg. The process, of development was supposed to consist of the expansion or unfolding of this preformed embryo. The process was commonly illustrated by reference to flower buds. "Just as already in a small bud all the parts of the flower, such as stamens and colored petals, are enveloped by the green and still undeveloped sepals,—just as the parts grow in concealment and then suddenly expand into a blossom, so also in the development of animals it was thought that the already present small but transparent parts grow, gradually expand, and become discernible." From the feature of unfolding this was called in the eighteenth century the theory of evolution, giving to that term quite a different meaning from that accepted at the present time.
This theory, strange as it may seem to us now, was founded on a basis of actual observation—not entirely on speculation. Although it was a product of the seventeenth century, from several printed accounts one is likely to gather the impression that it arose in the eighteenth century and that Bonnet, Haller and Leibnitz were among its founders. This implication is in part fostered by the circumstance that Swammerdam's 'Biblia Naturæ,' which contains the germ of the theory, was not published until 1737—more than a half century after his death—although the observations for it were completed before Malpighi's first paper on embryology was published in 1672. While it is well to bear in mind that date of publication, rather than date of observation, is accepted as establishing the period of emergence of ideas, there were other men, such as Malpighi and Leeuwenhoek, contemporaries of Swammerdam, who published in the seventeenth century the basis for this theory.
Malpighi supposed (1672) the rudiment of the embryo to preexist within the hen's egg, because he observed evidences of organization in the unincubated egg. This was in the heat of the Italian summer (in July and August, as he himself records), and Dareste suggests that the developmental changes had gone forward to a considerable degree before Malpighi opened the eggs. Be this as it may, the imperfection of his instruments and technique would have made it very difficult to have seen anything definitely in stages under twenty-four hours.
In reference to his observations he says that, in the unincubated egg, he saw a small embryo enclosed in a sac which he subjected to the rays of the sun. "Frequently I opened the sac with the point of a needle so that the 'animals contained within might be brought to the light, nevertheless to no purpose: for the individuals were so jelly-like and so very small that they were lacerated by a light stroke. Therefore it is right to confess that the beginnings of the chick preexist in the egg and have reached a higher development in no other way than in the eggs of plants." ("Quare pulli stamina in ovo præexistere, altiorémque originem nacta esse fateri convenit, haud dispari ritu, ac in Plantarum ovis")
Swammerdam (1637-1680) supplied a somewhat better basis. He observed that the parts of the butterfly, and other insects as well, are discernible in the chrysalis stage. Also, on observing caterpillars just before going into the pupa condition, he saw in outline the organs of the future stage, and very naturally concluded that development consists of an expansion of already formed parts.
A new feature was introduced through the discovery, by Leeuwenhoek about 1677, of the fertilizing filaments of eggs. Soon after, controversies began to arise as to whether the embryo preexisted in the sperm or in the egg. By Leeuwenhoek, Hartsoeker and others the egg was looked upon as simply a nidus within which the sperm developed, and they asserted that the future animal existed in miniature in the sperm. These controversies gave rise to the schools of the Animalculists, who believed the sperm to be the animal germ, and of the Ovists, who contended for the ovum in that rôle.
One of the curiosities of this period is shown in Fig. 5, taken from an old Dutch edition of Leeuwenhoek's works, in which he undertakes to represent predelineation of both sexes within the sperm.
Fig. 5. Sketches illustrating Pre-delineation of the Embryo within the Sperm. From an old edition of Leeuwenhoek's Works.
It is interesting to follow the metaphysical speculations which led to another aspect of the doctrine of preformation. There were those, notably Swammerdam, Leibnitz and Bonnet, who did not hesitate to follow the idea to the logical consequence, that, if the animal germ exists preformed, one generation after another must be encased within it. This gave rise to the fanciful idea of encasement or emboîtement which was so greatly elaborated by Bonnet and, by Leibnitz, applied to the development of the soul. Even Swammerdam (who, by the way, although a masterly observer, was always a poor generalizer) conceived the mental picture of the germs of all forthcoming generations having been located in the common mother Eve, all closely encased one within the other, like the boxes of a Japanese juggler. The end of the human race was conceived of by him as a necessity, when the last germ of this wonderful series had been unfolded.
His successors, in efforts to compute the number of homunculi,
which must have been condensed in the ovary of Eve, arrived at the amazing result of two hundred millions.
Work of Wolff.—Wolff, as a young man of 26 years, set himself against this grotesque doctrine of preformation and encasement, in his 'Theoria Generationis,' published in 1759. This consists of three parts: One devoted to the development of plants, one to the development of animals and one to theoretical considerations. He contended that the organs of animals make their appearance gradually, and that he could actually follow their successive stages of formation.
The figures in it illustrating the six, and his twenty figures on the development of the heart are more detailed than any of Wolff's. When the figures represent similar stages of development, a comparison of the two men's work is favorable to Malpighi. The latter shows much better, in corresponding stages, the series of cerebral vesicles and their relation to the optic vesicles. Moreover, in the wider range of his work, he shows many things — such as the formation of the neural groove, etc.—not included in Wolff's observations. Wolff, on the other hand, figures for the first time the primitive kidneys, or 'Wolffian bodies,' of which he was the discoverer.of the chick, some of which are shown in Fig. 6, are not, on the whole, so good as Malpighi's. Wolff gives in all seventeen figures, while Malpighi published
Although Wolff was able to show that development consists of a gradual formation of parts, his theory of development was entirely mystical and unsatisfactory. The fruitful idea of germinal continuity had not yet emerged, and the thought that the egg has inherited an organization from the past was yet to be expressed. Wolff was therefore in the same quandary as his predecessors when he undertook to explain development. Since he assumed a total lack of organization in the beginning, he was obliged to make development 'miraculous' through the action on the egg of a hyperphysical agent. From a total lack of organization, he conceived of its being lifted to the highly organized product, through the action of a 'vis essentialis corporis'
He returned to the problem of development later, and, in 1768-69, published his best work in this field on the development of the intestine. This is a very original and strong piece of observational work. While his observations for the 'Theoria Generationis' did not reach the level of Malpighi's those of the paper of 1768 surpassed it and held the position of the best piece of embryological work up to that of Pander and Von Baer. This work was so highly appreciated by Von Baer that he said: 'It is the greatest masterpiece of scientific observation which we possess' In it he clearly demonstrated that the development of the intestine, and its appendages, is a true process of becoming. Still later, in 1789, he published further theoretical considerations.
But all Wolff's work was launched into an uncongenial atmosphere. The great physiologist, Haller, could not accept the idea of epi genesis, but opposed it energetically, and, so great was his authority, that the ideas of Wolff gained no currency. This retarded progress in the science of animal development for more than a half century.
In 1812, the elder Meckel, recognizing the great value of Wolff's researches on the development of the intestine, rescued the work from neglect and obscurity, by publishing a German translation of the same, and bringing it to the attention of scholars. From that time onward Wolff's work began to be fruitful.
His 'Formatione Intestinorum' embodies his greatest contribution to embryology rather than his 'Theoria Generationis'; not only is it a more fitting model of observation, but in it he foreshadows the idea of germ-layers in the embryo, which, under Pander and Von Baer, became the fundamental conception in structural embryology. Throughout his work, both early and late, he likens the embryonic rudiments, which precede the formation of organs, to leaflets. In his work of 1768, he describes in detail how the leaf-like layers give rise to the systems of organs: Showing that the nervous system arises first from a leaf-like layer, and is followed, successively, by a flesh-layer, the vascular system and, lastly, by the intestinal canal—all arising from original leaf-like layers.
In these important generalizations, although they are verbally incorrect, he reached the truth as nearly as it was possible at the time, and laid the foundation of the germ-layer theory.
Wolff was a man of great power as an observer, and although his influence was for a long time retarded, he should be recognized as the foremost investigator in embryology before Von Baer.
The little known of his life is gained through his correspondence and a letter by his amanuensis. Through personal neglect, and hostility to his work, he could not secure a foothold in the universities of Germany, and, in 1764, on the invitation of Catharine of Eussia, he went to the Academy of Sciences at St. Petersburg, where he spent >the last thirty years of his life.
His sincere and generous spirit is shown in his correspondence with Haller, his great opponent. "And as to the matter of contention between us, I think thus: For me, no more than for you glorious man, is truth of the very greatest concern. Whether it chance that organic bodies emerge from an invisible into a visible condition, or form themselves out of the air, there is no reason why I should wish the one were truer that the other, or wish the one and not the other. And this is your view also, glorious man. We are investigating for truth only: we seek that which is true. Why then should I contend with you?"
I have not been able to locate a portrait of Wolff, although I have sought one in various ways for several years. The Secretary of the Academy of Sciences at St. Petersburg writes that no portrait of Wolff exists there, and that they will gratefully receive information regarding any existing portrait of the great academician.
The Period of Von Baer.
aspect of the field in which they work and all later growth is influenced by their labors."
The greatest classic in embryology is his 'Entwickelungsgeschichte der Tiere—Beobachtung und Reflexion,' the first part of which was published in 1828, and the work on the second part completed in 1834, although it was not published till 1837. This second part was never finished according to the plan of Von Baer, but was issued by his publisher, after vainly wailing for the finished manuscript. The final portion, which Von Baer had withheld, in order to perfect in some particulars, was published in 1888, after his death, but in the form in which he had left it in 1834.
The observations for the first part began in 1819, after he had received a copy of Pander's researches and covered a period of seven years of close devotion to the subject, and the observations for the last part were carried on at intervals for several years.
It is significant of the character of his 'Reflexionen' that, although published before the announcement of the cell-theory, and before the acceptance of the doctrine of organic evolution, they have exerted a moulding influence upon embryology to the present time. The position of Von Baer in embryology, is due as much to his sagacity in speculation, as to his powers as an observer. "Never again have observation and thought been so successfully combined in embryological work" (Minot).
Von Baer was born in 1792, and lived on to 1876, but his enduring fame in embryology rests on work completed more than forty years before the end of his useful life. After his removal from Königsberg to St. Petersburg, in 1834, he very largely devoted himself to anthropology in its widest sense, and thereby extended his scientific reputation into other fields.
If space permitted, it would be interesting to give the biography of this extraordinary man, but here, it will be necessary to content ourselves with an examination of his portrait and a brief account of his work.
Several portraits of Von Baer showing him at different periods of his life have been published. A very attractive one, taken in his early manhood, appeared in Harper's Magazine for 1898. The expression of the face is poetical, and the picture is interesting to compare with the more matured sage-like countenance forming the frontispiece of Stieda's 'Life of Von Baer.' This, perhaps best of all his portraits, shows him in the full development of his powers. An examination of it impresses one with confidence in his balanced judgment and the thoroughness and profundity of his mental operations.
The portrait of Von Baer at about seventy years of age, reproduced in Fig. 7 is destined to be the one by which he is commonly known to embryologists, since it forms the frontispiece of the great cooperative 'Handbook of Embryology' now appearing under the editorship of Oskar Her twig.
Apart from special discoveries, Von Baer greatly enriched embryology in three directions: In the first place, he set a higher standard for all work in embryology and thereby lifted the entire science to a higher level. Activity in a great field of this kind is, with the rank and file of workers, so largely imitative that this feature of his influence should not be overlooked. In the second place, he established the germ-layer theory, and, in the third, he made embryology comparative.
In reference to the germ-layer theory, it should be recalled that Wolff had distinctly foreshadowed the idea, by showing that the material out of which the embryo is constructed is, in an early stage of development, arranged in the form of leaf-like layers. He showed specifically that the alimentary canal is produced by one of these sheet-like expansions folding and rolling together.
Pander, by observations on the chick (1817), had extended the knowledge of these layers and elaborated the conception of Wolff. He recognized the presence of three primary layers, an outer, a middle and an inner, out of which the tissues of the body are formed.
But, it remained for Von Baer, by extending his observations into all the principal groups of animals, to raise this conception to the rank of a general law of development. He was able to show that in all animals except the very lowest, there arises in the course of development leaf-like layers, which become converted into the 'fundamental organs' of the body.
Now, these elementary layers are not definitive tissues of the body, but are embryonic, and therefore, may appropriately be designated 'germ-layers.' The conception that these germ-layers are essentially similar in origin and fate, in all animals, was a fuller and later development of the germ-layer theory, which dominated embryological study until a recent date.
Von Baer recognized four such layers: the outer and inner ones being formed first, and, subsequently budding off a middle layer composed of two sheets. A little later (1845) Remak recognized the double middle layer of Von Baer as a unit, and thus arrived at the fundamental conception of three layers—the ecto-, endo-and mesoderm—which has so long held sway. For a long time after Von Baer, the aim of embryologists was to trace the history of these germ-layers—and so in a wider and much qualified sense it is to-day.
It will ever stand to his credit, as a great achievement, that Von Baer was able to make a very complicated feature of development clear and relatively simple. Given a leaf-like rudiment, with the layers held out by the yolk, as is the case in the hen's egg, and it was no easy matter to conceive of how they are transformed into the nervous system, the body wall, the alimentary canal and other parts, but, Von Baer saw deeply and clearly that the fundamental anatomical features of the body are assumed by the leaf-like rudiments being rolled into tubes. Fig. 8 shows four sketches taken from the plates illustrating Von Baer's work. At A is shown a stage in the formation of the embryonic
envelope, or amnion, which surrounds the embryos of all animals above the class of amphibia. At B, another figure of an ideal section, shows that long before the day of microtomes, Von Baer made use of sections to represent the relationships of his four germ-layers. At C and D is represented, diagramatically, the way in which these layers are rolled into tubes. He showed that the central nervous system arose in the form of a tube, from the outer layer, the body-wall in the form of a tube, composed of skin and muscle layers, and the alimentary tube from mucous and vascular layers.
The generalization that embryos in development tend to recapitulate their ancestral history is frequently attributed to Von Baer, but the qualified way in which he suggests something of the sort will not justify one in attaching this conclusion to his work.
Von Baer was the first to make embryology truly comparative, and to point out its great value in anatomy and zoology. By embryological studies, he recognized four types of organization—as Cuvier had done from the standpoint of comparative anatomy. But, since these types of organization have been greatly changed and sub-divided, the importance of the distinction has faded away. But as a distinct break with the old idea of a linear scale of being it was of moment.
Among his especially noteworthy discoveries may be mentioned that of the egg of the human being and other mammals, and the notochord as occurring in all vertebrate animals.
Von Baer has come to be dignified with the title of the 'Father of modern embryology.' No man could have done more in his period, and it is owing to his superb intellect, and talents as an observer, that he accomplished what he did. As Minot says: He 'worked out, almost as fully as was possible at this time, the genesis of all the principal organs from the germ-layers, instinctively getting at the truth as only a great genius could have done.'
After his masterly work the science of embryology could never return to its former level; he had given it a new direction, and through his influence a period of great activity was inaugurated.
The Period from Von Baer to Balfour.
In the period between Von Baer and Balfour there were great general advances in the knowledge of organic structure which brought the whole process of development into a new light.
Among the most important advances are to be enumerated: the announcement of the cell theory, the discovery of protoplasm, the beginning of the recognition of germinal continuity and the establishment of the doctrine of organic evolution.
The Cell Theory.—The generalization that the tissues of all animals and plants are structurally composed of similar units—called cells—was given to the world through the combined labors of Schleiden and Schwann. Schleiden, the botanist, in 1838, and Schwann, the anatomist, in the following year, published the observations on which this truth rests. The investigations stimulated by the announcement of this theory soon resulted in showing that the conception of the cell entertained by the founders was very imperfect, and, by 1860, the original theory had been molded into the protoplasm doctrine of Max Schultze.
The modification of the cell theory did not, however, affect the original conception that the cell is a unit of organic structure, but showed that the unit is, essentially, a globule of protoplasm containing a nucleus, and not simply a box-like compartment as Schleiden and Schwann had suggested.
The broad-reaching effects of the cell-theory may be easily imagined since it united all animals on the broad plane of similitude in microscopic structure. Now, for the first time, the tissues of the body were analyzed into their units; now, for the first time, was comprehended the nature of the germ-layers of Von Baer.
Among the first questions to emerge in the light of the new researches were: What is the origin of the cells in the organs, the tissues and the germ-layers? The road to the investigation of these questions was already opened, and it was followed, step by step, until the egg and sperm came to be recognized as modified cells. This position was reached, for the egg, about 1861, when Gegenbaur showed that the eggs of all vertebrated animals, regardless of size and condition, are in reality single cells. The sperm was put in the same category about 1865.
The rest was relatively easy—the egg, a single cell—by successive divisions produces many cells, and the arrangement of these into primary embryonic layers brings us to the starting point of Wolff and Von Baer. The cells, continuing to multiply by division, not only increase in number, but also undergo changes through division! of physiological labor, whereby certain groups are set apart to perform a particular part of the work of the body. In this way arise the various tissues of the body—which are, in reality, similar cells performing a similar function. Finally, from combinations of tissues the organs are formed.
But the egg, before entering on the process of development, must be stimulated by the union of the sperm with the nucleus of the egg, and, thus, the starting point of every animal and plant, above the lowest group, proves to be a single cell with protoplasm derived from two parents. While questions regarding the origin of cells in the body were being answered, the foundation for the embryological study of heredity was also laid.
Advances were now more rapid and more sure, flashes of morphological insight began to illuminate the way, and the facts of isolated observations began to fit into a harmonized whole.
Apart from the general advances of this period, mentioned in other connections, the work of a few individuals requires notice.
Bathke and Remak were engaged with the broader aspects of embryology as well as with special investigations. To Rathke is owing great advances in the knowledge of the development of insects and other invertebrates, and Remak is notable for similar work with the vertebrates. As already mentioned, he was the first to recognize the middle layer as a unit—through which the three germ-layers of later embryologists emerged into the literature.
Koelliker, the veteran embryologist, still living in Würzburg, carried on investigations on the segmentation of the egg. Besides work on the invertebrates, later, he followed with care the development of the chick and the rabbit—he encompassed the whole field of embryology—and published, in 1861 and later, in 1876, a general treatise on
vertebrate embryology of high merit. His portrait is shown in Fig. 9.
Huxley took a great step towards unifying the idea of germ-layers throughout the animal kingdom, when be maintained, in 1849, that the two cell-layers in animals like the hydra, and oceanic hydrazoa, correspond to the ectoderm and endoderm of higher animals.
Kowalevsky, whose portrait is shown in Fig. 10, made interesting discoveries of a general bearing. In 1866 he showed the practical identity, in the early stages of development, between one of the lowest vertebrates (Amphioxus) and a tunicate. The latter had up to that time been considered an invertebrate, and the effect of Kowalevsky's work was to break down the sharply limited line, supposed to exist between the invertebrates and the vertebrates. This was of great influence in subsequent work. Kowalevsky also founded the generalization that all animals in development pass through a gastrula stage,—a doctrine associated, since 1874, with the name of Haeckel under the title of the gastræa theory.
Beginning of the Idea of Germinal Continuity.—The conception that there is unbroken continuity of germinal substance between all Fig. 10. A. Kowalevsky 1840-1901. living organisms, and that the egg and sperm are endowed with an inherited organization of great complexity, has become the basis for all current theories of heredity and development. So much is involved in this conception, that, in the present decade, it has been designated (Whitman) 'the central fact of modern biology.' The first clear expression of it is found in Virchow's 'cellular Pathology' published in 1858. It was not, however, until the period of Balfour, and through the work of Fol, Van Beneden (chromosomes, 1883), Boveri, Hertwig and others, that the great importance of the fact began to be appreciated, and the conception began to be woven into the fundamental ideas of development.
Influence of the Doctrine of Organic Evolution.—This doctrine, although founded in its modern sense by Lamarck, in the early part of the nineteenth century, lay dormant until Darwin, in 1859, brought a new feature into its discussion, by emphasizing the factor of natural selection. The general acceptance of the doctrine, which followed after fierce opposition, had, of course, a profound influence on embryology. The latter science is so intimately concerned with the genealogy of animals and plants, that the newly accepted doctrine, as affording an explanation of this genealogy, was what was most needed. The development of organisms was now seen in the light of ancestral history; rudimentary organs began to have meaning as hereditary survivals, and the whole process of development assumed a different aspect. This doctrine supplied a new impulse to the interpretation of nature at large, and of the embryological record in particular. The meaning of the embryological record was so greatly emphasized in the period of Balfour, that it will be commented upon under the next division of our subject.
The period between Von Baer and Balfour proved to be one of great importance on account of the general advances in knowledge of all organic nature. Observations were all moving towards a better and more consistent conception of the structure of animals and plants. A new comparative anatomy, more profound, and richer in meaning than Cuviers, was arising. The edifice on the foundation of Von Baer's work was now emerging into recognizable outlines.
The Period of Balfour, with an Indication of Present Tendencies.
The workers of this period inherited all the accumulations of previous efforts, and the time was ripe for a new step. Observations on the development of different animals—vertebrates and invertebrates—had accumulated in great number, but they were scattered through technical periodicals, transactions of learned societies, monographs, etc., and there was no compact science of embryology with definite outlines. Balfour reviewed all this mass of information, digested it, and molded it into an organized whole. The results were published in the form of two volumes with the title of 'Comparative Embryology.' This book of 'almost priceless value' was given to the world in 1880-81. It was a colossal undertaking, but Balfour was a phenomenal worker. Before his untimely death at the age of thirty-one, he had been able to complete this work and to produce, besides, a large number of technical researches. The period of Balfour is taken arbitrarily in this paper, as beginning about 1874, when he published with Michael Foster 'The Elements of Embryology.'
Balfour was born in 1851. During his days of preparation for the university he was a good student, but did not exhibit in any marked way, the powers for which later he became distinguished. At Cambridge, his distinguished teacher, now Sir Michael Foster, recognized his great talents, and encouraged him to begin work in embryology. After his work in this field was once begun, he threw himself into it with great intensity. He rose rapidly to a professorship in Cambridge, and so great was his enthusiasm and earnestness as a lecturer, that in seven years 'voluntary attendance on his classes advanced from ten to ninety.' He was also a stimulator of research, and at the time of his death there were twenty students engaged in his laboratory, on problems of development.
He was distinguished for personal attractiveness, and those who met him were impressed with his great sincerity, as well as his personal charm. He was welcomed as an addition to the select group of distinguished scientific men of England, and a great career was predicted for him. Huxley, when he felt the call, as a great personal sacrifice, to lay aside the more rigorous pursuits of scientific research, and to devote himself to molding science into the lives of the people, said of Balfour: 'He is the only man who can carry out my work.'
But that was not destined to be. The story of his tragic end need be only referred to. After completing the prodigious labor on the 'Comparative Embryology' he went to Switzerland for recuperation, and met his death, with that of his guide, by slipping from an Alpine height into a chasm. His death occurred in July, 1882. His portrait is shown in Fig. 11.
The memorial edition of his works fills four quarto volumes, but the 'Comparative Embryology' is Balfour's Fig. 11. Francis M. Balfour (1851-1882). monument, and will give him enduring fame. It is not only a digest of the work of others, but contains, also, general considerations of a far-seeing quality. He saw developmental processes in the light of the hypothesis of organic evolution. His speculations were sufficiently reserved and nearly always luminous. It is significant of the character of this work to say that the speculations contained in the papers of the rank and file of embryological workers, for more than two decades, and often fondly believed to be novel, were for the most part anticipated by Balfour, and also better expressed, with better qualifications.
The reading of ancestral history in the stages of development is such a characteristic feature of the embryological work of Balfour's period that some observations concerning it will now be in place.
Interpretation of the Embryological Record.—Perhaps the most impressive feature of animal development is the series of similar changes through which all pass in the embryo. The higher animals, especially, exhibit all stages of organization from the unicellular fertilized ovum to the fully formed animal so far removed from it. The intermediate changes constitute a long record, the possibility of interpreting which has been a stimulus to its careful examination.
Meckel, in 1821, and later Von Baer, indicated the close similarity between embryonic stages of widely different animals; Yon Baer, indeed, confessed that he was unable to distinguish positively between a reptile, bird and mammalian embryo in certain early stages of growth. In addition to this similarity—which is a constant feature of the embryological record—there is another one that may be equally significant, viz., in the course of embryonic history, sets of rudimentary organs arise and disappear. Rudimentary teeth make their appearance in the embryo of the whalebone whale, but they are transitory and soon disappear without having been of service to the animal. In the embryos of all higher vertebrates, as is well known, gill-clefts and gill-arches, with an appropriate circulation, make their appearance, but disappear long before birth. These indications, and similar ones, must have some meaning.
Now whatever qualities an animal exhibits after birth are attributed to heredity. May it not be that all the intermediate stages are also inheritances, and, therefore, represent phases in ancestral history? If they be, indeed, clues to ancestral conditions, may we not, by patching together our observations, be able to interpret the record, just as the history of ancient peoples has been made out from fragments in the shape of coins, vases, implements, hieroglyphic inscriptions, etc.?
The results of reflection in this direction led to the foundation of the recapitulation theory, according to which animals are supposed, in their individual development, to recapitulate to a considerable degree phases of their ancestral history. This is one of the widest generalizations of embryology. It was suggested in the writings of Von Baer and Louis Agassi z, but received its first clear and complete expression in 1863, in the work of Fritz Müller.
Although the course of events in development is a record, it is, at best, only a fragmentary and imperfect one. Many stages have been dropped out, others are unduly prolonged or abbreviated, or appear out of chronological order, and, besides this, some of the structures have arisen from adaptation of a particular organism to its conditions of development, and are, therefore, not ancestral at all, but, as it were, recent additions to the text. The interpretation becomes a difficult task which requires much balance of judgment and profound analysis.
The recapitulation theory was a dominant note in all Balfour's speculations, and in that of his contemporary and fellow-student, Marshall. It has received its most sweeping application in the works of Ernst Haeckel.
Widely spread through the recent literature is to be noted a reaction against the too wide and unreserved application of this doctrine. This is to be naturally expected, since it is the common tendency in all fields of scholarship, to demand a more critical estimate in research, and to undergo a reaction from the earlier crude an I sweeping conclusions.
Improvement in Methods.—Another feature of the work in Balfour's period was increasing attention to methods of preparing material for study. The great problem is to bring tissues under observation, with the normal relations as little disturbed as possible,* so that the prepared material will represent the conditions existing in life and no others. "Many of the most important elements of cellstructure are invisible in life, and can only be brought to view by means of suitable fixation, staining and clearing." One great danger is that pseudo-structures will be artificially formed by the action of reagents. On this account great attention has been given to every feature of technique, and the success of an investigation may depend, very largely, on the care exercised in the use of reagents and dyes, and the mechanical part of getting sections in shape for observation. Investigations of an earlier period were now repeated with greater refinement of technique, and the result was, a change not only in interpretations, but often in the points of observation.
Establishment of Marine Biological Laboratories.—Among other influences which have contributed to the advancement of embryology—as well as to all biology—has been the establishment of fully equipped sea-side laboratories. These have supplied facilities for working where developing forms are most abundant and most diversified. Also, as distributors of prepared material, they have made a wide range of forms available to investigators. The famous 'Stazione Zoologica' founded by Dohrn in 1872, and still under his direction, has exercised a powerful influence. Not only have numerous researches in embryology been carried on there, but, also, prepared material has been shipped to investigators in all parts of the world. Balfour was one of the earliest to avail himself of the opportunities at the Naples Station. The Marine Biological Station at Wood's Hole, Mass., of which Whitman has been director since its foundation, in 1886, is to be mentioned as second only to that of Naples for the extent of its influence and quality of its work. The many other similar laboratories in this country and abroad have aided in the great advance along embryological lines.
Nearly all problems in anatomy and structural zoology are approached from the embryological side, and, as a consequence, the work of the great army of anatomists and zoologists has been in a measure embryological. Many of them have produced beautiful and important work, but the work is too extended to admit of review in this connection.
Oscar Hertwig, of Berlin, whose portrait is shown in Fig. 12, is one of the representative embryologists of Europe, and lights of the first magnitude in this country are Brooks, Minot, Whitman and E. B. Wilson.
Although no attempt is made to review the researches of the recent period, we can not pass entirely without mention the discovery of chromosomes and of their reduction in the ripening of the egg and in the formation of sperms. This has thrown a flood of light on the phenomena of fertilization, and has led to the recognition of these bodies as, probably, the bearers of heredity.
The work of the late Wilhelm His, whose portrait is shown in Fig. 13, is also deserving of especial notice. His luminous researches on the development of the nervous system, the origin of nerve fibers, and his analysis of the development of the human embryo are all very important.
Recent Tendencies. Experimental Embryology.—Soon after the publication of Balfour's great work on 'Comparative Embryology' a new tendency in research began to appear which led onward to the establishment of experimental embryology. All previous work in this field had been concerned with the structure or architecture of organisms, but now the physiological side began to receive attention. Whitman has stated with great aptness the interdependence of these two lines of work as follows: "Morphology raises the question, How came the
Since physiology is an experimental science, all questions of this nature must be investigated with the help of experiments. Organisms undergoing development have been subjected to changed conditions, and their responses to various forms of stimuli have been noted. In the rise of experimental embryology we have one of the most promising of the recent departures from the older aspects of the subject. The results already attained in this attractive and suggestive field make too long a story to justify its telling in this paper. Roux, Herbst, Loeb, Morgan, E. B. Wilson and many others have contributed to the growth of this new division of embryology. Good reasons have been adduced for believing that qualitative changes take place in the protoplasm as development proceeds. And a curb has been put upon that 'great fault of embryology, the tendency to explain any and every operation of development as merely the result of inheritance.' It has been demonstrated that surrounding conditions have much to do with individual development, and that the course of events may depend largely upon stimuli coming from without, and not exclusively on an inherited tendency.
Cell-Lineage.—Investigations on the structural side have reached a high grade of perfection in studies on cell-lineage. The theoretical conclusions embodied in the germ-layer theory are based upon the assumption of identity in origin of the different layers. But the lack of agreement among observers, especially in reference to the origin of the mesoderm, made it necessary to study more closely the early developmental stages before the establishment of the germ-layers. It is a great triumph of exact observation that, although continually changing, the consecutive history of the individual cells has been followed, from the beginning of segmentation, to the time when the germ-layers are established. Some of the beautifully illustrated memoirs in this field are highly artistic. Blochman (1882) was a pioneer in observations of this kind, and, following him, a number of American investigators have pursued studies on cell-lineage with great success. The work of Whitman, Wilson, Conklin, Kofoid, Lillie, Mead and Castle has given us the history of the origin of the germ-layers, cell by cell, in a variety of animal forms. These studies have shown that there is a lack of uniformity in the origin of, at least, the middle layer, and therefore there can be no strict homology of its derivatives. This makes it apparent that the earlier generalizations of the germ-layer theory were too sweeping, and this theory is retained in a much modified form.
Theoretical Discussions.—Certain theoretical discussions, based on embryological studies, have been rife in recent years. And it is to be recognized without question, that discussions regarding heredity regeneration, the nature of the developmental process, the question of inherited organization within the egg, or germinal continuity, etc., have done much to advance the subject of embryology.
Embryology is one of the. three great departments of biology which, taken in combination, furnish us with a knowledge of living forms along lines of structure, function and development. The embryological method of study is of increasing importance to comparative anatomy and physiology. Formerly it was entirely structural, but is now becoming, also, experimental, and will be of more service to physiology. While it has a strictly technical side, the science of embryology must always remain of interest to intelligent people as embracing one of the most wonderful processes in nature—the development of a complex organism from the single-celled condition, with a panoramic representation of all intermediate stages.
- Notwithstanding the deserved praise of Malpighi as an observer, it may be remarked, in passing, that he was not the leader of his period in pure observation and description. Swammerdam showed even greater powers for critical and finished work in this direction. (See 'Malpighi, Swammerdam, and Leeuwenhoek,' Pop. Sci. Mo., April. 1901.
- As Whitman has pointed out, Aristotle taught epigenesis as clearly as Harvey and is, therefore, to be regarded as the founder of that conception.
- Fabricius supposed that the chick developed from the twisted cords of the white of the egg.
- The earliest figures on the development of the chick are probably those of Coiter, 1573.
- For a reproduction of that portrait see Pop. Sci. Mo., Vol. LVIIL, 1901, p. 563.
- See further under the period of Wolff.
- O. Hertwig.
- The discovery is also attributed to Hamm, a medical student, and to Hartsoeker, who claimed priority in the discovery.
- 'De Formatione Intestinoram,' Nova Commentar, Ac. Sci. Petrop., St. Petersburg, XII., 1768; XIII., 1769.
- Besides biographical sketches by Stieda, Waldeyer and others, we have a very entertaining autobiography of Von Baer, published in 1864, for private circulation, but afterwards (1866) reprinted and placed on sale.
- It is of more than passing interest to remember that Pander and Von Baer were associated as friends and fellow students, under Döllinger at Würzburg. It was partly through the influence of Von Baer that Pander came to study with Döllinger, and took up investigations on development. His ample private means made it possible for him to bear the expenses connected with the investigation, and to secure the services of a fine artist for making the illustrations. The result was a magnificently illustrated treatise. His unillustrated thesis in Latin (1817) is more commonly known, but the illustrated treatise in German is rarer. Von Baer did not take up his researches seriously until Pander's were published. It is significant of their continued harmonious relations that Von Baer's work is dedicated 'An meinen jugendfreund, Dr. Christian Pander.'