Book - Russian Embryology (1750 - 1850) 23
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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 23. Baer's Teratological Works and his Embryological Reports, Related to the Period of his Work in Petersburg
Baer was not a pioneer of teratological investigations in Russia. By the middle of the eighteenth century the Anatomical Museum of the Academy of Science had available a significant collection of preparations demonstrating the abnormal development of man and different animals. Before the arrival of K. F. Wolff in Russia, the materials of these collections had been little investigated, excepting the more or less accidental descriptions performed by A. Kau-Burgav. Wolff very energetically undertook the investigation and description of the abnormalities in the Academy's collection, connecting to them the material personally collected by him. However, because he was involved with vast anatomical investigations, he published only some of his observations e A great part of them were to have been included in the great work which was left unfinished due to his unexpected death, and which consequently did not appear. At the beginning of the nineteenth century the teratological collections were put in the reliable hands of Academician P. A. Zagorsky, organized, and again studied intensively. Zagorsky had time to investigate and describe only a small number of interesting teratological cases.
Eventually Baer settled in Petersburg. Despite his varied activities connected with investigations in different spheres, and with administrative, educational, and experimental work, he found time for teratological investigations. At this time he was not a beginner in this sphere. He was still a professor in Konigsberg when he published some small articles, from which information about deformed swine embryos has been mentioned.
In 1827 Baer published an article on a double chicken embryo at the beginning of the third day of incubation. 1 The purpose of this work was to solve the disputed question of the origin of double monsters. "I never could bring myself to accept the idea," Baer wrote, "that double monsters arise from the union of two individuals, although previously many physiologists insisted on this opinion. It seems to me that here one difficulty has replaced another" (p. 576) . The arguments proposed by Baer against the theory of union were very close to those used by Wolff in his "Description of Two-Headed Calf, accompanied by general illustrations on the origin of monsters." 2 In order for two embryos to unite in an earlier stage of development, it would be necessary for them to be completely movable and for there to be pressure upon them from two sides, pressing them together. But the main difficulty lay in the fact that it is impossible to imagine how the corresponding parts of the two individuals meet and why the union takes place without disturbing the topography of the parts. More naturally, Baer supposed this case to represent the splitting of a formerly unified rudiment. This explanation did not eliminate all the difficulties; it was suitable for the interpretation of lateral splitting, but left unexplained the cases of divergence of surfaces and union of twins by the ventral sides.
Baer thought that the solution to this interesting question could be achieved only through the study of the early stages of development of double monsters. He described one of these cases, in a developing chick at the beginning of the third day of incubation. The transparent zone CAREA PELLUDICA) had a cruciform shape with two long and two short ends; the embryos were situated in the long ends, the posterior ends of which moved apart and the anterior united in a general mass. Both the embryos were equally developed. The spinal plates, containing the foundation of the vertebrae (Pander's PLICAE PRIMITIVAE) were closed, enveloping the spinal cord; the abdominal plates (Wolff's LAMINAE ABDOMINALES) were situated nearly horizontally, i.e. the body of the embryo was still opened from below. The spinal plates passed without discontinuity into the common head, and the spinal cord into the common brain.
1. Baer, "Uber einen Doppel-Embryo vom Huhne aus dem Anfange des dritten Tages der Bebrutung," ARCH. ANAT . PHYSIOL. (1827) , pp. 576-586.
2. K. F. Wolff, "Descriptio vituli bicipitis . . . ," NOVI COMMENT. ACAD. SCIENT., PETRPOL.,32 (1773),
pp. 504-573 (see Chapter 5) . Baer did not refer to this work by Wolff. Baer's exceptional honesty, which appears in all his works, suggests that Wolff's paper on the double-headed calf was for some reason unknown to Baer.
Baer thought it improbable that these partners were at first divided, and then united. In the period up to the third day of incubation, i.e. when the beginning of the process of union could be expected, the embryos cannot move nearer to each other because they are still situated in the convexity of the blastoderm. If it is assumed that the first foundation of the double embryo was single, that the foundation of the head was formed not at its end but in its center, then the formation of the double embryo becomes understandable. The dorsal region of the cephalic end of the embryo grows more quickly than its other parts; therefore in the single embryo the head is curved downwards, and in the double embryo the frontal parts moved away from each other. The cruciform embryonic structures meant that the basis of double-embryo formation occurred in the ovum very early.
To the question of the origin of double monsters Baer returned in two works published in Petersburg ten years later, The first article^ was very brief and was, in fact, a preliminary to the second, which was a vast work. 4
3. Baer, "Uber doppelleibige Missgeburten," MEM. ACAD. SCIENCES ST.-PETERSB. , VI Ser., v. 3, book 2BULL. SCIENT.,No. 2 (1835), pp. I-II.
4. Baer, "Uber doppelleibige Missgeburten oder organische Verdoppelungen in Wirvelthiern," MEM. ACAD. SCIENCES ST. PETERSB.,TV Ser. Sc. nat., v. 4, Zool. et physiol. (1845) , pp. 79-194.
Baer first referred to his findings in 1827 on the double chicken embryo, where only its description had been carried out. He again raised the question of how these monsters were formed, disagreement about which had long existed in the literature on this subject. Haller and Meckel in their own day stated their opinion that double monsters develop by way of union. Burdach later subscribed to this interpretation. Isidore Geoffroy Saint-Hilaire was a more active supporter of the theory of union. Baer decisively raised an objection to this point of view. So he was astonished to find that Barkov, in his thorough work, "Double Deformities among Animals," claimed that Baer's description of the embryo "proved directly the union of separate parts of the accreted embryo." How could it happen, Baer wrote, that, "considering myself a decisive opponent of accretion, I have apparently recognized the accretion of parts of different embryos?" (p. 81) . From the moment of the appearance of Barkov' s work, Baer collected additional data on double monsters, on the basis of which he reconfirmed the accuracy of his previous opinion.
Speaking of investigations of monsters generally, Baer stated that their development must be studied as exactly as the course of normal development. The achievements of embryology, making the controversy of spermatists and ovists meaningless, was a consequence of that. From elucidation of the primary conditions of the processes of reproduction and development we arrived at the fundamental study of the most formative processes.
Before turning to the description of many cases of double monsters, Baer outlined his order of presentation. In the beginning he investigated what was formed; later, by comparison with other cases and with normal development, he elucidated the question of how these monsters could be formed. Concerning why these double monsters originate, only suppositions are stated.
DOUBLE EMBRYOS OF FISH IN VERY EARLY STAGES OF DEVELPMENT. Baer began with the description of two double monsters in perch which he found in 1835 in some eggs obtained from the Neva two days before the investigation. One embryo was double-heated. In the other the division was found in the trunk region. The division of the cephalic end of the first embryo was incomplete, since there were two mouths and four eyes but only two ears (Figure 34, A) . In the second embryo the posterior part of the body and tail was single and all the anteriorly situated parts were double. Both these cases, in Baer's opinion, totally excluded the possibility of regarding them as a result of the union of two individuals: from the moment of the appearance of the embryo not more than one day elapsed; thus during this short period the remaining undoubled parts could not have disappeared. In addition, the embryos were situated close to the yolk. They could not have been any closer in order to unite With each other. In an embryo with a partially doubled head, it can be assumed that the splitting occurred in a very late stage. In the second embryo the bodies were situated at an angle of 120Â°, indicating that the first foundation of the embryo, namely the primary streak, was divided from the front. The unusual width of the ovum supported this observation. A double embryo appeared after the division of the primary single rudiment. In connection with the description of these cases of double monsters in perch, Baer referred to examples of double embryos in fish described in the literature, beginning with Aldrovandi's MONSTRUORUM HISTORIA (1642) and ending with Heusner's "Double Monsters in Perch and Other Fishes". ^
DOUBLE CHICK EMBRYO AT THE BEGINNING OF THE THIRD DAY. In this section Baer gave a more detailed description of a divided chick embryo, the investigation of which was illustrated in 1827 (Figure 34, B) . Here Baer turned his attention to the reciprocal situation of the heads of the double embryo, excluding, in his view, the possibility of the union of two formerly divided embryos. He considered especially conclusive the reasons relating to the situation of dorsal and mainly ventral plates of both partners. "The ventral plate of one of them passed into the convexity of the blastoderm without interruption and without evidence of union in the ventral plate of the other. More exactly, one and the same continuous surface formed the future left ventral wall of one, and the future right ventral wall of the other partner .... If union had occurred during the first day of incubation, then the ventral walls could not unite, since they did not yet exist; hence they were isolated from the blastoderm only in the second day" (p. 110).
5. Heusner, "Descriptio monstrorum avium, araphibiorum, iscium." Diss, inaug. Berol. (1824) (cited by Baer) .
Later Baer described some cases of monsters in man. First he described twins which were joined at the forehead. In his opinion, this case was similar to the described in 1501 by Sebastian Minister. Baer cited an early description and illustration of a human double monster, according to data provided by Schmidt, published in the journal ISIS in 1825. He followed with general descriptions and drawings of "parasitic formations," as Burdach called them â€” i.e. additional parts attached to completely developed individuals, which were without an umbilicus and were nourished through its vessels. Baer referred to a similar case which he had observed in Konigsberg. The case involved a living baby whose incomplete individual hands and legs were attached to its chest but who lacked a head and a large part of the trunk .
Referring to analogous examples from other authors, Baer mentioned a case been earlier studied by Zagorsky.6 He noted that that famous Russian anatomists' s description, with the exception of some details, was completely correct. In this parasitic formation, as in other analogous specimens from the Academy's collection, the supply of blood occurred from arteria mammaria of the original individual. In addition to these cases,' Baer saw examples of formations of additional extremities, always, however, connected with the other additional organs. This category, called "parasitic formations," also included those cases where two additional legs are present with an atrophied foetus, attached toÂ»the pubic region and situated in the normal legs (Figure 34, D) . The genital organs and the internal organs of the pelvic cavity were also doubled. In a similar monster there was an additional leg without a mate; in the groin region of the extra leg were discovered an additional ovary and three elevations which undoubtedly were nipples. In this connection Baer thought the common phenomenon of supernumerary nipples in humans as being possibly inherited. He cited a case, described by Rober, in which a woman who had additional mammae on the right side of the thoracic cavity gave birth to a daughter with an additional mammae on the thigh. These additional mammae subsequently secreted so much milk that the woman could nurse, in addition to her own children, three additional babies for six years.
6. P. Zagorsky, "Foetus humani monstrosi, alii bene formato foetui adnati descriptio," MEM. ACAD. SCIENCES ST. PETERSB., VI sÂ£r. Sc. math, et phys., v. 2, book 2 (1833) , pp. 187-194.
Besides the cases mentioned of parasitic formations in human beings, Baer described an analogous case in a live adult cow? (Figure 34, C) . In the middle of its neck there was an appendage resembling in form the scaled-down underdeveloped posterior parts with two deformed legs and a tail. The immediate investigation showed, however, that the additional legs were not posterior but anterior, and the tail-shaped structure contained a rudiment of skeletal parts of the branchial girdle. Baer investigated in detail the nerves of the parasitic formation from the cervical part of the spinal cord.
This thorough teratological work of Baer's remained incomplete information intended for the second part was never published. Probably Baer intended to include at the end of the work some theoretical consideration of the origin of double monsters. However, that which did appear in his published work is enough to suggest his point of view on this subject. As previously observed, Baer decisively rejected the idea that double monsters result from the union of formerly separated individuals. He concluded that double monsters can be formed only from the fission of the formerly single embryo. Baer supposed that the separation would be more pronounced the earlier the fission took place. The cases of "parasitic formation" Baer was inclined to interpret, apparently, from this point of view. It must be noted that Baer's principle of explanation of double monsters was subsequently completely confirmed, especially when it was found that this kind of formation could be obtained experimentally by pretwisting the ova, turning the vegetative pole upwards, effecting temperature changes, and so on.
7. Information about this case was obtained by Velyaminov from the Caucasus, which Baer published earlier under the title, "Bericht uber eine ausgewachsene Missgeburt," BULL. SCIENT., ACAD. IMP. SCIENCES DE ST. PETERSB (1836), No. 1, p. 128.
In subsequent years Baer from time to time returned to teratological questions, describing interesting cases he discovered. The following cases present information of this nature :
1. "A new case of twins joined at the forehead, and comparison of it with analogous cases. "8 This concerned accreted twins, aged about eight months of uterine life, prepared for the Academy's collection. Both the foeti are female, well developed, joined to each other obliquely. Baer noted that similar kinds of union of accreted twins are seen more frequently in birds than in mammals and cited corresponding references.
2. "Notice sur un monstre double vivant "9 This case is that of a monster which attracted Baer's attention because the accreted double was delivered at full term and was born alive. The position of the twins was completely symmetrical, in contrast to the previous case, where they were at angles to each other ,
3. "Remarks on blind fish, an example of delay of development. "10 In these remarks Baer described a carp [Cyprinus gibelio) caught in a muddy pond in Kolomyaga. The eyes of the fish were rudimentary and sealed under the skin. In connection with this Baer posted the question, can darkness be the cause of underdevelopment of the eyes?
8. Baer, "Neuer Fall von Zwillingen, die an Stirnen verwachsen sind, mit ahnlichen Formen vergliechen. Mit einer Tafel," BULL. PHYS.-MATH. ACAD. SC. ST. PETERSB. (1845),
3, No. 8, pp. 113-128.
9. Baer, "Notice sur un monstre double vivant, compose de deux enfants f eminins , " IBID. (1856), 14, No. 3, pp. 34-37.
10. Baer. "Ein Wort liber einen blinden Fisch als Bildungs Hemmung," BULL. ACAD. SCIENCES (1862), 4, pp. 215-220.
Figure 34. Illustration from Baer, "On Double Monsters."
A â€” double-headed perch embryo; a â€” outer or upper plate of the rudiment corresponding to the abdominal wall of the embryo j b â€” Internal or ventral plate of the rudiment or allantoic sac. B â€” double-bodied embryo of a chicken in the embryonic sphere, at the beginning of the third day; ABCD â€” transparent embryonic sphere; E â€” the common head; F â€” approximate outlines of the heart; a â€” thickened place on the head between the skull and thickened edge of the abdominal plate . C â€” adult cow with additional growth in the left side of the neck. D â€” newborn baby with additional legs in the pubic region.
After Baer arrived in Petersburg he ceased his systematic study of embryology. This change in his scientific interests foreign biographers explained as due to difficulties in getting embryological material. Baer himself in his autobiography supported this version of the story of how he could not organize a supply of frogs, fish eggs, and mammalian embryos. Even N. A. Kholodknovsky, the author of an excellent biography of Baer, in spite of Academician V. F. Ovsyannikov's contrasting opinion, 11 also cited the same strange explanation of the striking fact that this great embryologist, who was making more scientific progress than all his predecessors, suddenly left this sphere of science in which he had enthusiastically worked for more than fifteen years.
The true cause of why Baer stopped his study of embryology appears in extracts of his correspondence reported by B. E. Raikov in the commentary to his translation of Baer's autobiography. 12 Raikov cited a long portion of Baer's letter of December 30, 1845 to T. L. Bischoff (published in ALLGEMEINE ZEITUNG, 1880, No. 325, appendix). In this letter Baer explained his departure from embryology as due to the lack of consideration and the unfair attacks with which his remarkable discoveries were met in Prussia. The Minister of Education, Altenstein, after the publication of DE OVI MAMMALIUM ET HOMINIS GENESI, declared that "the existence of the mammalian ovum had been known for a long time," and Plagge attempted to claim for himself the fame of the discovery of the ovum in the ovary. Other German biologists showed extreme indifference to his work, which offended Baer. Not finding sympathy in their midst and not receiving from the Prussian Ministry of Education the necessary material for continuation of embryological work, Baer, in his words, "decided to tear radically from his heart all scientific ambition." Baer spoke further about his promise to himself to cease his work on embryology and not even to read any embryological literature for nine years. This decision was taken undoubtedly because of nervous exhaustion caused by severe overwork and annoyance, and Baer himself certainly regretted it. "Generally I cannot decide," Baer wrote in the same letter, "whether I behaved correctly by making this vow, but I can confirm that I have injured myself deeply, perhaps even too deeply. Later it seemed to me that I had lost the best blood of my heart."
11. F. V. Ovsyannikov, "Brief biography of Baer read in December's general meeting of the Society of Naturalists," TR. SPB OBSHCH. ESTESTV. (1877), 8, pp. 97-107.
12. The materials stated below are cited with commentary 8 in Chapter 15 (pp. 519-523) .
Discussing the contents of this truly tragic letter, B. E. Raikov correctly concluded that "there is no basis for placing responsibility for the change in Baer's scientific studies upon conditions in Russia. Not the Russian, but the Prussian conditions of his life created in him this attitude which led him to a decision so unfortunate for science."
However, in the 1840s Baer's interest in embryology reawakened, undoubtedly in connection with his professorial activity in the Petersburg Medical -Surgical Academy, where he taught comparative anatomy and physiology, in which course embryology figured significantly (114) . In connection with teaching the science so near to his interest, Baer intended in 1843 to publish a Latin "essay on the history of development of men and other animals" (CONSPECTUS HISTORIAE EVOLUTIONIS HOMINIS ET RELIQUORUM ANIMALIUM) in twelve to fifteen papers with two tables of illustrations. In 1844 invited the conference of the Academy of Science to publish an atlas in Russian on this topic comprising forty papers and 150 drawings. A report by Baer on the plan of this publication was fully published in E. N. Pavlovskii's K. M. BAER AND THE MEDICAL -SURGICAL ACADEMY. The following parts of this document are interesting:
New investigations in 'developmental history' which have already been published are still little known in Russia .... Consequently it is reasonable now to wish for a textbook on one branch of physiology developing in recent times. This branch is riot only important for physicians, and for naturalists generally, but it also illuminates other parts of physiology and anatomy. Once I dared to hope that I possessed sufficient knowledge in this subject ....
The first priority is to become acquainted with all current results in addition to the necessary details of the experiments on which they were based ....
Further, it seems important also that the application of an explanation to congenital deformities must be not only mentioned, but also investigated in order that all educated physicians can learn how to recognize and describe similar cases for the further benefit of science. The main task must be, without any doubt, the history of the formation of the human embryo, including the deformities as well. But the first stages of this formation in man are still little noted, and that includes embryos in diseased conditions (to which all abortions are related) , without which we could not know the complete history, origin, or formation of any part if they were observed in mammals. The history of formation of mammals also cannot be understood at this stage if we do not have cases of observations on the formation of chicks. On this basis I find it necessary, before treating the history of human formation, to discuss as a preliminary the observations on other mammals and on the chicken in order to understand how the results are obtained and why and to what extent analogies can be drawn from them. It is necessary to discuss briefly the animals of other classes in relation to the history of formation, and especially to mention some general results ....
These extracts from the draft of Baer's work on the history of development of man and vertebral animals show what great educational significance Baer accorded to the field of embryology in the training of future physicians. Here also was briefly pointed out the fruitfulness of the comparative method in embryology, and also the importance of the detailed study of monstrosities, a description of which is necessary "for the future benefit of science."
The publication of Baer's projected textbook for some reason did not take place. The possibility cannot be excluded that Baer himself rejected this idea. Feeling the difficulty of the great work related to a sphere of science to which he was so close in the past, he was obliged to abandon it. Actually, it was not easy after a ten-year interval, during which Baer did not study embryology and did not follow the literature of this topic, to coordinate all that had been done by a complete galaxy of investigators â€” Rathke, Reichert, Kolliker, Bischoff, and others â€” who had followed in his footsteps, adding many new facts and raising new theoretical questions. It was also necessary to take a definite position coneming the cellular theory, which in the 1840s had spread its influence to embryology. In addition to this, and even in later years when Baer wrote his autobiography (i.e. in the 1860s), he did not consider it possible to speak decisively concerning this theory. Recognizing the importance of that wide biological generalization which the cellular theory considered, Baer could not agree with the discrete presentation of the organism which was proposed by some supporters of the cellular theory in the first period of its development, and later in the era of Virchow and Schultze.
However, Baer retained an interest in the problems of the history of animal development. In 1845-46 he made an attempt to return to research in embryology. Planning to study the embryonic development of invertebrates, Baer travelled to the Mediterranean coast and collected interesting material in Genoa, Venice, and mainly in Trieste, which consisted of fixed objects and drawings by an artist who accompanied him. In this last attempt to return to embryological investigations, misfortune pursued Baer. Part of the material was accidentally destroyed in Venice; another part of it was lost elsewhere.
But not all of the research carried out on the Mediterranean coast could be considered fruitless (115). The principal task of his trip to Genoa, Venice and Trieste, as Baer wrote in his report, 13 was the study of the possibility of artificial fertilization in different marine animals (116) .
13. "Auszug aus einem Berichte des Akademikers v. Baer aus Triest zom 1 (13) November 1845," BULL. PHYS.-MATH. ACAD. SC. ST. PETERSB. (1847), 5, No. 15, pp. 231-240.
Baer noted that since the time of Spallanzani it was known that it is easy to fertilize artificially the ova of frogs. Spallanzani (and later Rossi) inseminated a bitch by the artificial introduction of sperm. Baer's contemporaries, according to him, questioned these data, although without foundation, because Bischoff's work had made it apparent that in mammals and also in other animals, the separation of the ovum is due to its maturation and not a result of copulation.
Baer thought it evident that every mature ovum introduced into contact with sperm of the same species is fertilized and, if situated in suitable conditions, develops into an embryo. Sometimes experimental difficulties arise, which can, however, be overcome. His failures in Petersburg with artificial insemination of fish ova Baer explained by unsuitable temperature conditions.
In Genoa at the end of August 1845 Baer made the first experiments in the artificial insemination of the ova of Ascidians. "In the earliest hours," Baer wrote, "the division began, and before the day had passed larvae hatched in the form of large cercariae." The experiment of artificial insemination of mature ova of the sea urchin, carried out on the same day, was also crowned with success. Within sixteen hours freely moving embryos developed. "It was a very great success for one day," Baer stated: "a researcher rarely succeeds sufficiently to exclaim VENI, VIDI, VICI! After the first glitter, I was on top of the world" (p. 233). However, it was extremely difficult to keep the ascidian larvae alive. They died within hours after hatching; the sea urchin larvae did not live more than four days.
Gaining success in the artificial fertilization of the ova of sea urchins, Baer turned to study their development. He observed the early stages of development at first in Genoa
13. Extracts from this report were published in FRORIEP NOTITZEN (1846, No. 39, pp. 38-40) under the heading, K. E. v. Baer, "Neue Untersuchungen iiber die Entwickelung der Thiere . "
in the ova of Echinus hrevispinosus (esoulentus) 9 and then in Trieste on the smaller, more transparent ova of Echinus lividus (saxatilis) , with the first, his attention was attracted to the fact that inside the ovum, soon after fertilization but before the beginning of division, a light radiance appeared but shortly afterwards disappeared. Applying slight pressure upon the ovum, Baer could get an extended vesicle (nucleus) or two vesicles situated side by side. Noting the direction of the longitudinal axis of this extended light region, Baer observed that after the yolk (ovum) divided into two halves, the centers of these halves were situated in the same axis. From these first observations he concluded that "the processes in the internal region of the ovum precede the division of the yolk and predicate it," The study of the subsequent stages of division led Baer to the conclusion that the externally observable phenomenon of division always precedes the division of the transparent nucleus present inside. The details of the ovum structure, using intravital observations with little magnification, Baer of course could not investigate, and discussions preserved the terminological confusion of that time, when the relations between the nucleus of the immature ovum (embryonic vesicle, or Purkinje's vesicle), the ovum nucleus, and its nucleolus (Wagner's spot) were still not established. 14
For the first stage of development of the immature ovum, Baer wrote,
I consider the nucleus to be identical with that part which is usually called Wagner's spot . , . . Much later the part which is apparently considered an embryonic vesicle occupies a great part of the ovum. Because the small body shows, under pressure, a great resistance, the name "spot" is not so suitable. It seems to me very probable that the role which this nucleus (or embryonic spot) plays in the ovum of the sea urchin occurs in other animals in the embryonic vesicle. In addition to this, in the ovum of the sea urchin the part called embryonic vesicle disappears long before the end of maturation. (p. 238)
14. It must not be forgotten that even thirty years later O. Hertwig thought that during the maturation of the ovum the embryonic vesicle disappears, and the embryonic spot becomes the nucleus of the mature ovum (O. Hertwig, "Beitrage zur Kenntniss der Bildung, Befruchtung und Teilung des tierischen Eies," MORPH. JAHRB., I (1875) } III (1877) t IV, (1878) .
If the details of maturation remained unclear for Baer during the study of the processes of segmentation he could, despite his imperfect means of observation, see much and essentially move ahead of his contemporaries. According to Baer's description, after fertilization the ovum nucleus greatly submerges in the yolk and after some minutes it seems that it completely disappears. "However, under the microscope," Baer state, "the nucleus may be seen, although its boundaries are unclear due to uneven refraction of the surrounding granular yolk, Sometimes only a limited light radiance is seen. During the delivery of the ovum its boundaries, having the form of a circle, become more distinct" (p. 238).
Baer described the process of division itself in the following manner:
After the period of dormancy, the nucleus, having until then a spherical form, very quickly becomes elongated, and at the same time processes of protuberances appear from both of its sides. These ends of the nucleus swell, and its center becomes thin and is quickly torn off, so that two cometshaped nuclei with tails directed towards each other are formed. The tail-shaped appendage quickly extends inside its spherical mass, and then two nuclei are apparent .... Just before division the nucleus increases in size; at the time of division this increase progresses, so that each of the two new nuclei is approximately equal in size to the initial one .... Only after that, when both of the new nuclei are separated from each other, does the re twisting of the ovum begin, in consequence of which it is divided into two halves near to each other, each surrounded by its portion of yolk, (pp. 238-239)
Here Baer referred to Kolliker's paper. Since Kb'lliker did not doubt the complete disappearance of the embryonic vesicle, Baer, referring to his authority, left his own completely accurate observations in doubt and considered it necessary to check them again. Baer noted, however, that he could never ascertain the moment when the nucleus was completely absent. And what is more, he expressed confidence that in the ova of frogs the embryonic vesicle would be detectable in this period when, as it was believed, it completely disappears.
Further on, Baer passed to the description of the following stages of segmentation. Soon after the division of the ovum into two parts, "each nucleus begins by the way previously described to form processes, and changes by dividing in the middle into two new nuclei; the mass of yolk adjacent to it also is divided, so that all the ovum disintegrates into four masses. Each quarter becomes rounded, and in the center between them an empty space 16 is formed. The division of quadrants takes place completely similarly; moreover, the newly formed processes are situated at right angles to the previous ones. This continues also during the following divisions; moreover, for every new fragment of yolk a nucleus is formed by division of the earlier one,
Baer noted that the nucleus in the process of division is not always distinctly demarcated from the surrounding substance, "In the period of dormancy," Baer wrote, "a clear boundary line is seen under the microscope, but at the time of formation of the processes it cannot be observed with definition" (p. 239). Further, he drew from his observations another essential conclusion, concerning the method of formation of cells in the process of early embryonic development in the sea urchin. With great accuracy it can be established, and for subsequent divisions also, that when the nuclei are surrounded with a small layer of yolk the new cells are not formed "inside the maternal ones" ("yolk bodies," in Baer's terms) .
15. A. Kolliker, "Beitrage zur Entwickelungsgeschichte wirbelloser Thiere. 1: Uber die ersten Vorgange im befruchteten Ei," ARCH. ANAT . , PHYSIOL. (1843), pp. 68-141.
16. This rudiment of the division cavity (blastocoel) is called Baer's cavity by R. Remak.
Up to the stage of thirty-two blastomeres he could note directly the appearance of new blastomeres by division. Later on when they became numerous, it did not seem to be possible to trace the appearance of each, "However, here and there," Baer wrote, "in the yolk bodies, situated in the edge, the processes of division can be seen also as earlier. Even when the embryo leaves the ovum membrane and moves by the help of the cilia, every granule or each histogenic element (cell) possesses a very distinct nucleus" (p. 240), Baer could not trace the subsequent development, due to the movement of the embryo. Nevertheless, on the basis of the observations described above he reached the conclusion that later the histogenic elements (i.e. cells)result from earlier existing ones by means of the same kind of division. The general conclusion with which he ended the report stated: "The division of the yolk is considered only the beginning of the histogenetical separation, which continues uninterruptedly to the final formation of the animal. If this presentation is correct, then the question of the pre-existence of the new individual, before fertilization, is beyond any doubt. The unfertilized ovum is an embryo having latent life. The fertilization makes its life active" (p. 240),
This short paper of Baer's, as can be seen from its summary, contains an extremely rich content. There is no doubt that Baer saw with complete clarity all these processes of maturation and division of the ovum, which can be seen by intravital observations. He surpassed, by many decades, the views of his contemporaries, showing, first, that the nucleus of the fertilized ovum does not disappear, but becomes only less distinctly visible. He saw further the appearance of the radiant achromatic figures in the mitoses of division, the change at the time of their division, the disappearance of the nuclear membrane at the time of mitosis, and its appearance in interkinesis.
Drawings were not appended to the published report. If one looks at a picture of the division of the sea urchin ovum by intravital observation performed approximately a century later with an apochromatic objective and condenser, one will be surprised by the accuracy of Baer's observations and the insight of his discussion.
While his work on the history of development of invertebrates was not continued, Baer continued to the end of his life to be interested in the progress of the science of development and responded to every major event in this sphere.
The discovery, by N. P, Wagner of Kazan, of asexual reproduction in the larvae of diptera from the family Cecidomyidael7 evoked from Baer first a short report 18 and then a long paper. 19 Wagner found that from the ova laid by winged adults come large larvae, which, however, do not pupate. Inside each of these larvae a new generation of larvae develops, feeding on the fat body and other organs of the mother. Inside the larvae of the second generation the third generation originates and so on, resulting in an intermediate series of asexually formed generations. The larvae pupate and produce the dioecious winged cecidiums . The discovery of this unusual method of reproduction in dipters caused such great doubt that Siebold did not dare even to publish in the journal which he edited (ZEITSCHRIFT FUR WISSENSCHAFTLICHE ZOOLOGIE ) an article sent to him by Wagner .
17. N. P. Wagner, SPONTANEOUS REPRODUCTION OF CATERPILLARS AND INSECTS (Kazan, 1862) , 50 pp.
18. Baer, "Bericht (iber eine neue von Prof. Wagner in Kasan an Dipteren beobachtete abweichende Propagationsform," BULL. ACAD. SC. ST. PETERSB. (1863), 6, pp. 239-241.
19. Baer, "On the discovery by Professor Wagner of the asexual reproduction of larvae, on additional observations on this subject by G. Ganin, and on paedogenesis generally," Appendix to Vol. 10 of ZAPISOK IMP. AKAD NAUK (1866), No. 1, pp. 1-77.
Academicians Baer and Ovsyannikov personally were sure of the reliability of Wagner's observations, and he, after review by a committee that included Baer, Ovsyannikov, and Brandt, was awarded the Demidov prize. 20 i n the same year in Denmark Meinert21 confirmed Wagner's discovery, and proposed for cecidia, the larvae of which kill their mother during their development, the new generic name MIASTOR (Greek for killer, villain) . Meinert also agreed with Wagner that the larvae of the new generation arise from the fat body of the larva-mother. Pagenstecher22 also confirmed the fact of asexual reproduction among the larvae of cecidia, but he thought the larvae to be formed, not from the fat body, but from minute ova, but Pagenstecher could not establish the places of appearance of the ova.
The subsequent investigations of the reproduction of Miastors were performed in Leuckart's laboratory at Giessen; I. I. Mechnikov, who worked there, shared in them. In a preliminary report published in NATURALIST (No. 8, 1865), Mechnikov wrote that he, together with Leikart, found in the larvae of cecidia special organs â€” "small embryos, fissionable into separate compartments, swimming freely in the body cavity and producing new embryos. The development of the latter," Mechnikov continued, "I have also observed, and have found that the small embryos are formed from direct transformation of 'polar cells,' as in the genus Chironomus, as earlier blastomeres ." Baer especially underlined the importance of Mechnikov' s discovery, that "the small embryos or ovariesâ€” call them what you will" are formed from special polar cells. "If the formation of the small embryos from these 'polar cells' is confirmed, then it will be an important increase of our expanded knowledge about the process of animal development" (p. 10). 23
20. Spontaneous reproduction of caterpillars and insects. Prof. Wagner. Kazan (Thirty- third Award of P. N. Demidov prize, June 26, 1864, pp. 238-242) (review by Baer together with Ovsyannikov and Brandt) .
21. F. Meinert, "Weitere Erlauterungen liber die von Prof, N. Wagner beschriebene Insektenlarve, welche sich durch Sprossenbildung vermehrt," ZEITSCHR. WISS. ZOOL. (1864) , 14, pp. 394-399.
22. A. Pagenstecher, "Die ungeschlechtliche Vermehrung der Fliegenlarven," ibid. , pp. 400-416.
Later Baer reported in detail the investigations of M. S. Ganin, ^4 prosector at Kharkov University. His observations are "extremely careful and circumspect, representing great interest." Describing in detail the structure of larvae, Ganin turned to their embryonic development; moreover, he reached the conclusion that the development of the young larvae inside the older ones did not arise from the fat body. He found an ovary, in which ova developed, giving the beginning to the young generation. Ganin thought that the ova of viviparous diptera that he investigated are differentiated from the ova of those insects which are characterized in the imaginal stage by the absence of the nucleus (Purkinje's vesicle) (117). Leuckart soon confirmed Ganin's observations concerning the development of Miastor ova within special organs, but he called these organs small embryos and not ovaries as Ganin called them. The developing group of cells in these small embryos are separated and fall in the body cavity. In each of these groups one cell increases in size and serves for the formation of the embryo, while the others, as in the embryonic chambers of other insects, play the role of feeding cells.
Baer considered it correct to distinguish the embryos identical with ova which are developed from ovaries producing true ova, and approved Leuckart 's suggestion of calling the early embryos of the Miastor false ova (PSEUDOVA) (118).
23. In the following year Mechnikov published detailed reports on the embryonic development of Miastor ("Uber die Entwickelung der viviparen Cecidomyidenlarve, nebst Bemerkungen iiber den Bau und die Fortpflanzung derselben," ibid . (1866) , 16, p. 407). In this paper the development of small embryos from polar cells was traced in detail .
24. M. Ganin, "Neue Beobachtungen liber die Fortpflanzung der viviparen Dipterenlarven," ibid . (1865), 15, p. 375; ZAP I SKI IMP. ACADEMII NAUK, 1865, 8, pp. 36-56.
Returning to Wagner's discovery, Baer mainly emphasized the distrust with which it was first viewed, "This circumstance," he stated, "shows to what extent the aforesaid discovery was unexpected and how little prepared we were for it. Subsequently, its importance lay in its proof, and credit was given to him" (pp. 22-23) . In connection with this he mentioned the words of Humboldt: "A book which, upon its appearance, immediately meets with general approval cannot be worth the publishing, because it can only conclude what already completely predominates in everyone's convictions." Illustrating this, Baer noted the discovery made by Peyssonel in 1723 that corals are animals, which "the great Reaumur rejected as an absurdity," so that Peyssonel could only publish it thirty years later. Further, Baer noted the fate of Harvey's discovery of blood circulation, which was subjected to doubt because "they did not know where air goes, it was assumed to exist in the arteries. The correctness of his discovery was recognized twenty years later, that is to say after Harvey's death. More time elapsed before the general recognition of Copernicus' discovery. The earth made its orbit around the sun many times before popes could speak of this publicly" (p. 24).
Baer remarked later that he did not mean to compare the discovery of asexual reproduction of larvae with the basis of the heliocentric theory; he wanted only to call attention to the frequently repeated historical relation of new ideas. He cited Agassi z, that each newly appearing study must pass through three phases: first they will say that it is incorrect, then that it is against religion, and finally that it had been known for a long time. "Wagner's discovery," Baer stated, "without doubt, does not need proof that it is not contrary to religion, or rather, dogma, for no dogma is concerned with fly larvae" (p. 25) . This discovery had already reached the stage of general recognition, the stage of coordinating it with previous opinions, for which it was necessary to change them somewhat. The idea must be rejected that the reproduction of posterity by means of fertilization, which is characteristic of man and other vertebrates, is a rule, and that all other forms of reproduction are exceptions. This idea is the source of the usual anthropomorphism, "man always standing at the center of his mental as well as his physical horizon" (p. 26).
Sexual reproduction, which is considered a necessity for the majority of animals, especially the higher ones, is intrinsic also to the plant kingdom, but in the latter it is not so necessary and frequently is changed into different forms of vegetative reproduction.
Referring to corresponding examples (potato, weeping willow, bulbous plants, and so on), Baer turned to the invertebrates, noting that in them also (for example, in Infusoria, Bryozoa, Ascidia, and polyps) reproduction by division and budding is very usual. Other forms of reproduction without fertilization, namely parthenogenesis, is characteristic among insects. The main form of parthenogenesis was considered Wagner's discovery of the Miastor. Contrary to the earlier known parthenogenesis, in which the source of ova developing without fertilization was thought to be the sexually mature females, in viviparous cecidia the embryos originate in the organism of the unformed larvae incapable of fertilization, Baer considered it advisable to give this form of reproduction a special name, and by analogy with parthenogenesis he termed it paedogenesis.25 i n both terms, Baer wrote, the first half of the word shows the producing subject. In a footnote he mentioned the correctness of Leuckart's note that the word parthenogenesis actually means the genesis oÂ£ a virgin, and not genesis by a virgin; however, as this term became generally used, it was impossible to avoid analogy with it in the formation of a new record.
Comparing the phenomenon of parthenogenesis in Aphidae with paedogenesis in cecidia, Baer reached the conclusion that, as in Aphidae, the appearance of this or that form of reproduction (with fertilization or without it) is closely related to conditions of existence, in particular to intensity or nourishment; more abundant nutrition aids parthenogenetic and paedogenetic reproduction. The main conclusion to be drawn from the comparison of these forms of reproduction is that they both are an example of alternation of generations, or "alternating reproduction," during which sexual generation, i.e. by means of fertilization, alternates in one or more generations with reproduction without fertilization. To substantiate this conclusion Baer extracted material from Steenstrup's ftBER DEN GENERATIONSWECHSEL , 26 and other sources. He rated highly Steenstrup's work, in which the wide distribution of the phenomenon of alternation of generations is shown, which indicated its general biological significance. Baer turned his attention to the fact that Steenstrup had stated almost nothing about the alternation of generations in plants. Attempting to fill this gap, Baer cited many clear examples related to sporophytic and phanerogamous plants. These examples show that the alternation of two forms of reproduction â€” sexual and asexual (i.e. occurring without fertilization) â€” is inherent to the entire organic world. The alternation of generations, during which the ability to reproduce asexually appears (Baer related it not only to division, as in budding and other forms of vegetative reproduction, but also to development from unfertilized ova, i.e. parthenogenesis and paedogenesis) , he considered not an exception but a rule. The essential difference between sexual reproduction and all forms of asexual reproduction Baer concluded from the fact that sexual reproduction takes place only in the adult condition, while asexual reproduction occurs in different stages of ontogenesis. In particular, adult individuals are capable of parthenogenesis and reproduction by means of fertilization, while the paedogenesis of dipterous insects occurs only in larvae.
25. From the Greek P AIDES, meaning children,
The ability to reproduce in a sexually immature condition is characteristic also for other invertebrates â€” Coelenterata, parasitic Platyhelminthes, and Tunicata; therefore, all sexually immature forms of different animals â€” rediae, cercariae, brachiolariae, tornariae, strobiliae, scyphistomae, and so onâ€”must also be called larvae.
In comparing the individual life of the organism â€” i.e. its development with reproduction â€” with the life of the species,
26. J. J. Steenstrup, (JBER DEN GENERATIONSWECHSEL, ODER DIE FORTPFLANZUNG DURCH ABWEICKENDE GENERATIONEN, EIN EIGENTHUMLICHE FORM DER BRUTSFLAGE IN DEN NIEDEREN THIERCLASSEN (Copenhagen, 1842), 140 pp. This edition represents the translation of Forentsen from a manuscript published at the same time in Danish.
Baer was concerned with the question of the relation of the instinct for self-preservation to the sexual instinct, and in connection with this he touched upon a question which worried him, on the possibility of including the understanding of purpose in a sphere of strict scientific investigation. His considerations are discussed below. 27
In the concluding chapter of his work Baer again noted the importance of the discovery of paedogenesis for understanding the idea of parthenogenesis and the alternation of generations. Understanding of these phenomena required systematic examination of different forms of reproduction. "I sought," Baer wrote, "to look through all that had been written on this subject, as in a consultation with myselfâ€” a consultation which may prompt more young investigators to attempt to make similar comparisons that would also be useful for them. During this, difficult tasks appear which we must trace more closely" (p. 70) . To these tasks Baer related the experimental investigations of the external conditions upon which the parthenogenesis of Aphidae and the paedogenesis of cecidia occurs. By means of these experiments, Baer supposed, it could be elucidated why, in apparently related species of medusa, one species immediately forms sexually mature medusa from the swimming larvae and the other goes through the sessile stage which is the budded strobila.
To these facts and considerations Baer gave the following resume:
Organic bodies possess the ability to self-develop by action intrinsic to them, as soon as they have the possibility of receiving the necessary material. In addition, they still have the ability to produce new individuals. (p, 72)
Self -development moves from absolutely simple forms and elementary parts by gradual transformations . â€ž . . The sequence of these changes is called in scientific terms development, and in ordinary term life, growth;
27. See Chapter 24
in addition, it means mainly the increase of body. But this increase in every separate vital process ceases early or late, although the tendency of self-preservation continues to exist, but the changes lead the organism to final dissociation. The capacity of reproduction, on the contrary, creates new individuals of this species, (p. 73)
The ability to reproduce is present in two forms. First, it may be considered a result of the necessary influence of one on the other of two materials .... This reproduction is called sexual , even in the case in which male and female sexual organs are present in the same individual. These individuals are called hermaphrodites. The second kind of reproduction is termed asexual reproduction. It is frequently seen in plants and the lower animals. Asexual reproduction in many organisms is found together with sexual reproduction .... This union of both kinds of reproduction can be called, according to Owen, metagenesis, or, according to Van Beneden, digenesis. The expression "alternation of generations". . . must be used only in those conditions when these forms of reproduction are alternated one with the other .... Sexual reproduction can never appear at the beginning of individual development, because the sexual organs must be formed early and secrete their products. The embryo produced by fertilization must always pass through all the stages of development intrinsic to this organism. When asexual reproduction takes place in the mature condition of the female individual, then it is called parthenogenesis. Similar reproduction in the immature condition we suggest calling paedogenesis. The latter may occur in all the different periods of development and may appear in very different forms, and also begins the course of development each time from the beginning, or may only continue it. (pp. 75-76)
From this it is clear that Baer established an orderly classification of the methods of reproduction, including all the organic world from the lower water plants to man, showing, in fact, the form of reproduction for evolution.
N, P, Wagner's discovery of the phenomenon of paedogenesis, which served as the starting point for Baer's discussions and also M, S. Ganin's detailed investigations in this topic, attracted Baer's intense attention. Because these works were considered evidence of the successes of Russian embryology, Baer always showed great care for its development. He energetically popularized works by Russian authors in his reports, published statements and letters. He was glad of the recognition of the importance of these works by influential investigators abroad and expressed the conviction that "there is yet no single route to us via the printing press and book trades" (p. 22).
It is interesting historically, however, that Baer's other great work, in which he posed the question, "Do the larvae of simple ascidiae develop initially like types of vertebrates?"^^ wa s very controversial.
The cause of his writing this paper was sensational . As Baer wrote, A. 0, Kovalevsky discovered that ascidians, which in their adult condition are so strongly differentiated from vertebrates, at the beginning develop similarly to vertebrates, "If this conclusion," Baer stated, "could have been substantiated, the sensation would have been completely valid, since Darwin's bold hypothesis that the higher organisms have evolved over the course of time from the lower, the lowest form differing absolutely from the highest, received great support" (p. 1) .
29 Outlining the content of Kovalevsky' s work, Baer again returned to the idea that "rigorous comparison of ascidian
28. Baer, "Entwickelt sich die Larve einfachen Ascidien in der ersten Zeit nach dem Typus der Wirbelthierefc" MEM. ACAD, IMP. SCIENCES ST. PETERSB, ,VII Ser (1873), 19, No. 8, pp. 1-35.
29. A Kovalevsky, "Entwicklungsgeschichte der einfachen Ascidien," ibid . (1866), 10, No. 15, 16 pp.
larvae with the early stages of vertebrates . . . apparently removes the difference between the main groups of the animal kingdom and makes obvious what has been accepted by many authors subsequent to Darwin, that the transition is made from lower forms into absolutely higher forms" (p. 3). How great the attention attracted by Kovalevsky's paper was, Baer judged by London's QUARTERLY JOURNAL OF MICROSCOPICAL SCIENCE, which as a rule printed only original papers, but made an exception for Kovalevsky's paper and published a translation of it practically in full.
30 Baer then referred to the investigation of I. I. Mechnikov,
"also an experienced embryologist," and in particular to the
data of K. Kupffer which he stated in a letter to M. Schultze
and which were confirmed by the latter, and attached to
Kupffer 's detailed article, "Already the name itself," Baer
wrote, "'the genetic relationship between Ascidiae and
vertebrates, '31 indicates the importance of the results received.
The factual data and Kovalevsky's interpretations concerning
the coincidence of the development of ascidiae and vertebrates,
were confirmed and partially made more accurate" (pp. 4-5).
Further on Baer noted this relation to Kovalevsky's discovery,
which Darwin had mentioned in THE DESCENT OF MAN, and cited
the following passage from this work:
Kovalevsky has lately observed that the larvae of Ascidians are related to the Vertebrata, in their manner of development, in the relative position of the nervous system, and in possessing a structure closely like the CHORDA DORSAL IS of vertebrate animals .... Thus, if we may rely on embryology, ever the safest guide classification, it seems that
30. I. Mechnikov, "Observations on the development of some animals (Bothryllus and solitary ascidiae)," IZV.
PETERSB. AKAD. NAUK (1869), 13, pp. 284-300.
31. K. v. Kupffer, "Die Stammverwandschaft zwischen Ascidien und wirbelthieren," ARCH. MIKR. ANAT . (1870), 6.
we have at last gained a clue to the source whence the Vertebrata were derived. We should then be justified in believing that at an extremely remote period a group of animals existed, resembling in many respects the larvae of our present Ascidians, which diverged into two great branchesâ€” the one retrograding in development and producing the present class of Ascidians, the other rising to the crown and summit of the animal kingdom by giving birth to the Vertebrata. 32
Baer cited after this only one negative treatment of Kovalevsky's discovery, 33 an d with characteristic honesty he drew attention to its groundlessness. Baer's objectivity stands out here more clearly, since he himself did not agree with the opinions of Kovalevsky, Kupffer, and Darwin. In the next twenty pages of his article, he brought all his learning and all his authority to bear on the idea of the relationship between tunicates and vertebrates and attempted to prove their systematic nearness to the bivalves, agreeing in this with Cuvier and basing his ideas upon the situation of siphons, nervous ganglia, and so on. Baer confirmed in particular that the side of the body where the nervous ganglion of ascidians is present is not the dorsal, but the ventral; therefore the nervous system of ascidians could not be homologous to the central nervous system of vertebrates. Later he discussed the cord in the caudal part in the larvae of ascidians and also refused to recognize its homology with the spinal cord of vertebrates. All polemics were sustained in very correct tones. Of his scientific opponents Baer everywhere spoke in a tone of complete respect to their scientific services and high competence.
Afterwards Baer explained why he went into anatomical details in this article, where it might better have been limited to brief reminiscence. "I meant," Baer wrote, "to show many dilettantes unconditionally believing in the possibility of transmutation and inclined to believe that the nonrecognition of ascidians as ancestors of man is attributable to sheer vanity" (p. 35) .
32. Charles Drawin, THE DESCENT OF MAN, AND SELECTION IN RELATION TO SEX. rev. ed. (New York, 1883),
pp. 159-160. Russian translation edited by I. M. Sechenov, S0CH.,Vol. 5 (Izd. AN SSSR, 1953), pp. 268-269.
33. ARCH. ANAT. PHYSIOL. (1870), p. 762.
The source of all his interpretations which were basically erroneous was Baer's view of the theory of evolution, in particular in relation to Darwinism. The discussion of Baer's views on evolution is beyond the scope of the present book. They did not appear in the literature; however, they have not been investigated sufficiently. But here it can be mentioned that Baer considered the evolutionary development of the organic world to be without doubt, but with known limits, having their sources in the theory of types established by him and Cuvier.
Stating the presence of transitional forms between the systematic groups within each of the four types, which testifies to the community of origin of all representatives of the type, Baer doubted that the signs of one type could be seen in the development of another type (119) .
Proceeding from this belief, Baer could not imagine that the tunicates, not having in the adult condition the general typical signs of the vertebrates, could show at any stage of development the characteristics of this type. From here there was a persistent striving from his side to give Kovalevsky's discovery this explanation, which did not stand in conflict with the theory of types.
Being considered the founder of embryological science, Baer did not leave behind him what could be called a school, In Konigsberg there were some young people who performed under his guidance dissertations on teratology, touching to some extent upon the history of development (120) . One Konigsberg student of Baer's, A. E. Grube (1812-1886), was later recommended by him to the Department of Zoology in Dorpat University. Grube published many works on zoology and comparative zoology; in addition he mainly studied annelids.
The influence of Baer's scientific interest is recognized first in the work Grube performed in Dorpat and published in 1844 (see Chapter 25), and also in the investigations of Majewski and Tschernow, whose dissertations were also defended at Dorpat University, in 1858. Both these dissertations illustrated the embryo-physiological problem of the chemical composition of the foetal fluids in mammals. Majewski34 (121) investigated the composition of the amniotic and allantoic fluids in twenty-eight embryos of sheep (four to thirteen weeks old) and in sixteen embryos of cattle (twelve to twenty-six weeks old), adding to this data concerning some embryos of swine and man. In this work he determined the weight, the length of the embryos studied, the quantity of amniotic and allantoic fluids, their specific gravity and reaction, and also the amount of solid constituents, either organic or inorganic, namely albumin, sugar, urea, phosphoric and sulphuric acids. In the tables of this dissertation, numbers are given for separate observations and average numbers for different periods of development. Majewski established that, according to the extent of formation of the embryo, the amount of the solid constituents in amniotic fluid increases. In particular, in all stages of intra-uterine life in all investigated animals and in man, the albumin and sugar are present in the amniotic fluid; the quantity of albumin shortly before birth somewhat decreases, and the quantity of sugar in the process of development of the embryo gradually increases, reaching the maximum before birth. The quantity of fluid in the allantois, and also the content of solid matter in it, increases with the development of the embryo. The allantoic fluid always remains transparent and similar to the saturated urine, while the quantity of urea in it gradually increases; in it there are also albumin and sugar. On the basis of his investigations, Majewski reached the conclusion that the amniotic fluid does not serve to feed the embryo, but protects it from external harmful influences and that the feeding of the foetus takes place only through the placenta.
34. Adolphus Majewski, polonus, DE SUBSTANTIARUM, QUAE LIQUORIBUS AMNII ET ALLANTOIDIS INSUNT, DIVERSIS VITAE EMBRYONALIS PERIODIS (Dorpat, 1858), 44 pp.
35 The thesis of Tschernow " (122) is a natural continuation
of the investigations of Majewski, which dealt with herbivorous animals, while Tschernow performed the study of the chemical composition of foetal fluids on carnivorous animals, mainly cats (he investigated forty-four foeti, weighing from 0,074 to 107,3 gm) . In addition, in his collection there were embryos of four dogs, nine swine, and one human, and also one horse embryo, which was used for comparison with carnivorous animals. Tschernow established that in the amniotic fluid the relation of the quantity of water to the solid constituents remains constant. In the allantois, the quantity of fluid in the process of foetal development gradually decreases, and simultaneously the relative quantity of water diminishes (from 989.5 to 949.8%), The amount of solid material in the fluid of the allantois increased at the expense of the organic matter (albuminâ€” from 1.05 to 5.59%; sugarâ€” from 0.95 to 2.02%; ureaâ€” from 1.48 to 12.10%). The albumin of allantoic fluid, according to Tschernow, is not necessarily secreted by the capillary renal vessels, although this possibility is not excluded, as the vessels of the foetus probably possess different permeability properties than the born animals, in whose urine there are no traces of albumin. Tschernow thought that albumin passes in the fluid of the allantois from the amniotic fluid, where it is always present. The sugar in the allantoic fluid of carnivores is significantly lower than in herbivores (in sheep, according to Majewski, it averages 7.57%, and in horses, according to Tschernow 's observations, it averages 11%), On the other hand, the urea in the allantoic fluid in carnivores is greater than in herbivorous animals.
The tradition of embryological investigations in Dorpat University, which is connected with the influence of Baer, remained. Thus, M. Braun, who was present from 1880 to 1886, in the beginning as a prosector of comparative anatomy and then as professor of zoology, worked on the embryology of bivalves, tapeworms, reptiles, birds, and mammals (among these works, investigations on the development of the wavy parrot
35. Nikolaus Tschernow (Estonus) , DE LTQUORUM EMBRYONALIUM IN ANIMALIBUS CARNIVORIS CONSTITUTION CHEMXCA (Dorpat, 1858) , 35 pp.
were distinguished) . Yu. Kennel replaced him in the department in 1887, He studied the embryology of Prototracheata and published three works on this topic.
The true successors of Baer as embryologists were, of course, A, 0. Kovalevsky and I. I. Mechnikov. Not formally students of Baer, they nonetheless paid high tribute to his classical investigations. To Kovalevsky and Mechnikov belong that unquestionable service of further development of Baer's studies and the creation of the theory of embryonic membranes, making the basis for comparative and evolutionary embryology.
Therefore, it was not by chance that in connection with Baer 1864 jubilee, the Academy of Science established a prize in his name for the best works on biological sciences. The first award of Baer's prize was received by Kovalevsky and Mechnikov. Baer himself participated in the discussion of candidates for the prize and expressed a patriotic pride on the occasion of its being awarded to Russian investigators.
In the conclusion of this chapter, some words must be stated about Baer as historian of embryology. Although he was not studying systematically the history of embryology, Baer stated many profound ideas about the development of this science. His discussions on this question are scattered through many works. Besides this, Baer subjected the literature of his prominent predecessor in the field of embryology, K. F. Wolff, to special study, and published a report on this work, 36 Baer's research concerning Tredern has been mentioned in Chapter 11.
References to the history of embryology in the works of Baer are always intimately connected with his own works; however, they frequently include an unusually large number of quotations from the works of his predecessors, and represent fragments of investigations in the field of history of science, showing a deep, thorough acquaintance with the literature (always from original sources) , and briefly describe the scientific epoch or direction of scientific thought ,
36. Baer, "Uber den litterarischen Nachlass von C. F. Wolff, ehemaligem Mitglied der Akademie der Wissenschaften zu St. Petersburg," BULL. CL . PHYS.-MATH. AC. ST. PETERSB. (1846), 5, No. 9, 10, pp. 129-160.
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