Meyer - Essays on the History of Embryology 6

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Meyer AW. 1932 - Essays on the History of Embryology: Part I | Part II | Part III | Part IV | Part V | Part VI | Part VII | Part VIII | Part IX | Part X | Part XI | Arthur Meyer | Historic Embryology Papers

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Arthur William Meyer (1873 – 1966)
Arthur William Meyer (1873 – 1966)

Essays on the History of Embryology VI

By A. W. Meyer, M. D.

Stanford University

This is the sixth paper of a series of essays on this subject. Previous papers were printed in this journal as follows: Part I, in December California and Western Medicine, page 447; Part II, in January number, page 40: Part III, in February number, page 105; Part IV. in March number, page 176; Part V, in April number, page 241.
Author's note: I regret that my memory was truant in connection with a legend for the frontispiece from the Elzevir edition of Harvey, which appeared in the March issue of California and Western Medicine. My notes. accumulated in the course of years. contain a memorandum from Vierordt to the effect that the Elzevir edition. which appeared in Amsterdam in 1651, also was issued with a London title-page. This makes it seem as though two London editions had appeared in that year.

THE controversy between the old evolutionists or preformationists and the epigenesists probably was not solely responsible for a long sterile period in embryology. Although the theory of epigenesis or evolution dated back to Aristotle and got support from the work on the incubating hen egg, of Harvey and Malpighi, the latter rejected it. It owes its final triumph to the brilliant work of Caspar Friedrich Wolff, who, according to Meyer—Steinegg and Sudhoff, was encouraged to reinvestigate the question of epigenesis by Haller. The publication of Wolff’s “Theoria Generationis” as a thesis at the age of twenty-six years, in 1759 in Latin, and in 1764 in German, did not attract much attention, but a second publication of his entitled “Formatione Intestinorum,” which appeared in 1768, finally discredited the preformation theory after it became known through its translation into German by Meckel in 1812. It justified the old Heraclitean doctrine,” All things are forever becoming, nothing ever is.”

Wolff as Founder of the Germ Layer Theory

By a very careful series of observations on incubating hen eggs, Wolff showed that the different organs are not present in the egg in miniature, but result from the growth, folding and fusion of a few strata which we now know as germ layers. Wolff traced and represented the development of the amnion, stomach, intestine, and the neural tube, and was the real founder of the germ layer theory. It was he who placed the theory of epigenesis on an unshakable foundation of direct observation and converted it into a law for many forms of beings at least.

Wolff stated that the different organs and organ systems are formed after each other, and declared that some organs which have a totally different function may, nevertheless, arise from the same germ layer, saying:

“It appears as though at different periods, and many times become formed after one and the same type, and as if they might be on that account similar to one another, even though they are in reality different. The system which is first produced, which is first to take on a specific form, is the nervous system. When this is concluded, then the fleshy mass, which really makes up the embryo, is formed after the same type; then appears‘ a third, the vascular system, which certainly is not so unlike the first ones that the form described as common to all systems could not be easily recognised in it. After this follows the fourth, the intestinal canal, which, again, is formed after the same type, and appears as a completed independent whole, similar to the first three."

Wolff humbly and in a very exemplary way added that he was unable to see any of the causes which produce these changes, and that he therefore had nothing to say about them. Although his investigation of the development of the chick still is regarded as the most outstanding embryological investigation done up to that time, it attracted little attention at first, and the German edition of his observations, which was not published until forty-four years after the Latin edition, appeared some time after his death. As a result of Wolff’s work, the theory of preformation was finally abandoned at the end of the eighteenth century except by a few, and has had only an historical interest since.

Wolff’s earlier work, published as a thesis in his young manhood, has not yet received due recognition, for as stated by Gilis, he described globules in the homogeneous mass in which the embryo was formed, such as could be seen in young plants. This observation implied the discovery of the cell at least after the manner of Malpighi, and Wolf? also saw blood islands and veins form in the same mass.

Other Students of Embryology

Interest in embryology, as in other things, was not confined to any one country. John Hunter’s “Anatomia uteri humani gravida” appeared in London in 1775, and Semmering’s “Incones embryon humani” in 1799. Oken’s work “Ueber die Bildung des Darmkanals aus der Vesicula Umbi1icalis” appeared in 1806, and it was not long thereafter before another epoch-making discovery was made by Heinrich von Pander, a young German of means and a pupil of Diillinger. Pander recognized the three germ layers in the developing chick, and published his monograph, with beautiful drawings by d’Alton, in 1817. According to Pander, “Whatever noteworthy may subsequently occur, it is never to be regarded as anything else than a metamorphosis of the blastoderm and its layers . . . ” by folding.

Not long after Pander’s publication, Purkinje discovered the germinative vesicle or nucleus of the hen egg in 1825. Since he found the germinative vesicle absent in eggs taken out of the oviduct, he concluded that it had been ruptured and that its “lympha generatrix” was mixed with the germs, thus explaining the term “germinative vesicle” and inaugurating the study of the maturation of the ovum which, however, was still unknown as a cell.

  • Author's Note.— Through an oversight John Hunter instead of William was credited with the authorship of the “Anatomia uteri humani gravida” in this May installment of my essays. This is especially embarrassing because of John’s remark about stuffing him with Latin and Greek. My sympathy has always been with him, and it seems highly probable to me that he really made the discovery regarding the utero-placental circulation. which is announced in the royal folio by William. In my enlarged manuscript I attempt to accord John recognition so long withheld him, for among other things embryological, John made an unequivocal statement of the law of recapitulation, usually attributed to Fritz Müller about half a century latter.

Von Baer, Father of Modern Embryology

Pander’s publication was soon followed by that of another pupil of Dollinger, who is generally regarded as the father of modern embryology, Carl Ernst von Baer. He discovered the true mammalian ovum in 1827, displacing the Graafian follicle from its wrongful position, and clearing the way for an altogether better and different understanding regarding the sex cells. But we must not forget that the cell was not known until 1833, and that the recognition of spermatozoa and ova as cells was not realized until long thereafter. The cellular nature of the ovum was established by Gegenbauer in 1861, and although Kolliker had shown in 1847 that spermatozoa arise from cells, the spermatozoon was not recognized as a cell till four years later, in 1865, when Hertwig declared it to be such.

The first part of von Baer’s “Die Entwicklungsgeschichte der Thiere—Beobachtung und Reflexion,” dedicated to Pander, was published in 1828, the second, in 1837, in incomplete form, and the concluding part in 1888, twelve years after his death. Von Baer’s claim to fame rests not only on his discovery of the ovum. He carefully described the development of different organs from germ layers and showed that this form of development holds for many animals, thus raising this theory to the status of a law. Von Baer’s observations were so accurate and complete, and his reflections so comprehensive and so sound, that they won general recognition. He also expressed the idea that embryos of the different species in the same group are more alike than the adult forms of these, and that the resemblances between related forms is greater the younger the embryos are, which is not always the case. He described four germ layers, failing to see that the middle layer or mesoderm includes two similar strata, the splanchno- and somatopleura. However, Pander’s recognition of three layers was confirmed by Remak in 1845, and we speak of only three germ layers today—the ecto, endo, and mesoderm.

In the further development of the germ layer theory it was held that these layers are homologous throughout the animal kingdom, and that analogous organs in the adult must have arisen from the same layer. This idea stimulated an amazing amount of investigation in comparative embryology, and although the germ layer theory was found to apply in a very remarkable way to the ecto- and endoderm of most metazoa, there are exceptions even here, and the mesoderm which arises later than the other two layers does not have exactly the same origin in all forms.

The origin, identity, and the mechanics of the formation of the germ layers aroused a great deal of discussion. Much light was thrown on the problem by the study of the development of invertebrates and it was soon evident that these primary layers did not always arise in just the same way. Another interesting problem was the method of formation of the body cavity and upon this Balfour, Hertwig, Huxley, and Lankester threw much light by their studies upon invertebrates.

The Cell Theory

The fine advances made in morphology between the day of Wolff and von Baer could not well continue without the establishment of the cell theory and the development of technical procedures which enabled investigators to discriminate between cells of different kinds and trace their lineage and that of the differentiating organs which arose from them.

The discovery of the cell as the living structural unit of the plant and animal body in 1833 not only directed attention again to the nature of the sex cells, but raised anew the question of cell multiplication and division. Although Spallanzani, and especially Swammerdam had observed, and the latter also had represented, division of the fertilized frog egg, this process was first described more fully by two French investigators, Prevost and Dumas. However, the problem whether these cleavages represented cell formation remained unsettled for some time, for it was not easy to determine whether the germinal vesicle of Purkinje or the egg itself was the cell, and it may be recalled that von Baer compared the ovum with the vesicle of Purkinje. It is strange that Schwann, who had discovered the vegetable cell took no leading part in this matter, although this may have been due to his idea that cells were differentiated from a homogeneous layer or cytoblastema. However, the doctrine of Virchow, “omnis cellula e cellula,” was finally established by Nageli in botany and by Kolliker, Reichert, Remak, and Leydig in zoology. Although leading botanists held that the nucleus disappeared and then was re-formed with each new cell division, anatomists, such as von Baer, Gegenbaur, Johannes Muller, and van Beneden, held the opposite view. This was finally established and “omnis nucleus e nucleo” became a law in cytology as well.

It would be a mistake to conclude that the discovery of the cell resolved all difficulties. The problem of the origin of the cell itself remained as before. It will be recalled that Schwann thought that a place existed where cells were formed out of unformed material and that Kolliker showed that this was not the case in cephalopods. Although Swammerdam and Spallanzani had seen and represented cleavage in the frog egg, it remained for Reichert and Bischoff to show that the “segmentation spheres,” or blastomeres as we now call them, really are cells, and the former showed that the different organs arise from these cells. Later Biitschli, Fol, Hertwig and Strassburger and others, confirmed the observation of Auerbach that changes occur also in the nucleus. Hence it became clear that cells also have parentage in other cells, but this does not imply that the sex cells themselves were henceforth regarded as cells. Although Coste had seen the segmented ovum in the oviduct in 1848, the real nature of the sex cells was not established for some time. Nor was it easy to establish that the cicatricula of Fabricius or blastoderm, alone gave origin to the embryo. It is true that Pander had recognized two layers in the cicatricula, but it remained for Remak to establish that it was in fact the source of all the germ layers, and that none of them arose from the yolk.

The Theory of Recapitulation

Although the idea that the early stages in the development of mammals are very similar was an old one, it could not be established without a long series of careful observations and a painstaking analysis on many forms. The existence of a certain amount of parallelism in the development of all mammals studied and the fact that the higher animals seem to pass through stages similar to those in which some of the lower stop, was emphasized particularly by Meckel in 1812, and led to the formulation of the theory of recapitulation by Johannes Miiller in 1863. The idea that the course of development of the higher forms repeats that of the lower had been suggested by von Baer and Agassiz, and greatly stimulated investigation in comparative anatomy and embryology. It resulted in the accumulation of an amazing number of observations, many of which were brought together by a young British genius, Balfour, in his comparative embryology published in 1880-1881. This undertaking was enlarged in the extensive treatise on comparative embryology by Hertwig, published in 1906, which contains a fine historical introduction and a general review of the literature in embryology.

Although the idea that ontogenesis exactly repeats phylogenesis had to be greatly modified later on, it was one of the most stimulating ideas, and also helped to explain variations commonly known as atavisms—the survival of structures possessed by an alleged remote ancestor, such as the milk line, the gill slits, the nucleus pulposus, a tail, a Darwinian tip, etc.

Attractive as the theory of recapitulation may seem, were it uniformly valid it would imply that our ancestors must have been bisexual, for the male possesses rudimentary female organs, such as nipples, and the female, rudimentary male organs, such as the clitoris. However, the ductus Botalli, the gill slits, the milk ridge, the yolk sac, etc., in man and such structures as the embryonic teeth in whalebone whales, seem to be examples of recapitulation, for the presence of these things in embryonic stages of higher forms is difficult to explain except upon the assumption that they represent vestiges of organs that were present in some ancestral form.

Work of His, Born, Schultze, and Spalteholz was of Importance

Although development of the outer form of embryos could be followed easily by inspection, or by a record made by hand or by photography or by anthropometry, the development of the internal organs and their relationships was too intricate a matter to be followed satisfactorily in this way. Hence relatively little progress was made with these difficult problems in morphology until a sufficient advance had been made in the technique of preservation, embedding, sectioning, and mounting serial sections. These things alone made it possible to unravel the inner architecture of embryos, and the ingenious method of cutting specimens by the microtome devised by His and of reconstruction devised by Born were most important advances, for they enabled anyone to reproduce both the external form and the internal structure of entire embryos or of any part thereof with great accuracy. Later photographic methods of reconstruction and the invention of the Schultze and Spalteholz methods of clearing embryos and fetuses placed further very useful means in the hands of embryologists. Without these technical advances recent rapid progress in morphologic embryology would have been impossible. Although we have been unable to gain much insight into the causes of development, we now know a great deal about how things happen in a large number of forms. The chief obstacle at present often lies in the expense or difficulty, or both, of obtaining the necessary early stages, and this remains true especially regarding man himself.

The later and more recent contributions to embryology have been many indeed, and have given us a fairly detailed knowledge of the story of development of most of the organs in a number of different forms. The comprehensive treatise on the embryology of the vertebrates by Hertwig, and that of Korschelt and Heider, in two languages, on invertebrates, furnishes a fair idea of the status of that subject at the time of its publication, and the “Manual of Human Anatomy” by Keibel and Mall furnishes similar information regarding the status of human embryology at that time. Since these works were published, much progress has been made, although the attention of most anatomists has of late been devoted to other things than morphologic embryology. Although the earliest stages in human development remain unknown to us, clinicians are now alive to the situation and are cooperating with anatomists in such a way as to suggest that these stages will probably be known before many years, and no one doubts that they will closely parallel similar stages in the higher mammals.

A considerable number of young human embryos are known by letters or names, and really are famous in embryology. We speak of the Coste, Reichert, Graf Spee, Peters, BryceTeacher, Leopold, Thompson, Etemod, Miller, Mall, and Mateer embryos, for example, because the individuals named obtained or described them carefully.

One of the important questions connected with all these embryos was that of their age, and practically all of them were at first believed to be younger than they actually were. This is due to the fact that the exact times of ovulation and fertilization are unknown. We do not yet know how long a time supervenes between ovulation and fertilization and between the latter and implantation or conception. Although the true age of an embryo begins with the zygote, it has been customary to think of it as beginning with implantation. We have data regarding these matters in a number of laboratory animals, but they remain undetermined regarding man. Because of this lack, an attempt was .made to extend the curve of prenatal growth in man downward to the beginning, but this could only be serviceable if the rate of growth during prenatal life were constant, which is not the case.

It is extremely regrettable that it is so difficult and so expensive to obtain embryological material from man’s nearest congeners, the chimpanzee, gorilla, and the orang-ou-tang; or to use these animals for experimental purposes in embryology. However, now that it has been found that the chimpanzee can prosper in a northerly climate, it is possible that it will contribute much in the near future toward the solution of problems in human embryology.

Stanford University.

(To be continued)

Historic Disclaimer - information about historic embryology pages 
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Meyer AW. 1932 - Essays on the History of Embryology: Part I | Part II | Part III | Part IV | Part V | Part VI | Part VII | Part VIII | Part IX | Part X | Part XI | Arthur Meyer | Historic Embryology Papers

Cite this page: Hill, M.A. (2024, April 19) Embryology Meyer - Essays on the History of Embryology 6. Retrieved from

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