Book - Russian Embryology (1750 - 1850) 15

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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).

Historic Russian Embryology TOC: 1. Beginning of Embryological Investigations Lomonosov's Epoch | 2. Preformation or New Formation? | 3. Kaspar Friedrich Wolff - Theory of Epigenesis | 4. Wolff: "Theory Of Generation" | 5. Wolff: "Formation of the Intestine" | 6. Wolff's Teratological Works | 7. Wolff: "On the Special Essential Tower" | 8. Ideology of Wolff | Chapter 9. Theory of Epigenesis End of 18th Century | 10. Embryology in the Struggle of Russian Empirical Science Against Naturphilosophie | 11. Louis Tredern - Forgotten Embryologist Beginning of 19th Century | 12. Embryonic Membranes of Mammals - Ludwig Heinrich Bojanus | 13. Embryonic Layers - Kh. I. Pander | 14. Karl Maksimovich Baer | 15. Baer's - De Ovi Mammalium Et Hominis Genesi | 16. Baer's Ober Entw I Cklungsgesch I Chte Der Thiere | 17. Baer Part 1 - Chicken Development | 18. Baer Part 2 - History of Chicken Development | 19. Baer Vol 2 | 20. Third Part of the Bird Egg and Embryo Development | 21. Third Part - Development of Reptiles, Mammals, and Animals Deprived of Amnion and Yolk Sac | 22. Fourth Part - Development of Man | 23. Baer's Teratological Works and Embryological Reports in Petersburg | Chapter 24. Baer's Theoretical Views | 25. Invertebrate Embryology - A. Grube, A. D. Nordmann, N. A. Warnek, and A. Krohn
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This historic textbook by Bliakher translated from Russian, describes historic embryology in Russia between 1750 - 1850.



Publishing House of the Academy of Science USSR

Moscow 1955

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

NAMRU-3, Cairo

Arab Republic of Egypt

Published for

The Smithsonian Institution and the National Science Foundation, Washington, D.C, by The Al Ahram Center for Scientific Translations 1982


Published for

The Smithsonian Institution and the National Science Foundation, Washington, D.C by The Al Ahram Center for Scientific Translations (1982)


Also available online Internet Archive


Historic Embryology Textbooks

Historic Disclaimer - information about historic embryology pages 
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Pages where the terms "Historic Textbook" and "Historic Embryology" 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 and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 15. Baer's Treatise De Ovi Mammalium Et Hominis Genesi

From the time of Hippocrates and Aristotle to the seventeenth century, the idea predominated that the source for development of a new individual is some "generative material" which is present in the female sexual organs. It was considered analogous to the male sperm and was frequently called the female sperm. By extension, the ovaries were named the "female testicles." Following Harvey's statement that "all that is living comes from the ovum," the idea that the source of development for all animals, including mammals, is the ovum, received a wide distribution. Johann van Horn and Nicholas Stensen claimed that the ova of mammals are those early-opened follicles of the ovary. They contended that the ova (follicles) represent generative material for future generations, an idea supported in particular by the anatomist Ruysch. In accordance with the new point of view, the female sexual gland was named the ovary.

In 1672 (Rene) de Graaf played a very important role with his detailed study of the structure of the ovary. He actually discovered the ovarian cycle, i.e. the interrelation between the follicle and the corpus luteum. De Graaf thought that the ovum exists below the membranes of the mature follicle (Graafian vesicle) ; however, the nature of the latter remained unknown to him. From his investigations, de Graaf concluded that: "All animals in general, and in the same respect man himself, get their beginning from the ovum which is present (in the female testicles) prior to copulation." This statement formed the anatomical basis of Harvey's thesis, and at the same time was in agreement with predominating earlier ideas.


1. (Ed.-. Baer, DE OVI MAMMALIUM ET HOMINIS GENESI

(Leipzig, 1827) ; translated by Charles Donald O'Malley, ON THE GENESIS OF THE OVUM OF MAMMALS AND OF MAN (Cambridge, Mass.: History of Science Society, 1956).)


Even within 150 years after de Graaf ' s work, when Baerbegan to study the initial stages of development, nothing had been added to the understanding of the nature of the ovum.

As mentioned above, Baer's report DE OVI MAMMAL IUM was published in 1827 in Latin (Figure 25) as a letter to the Petersburg Academy of Science. The next year Baer published it in a reworked form in German. 2

In the introduction to the jubilee edition in 1927 } the translator (Ed.: into Russian), B. Ottov, confirmed that "among all the results of Baer's investigations, the discovery of the mammal ium ovum occupies the most significant position." It is impossible to agree with this contention only because the classical work UBER ENTWICKLUNGSGESCHICHTE has still more significance. In the latter work, Baer actually established the basis for comparative vertebrate anatomy. Yet this assessment, of course, does not in any degree depreciate the value of the wonderful discovery of the true ovum in the mammalian ovary.

Baer gave a detailed account in his autobiography of the history of his embryonic investigations, explaining how he discovered the mammalian ovum. The main method of his work, which he frequently substantiated in his reports, was to trace the process of development in reverse chronological order; that is, to work from the already formulated condition of any system or organ back to its more primitive beginning. With his approach to the whole embryo, he could detect in the incubated bird egg the already developing rudiment, which had been named by previous embryologists the cock's trace or the cover (in recent terminology, the embryonic disk) . He admitted that "how the cock's trace or rudiment is formed did not become completely clear to me, and it is, so far as I know, still not. "3


Baer, "Commentar zu der Schrift De ovi animalium et hominis genesi," ZEITSCHRIFT FUR ORGANISCHE PHYSIK, v. 2, pp. 125-192. (85)

Baer, NACHRICHTEN, p. 413 (309) (Ed.: p. 300 in German 2nd ed.).


Trying to clarify differences in development of different animals, after detailed studies of chick embryology, Baer turned to the study of development in invertebrates (isopodous Crustacea, fresh-water and land molluscs) and also the lower vertebrates (salamanders and frogs) . Baer wrote that "the development of mammals had especially attracted me in regard to the formation of the embryo itself as well as in regard to the formation of the ovum. "4

Working first on dog embryos and concentrating on the early stages, Baer found very small semi-transparent vesicles at the end of the oviducts. Under the microscope he could detect rounded patches like that of the cock's trace, or even smaller non- transparent rounded bodies of a granular structure. "Thus I was led to find the ovum in this form as it lies in the ovary before fertilization. In order to show the importance of detecting the ovum, and in order to explain why I could not start the investigation from this end, I am allowing myself to refer to an older investigation. "^

Recalling the greatest anatomist and physiologist of the eighteenth century, Albrecht von Haller, "a man of very extensive knowledge and almost incomprehensible diligence," Baer gave him credit for the investigation of heart and vascular development and also for the study of the formation of the skeleton. Haller' s study of development of the mammalium ovum was unsuccessful, however; the investigations of Haller' s student, the Russian physician J. C. Kuhlemann, conducted on forty sheep, were also without results. 6 From these unsuccessful investigations Haller concluded that "the formation of the embryo takes place by coagulation of the fluid similar to crystallization." This point of view persisted because of Haller's almost unquestioned authority.

In the 1820s, some investigators supported the previously dominant idea that the origins of the primary ova are the Graafian vesicles, which are entirely separated from the


4. Ibid. , p. 421 (313) (306) .


5. Ibid . , p. 422 (314) (307-308).

6. Ibid . (308). Johann Christian Kuhlemann, DISSERTATIO INAUGURALIS ANAT . -PHYSIOLOGICA EXHIBENS OBSERVATIONES QUASDEM CIRCA NEGOTIUM GENERATIONS IN 0VIBUS FACTAS (GSttingen, 1753), 60 p.


ovary and received in the oviducts. (86) Other observers considered this incorrect but could not find an accurate solution. Also, in 1797 (William) Cruikshank (87)7 reported that he saw the ovum in the oviduct of a rabbit within three days after mating. There, the ova were much smaller than the Graafian vesicles. At first, he was not believed, and Prevost and Dumas** (88) in 1824 failed to confirm his observations on dogs and rabbits. Baer highly regarded the work of the French embryologists," but he contended that they were not concerned with the ova but with the early embryos.

Already in 1826, Baer had seen many times small transparent ova (1 - 3 mm in diameter) in the horns of the uterus and in the oviducts; in spring 1829, he observed significantly smaller and less transparent ova. He did not doubt that these bodies actually were ova, because he assumed that the yolk of the mammalian egg is non- transparent as it is in birds. Thus, Baer described how he accomplished this last step:


William Cruikshank, "Experiments in which, on the third day after impregnation, the ova of rabbits were found in the fallopian tubes, and on the fourth day after impregnation in the uterus itself? with the first appearance of the foetus," PHILOSOPHICAL TRANSACTIONS of the Royal Society of London, 87 (1797), pp. 129 - 137. J. L. Prevost et J. A. Dumas, " Nouvelle theorie de la generation . " Deuxieme memoire , "Rapports de 1 ' oeuf avec la liqueur fecondante. Phenomenes appreciables , resultant de leur action mutuelle. Developpement de l'oeuf des batraciens," ANN. SCI. NAT., 2 (1824), pp. 100 - 121, 129 - 149. Troisieme memoire, "De la generation dans les mammiferes et des premiers indices du developpement de l'embryon," ibid . , 3, pp. 113 - 138.

However, in the introduction to the second volume of UBER ENTWICKLUNGSGESCHICHTE, Baer talked about the mistakes of Prevost and Dumas in a fairly ironical tone.



Figure 25. Title page of Baer's treatise "De ovi."


In April or the first days of May, I discussed with Burdach that I could no longer doubt the origin of the ova from the ovary and that I very much wanted to obtain a bitch mated a couple of days earlier for the investigation .... Burdach had such a bitch in his house and sacrificed it. Opening it, I found some burst Graafian follicles and others that were close to bursting. On examining the ovary, I noticed in one follicle a yellow spot, and then saw the same in many others, but always only one spot. It was wonderful I I thought, what could this be? I opened a follicle and carefully carried the spot by a knife to a watch glass filled with water, then put it under the microscope. Looking into it, I jumped; how surprising the discovery was, because evidently I had seen a very small, distinctly formed, yellow yolk ball. I had to take a breath before being able to look again into the microscope, because I was afraid that my vision may have deceived me .... I saw in front of me a sharply outlined, regular small ball within a thick membrane, which was distinguished from the bird's yolk only by this rough shape, somewhat falling behind the external membrane. And thus the primary ovum of the dog was found! It does not float in an undefined position in the fairly viscous fluid of the Graafian vesicle, but is attached to its wall and held to it by a wide row of very large cells. 10

With his report of this discovery, Baer wrote to the members of the Petersburg Academy of Science to express his happiness that he could publish his discovery under the auspices of the Petersburg Academy.

It is for whoever does not know how far Your Academy exceeds all others in its services to the study of the secrets of nature related to the formation of new organic bodies. The investigators who established the first sound basis for the history of development of animals were members of Your Academy. First was Caspar Friedrich Wolff, the father of eternal glory, of whom in all the world I have seen very few so clever, nor have I seen his equal in decisiveness in investigating the most delicate things. I cannot pronounce his name without such reverence as we feel when we talk about the idea of the origin of good. Next, Christian Pander: it has always been my pride that I could give, though insignificant, a push to his wonderfully illuminating investigations on chick development. (p. 1).


10. Baer, NACHRICHTEN, pp. 427-428 (311-312).


In § 1, entitled "The Origins of the Canine Fetus," Baer gave a description of a three-week-old dog embryo, accompanied by life-size illustrations (Figure 26, 7), in lOx magnification from the side (Figure 26, VII a) , and in transverse section (Figure 26, VII b) . The description is accompanied by a comparison with the analogous stages of bird, reptilian, and amphibian development. Baer also referred to Wolff's treatise on the development of the intestine, to the German text of Pander's dissertation, and to the work of Prevost and Dumas.

In § 2, entitled "Primary Development of the Canine Ovum," he described still earlier ova extracted from the uterus.

Section 3 he entitled "Ovules in the Ovary of Dogs." It must be noted that Baer called the early rudiment of the developing embryo by the term ovum, still having a follicle form. The previous section was particularly concerned with those "ova." That which in modern literature is frequently called the ovum-cell Baer designated by the term small ovum, or ovule. First, he expressed his belief that the very ovules which he observed in the oviducts and uterus could not be made up from the ovary by the Graafian vesicles. He considered it unlikely that very compact bodies develop in the tubes as a result of coagulation of the fluid which comes out of the Graafian vesicles.

By examining the ovary with the naked eye, Baer saw in the intact Graafian vesicles a pale yellowish spot which, it seemed to him, swam freely in the fluid of the follicle and could be disturbed by pressing with a probe. His experiences are associated with the detection of the ovule, which he explored by looking at it under the microscope, and Baer described it alive subsequently in his autobiography. Here, he only talked about how he was surprised at seeing the ovum in the ovary, which he already had observed in the ducts "so clearly that it can be seen by a blind man" (p. 12). The ovule had a diameter of one-thirtieth to one-twentieth of a Parisian line. On the surface of the ovule, there is a ring-shaped plate, which Baer called the discus pvoligevus.

In § 4, "How the Graafian Vesicles are Constructed and General Considerations on the Mammalian Ovule," Baer stated that, besides the ovaries of dogs, he had investigated the ovaries of cows, pigs, sheep, rabbits, hedgehogs, dolphins, and man. He was convinced that in all the mammals studied > the Graafian vesicles have an identical structure. Each Graafian vesicle has two parts — an enveloping part, or the shell (putamen) , and the part included inside its nucleus. The first is composed of membranes, not related to the Graafian vesicle but to the ovary, and only a little raised by the follicle.

The membrane (indusium) covers only part of the follicle; it is composed of peritoneal epithelium (Figure 26, IX, 1) and of what is called cellular tissue, termed by some authors albuginea. This theca consists of two layers, an external and an internal layer. The external is thin and transparent, composed of compact cellular tissue. It receives the blood vessels and sends their terminal branches into the next layer. The internal layer is thicker, softer and darker; its internal surface is covered with delicate fibers. The external surface is firmly connected with the external layer. In the place of the future rupture, a thinning of the theca occurs as a transparent spot.

Related to the nucleus are the following: granular membrane, follicle fluid, embryonic disk, and the ovule itself. The granular membrane is composed of a thin layer of granules and includes the fluid content of the Graafian vesicles. The humor consists of fluid and granules swimming in it. In natural follicles, the fluid is yellow-colored, perhaps because of the yellow color of the granules. Boiling the fluid and the effect of alcohol cause the fluid to coagulate, which indicates its protein nature. The embryonic disk and the cumulus are in the fluid or on its surface. They consist of white granules, distinguished by this color from the yellow granules of the fluid. With the maturation of the follicle, the embryonic disk gets closer and closer to the periphery. Finally, in the cumulus and in the embryonic disk it is sometimes possible to see the ovule itself. In all animals the round formed mass of the ovule has a dark center, and the transparent periphery is surrounded by a membrane (membrane corticalis) . The size of the ovule in different mammals is different: the largest are in cows, sheep and swine; they are smaller in the rabbit and dogs; the smallest are in humans.

It is difficult not to be amazed by the accuracy of Baer's microscopical studies, which he accomplished not by the modern use of paraffin sections and selective stains, but by means of preparations with needles and examinations under the microscope. Comparison of Baer's data with modern ideas about the structure of the Graafian follicle leads to the conclusion that he succeeded in seeing all the constituent parts of the mature follicle. As a matter of fact, the terminology has changed only insignificantly. Baer's cellular membrane (indusium) is now called the external theca; the internal layer of Baer's theca, which contains the vessels, is called the internal or vascular theca. The transparent layer of Baer's theca presently is called the hyaline membrane. Baer claimed that the cumulus swims on the surface of the follicular fluid, while now it is known that it represents one unit with the granular membrane forming its thickening. The membrane, directly covering the ovum itself, which Baer called the cortical membrane, is now named the transparent zone.

Section 5 is called "A Brief Review of the Development of Mammals," since Baer considered it important to compare the development of mammals with the development of other animals. Because the ovule is invisible in the early stages of formation of the Graafian vesicle, and since investigations during that period are very difficult, Baer was limited to a hypothetical conclusion that the ovule exists in the Graafian vesicle before it is possible to detect it. He assumed that the ovule of mammals could be comparable with the Purkinje vesicles of other animals, such as molluscs and worms, in which features preceding the development of the ovum are observable.


Fig. 26. Illustrations from Baer's article "About the formation of the ovum" .


Key:

1* - ovum from the ovary of bitch with embryonic disc

(enlarged in 30 times) :

6- 12th-day ovum of bitch with embryo (natural size) (from Prevo and Dum) :

VI - enlarged embryo of the same ovum;

7- nearly three weeks ovum of bitch (natural size) ;

VII a- embryo of the bitch; located on left side, after removal of right half of cephalic vagina and right half of intestinal sac (zaccus intestinalis) : ab- passage of mucous plate of the intestinal sac in the cephalic vagina; acde- cut cephalic vagina; ef- right descending vein, recurved backwards; f"- its original position; e g- auricle of the heart;

h- ventricle; ii- pericardium; k- bulb of the aorta; 1- aorta; mno- brain; m- ear and medulla oblongata; ep- commissure or groove of middle intestine; arep- left side of the intestine; step- its right side; gr- angle, in which the left wall of the intestine passes to the intestinal sac; uvw- vascular area; grw- transparent area; us- ascending vein; xy**- amnion; 2- urinary sac or allantois:

VII b- transverse section of the same embryo in the middle of the back: -a- upper part of the back; ab- spinal (dorsal) plate; be- ventral plate; d- intestinal sac; e- passage of the wall of intestine in the wall of intestinal sac; de- limb, according to Wolff; f- intestinal commissure, by Wolff; g- laying of spinal column; h- notochord; i- amnion; k- spinal chord;

IX- middle sizes of swine Graafian follicles (enlarged 10 times) : 1- peritoneal epithelium; 2- formative cover of the theca; 3- external layer of the theca; 4- internal layer of the theca; x- stigma; 5- granular membrane of the nucleus; 6- fluid of the nucleus; 7- embryonic disc; 8- ovary:


In lettering on table of Baer, there is an explanation of some parts of his illustrations only.



X- Embryonic disc of the swine; XII- ovary of cow with embryonic disc magnified by 10 times) ;

XIII- ovary of woman with embryonic disc and granular membrane (magnified by 10 times) ;

XIV- corpus luteum of bitch (magnified by 10 times) : 1- peritoneal epithelium; 2- formative cover ; 3- external layer of the theca; 4- corpus luteum; 5- its opening;

6- albuminous mass; 7- scar of corpus luteum;

XV- stigma of Graafian follicle (magnified by 10 times) ;

XVI- embryonic layer from mature ovum of grass-snake (magnified by 5 times) ;

XXIV- nearly mature ovum of Eana tempovavia , sealed in diluted nitric acid and cut by longitudinal axis (magnified by 10 times) ;

XXV- mature ovum of Rana temporaria with moved embryonic vesicle (magnified by 10 times) ;

XXVI- the same ovum with driven out embryonic vesicle (magnified by 10 times) .


At a later stage of development of the Graafian vesicle, its fluid content corresponds to the compact part of the protein in the chicken egg. Then polar differences in the fluid appear: on one side it is more transparent, and on the other there appears an aggregation of granular material. Thus, the cumulus is formed with the embryonic disk, and the ovule situated there becomes increasingly separated by the forming membrane. Next, the central area appears in the ovule since its granules become attached to the periphery. In this, according to Baer, the general principle of development in the centrifugal direction becomes apparent.

On the question about the origin of the corpus luteum, Baer disputed the current opinion that this was a new formation, unrelated to the Graafian vesicle. Instead, he tried to establish an indication that the corpus luteum resulted from growth of the internal layer of the theca in the empty Graafian vesicle. This completely corresponds with modern ideas, but now it is known that the hypertrophy of the corpus luteum walls takes place at the expense of the follicular epithelium, and not at the expense of the internal theca as Baer thought. The elements of the internal theca are only preserved in the radial connective tissue partitions, which correspond to the folds of the rupturing vesicle.

The ovule separated or released from the Graffian vesicle falls into the funnel in the fallopian tube and then into the uterus, where it is covered with fibers. The development of the embryo in the mammalian ovum, according to Baer, "was in the same manner as in birds." First of all, he claimed, the spina dorsalis appears. H From it grow the spinal plates (laminae dorsalis) , which correspond to the primary or primitive folds (plicae primitivae) of Pander, and soon the ventral or abdominal plates (laminae ventrales) . All vertebrates undergo one type of development, namely growing centrifugally; hence the two plates are directed upwards and form a plane for the spinal and head brain, and two plates are directed downwards. A plane for the internal parts and vascular stems forms there.


11. By this term Baer designated that formation to which, in UBER ENTWICKLUNGSGESCHICHTE, he gave the name accepted in modern embryology, primary region.


Baer followed these phenomena also in frogs, snakes, lizards, and birds. Although in fish he could not see the early stages, he nevertheless claimed that they do not constitute an exception. With Burdach, Baer observed development extending from the abdominal side to the spinal side in fish. These data Baer did not publish at the time, but in 1824, under the heading, "What Did the Notebooks of My Listeners Indicate?" (p. 24).

The last paragraph (§ 6) is entitled "Comparison of the Mammalian Ovule with the Ova of Other Animals." Regardless of the differences seen in the ova of different animals, there may also be significant similarities, according to Baer. It is common, for example, for the the ova of all animals to have a nucleus, or, as Baer called it at that time, the Purkinje vesicle, named for the famous Czech physiologist who discovered this formation. Purkinje had described this formation in his HISTORY OF THE BIRD'S EGG BEFORE INCUBATION, which he dedicated to J. F. Blumenbach in connection with his fiftieth anniversary of scientific activity (89) .

Baer indicated that in all the ova investigated, the Purkinje vesicle persists until the end of ovum development. In mammals, the vesicle is included in the cumulus.. In the deposited ova and in the ova located in the oviduct, he never observed the follicle. According to Baer's observations, the follicle disappears earlier. For example, in insects it disappears while it is still in the ovary. Concerning the significance of the Purkinje vesicle, Baer suggested that "the Purkinje vesicle is the effective part of the ovum, by which the female faculty exerts its power, as the male faculty resides in the male sperm." Concerning the disappearance of the Purkinje follicle, which Baer designated as "throwing out" and "dissolving," it is "dependent upon the maturity of the ovum and upon the irritation .... After fertilization the blastoderm grows in the same place where the fluid content of the follicle was poured off" (p. 29).

Baer's words appear striking, especially his anticipation of the discovery of division of maturation and the theory of polarity of the mature ova. Actually, the expression "throwing out" of the Purkinje vesicle successfully describes the theory of separation of the polar bodies which actually acts as if they are thrown out of the ovum. The expression "dissolving" of the vesicle indicates the disappearance of the nuclear membrane in the nucleus of the ovum in the metaphase of the division of maturation.

Baer's contention that the blastoderm develops in the place of the release of the Purkinje follicle is also quite correct. In the overwhelming majority of cases, the place of separation of the polar bodies is close to the animal pole of the ovum, i.e. to the same place where, in particular in the mesoblastic ova, the first blastoderms are settled.

Less accurate area Baer's discussions concerning the nature of the Graafian follicle. He called it an ovum and equated it to that follicle-shaped stage which is already observed in the fallopian tube or uterus, identifying the ovule as the Purkinje vesicle. However, on describing the ovule, Baer mentioned in one place (p. 18) the darker central and transparent peripheral parts of the ovule; in another place (p. 19), he said that a central area appears in the ovule which is free from granules, which remain at the periphery. From these descriptions it is clear that Baer had seen the nucleus in the mammalian ovum but was not sure if this formation were identical to the Purkinje vesicle of other animals. *â– *


12. The modern terminology was introduced by.,P. G. Svetlov in a commentary on the second volume of UBER ENTWICKLUNGSGESCHICHTE (note 139, pp. 476 - 480).



Historic Russian Embryology TOC: 1. Beginning of Embryological Investigations Lomonosov's Epoch | 2. Preformation or New Formation? | 3. Kaspar Friedrich Wolff - Theory of Epigenesis | 4. Wolff: "Theory Of Generation" | 5. Wolff: "Formation of the Intestine" | 6. Wolff's Teratological Works | 7. Wolff: "On the Special Essential Tower" | 8. Ideology of Wolff | Chapter 9. Theory of Epigenesis End of 18th Century | 10. Embryology in the Struggle of Russian Empirical Science Against Naturphilosophie | 11. Louis Tredern - Forgotten Embryologist Beginning of 19th Century | 12. Embryonic Membranes of Mammals - Ludwig Heinrich Bojanus | 13. Embryonic Layers - Kh. I. Pander | 14. Karl Maksimovich Baer | 15. Baer's - De Ovi Mammalium Et Hominis Genesi | 16. Baer's Ober Entw I Cklungsgesch I Chte Der Thiere | 17. Baer Part 1 - Chicken Development | 18. Baer Part 2 - History of Chicken Development | 19. Baer Vol 2 | 20. Third Part of the Bird Egg and Embryo Development | 21. Third Part - Development of Reptiles, Mammals, and Animals Deprived of Amnion and Yolk Sac | 22. Fourth Part - Development of Man | 23. Baer's Teratological Works and Embryological Reports in Petersburg | Chapter 24. Baer's Theoretical Views | 25. Invertebrate Embryology - A. Grube, A. D. Nordmann, N. A. Warnek, and A. Krohn

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