Book - Russian Embryology (1750 - 1850) 18
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Blyakher L. History of embryology in Russia from the middle of the eighteenth to the middle of the nineteenth century (istoryia embriologii v Rossii s serediny XVIII do serediny XIX veka) (1955) Academy of Sciences USSR. Institute of the History of Science and Technology. Translation Smithsonian Institution (1982).
Publishing House of the Academy of Science USSR
Translated from Russian
Translated and Edited by:
Dr. Hosni Ibrahim Youssef # Faculty of Veterinary Medicine Cairo University
Dr. Boulos Abdel Malek
Head of Veterinary Research Division
Arab Republic of Egypt
The Smithsonian Institution and the National Science Foundation, Washington, D.C, by The Al Ahram Center for Scientific Translations 1982
The Smithsonian Institution and the National Science Foundation, Washington, D.C by The Al Ahram Center for Scientific Translations (1982)
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Chapter 18. Second Part Of Uber Entwicklungsgeschichte - Scholia and Corollaries to the History of Chicken Development in the Egg
Baer called the second part of the first volume of his work "Scholia and Corollaries" (95). In spite of that name familiar to medieval students, it does not contain dogmatism originating from any preconceived point of view, but penetrating thoughts on the basic regularities of chick development. These represent generalizations of those observations stated in the first part.
The first scholium, "On the certainty of observations on embryos," poses a question basic to arguments by embryo logists from the seventeenth century. Specifically, is it possible to confirm that the embryo exists only from that time when it, as a whole, and its component parts become accessible for observation? Because of the narrowness of investigative means, Baer asked "whether all the embryo with all its parts can be there, but so finely structured that it is not accessible to the knife and microscope" (I 212 (143)).
With the example of muscle development, Baer proved that the embryo as a whole does not acquire parts with minute structure? and dimensions are beyond the limits of microscopic powers. While the muscular fibers of the adult chicken can be split into very minute fibers, seen only at very high magnification, the embryonic muscle fibers are accessible for observation under lenses, although they are still not formulated and can be separated from each other only with difficulty. The same thing applies to the nerve fibers and other component structures of the embryo. Baer's general conclusion opposed the opinions of the preformationists, leading to the assumption that the large size of the embryonic component elements "undoubtedly makes impossible the existence of the embryo in a preformed shape in the second and third generations" (I Sch la 214 (145)).
The second scholium, "The formation of the individual in relation to its surrounding," discusses two questions, concerning the essence which manages the development of the animal, and about the most essential result of development, the progressive independence of the embryo. Desiring to "clarify the essence of development," Baer stated that "although each new step in development becomes possible only because of the pre-existing condition, nevertheless the entire development is directed towards the prevailing essence of the animal" (I Sch Ila, p. 217 (147)).
This situation is illustrated by the significantly greater variability of the embryo than of the adult organisms of the same species. Noticing the frequently deep differences between embryos of one age, Baer wrote that "one must conclude that the differences are compensated for, and each abnormality, as much as possible, will return to the norm" (I Sch Ila, p. 218 (148)).
From the fact of this remarkable regularity which is inherited by developing embryos, Baer concluded that "not every stage is like the others with all its particularities determining the future stages of development, but here more general and higher relationships predominate." And further: "Natural science, which is readily identified in that it feeds and supports materialistic ideas, can, as a result of observations, disprove the strict materialistic studies and lead to the evidence that not the material but the essence (the idea according to the new school) of the developing life form governs the development of the fetus" (I Sch Ila 148).
Commenting on this point in the Russian version of Baer's work (pp. 218 - 219 and note 43, pp. 450 - 451), B. E. Raikov considered that "Baer's presentation clearly reveals the idealistic character of his view about the factors of development." This requires an explanation, however, in order to reject the charge of the influence of Naturphilosophie and followers of Schelling on Baer during the early years when this science was very popular. Objecting to the "strict materialistic studies" of life phenomena, Baer addressed the materialism of the beginning of the nineteenth century, which is now called mechanical or metaphysical materialism. It is not surprising that Baer's deep intellect could not be reconciled with the primitive simplified ideas about development coming out of the general principles of this materialism.
Baer's negative relationship to materialism was characterized by his concern about complex life processes, for which he sought other than dead mechanical schemes. The possibility of real scientific solution to the questions interesting him in the first half of the nineteenth century was not yet available, however. Instead, Baer was faced with idealistic ideas, diligently propagated by the departments of German universities. From Baer's endorsement of idealism comes the beginning of his views about "The general and superior attitudes" about the "essence" or "notion" governing the development of individuals. Baer's idealistic discussions frequently go against his specific ideas about ontogenic development, which are based on strict and accurate observations.
On a much greater level, Baer's idealistic opinions on the evolution of the organic world remain consistent. Admitting the natural origin and development of living creatures, because otherwise a miracle would be required, Baer limited the evolutionary process to lower systematic groups and did not extend the idea to his types. In relation to the mobile powers of evolution, Baer, especially in later years, decisively objected to Darwin's materialistic point of view. His question about evolutionary opinions extends, in general, beyond the limits of the present book and requires further specialized study.
The second question discussed in Baer's second scholium concerns the most important result of development as a whole, "the increasing independence of the developing animal."
The main stages of chick embryo development, discussed in detail in the first part of the work, include the following: 1) growth of the rudiment (blastoderm) , whose unlimited part constitutes the embryo; 2) separation of the embryo from the remaining rudiment, but with the embryo remaining subordinated to and supplying the rudiment with nutrient materials; 3) the separation of the embryo from the egg parts outside the embryo, but with the embryo still supplying nutrition and constituting one unit with those extra- embryonic parts; 4) the inclusion of the individual extra- embryonic parts of the rudiment within the embryo (by immersing the remaining yolk in the embryo's abdominal cavity) and the separation of the embryo from the remaining parts (amnion and allantoides) upon hatching. By this, according to Baer, "the last stage of the growing independence" is attained.
His comparison of the development of birds amphibia, fish and mammals led Baer to conclude that in amphibia from the beginning, the embryo is very large in comparison to the yolk mass; to achieve independence, it does not require separation (unlacing) as other embryos do. Bony fish hold an intermediate position between the birds and amphibia in this respect, with only insignificant unlacing. In mammals, the separation and the covering with membranes takes place more quickly than in birds and results in an especially long umbilical cord in the most highly organized representative of mammals, the human.
This early separation of the mammalian embryo from the yolk sac does not contradict the truth that for all vertebrates the embryo constitutes a single unit with the blastoderm and yolk. Fertilization of the egg, which was previously a part of the mother, produces an independent unit, similar to the parents; the features of this unit appears during the process of development.
In lower animals, according to Baer, there is no separation of sexes, and reproduction occurs only by growth beyond the limits of the individual. Baer developed this erroneous idea in the special supplement to his second scholium in "Corollaries about reproduction." There he tried to discredit the data about parthenogenesis in vertebrates, and with perplexity he dealt with facts about the parthenogenetic development in the plant louse and certain butterflies. The difficulties which Baer met in the interpretation of features of fertilization he explained with the standard knowledge of the time.
The third scholium, "Interior transformation of individuals," concerns those routes along which the embryo develops. Baer stressed that individual development at all stages proceeds from the homogeneous to the heterogenous, from the general to the specific. Baer distinguished three forms of transition from the general to the specific in ontogenetic development, or three forms of differentiation: the primary, histological, and morphological differentiations.
The first type, primary differentiation, comprises the division of the blastoderm into layers, which Pander had called plates. At first, two layers appear. The external layer Baer called the animal layer, and the internal layer he called the vegetative, or the plastic layer. In the animal layer, a skin layer and muscular layer become differentiated; they are composed of the spinal and abdominal plates "which include in a nondifferentiated shape the bony, vascular and muscular systems with the corresponding nerves." The vegetative part also differentiates into two layers, the vascular and the mucous.
In an article supplementing the translation of the first volume of UBER ENTWICKLUNGSGESCHICHTE, B. E. Raikov* successfully compared the subdivision of the blastoderm into embryonic layers as suggested by Pander, Baer, and later by Remak; illustrating his comparison with a table^ (96) . The comparison reveals that Baer's identification of the rudiment layers with the embryonic layers, in the modern meaning of that term, constitutes an incorrect interpretation of primary differentiation. BischoffS and Filipchenko also discussed this issue. 4
It must be added that Baer's designation of two embryonic layers between the skin layer (ectoderm) and the mucous layer (endoderm) , and not only one "mobile-rudiment layer," as Remak later identified it ("mesoderm," according to modern embryology), indicate Baer's cleverness in y observation. He was capable of distinguishing the two layers of the lateral plates of the mesoderm, which are now named somatic and splanchnic mesoderm. (97)
1. B. E. Raikov, "On the life of scientific activities of K. M. Baer" (Ozhizni inauchnoi deyatelnosti K. M. Baer) , in K. M. BAER (1950), pp. 383 - 438.
2. Ibid ., p. 419.
3. Theodore Bischoff. ENTWICKLUNGSGESCHICHTE DER SAUGETHIERE UND DES MENSCHEN (Leipzig: Voss, 1842) (Vol. VII of "Anatomie" edited by Zemmering).
4. Translator's note: K. M. Baer, SELECTED WORKS, translation with introduction and comments by Yu. A. Filipchenko.
Baer also related the separation of the vertebrate central nervous system rudiment into layers to primary differentiation. Baer remarked that the skin and nervous layers originate from the upper surface of the embryo, so his point of view concerning the source of the central nervous system remains unclear.
The second type of differentiation, histological differentiation, already occurs inside the layers, as a result of which the skeleton, muscles, nervous system, and blood form.
The third type of differentiation, the morphological, produces the external shape of the embryo. After that occurs the embryonic layers are transformed into tubes; the separated parts of these tubes acquire different configurations and serve as a source for the organs. Thus, the neural tube subdivides into the spinal and cephalic membranes and the organs of sense. The mucous tube gives rise to the mouth cavity, ^ the esophagus, stomach, intestine, respiratory organs, liver, allantoides, and so on. Morphological differentiation takes place, according to Baer, due to the irregular growth of individual parts of the nervous, mucous and other tubes. By irregular growth, Baer could explain the apparent demarcation of one section of the tube from another; for example, the divisions of the brain from each other, the stomach from the intestine, and also the localized protrusions giving rise to the sensory organs, respiratory organs, liver, and allantoides.
The three forms of differentiation described represent, according to Baer, a source of heterogeneity of the organism. Early in development, heterogeneity of the organs and their constituent histological elements is less marked. "Observations," Baer said, "show more than any description can that all individuality is at first contained in the general" (I Sch Ilia 156). This pre-existence of individuality in the general, though not of course in the sense of primitive preformation, others might consider an argument against studies of new formation. But Baer disputed this point of view.
5. It was not known to Baer that the mucous membrane of the mouth cavity originates from the ectoderm.
Study of histological differentiation shows that new structures form not at an empty place, but by transformation of the previously existing simpler and homogeneous formations. The modern embryologist could describe the phenomenon of cartilage formation ir. the extremities, for example, as the transformation by condensation of mesenchymal cells into cartilage. The same also relates to morphological differentiation, because "each organ is a changing part of the more general organism." Baer illustrated this with the example of respiration development. When the lungs develop as outgrowths of the mucous tube, there is already an elevation between them which, after stretching, forms the trachea.
Another example concerns the development of the extremities. Their first appearance could be called a new formation; however, the skin layer which is separated from the spinal and abdominal plates in the region of the future extremities already contains primitive structures which initiate the formation of extremities.
In opposition to Serres, who considered that organic growth is represented by the union of initially isolated elements, as with crystal development, Baer confirmed that the formation of an organ, like the formation of the whole embryo, involves a sense of preformation. "The absolute beginning of the process," he asserted, "is always imperceptible" (I Sch II If, 157 - 158).
The development of the whole embryo and its organs universally occurs, in Baer's opinion, from the center to the periphery, but the universality of this idea seemed similar to the constructions of Naturphilosophie which were in general so foreign to his thinking. In his next scholium, Baer objected to this centrifugal character of development. This fourth scholium, with its two corollaries, is called "On the scheme which vertebrate development follows."
Turning to the first step of development (Sch. IV, Â§ 1), Baer remarked that the breakdown in the rudiment and embryo follows one plan, but in different directions. Thus, in the rudiment and the rudiment membrane, differentiation of thickness develops along the surface and the length. Finally, the fetal area is most developed in the anterior region, and the vascular in the posterior region.
The differentiation in thickness leads to the separation of the animal part of the embryo, and also the vascular and mucous layers. Differentiation along the surface delimits the embryonic body and the vascular and yolk fields from each other; in length such important parts as the brain and skull, heart and intestines becomes separated. The transformation of the layers of the rudiment into the embryonic body, according to Baer, proceeds from the surface to the interior. At first, parts of the serous layer are differentiated, then formation of the heart and aorta begins in the vascular layer, and only later the mucous sheet begins to form the digestive tube. Along the length and breadth of the embryo, differentiation proceeds in defined succession from the anterior backwards and from the middle to the periphery. The successive processes of separation of the embryo occur in the same order. The conversion of the layers into tubes is accomplished in vertebrates by a double symmetric development from one axis (I Sch. IV 2a, 143).
Baer first stated the idea basic to comparative embryological investigation, that the three methods of differentiation just mentioned are apparently inherent to all animals except the simplest. Also, for the vertebrates, in his opinion, it is characteristic that the processes of formation follow the principle of double symmetry in relation to the longitudinal axis of the body. The upper or animal layer forms a tube by rolling up this layer above the axis of the body. Describing this idea in the chick embryo, he also confirmed it in the frog embryo and expressed his certainty that in the other amphibia, fish and mammals development follows that same scheme.
After formation of the spinal tube under the cord, which lies along the axis of the body, the abdominal tube forms by the union of two symmetrical halves. In transverse section, the vertebrate embryo assumes the form of a figure eight.
In the process of fusion of the upper as well as the lower parts of the embryo, the layers separate and acquire the form of tubes inserted into each other, which Baer called the basic of fundamental organs.
The mucous layer forms the internal tube of the abdominal half of the embryo. This fundamental organ Baer called the ir;L coder ma] tube. It begins material exchange with the external environment, specifically with the organs of digestion and respiration. The vascular layer in the abdominal half covers the mucodermal tube and forms two tubes: one lies above the intestine and the other surrounds the intestine. From this duplicated vascular-dermal tube, the blood vessels form. The lower muscular layer of the animal layer also forms two tubes: the spinal one includes the neural tube, and the abdominal includes the previously mentioned two vascular-dermal tubes. By means of histological differentiation, the muscular layer divides into the dermofibrous and the specific muscular layer. The central nervous system and skin also form tubes: the skin tube envelops both muscular tubes. Both have a common origin in the separated upper layer of the animal part of the germ. Part of this initially single formation appears in the interior (the neural tube) and part remains at the periphery.
The fact that these tubular basic organs are located one inside the other, but become separate as different parts undergo different activities, Baer tried to explain by the polar opposition of their vital properties. This idea of opposition as a source of formation undoubtedly has a naturephilosophie character, and Baer considered it an echo of the effect of Schelling and Cken on him. Baer, being sufficiently sceptical about Naturphilosophie, nonetheless tried to find in such ideas a rational seed and to use them for the discussion of the phenomena he sought to understand.
Baer used his scheme of embryonic development from systems of tube-shaped basic or fundamental organs to establish a general plan for the organization of vertebrates. Still more important for this plan of organization Baer considered the axes and planes of symmetry. Referring to his schematic drawings, Baer described the relation of the basic (fundamental) tubes to the main axis of the body.
Baer considered the irregular growth of fundamental organs as the basic differentiating factor (Â§ 3 (173)), and he illustrated this idea in particular with examples of development of the skull, cephalic brain and heart. The source of irregular growth Baer saw as in the direction of the flow of nutritional materials, on which depend the separation of: surface layers of the blastoderm from the layers adjacent to the yolk, the central parts from the peripheral parts, and the cephalic part of the embryo from the caudal or tail part. In judgments of this kind, Baer showed his inherent scientific carefulness and stipulated that he did not so much search for the reasons of development as he paid attention to coordinating the processes of development, "that it is for us in the beginning more important to gain knowledge of the deepest basis of the process, because the latter at the beginning of investigations is difficult to clarify in all completeness." This carefulness, moreover, gives credit to Baer that he, in all discussions of the dependence of development on movement of nutritional materials, moved beyond the wrong impression that the nutrition of the embryo passed through a canal which connects the central yolk cavity with the blastoderm lying on it.
Besides the separation of the basic or fundamental organs in the process of development, formations appear which connect the main organs with each other. According to this plan, the sensory organs develop (they connect the nervous system with the skin) and the derivative mucodermal tubes pass outwards (mouth, anus and branchiate slits).
About histological differentiation, Baer said very little, considering only the differentiation of bones and nerves from muscles. He reasonably objected to Serres, who suggested that nerves grow from the periphery to the center. Yet he also adopted the inaccurate view that nerves are formed at once along their length "from the muscular layer by means of histological separation." Baer did not promote this idea, remarking that the essence of the histological separation remained unknown to him.
The fifth scholium, entitled "On the relation of forms which the individual takes at the different stages of its development," is most widely known, and most frequently cited.
Baer began (Â§ 1) with the formulation of the contemporary idea that "the embryo of the higher animals passes through the permanent forms of the lower animals." He underlined
that the higher animals in single stages of individual development, from the first origin to completed development, respond to the permanent forms of the animal series, and that the development of individual animals follow the same laws as that of the whole animal series. Therefore, the more highly organized animal in its individual development passes through the most important permanent stages lower than its own, so that the periodical differences of the individual may be related to the differences in the permanent animal forms. (I Sch. V la, 286 - 287 (199))
The development of this idea Baer attributed to the time when systematic investigations of early embrycgenesis were absent, with the exception of the work of Malpighi and Wolff. It was developed by Johann Friedrich Meckel, Junior, whom Baer did not name and only mentioned as a person who "acquired a highly serious knowledge about the history of development of the higher organisms" (I Sch. V la, 200) . Baer remarked next on those evolutionary conclusions based on the similarity of embryos of higher animals to the adult lower animals. Ridiculing these hurried evolutionary conclusions, Baer caricatured the transformation of fish into land vertebrates and the elongation of the heron's neck as a result of its trying to reach to catch fish. His objections were directed not so much against the principle of evolution in general, or even against Lamarck's principle cf inheritance of acquired characteristics, as against "the permanent arrangement of animal forms in one series" (I Sch. V la,b, 200).
In section 2, therefore, Baer turned to his doubts and objections to the repetition of the phylogenetic history in ontogenesis. The comparative study of adult animal forms made him sceptical of the idea of ontogeny following phylogeny. Insofar as his first embryo logical investigation led Baer to conclude that in chick development the features of the vertebrate animal appear very early, he had already referred to his doubts in his dissertation published five years earlier.'
The similarity between the individual embryonic stages and the adult stages of others Baer considered as unquestionable, but without major significance. Against the view that development of the individual passes through constant forms, which are characteristic of lower animals, Baer promoted the following objections. If it were true, there should not be in the embryo any characters that are absent in lower adults (for example, the reserve of nutritional materials in the yolk or the intestinal loops hung externally). Besides, there might be similarity in just one feature or aggregate of features. According to the structure of the heart and extremities, bird embryos are similar to those of fish; but the other series of features inherent to early fish development are not present. The similarities of embryos with animals which are more highly organized also argues against the recapitulation idea . Finally, those organs which are inherent only tc the higher animals appear very early. For example, the foundation of the vertebral column appears when the vertebrate animal, according to the idea of repetition of the species' history, would only have passed the stage of invertebrates (I, Sch. V 2b, 204 - 206).
All of Baer's argument against the repetition of the species history in ontogenesis, he mobilized for his theory of types. The theory of types played, in the history of science, a dual role: a positive one, insofar as it represented the basic natural classification system, and a negative role, insofar as the theory of tyr^s represented an antithesis to the theory of one origin, the evolution of the organic world. To the theory of types, Engels' familiar words relate directly: "... from that time, when they began working in biology in the light of evolutionary theory, one rigid boundary line for classification after another has been swept away in the domain of organic nature ... the distinguishing characteristics, which are almost becoming articles of faith, are losing their absolute validity . . . ."Â°
7. Baer, DE FOSSILIBUS ANIMALIUM RELIOUIIS IN PRUSSIA REPERTIS DISSERTATIO. Regiomontii , 1823, 40 pp. (98) .
Baer's theory of types was more flexible than Cuvier's, because Baer assumed, though very cautiously, that all animals develop from one general original form, and, more confidently, that there is variability of organisms within the limits of each type. Although it is impossible to consider Baer a consistent evolutionist, his services in the preparation of the evolutionary idea are unquestionable.
Engels further defined Baer's historical significance. Remarking that the first assault on the theory of species constancy and proclamation of evolution was accomplished in 1759 by K. F. Wolff, Engels continued: "But what he had was a genius' anticipation which took a defined form in Oken, Lamarck and Baer, and was victoriously carried through by Darwin in 1859, exactly one hundred years later. "9 In another place, on enumerating the gaps opened by science, Fngels wrote: "(Natural science, at the outset revolutionary, was confronted by an out-and-out conservative nature) in which everything remained today as it was at the beginning of the world, and in which, right to the end of the world, everything would remain as it had been at its beginning. "10
The urge to find out the most essential in the processes of individual form-production led Baer to establish the true relationships in the organization of different animals; however, he suggested distinguishing the degree of development of the animal body and the type of organization^ (Sch. V 3 a,b, 207, 208).
8. F. Engels, ANT I -DUHRINC , Gospolitiedat, p. 13, 1952. (Ed.: English page numbers are from ANTT-Dt)HRING, Moscow? Foreign Language Publishing House, 1959, p. 21.)
9. F. Engels. DIALECTICS OF NATURE. Gospolitisdat, p. 11, 1952. (Ed.: English version page numbers are from DIALECTICS OF NATURE, New York: International Publications, 1940, ed. and trans, by Clemens Dutt, p. 13.)
10. F. Engels. DIALECTICS OF NATURE, pp. 153-154 (186).
By degree of differentiation Baer meant the degree of heterogenity of its component parts or the degree of histological and morphological differentiation. Baer considers a higher organization where "the different divisions of systems of organs are not similar to each other, and each part has a more remarkable individuality," in contrast to forms in which "all the organization as a whole is more homogeneous." Comparing the lower vertebrates (fish) with the higher arthropods (insects) reveals a greater heterogeneity of structure in the latter, although fish are a higher type in organization.
By type, Baer meant the character of distribution of organic elements and organs, which represent an expression of "the main relations between individual features of life of the organisms." The possibility of distinguishing "type" from "degree of differentiation" leads to the conclusion that one type can cover different degrees of development, and, on the contrary, one stage of development can be reached by different types. The combination of the type with a particular degree of differentiation or degree of development produces the major groups of the animal kingdom, the classes.
The confusion of the type and degree of development represented, in Baer's opinion, the source of incorrect classification. Clear demarcation leads to the conclusion that the different forms of animals could not be put in one line of development "from the monad to the human."
Between the irain types, in Baer's words, there are "intermediate forms, in which the characteristic features of the main types are combined in one middle type, or in which one half of the body represents one type, and the other half represents another." But Baer did not review these intermediate forms in the present work, in order to set off the peculiarities of the four main types ("Haupt-Typen") : the peripheral or radiate, the articulated or elongated, the massive or mollusc, and the vertebrate.
11. On the necessity of differentiating these two understandings , Baer remarked in a more easily understood report â€” "Beitrage zur Kenntniss der niedern Thiere . " (NOVA ACTA PHYSICO-MEDICA ACADEMIAE CAESAREAE LEOPOLDINA CAROL INAE NATURAE CURIOSORUM, 13 (1827) , pp. 525-762) , especially in the article "liber die Verwandschaftsverhaltnisse der niedern Thierformen" (pp. 731-762).
Baer studied the types of organization for analysis of individual development (219 4a) . Embryonic development involves an increase of the "degrees of development, or histological and morphological differentiation. It is possible to show that between the embryos of higher animals and the constant forms of the lower animals there exists a certain relationship. This, however, does not represent evidence that "each embryo of the higher animal form gradually passes through the form of the lower animals," according to Baer. On the contrary, "the type of each animal from the beginning is fixed in the embryo and governs all development."
All work on chick development Baer considered "only a long comment to this confirmation." Before anything else the spinal column is separated, then the spinal and abdominal plates, and also the spinal cord. All these rudiments appear very early, and after their appearance "there could not be any speech about the correspondence of the embryo to any invertebrate animal." On the contrary, the first recognized are the properties characteristic of the vertebrate animal. This is true not only for the bird embryo but also for all classes of vertebrates, from which Baer concluded that "the embryo of the vertebrate animals is from the first a vertebrate." Because no adult vertebrate animals exist which could be characterized with no little histological and morphological differentiation as the young chicken embryo, Baer concluded that "the vertebrate embryo in its development does not pass through the stages ... of adult animal form."
In the formation of individuals, it is possible to establish a general regularity characteristic of the type. Such a rule is represented by the greater early similarity of embryos of different vertebrates and the approach of a later moment of original development. The peculiarities, characteristic of the large divisions of the vertebrate type, appear earlier, and the features of the smaller systematic groups appear later. In other words, "from the more general type the more specific is formed." This same regularity is true for the development of the vertebrate, as well as invertebrate animals, such as the Crustacea.
At the very early stages, both vertebrates and invertebrates have what is called the primary zone. This raises a question, "are not all animals basically identical at the beginning of their development, and is there not for all of them one general primary form," Baer confirmed that such a general form actually exists, represented by the vesicular stage. Birds constitute no exception because the blastoderm, gradually growing over the yolk, is completed by the yolk membrane, and in mammals the vesicle surrounds the yolk from the first.
Yet in the German commentary to the treatise on the origin of the mammalian and human ovum,^^ Baer confirmed that "the simple form of the vesicle is the general basic form, by which all animals develop, not only in idea, but also historically." This vesicle-shaped embryonic stage, which Baer considered common to all animals, corresponds in modern terminology to the blastula. It is difficult to say with confidence whether this echoes Oken's idea about the round- shaped original form of all the bodies of nature, or if it indicates Baer's unusual perspicacity which was capable of discovering this important regularity through comparative anatomy.
12. "Commentar zu der Schrift: DE OVI MAMMALIUM ET HOMINIS GENE SI," p. 173.
Bringing in his results about the similarity of early embryos of different animals and about the divergent character of their subsequent development, Baer formulated four fundamental conclusions:
1. "In each large group the general is formed earlier than the special." The most general for all animals is considered to be that opposition between the external surface, directed towards the surrounding media, and the internal surface. Therefore it is absolutely natural that the general original form represents a hollow circle, or vesicle.
2. "From the general the less general is formed and so on, until at the end the very special appears. "
3. "Each embryo of a certain animal form, instead of passing through other defined forms, deviates from them."
4. "The embryo of higher forms is similar not to the other animal form, but only to its embryo." (320 - 321 (224))
The superficial similarity of the embryos of higher animals to the adult lower animals depends on the fact that the latter are little differentiated and therefore not so far from the embryonic condition.
The transition in individual development by means of divergence from the more general form to the more special Baer illustrated by his schema, "Illustration of the course of development" (227) .
0) ^ +->
In the egg occurs
Radiant development (?)
Animals of peripheral
Animals of massive
Animals of elongated
Vertebrate animals have vertebral column, spinal
plates, nervous tube,
branchiate slits and
Branchiate fringe; the
Do not form actual lungs
Skeleton does not ossify
Bony fish Amphibia
Growing urinary sac
No umbilical cord
There is umbilical cord
No wing and air sacs
There are wing and air
Baer's table continues also to the left, where it shows that the ovum is a consequence of the dichotomy of asexual and sexual multiplication, and to the right, where it presents the divergence of the amphibia and mammals according to peculiarities of later development. The text (p. 321 and on (225 ff.)) gives a detailed commentary to this table. One of its main purposes is to show clearly that the embryo in the course of development sometimes does not pass from one main type to another. From the point of view of modern embryology, Baer's scheme could have included a series of corrections, such as that spiral development is inherent not only to molluscs (to the massive type), but also to the annelid worms (representatives of Baer's elongated type), and so on. Baer himself stipulated the imperfection of his scheme, as also all others, but his main idea about the divergent character of development need not fluctuate with particular corrections.
Baer considered it absolutely natural that the erroneous point of view that transmission to embryonic stages correspond to simply constructed animals had acquired many supporters. Since fish deviate less from the basic type than mammals, it is natural that the embryo of mammals is more similar to fish than the embryo of fish is to mammals.
To the fifth scholium Baer attached four corollaries, which begin with the wonderful aphorism so frequently cited thereafter: "The history of development is a real light for the study of organic bodies. At each step it finds its application, and all ideas which we have about the reciprocal relations of organic bodies will experience the influence of our knowledge of the history of development" (p. 328 (231)).
The first corollary is entitled "The application of this 1 _ scholium to the study of arrested development (Hemmungsbildung) ." It begins with the confirmation that arrested development can be understood only by knowing the process of normal development. After that comes a controversy with those authors who considered that arrested development illustrates the repetition of ancestral features in ontogenesis. If similarity is recognized between the developmental stage and the adult stages of lower organized animals, then this, Baer considered, occurs most frequently because the given adult form is closer to the original type. As a result, arrested development of the more highly organized animal will certainly draw these forms closer to each other.
13. (Ed.: "Hemmungsbildung" means structural defects, malformations, or arrested development, but Baer seems to have meant the last.)
The second corollary, called "Application of the present opinion to the determination of individual organs in different animal forms," concerns rational nomenclature for comparative anatomy, which can be constructed only with consideration of the history of development. Thus, the series of abdominal nerve knots of articulated animals cannot be called a spinal cord and compared to the nodes of the vertebrate sympathetic nervous system. The anterior pair of nodes in articulated animals cannot be called the cephalic brain. The supporters of such suggestions refer to these nodes, but Baer showed, illustrating his idea by a simple graphical scheme, that these nodes lie not above but anteriorly to the gullet. Thus the nerve ring near the gullet represents a secondary formation, dependent on the breakdown of the mouth at the abdominal surface of the body. Equally, one cannot compare the respiratory tubule of insects with the respiratory tube of vertebrates, because the latter develop from the mucodermal tubes, and the former represent a result of the protrusion of the external covers. At the same time, in a number of types there are organs of identical purpose and origin. Thus, "the digestive canals in all animals are formed from the membrane facing the yolk." This example and a number of others, Baer put as basic to the study of analogous and homologous organs; the demarcation of these understandings are attributed usually to a later period of the history of biology and connected with the name Richard Owen.
In the third corollary, "Application to the knowledge of animal affinities," Baer again returned to the idea of the linear or stair-like succession from the cephalopods or Crustacea to fish, from the echinodermata to the molluscs, the impossibility of knowing which stands higher â€” the articulated animals or the molluscs â€” and so on. If the current idea about the ladder of animals were renounced, every form could be considered a type -change from. a more general form; even the last is a modification of the basic type. Hence in individual representatives of a type, some organs are more developed, and in others, other organs. Baer's rejection of the linear succession of developing individual organs led him to deny the possibility of reverse development. In some undefined form he stated, at the end of this corollary, an idea about the fact that development is always progressive and in the animal world leads to the foremost developed system, the cephalic brain.
The extensive fourth corollary, entitled "The division of animals according to their method of development," is devoted first, to the developmental difference between plants and animals, and secondly, to the differences of individual groups of animals based on processes of development. Such differences correspond to the animal type. Thus, for the vertebrates it is characteristic, according to Baer, to have a double symmetrical development, which he himself studied in detail. Typical for vertebrate development is the formation of two tubes, which are closed at the spinal and abdominal ends and divided at the longitudinal axis by the spinal or vertebral cord. To the elongated animal the symmetrical type of development is inherent, which leads to the formation of one symmetrical tube closed along the abdominal side to the spinal side. It is right to compare symmetrical development with double symmetrical development, but not to deduce one type from the other.
The preliminary data, obtained by Baer from his embryological study of bivalved molluscs and snails, led him to conclude that this development proceeds according to the principle of transference of the developing parts into spirals; therefore the molluscs have a waved form of development. Baer had little data concerning development of the peripheral or radiate type.
From comparing the methods of development of animals of the four main types, Baer concluded that "each main type follows a peculiar plan of development." The connection between the character or plan of development he designated by the following aphorism: "The plan of development is nothing other than the emerging type, and the type the result of the plan of development." Baer continued, "the type can be known best by its method of development.- This conveys the difference in the existing relationship of the initially agreeing germs" (Sch. V C4k, 257 - 258) .
The given confirmation must be considered as recognitiion of the initial unity and general origin of all the animal world. Here Baer returned to the idea about a single method of acquisition of independence for all animals, by formation of the vesicle-shaped stage in which the most general character is recognized. The unity of the original form, inherent to each of the four types, has as a consequence the similarity between the representatives of the different types, which are preserved throughout life. Baer specifically said that the initial similarity of the embryos of all animals does not disappear absolutely in the adult form.
In Baer's study of differences but also of similarities in the types of development, one can perceive reflections of Naturphilosophie. In distinction from the principles of German Naturphilosophie, which originated almost completely from a priori understandings, Baer's opinion depended on thoroughly checked observations; therefore they stand in close connection with the evolutionary ideas of the second half of the nineteenth century.
From the assertion that the type of animal depends on that form of development which is inherent to its type, Baer concluded that a rational classification of animals should be based on embryological data. This idea he illustrated by examples. That the insects are organized higher than the arachnids and Crustacea, and that amphibia and reptiles are considered differentiated from each other by classes can only be established by studying the history of their development.
The last, sixth scholium briefly sums up the others. In the process of multiplication, a part becomes a whole, and in the process of development the independence of the organism from its surrounding medium increases. The definition of its form also increases; internally, from the most general parts, the most specific develop and their originality becomes more distinct. All these conclusions Baer generalized in the following words: "The developmental history of individuals is the history of growing individuality in every respect" (Sch. VI, 263).
The first volume of OBER ENTWICKLUNGSGESCHICHTE established the bases of embryology. Baer's contemporaries were not, however, in a position to evaluate such an immense contribution to science. Evidence of how far Baer preceded his times is clear from the almost complete silence with which his book was received. In his autobiography, written thirty-five years later, Baer could not cite any single serious objection to his work. He wrote only that within three years (actually within eight years) a French edition of his book appeared in a translation by Breschet.14 And only after a quarter of a century did Huxley publish an English translation of the fifth scholium. 15
In a supplement to the German edition of his NACHRICHTEN, commenting on the purpose of his major work, Baer wrote:
Soon after my publishing of this work, Oken's critique appeared in his ISIS (1829, pp. 206-212) which pleased me greatly. Notwithstanding many friendly acknowledgements, it becomes very lively and pointed as soon as a statement deviates from his prevailing opinion. This in particular relates to the presentation about the development of the intestine, which in Oken's opinion is completely formed from the yolk sac and grows in the direction of the embryo. In the supplements (Baer means "the studies and corollaries." â€” L.B.) , I made it my main task to correct the currently dominant opinion that the more highly organized forms, during their formation, gradually pass the stages of the lower forms, correcting this assertion with the idea that the early stages correspond more to the intermediate forms , from which all the peculiarities of the different classes, families; genera, and species have gradually appeared. The earlier idea was especially created by Meckel and Oken. The examples and expressions selected for characterizing this opinion I took directly from Meckel without designating him by name. Oken believed himself directly insulted and courageously defended his point of view, which rested only on assertations. *â– &
14. Baer, HISTOIRE DU DEVELOPPEMENT DES ANIMAUX, Part I, trans, by G. Breschet, Paris, 1836. Complete, 1846.
15. Thomas Henry Huxley in SCIENTIFIC MEMOIRS (full citation Chapter 16, fn. 8) .
It must be thought that Baer was particularly cheered by Oken's notice.
Baer, of course, did not accidentally pause in his memories at Oken's article. When UBER ENTWICKLUNGSGESCHICHTE had just gone into print, he waited with impatience to see how his colleagues would accept his work. During the following years, apparently, he did not lose the feeling of disappointment and injury that his main life effort was not evaluated and credited to him during his lifetime. 17
16. Baer, NACHRICHTEN UBER LEBEN UND SCHRIFTEN . . ., p. 609 (447).
17. See Chapter 23.
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