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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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History of the Germ-Layer Theory
THE fundamental facts of the sheet-like structure of the vertebrate body, which have been treated of in the two preceding chapters, are epitomised as the doctrine of the germ-layers, or the germ-layer theory. Since this theory is of the most far-reaching significanc for the comprehension of the evolution of form in animals, and can be placed side by side with the cell-theory as coequal with the latter, I devote a separate chapter to its history.
The very earliest establishment of the germ-layer theory is associated with the most celebrated names in the field of embryology : CASPAR FRIEDRICH WOLFF, PANDER, and CARL ERNST VON BAER.
CASPAR FRIEDRICH WOLFF, the discoverer of the metamorphosis of plants, who, even before GOETTE, had clearly and distinctly stated that the various organs of the plant, as, for example, the separate parts of the flower, have been developed by various modifications of leaf-like fundaments, also established the metamorphosis of animals, for which he endeavoured to found a similar law of development.
He showed in his important work on the formation of the intestinal canal of the Chick, that it originally appeared in the egg as a leaf -like structure, and that this afterwards became folded into a groove, and finally converted into a tube.
He conjectured that the remaining systems of organs might arise in a similar way, and appended to the account of the development of the intestinal canal the significant assertion : " It appears as though at different periods, and many times in succession, various systems might 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'S article, written in Latin, made no impression on his contemporaries ; it had to be rescued from oblivion by MECKEL, who published a German translation of it in 1812. It was probably by means of this translation that the attention of PANDER was directed to WOLFF. PANDER, under the stimulus and direction of his celebrated teacher, DOLLINGER, further developed the doctrine, the germ of which was contained in WOLFF'S paper.
In his publication, " Beitrage zur Entwickluiig des Huhnchens im Ei," issued in the year 1817, PANDER distinguished in the blastoderm, as early as the twelfth hour of incubation, two thin separable lamella? as the serous layer and the mucous layer, and maintained that subsequently a third, the vascular layer, was developed between them. " Whatever noteworthy may subsequently occurmhe remarks, " it is never to be regarded as anything else than a metamorphosis of the blastoderm and its layers, endowed as they are with an inexhaustible store of formative energy." A few years later the germ-layer theory reached at the hands of CARL ERNST VON BAEII a preliminary completion, which served for some time, vox BAER, likewise a pupil of DOLLINGER, had observed in Wurzburg the beginning of the investigations of his young friend, PANDER. In laborious studies pursued for many years, BAER followed with wonderful accuracy the origin of the germ-layers and their metamorphosis into the individual organs of the adult body, principally in the case of the Chick, but also in the case of some other Vertebrates, and recorded his investigations in his classical work, " Ueber Entwicklungsgeschichte der Thiere, Beobachtimg und Reflexion," which is unsurpassable both in observations and in its general standpoints.
BAER differs from PANDER in maintaining that each of the two primary germ-layers, which he distinguishes as animal and vegetative, subsequently divides into two sheets. The animal germ-layer divides itself into dermal lamella and sarcous lamella (Hautschicht, Fleischschicht), the vegetative into mucous lamella and vascular lamella, so that now four secondary germ-layers have arisen. The individual organs are developed out of the germ-layers by morphological and histological differentiation.
A further advance beyond that of BAER could not be attained until, with the establishment of the cell-theory, entirely new points of view were introduced into morphology and, with improved construction in microscopes, methods of investigation were refined. It is chiefly REMAK and KOLLIKER who have promoted the germlayer theory in this direction.
REMAK took in hand successfully in his noted investigations on the development of Vertebrates the very important question, how the originally similar cells of the germ-layers are related to the tissues of the completed organs. He showed that out of the lowest of the four germ-layers there proceed only the epithelial and glandular cells of the intestinal tube and its appendages, that from the uppermost layer the epithelial cells of the epidermis, the sensory organs, and the nervous tissue arise, whereas the two middle layeis furnish the mechanically sustentative substances arid the blood, the muscular tissue, and the urinary and sexual organs.
In regard to the manner in which the four secondary germ-layers arise, REMAK differs from BAER. Out of the two primary germlayers he first makes a third one, the middle germ-layer, arise, and indeed he derives it exclusively from the lower germ-layer by a process of fission. He designates the three layers as the upper or sensorial, the middle or motor-germinative, and the lower or trophic. The four secondary germ -layers of VON BAER come into existence subsequently by a repetition of the fission, whereby the middle germlayer is split, at least in its lateral portions (lateral plates), into the dermo-fibrous layer and the intestine-fibrous layer (Hautfaser- imd Darmfaserblatt), between which arise the thoracic and body-cavities.
REMAK in his account approximates the true state of affairs, as detailed in the preceding chapters, more nearly than VON BAER ; however, both made the same mistake of interpreting the formation of the germ-layers as always a process of disassociation or fission. That is also the rock on which were wrecked the researches of numerous other investigators, who in the decennary succeeding REMAK dealt with the important question of the origin of the germ-layers. It was difficult to decide this question for the higher Vertebrates, which have been most frequently investigated ; so that very contradictory opinions were expressed relative to the development of the middle layer whether it was exclusively from the lower (REMAK), exclusively from the upper, or from both layers.
This question could be clearly understood only upon the establishment of new general standpoints. These could be acquired only by the comparative method, and by the study of lower Vertebrates and the Invertebrates.
Two fundamental processes needed to be better comprehended :
- How are the two primary germ-layers developed ?
- How are the two middle germ-layers developed ?
By means of the comparative developmental method, one question has been brought nearer to a solution in the gastrcea-theory, the other in the ccelom-theory.
In the study of the first problem, which was the earlier solved, HUXLEY and KOWALEVSKY, HAECKEL and RAY LANKESTER, have shown especial merit. They demonstrated, partly through anatomical, partly through embryological studies, that, with the exception of the Protozoa, the body of every invertebrated animal is constructed of layers, which may be compared with the primary germ-layers of Vertebrates.
The highly gifted English zoologist HUXLEY distinguished as early as the year 1849 two membranes in the Medusae, an outer and an inner layer, out of which alone their bodies are constructed ; and at the same time expressed the happy idea that physiologically they were equivalent to the serous and the mucous layers of BAER. Soon after this (1853) ALLMAN introduced for the layers of the Coelenterates the names, which are now so much employed, ectoderm and entoderm ; subsequently use was also made of these for designating the embryonic layers.
The germ-layer theory was promoted to a still greater degree by the Prussian zoologist KOWALEVSKY, who made us acquainted in numerous excellent detailed investigations with a profusion of important facts concerning the embryology of Worms, Coelenterates, Molluscs, Brachiopods, Tunicates, and Arthropods. He produced evidence that in all the Invertebrates which he investigated two germ-layers are formed at the beginning of development, and that in almost all cases, when the process of cleavage is at an end, a cellular sac arises, and that this, by the infolding of a part of the wall, becomes converted into a double cup, the cavity of which, enclosed by two germ-layers, communicates with the outside by means of an opening. He succeeded in establishing the existence of this very important cup-shaped larva (gastrula) in many branches of the animal kingdom.
In this connection should be mentioned the services of several other embryologists, who at a still earlier period had observed in isolated cases the cup- shaped larva and its origin by means of invagination. RUSCONI and KEMAK had described the cup-shaped larva of Amphibia, GEGENBAUR that of the Sagittse or arrow-worms, MAX SCHULTZE that of Petromyzon.
Whereas KOWALEVSKY by his series of investigations enriched our knowledge of material facts, HAECKEL first sought to utilise the same for a general theory, since by the process of morphological comparison he brought into association hitherto disconnected observations. Starting from the development and the anatomy of the Sponges, he compared the layer-like structure of the embryos of all animals with the layer-like structure of the Coalenterates, and produced as the fruit of this study the celebrated gastrcea-theory, which, attacked on many sides at the time of its publication, has now found in its essential substance general acceptance, and has given the impetus to numerous investigations. HAECKEL showed that in the development of the various classes of animals from the Sponges up to Man a single form of the germ makes its appearance, the gastrula, which consists of two cell- layers, and jthat the two cell-lajers of the various embryonic forms are comparable to one another or The gastrula in its simplest condition presents, as lie endeavored to establish, the form of a double cup with a ccelenteric cavity and a primitive mouth, but may be greatly altered, as in the most of the Vertebrates, by the deposition of yolk-material in the egg, so that the original fundamental form is scarcely recognisable. Consequently he distinguished, according to the kind of modification, different forms of the gastrula, as bellskaj}ed, cap-slmped, disc-shaped, and vesicular yastrulce. He made the various forms arise by a process of invagination from a still simpler fundamental form, the blastula, which is the final result of the cleavage process.*
HAECKEL published his excellent gastrrea-theory in two articles in iheJenaischeZeitsc/trijt: (1) " Die Gastrreatheorie, die phylogenetische Classification des Thierreichs, und die Homologie der Keimblatter," (2) "Nachtrage zur Gastrreatheorie." At the same time with HAECKEL, RAY LANKESTER in England was led to a similar theory, which he had worked out in a paper full of new ideas : " On the Primitive Cell-layers of the Embryo as the Basis of Genealogical Classification of Animals." Both HAECKEL and LANKESTER failed to point out how the formation of the gastrula takes place in some of the divisions of Vertebrates in Fishes, Reptiles, Birds, and Mammals. Essential service in the establishment and explanation of numerous questions of detail, which remained unsettled in the gastrcea-theory, has been rendered by BALFOUR, VAN BENEDEN, GERLACH, GOETTE, HOFFMANN, ROLLER, RAUBER, RUCKERT, SELENKA, DUVAL, and others.
Thus through HAECKEL'S gastrpea-theory the following points were gradually cleared up : (1) The two primary germ-layers, which form the foundation for the development of both Invertebrates and
- It should be here stated that even OKEX and C. ERNST v. BAER had set forth, although in a very indefinite manner, the importance of the vesicular form for the development of the animal bod}*. OKEN was an opponent of the germ-layer theory of WOLFF. In a criticism of PANDER'S investigations he exclaimed with emphasis and a certain justice : " The facts cannot be so. The body arises out of vesicles and never out of layers," and he added the very pertinent remark : " It appears to me as if it had been entirely forgotten that the yolk and the yolk-membrane, which is a vesicle, belong essentially to tlie lody of the germ ; that the embryo does not swim upon it like a fish in the water, nor lie upon it like a funnel on a cask."
In a similar manner BAER remarks, but without further expounding the relation to the germ-layers : " Since the germ is the undeveloped animal itself, one can affirm, not without reason, that the simple vesicular form is the common fundamental form, out of which all animals are developed, not only ideally, but historically."
Vertebrates, arise, not through disassociation or fission, but through infolding of an originally simple cell-layer.* (2) These are comparable with one another or homologous, because they are developed according to the same process, and because the two fundamental organs of the body, the layer which limits the body externally (the ectoderm) and the layer which lines the digestive cavity (the entoderm), arise from them. (3) The intestinal canal of all animals arises_byjn va gi nat ion .
In the question as to the development of the middle germ-layer HAECKEL remained at the traditional standpoint, and inclined most to C. E. VON BAER'S view that the parietal lamella arose by fission from the outer primary layer, and the visceral lamella from the inner germ-layer. Most embryologists, who worked on the development of Vertebrates, entertained, on the contrary, REMAK'S view, and made the whole middle germ-layer arise from the inner by fission.
They regarded the body-cavity as a fissure in the middle germlayer, and compared it with other lymphatic spaces, such as occur in the connective tissue at various places in the body.
The correction of this view was undertaken by various persons in the same manner as in the case of the primary germ-layers. By detailed study of the formation of the germ-layers in the Chick and Mammals, KOLLIKER found that the middle germ-layer did not simply split itself off from the inner, but that it arose from a limited region of the blastoderm, namely, from, the primitive groove, where the two primary germ -layers are continuous. He maintained that from this region it grew out between the two primary germ-layers as a solid cell-mass, and that subsequently the body-cavity appeared in it by means of its fission into two layers. This was an essential advance in the representation of the actual state of affairs.
But a deeper insight into these embryonic processes in Vertebrates was first acquired in this case also through the study of Invertebrates, especially through the important discoveries of METSCIIXIKOFF and KowALEVSKY_concerning the formation of the body-cavity in Echinoderms, Balanoglossus, Chretognathi, Brachiopods, and Amphioxus. The former found that in the larvae of Echinoderms and in Torn aria, the larva of Balanoglossus, the walls of the body-cavity are formed from evaginations of the intestinal canal. But a still greater sensation was created when KOWALEVSKY in 1871 published his " Embiyology of Sagitta," and showed how the coelenteron of the gastrula was divided by two folds into three cavities, into the secondary intestinal cavity and into the body-cavities : this discovery was afterwards fully confirmed by the investigations of BUTSCHLI and the author. After a short interval, KOWALEVSKY'S account of the development of Sagitta was followed by his work on Brachiopods, in which he again enriched science with the new and important fact, that in this class also the body- cavity was formed in the same way as in the case of the Chretognaths. This was followed by his fundamental work on Amphioxus.
- It is still affirmed by several authors for certain Invertebrates that the inner germ-layer develops, not by infolding, but by a splitting off or delarnination from the outer germ-layer.
Through the important discoveries made on Invertebrates, HUXLEY, LANKESTER, BALFOUR, my brother and I were stimulated to theoretical speculations concerning the origin of the body-cavity and the middle germ-layer in the animal kingdom.
HUXLEY distinguished three kinds of body-cavity according to their origin : (1) an enter occd, which arises as in Sagitta, etc., from evaginations of the coelenteron ; (2) a schizoccel, which is developed by means of fission in a mesodermal connective substance lying between the integument and the intestine ; (3) an epicoel, which is formed by an invagination of the surface of the body like the perithoracic space of the Tunicates. The last kind, 'HUXLEY thinks, may perhaps correspond to the pleuroperitoneal cavities of the Vertebrates.
LANKESTER makes HUXLEY'S paper his starting-point. He gives preference to the hypothesis of the common origin of the bodycavity in all animals until decisive proof of diverse origins is produced ; and, in fact, he makes the schizocoel arise out of the eiiteroccel in the following manner. Evaginations of the coelenteron have lost their lumen, and therefore are begun as solid cell-masses, which only subsequently acquire a cavity. While LANKESTER in this, as well as in a second publication, overlooks existing differences in his effort to reduce everything to a single scheme, BALFOUR in various essays takes more fully into account in his speculations the actual condition of affairs ; he also limits himself chiefly to the explanation of the conditions in Vertebrates. In investigating the development of Selachians, he made the important discovery that the middle germ-layer arises from the lateral margins of the primitive mouth, and at first consists of two separate masses of cells, which grow out forwards and laterally into the space between the two primary germ-layers. Since in each cell-mass a separate cavity soon makes its appearance, he designates the body-cavity as from the beginning a .paired structure, and compares it to the body-sacs which are developed in Invertebrates by evagination from the coeleiiteron. BALFOUR justly alleges that the originally solid condition of the two fundaments can have no weight against his interpretation, since in numerous instances organs which ought properly to contain cavities are developed solid, and subsequently become hollow, as, for example, in many Echinoderms one encounters solid cell-masses in place of hollow evaginatioiis of the ccelenteron.
Led by theoretical considerations similar to those of the English morphologists, my brother and I, by a thorough comparison of developmental and anatomical conditions, and with due regard to the morphological and histologicai structure of organisms, then endeavored to bring to a solution this question of the day, the question of the development of the body-cavity and the middle germ-layers, by systematic investigations (published in " Studien zur Blattertheorie "), which extended over Invertebrates and Vertebrates. The results of these series of investigations were published in two articles: (1) in the " Coelomtheorie, Yersuch einer Erklarung des mittleren Keimblattes," and (2) in the " Entwickhmg des mittleren Keimblattes der Wirbelthiere."
In the first paper, in order to prepare the way, we were compelled to give the term germ-layer a more precise definition. We designated as such a layer of embryonic cells which are arranged like an epithelium and serve for the limitation of the surfaces of the body. At the close of segmentation there is only one germ-layer present; namely, the epithelium of the blastula. The remaining germ-layers arise from it by the processes of invagination and evagination. The inner germ-layer is formed by means of gastrulation, the two middle germ-layers by the formation of the body -cavities, in that two body -sacs are evaginated from the ccelenteron, and grow out between and separate the two primary germ-layers. There are, in the first place, animals which are formed of t\vo germ-layers, and possess in their bodies only one cavity, a coeleiiteron, produced by invagination (Ccelenterata and Pseudocoelia), and, secondly, animals with four germ-layers, a secondary intestine,' and a body-cavity derived from the ccelenteron an enter ocoel. To the two-layered animals belong the Coelenterates and the Pseudoecels, but all four-layered animals are Enteroccels.
From this standpoint we endeavored to prove that hitherto there had been confused under the conception "middle germ-layer" two things which are genetically, morphologically, and histologically entirely different.
Besides the cell-layers which arose by invagination there had been assigned to the middle germ-layer cells which detach themselves individually from the primary germ-layers, and give rise between the epithelial layers of the body to the snstentative substances, and also to the blood, when such exists. Embryonic cells of that kind, which are formed by emigration into the space surrounded by the germ-layers, we named the mesenchymatic germ, and the tissue produced from them mesenchyme. This occurs as well in twolayered as in four-layered animals. In our opinion a sharp distinction must be made between the formation of germ-layers, which is correlated with the morphological differentiation of the body, and the formation of mesenchyme, which will especially engage our attention in one of the next chapters, if clearness and a uniform principle are to be introduced into the whole germ-layer theory.
In the second article it was our aim to show that in the Vertebrates a middle germ-layer is developed by infolding. For that purpose the development of Amphibia, Fishes, Reptiles, Birds, and Mammals was compared with the development of Amphioxus, and thus was acquired the foundation upon which is based the account of the development of the middle germ-layer given in the preceding chapter.
After the publication of these two papers, there appeared numerous articles by VAN BENEDEN, DUVAL, HEAPE, HOFFMANN, KOLLIKER, KOLLMANN, RABL, RUCKERT, STRAHL, WALDEYER, and others, through which valuable facts concerning the development of the middle germlayer in the different classes of Vertebrates have been made known. In some of these the chief points of view of the coelom-theory were in general recognised as correct, attempts were made to modify details, but especially was the question of the formation of the mesenchyme of the Vertebrates actively discussed.
The mechanical principle of the process of development, by means of which the germ-layers are formed, and out of these the separate organs, is appreciated in its full significance by only a few, and in text-books particularly has not been adequately presented.
Among the founders of the germ-layer theory, PANDER best comprehended this principle. " The blastoderm," he says in one place, " forms, exclusively through the simple process of folding, the body and the viscera of the animal. A delicate thread attaches itself as the spinal cord to it, and scarcely has this taken place, when the blastoderm sends the first folds, which themselves necessarily designate the position of the spinal cord, as an envelope over the exquisite filament, thus forming the first foundation of the body. Hereupon it produces new folds, which, in contradistinction to the first, give shape to the abdominal and thoracic cavities, together with their contents. And for the third time it sends out folds to envelop in suitable membranes the fretus, which is formed out of it and by means of it. Therefore it need not surprise any one if, in the course of our narration, so much is said about folds and envelopes." And in order to avoid misunderstandings he adds in another place the important statement that " wherever anything is said about the folds of the skin, one is not to imagine a lifeless membrane, whose mechanically produced folds would necessarily spread themselves over the whole surface, without allowing themselves to be limited to a definite space. The folds which cause the metamorphosis of the skin are rather themselves of organic origin, and are produced at the appropriate place, either through increase in the size of the spherules already present there, or through an accession of new spherules, without the remaining part of the blastoderm being thereby altered."
PANDER'S successors have expressed themselves concerning the mechanism of foldings much less clearly ; the most of them, indeed, not at all. The whole doctrine was in fact condemned by RUDOLPH WAGNER as positively erroneous. " It will occur to no one," he says in his " Lehrbuch der Physiologie," " to imagine the three germlayers to be like the leaves of a book. No one will entertain the mechanical conception that the embryo arose by a folding process of these three layers." After PANDER, LOTZE was the next to be occupied with the " Mechanik der Gestaltbildung," as has been pointed out by EAUBER in a meritorious history of this topic. He designates "unequal growth "or " unequal vegetation " as the cause of the changes of place, which in part only appear to be shif tings, out-pocketings, imaginations, or extensions, but in part are .actually such, being brought about in this way by mechanical traction and pressure.
In very recent times His has prosecuted the study of embryology from the mechanico-physiological standpoint more intensely than all his predecessors, and has also particularly emphasised the significance of the process of folding for the formation of the body. The two principal writings of His in this connection are : " Untersuchungen iiber die erste Anlage des Wirbelthierleibes " (1868), and " Unsere Korperform und das physiologische Problem ihrer Entstehung " (1874). While I refer for details to the original papers, I remark that, notwithstanding manifold agreements, I cannot in important points assent to His's view. When, for example, His (1874, p. 50) seeks to reduce the mechanics of form to the simple problem of the form-changes in an unequally stretched elastic plate, in my opinion he overlooks the fact that a plate composed of cells, even if it possess elastic properties, is, nevertheless, a much more complicated structure, and that the processes of folding and evagmation are primarily produced by the energy of the growth of special groups of cells, and are therefore not to be compared with the bendings and stretchings of elastic plates. As PANDER has already emphatically stated, one is not to imagine in the folding processes a lifeless membrane, but rather the folds are themselves of organic derivation, called forth at the proper place by a cell-multiplication at that place. For this reason, too, HAECKEL in his polemic, " Ziele und Wege der heutigen Entwicklungsgeschichte," has attacked this method of treating embryology, introduced by His.
That the morphological differentiation of the animal body primarily rests upon a process of folding of epithelial lamellre, my brother and I have endeavored, by means of an abundant series of observations, to demonstrate in a still more exhaustive manner than our predecessors. In our " Studien zur Blattertheorie " we have, in the first place, directed attention to the Ccelenterates as the animal organisms in which the principle of the formation of folds is most clearly shown throughout the whole organisation, even into details ; and, secondly, we have endeavored to establish for Vertebrates that organs like the body-cavity, chorda, and primitive segments, which it was claimed arose by a separating and splitting of cell-layers, likewise come into existence through the typical process of foldings and constriction.
Finally we have endeavored to point out a physiological cause for the unequal growth of a cell-membrane, and have found such in the Ccelenterates in the unlike functional activity of its various regions. Parts of a membrane will grow more rapidly and must become * infolded, when in consequence of their position they are called upon to accomplish more than neighboring regions.
In concluding this historical sketch attention should be called to the fact that C. E. VON BAER, in the general discussion of embryological processes, was the first to distinguish clearly between the events of morphological differentiation, which take place in the beginning of development, and those of physiological differentiation, which occur later.
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Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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