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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 1955: 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

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

Chapter 25. Investigations on Invertebrate Embryology - Work of A. Grube, A. D. Nordmann, N. A. Warnek, and A. Krohn

During the first twenty to thirty years of the nineteenth century, embryology remained chiefly the study of the embryonic development of vertebrates; the comparative peculiarities of development of different animals was studied only at the limits of this most studied group. By the end of the twentieth year of that century, i.e. the period of Baer's active work in embryology, the first investigations into the development of invertebrates appeared. Baer himself, as mentioned above, 1 turned his attention to the characteristic peculiarities of development of arthropods, noting in particular that their blastoderms are situated on the abdominal side of the egg and distributed from here in the dorsal direction. Baer was not able to explain the development of arthropods more clearly. Even the investigations of his predecessor Herold, which were limited to the study of the late stages of development of butterflies and spiders, 2 could not give Baer material for well-grounded comparative embryological conclusions.

The aspiration to apply embryological principles to arthropod development was actualized by Baer, first in the study of embryonic layers, reflected in the works of

1. See Chapter 15.



(Marburg, 1824) , x + 63 pp.

M. H. Rathke. A native of Danzig, Rathke in 1829 arrived in Russia, where he was professor for six years at Dorpat University. Before that, he published many embryo logical works, including a valuable work on the development of crayfish. 3 During his tenure at Dorpat, Rathke visited Moscow and Petersburg and also travelled in the Crimea to investigate the fauna of the Black Sea. On the Black Sea coastline, Rathke collected comparative embryological material which was later used in ON MORPHOLOGY: TRAVEL NOTES FROM TAURIA.4 in this collection, in addition to brief information about the embryonic stage of actinia, there is also an essay on the embryology of the Crimean scorpion and investigations concerning the development of nine species of Crustacea of different orders (copepods, araphipods, decapods, and isopods) . Rathke 1 s work on crayfish development, and also his investigations on the development of other arthropods (124), represent a clear interest in describing the phenomenon of embryonic development in arthropods according to the ideas of Pander and Baer. Rathke spoke of the embryonic disk or blastoderm, of the primary cavity, and of the two embryonic membranes (serous and mucous) into which the blastoderm is divided. The first stages of development, the division of the ovum and the first processes of separation of the rudiments, remained untraced.

It must be borne in mind that a clear presentation about the essence of the processes which take place in the early stages of embryonic development — i.e. first of all the process of division — was not yet established in the first forty years of the last century. Thus, Reichert (125) studied the development of the frog ovum but reached an incorrect conclusion about the structure of the still-undivided ovum, supposing that it consisted of many cells (by cells he meant the round accumulations of the yolk plates) . This point of view was raised by Reichert both in his DIE ENTWICKELUNGSLEBEN IM WIRBELTHIRREICH and in an article published one year before, "On the Process of Division in the Ova of Amphibia, "5


4. Rathke, ZUR MORPHOLOGIE. REISEBEMERKUNGEN AUS TAURIA (Riga und Leipzig, 1827) , 192 pp.

5. K. B. Reichert, DIE ENTWICKELUNGSLEBEN IM WIRBELTHIERREICH (Berlin, 1840), x + 261 pp.? "Uber den Furchungsprozes der Batrachier," ARCH. ANAT . PHYSIOL. (1841), pp. 523 541.


in which he wrote: The process of division of the amphibian ova is nothing more than successively accomplished generic action (GEBURTSACT) of the maternal cells, repeatedly invested in each other.

T. Bischoff, working on the embryology of mammals and publishing in the period from 1842 to 1852 a monograph about the development of man, rabbit, dog and guinea pig, also did not reach a clear understanding of processes of division and did not recognize the spheres resulting from division as cells; since in his opinion, the cell must possess a cavity, but the spheres resulting from division are filled with yolk, the nuclei of the blastoderms were taken by Bischoff as fat droplets.

A. K8 Hiker** went much further in the analysis of the process of ovum division, admitting the direct continuity of the blastoderms and those cells from which the embryo is built in later stages.

In all these cases the discussion was about the forms of development, complicated by the great quantity of yolk in the centrolecithal and telolecithal ova. The nature of the occurrence there of superficial and discoidal division was explained much later, after the introduction of sectioning in embryology.

A more distinct presentation on the phenomena of division was stated by Baer in his work on the development of amphibia, 7 but especially in the work noticed by his contemporaries and later forgotten on the development of the ova of the sea urchin. 8

Certain embryologists of the thirties and the forties nearly approached the correct interpretation of the phenomena of the ovum division. They include KClliker, then Loven,9


7. See Chapter 21 .

8. See Chapter 23.

9. S. L. Loven, "Bidrag til kannedomen af Molluskenas untveckling," K. VET. AKAD . HANDLINGAR (1839), pp. 227 - 241.

Sars,10 van Beneden,H and Quatrefages-^ and must be mentioned. The first four investigated mollusc development, and the last studied the development of annelids. They presented some stages of ovum division, but did not broach the subject of the internal processes occurring in it nor of the fate of the spheres of division.

A great part of the works of that time concerning the development of invertebrates was illustrated by the study of different types of reproduction, and also by the description of the structure and the transformation of different larval forms which sometimes did not yield to systematic determination and figured under different specific names (126) . The investigations of forms of reproduction became especially popular after Steenstrup showed the wide distribution of the alternation of sexual and asexual generations of many invertebrates; the application of this empirical regularity to groups of animals not investigated before this time constituted the majority of works in the present sphere. The single base for generalization concerning early embryonic development is thought to be the cellular theory formed shortly before this. Using extremely imperfect microscopic techniques, the embryologists of the first half of the nineteenth century posed the question, can the spheres of division be called cells, and are the cells of which the embryo consists the direct descendants of the primary spheres of division? They attempted to trace the fate of those existing in the unfertilized ova "embryonic vesicle" (and "embryonic speck"), i.e. to explain whether these formations disappear after fertilization without a trace or whether they stand in continuous genetic connection with the nucleus (and the nucleolus) of the cells of the embryo. On the foundation of a one-sided and primitive understanding of the cellular theory, fantastic presentations sometimes grew, like the ideas of Reichert, which resurrected the long-buried theory of preformation.

10. M. Sars, "Beitrag zur Entwickelungsgeschichte der Mollusken und Zoophy ten," ARCH. NATURG . , 6 (1840), pp. 196 - 219.

11. P. J. van Beneden et Ch. Windischmann, "Recherches sur l'embryogenie des Limaces," ARCH. ANAT . PHYSIOL. (1841) , pp. 176 - 195.

12. A. de Quatrefages, "Sur l'embryogenie des Annelides," ANN. SC. NAT., 3 Ser. Zool. 8 (1847), pp. 99 - 102.

For the purposeful coordination of the efforts of zoologists studying invertebrate development, the theory of embryonic layers, which still had not become a broad scientific generalization, could not serve. Its correctness was proven only for vertebrates, but the application of the theory of embryonic layers to invertebrates was believed by nearly no one. Rathke's old data concerning crayfish were known. More than a quarter of a century later Zaddach also reported on the embryonic layers of insects, admitting to his descriptions crude morphological mistakes.

The data related to the development of different types of invertebrates were accumulated relatively slowly, because a theoretical conception for which this material could be used did not exist. The study of the types of structure and development of animals, taking its beginning from the opinion of Cuvier and Baer, fell into decay. The formally opposite theory of types, that of Naturphilosophie, the idea of unity of planes, caused little enthusiasm. Only after fifty years, after the appearance of Darwin's theory, did this period of mental stagnation end. Then the naturalists were divided into two camps — the hearty supporters and the violent opponents of the theory of evolution. Practically none of the contemporaries of the great reformer of biological science could keep an olympian calm and sustain neutrality for any length of time.

The investigations preceding the publications of the ORIGIN OF SPECIES played, however, its historical role. Factual material was collected which Darwin and his early followers used. In this period came the important works of the Russian embryologists A. Grube, A. D. Nordmann, N. A. Warnek, and A. Krohn.

To Dorpat University professor A. Grube belonged serious investigation on the development of annelids; he studied the embryonic and partly post -embryonic development of the Proboscidea leeches Clepsine oomplanata and C. biooulata (genus Glossosiphonia, according to the current terminology). 13

Adolf Edward Grube* 4 was born in 1812 in Konigsberg, where he graduated from the university. For thirteen years C1843 - 1865) he was professor of zoology and comparative anatomy in Dorpat University, and to this period of Grube's life are related his most important works in systematics, anatomy, and embryology of the annelids.

Grube's work undoubtedly possesses remarkable significance. Because it was undeservedly forgotten and not mentioned even in the detailed reports and special works on the development of leeches (127), we must dwell here on its content.

In the introduction to his work, Grube turned to memories of his years of study in Konigsberg, where Baer was working. Actually the great embryologist at that time read few lectures, being engrossed in the investigations of development of fish and amphibia. He was helped by Grube's friend, the clever graphic artist Burov, who, by the way, encouraged Grube's attraction to the laboratory, where the latter acquired a taste for embryological investigations. Grube was aware of the difficulty of these observations, but he did not abandon his idea of devoting himself to embryology.


ii + 56 pp.

14. The author expresses heartfelt gratitude to the head of the Department of Zoology at the University of Tartu, Professor Kh. Kh. Riikoya, for the photo reprinted here of A. E. Grube.

Shortly after graduation from the university, Grube left for a trip to the Mediterranean seacoast. "Baer's observations on the history of animal development," Grube wrote, "accompanied me on the trip to Italy, and along with the enjoyments of nature and art I continuously sated my interest in this sphere of science" Cp. 1). On the coast Grube zealously collected zoological materials and made dissections, considering these studies important for his future embryological investigations. His attention was attracted mainly to annelids, the study of which became his basic zoological specialty. Only in 1839 did Grube turn to work directly on annelid development, beginning with the study of the very small ova of the Saenuris vaviegata described by Hofmeister. In addition he reported that their development differs in many relations from the corresponding phenomena in medical leeches which had been very superficially described by E. Weber. The following spring, Grube studied the embryology of the leech and confirmed the reality of the formerly discovered differences. Preparing the results of his observations for publication, Grube conscientiously studied the works of the authors whom he considered his predecessors. The work of Filippi on the anatomy and embryology of Proboscidae leeches,! 5 recently published, especially interested him. Grube found that in Filippi' s investigations there was only scanty information about the division of the ovum, as the Italian author saw only "six lobes, situated in one and the same plane around the seventh, situated in the center." To this description he added that these spherical segments disintegrate into smaller spheres (GLOBULI ORGANICI), sharing in the formation of the embryo. Filippi apparently studied the developing ova through the membrane of a cocoon, which can explain the indistinct results of his observations. The first foundation of the embryo Filippi described with an indefinite expression-— cutis (LA CUTE) . He also erroneously considered, that the hatched young is nearly similar to the adult leech and at once is attached to the mother by the help of posterior suckers (in fact they are not present at this time) .

Grube could trace the development of Clepsine in incomparably more detail; his success was aided by the application of concentrated reagents, in particular diluted nitric acid. In the first section of his work, Grube described the structure of the female and male sexual organs, copulation, the act of deposition of eggs, and also the structure of the ova at the time of maturation and directly after deposition.


The still-undeposited egg, hung in a special pocket in the oviduct, consists of a fine-grained substance (molecular bodies) and yolk (fatty bodies) ; it is provided with a nucleus (embryonic vesicle) . Before the deposition, the ovum, which is separated from the oviduct and is freely situated in the egg-reservoir, has the same structure; the nucleus already cannot be seen in a beam of light and can be seen only during the crushing of the ovum. Later the embryonic vesicle disappears completely.

The second section of the work was devoted to the description of embryonic development. In the deposited ovum for one hour no change can be seen, and then at its poles the following phenomena can be observed. At the beginning, on one of them, a white spot appears; this increases and turns into a disk with a grey spot in the center (Figure 36, a). Then this grey center increases and a white spot appears in it, and the external white disk is transformed into a ring, which Grube called the polar ring (Figure 36, b) . The field of formation of the polar ring he called the active pole, since according to his observations, it is here at the time of development of the embryo that the most noticeable changes take place. On the opposite inactive pole a white ring appears, but less distinctly delimited. The polar ring ascends over the surface of the ovum in the form of a papilla, so that after its infiltration its condensation can be prepared. Grube considered that the formation of polar rings is the result of the displacement of an internal substance, during which the "molecular bodies" are gathered in the form of rings on the poles.

Following the appearance of polar rings, or, in Grube* s expression, "the process of formation of fissures (DURCHFURCHUNG) or, rather, cleft (ZERKLUFTUNG) . . . . those fissures not only occur on the surface, but penetrate deep into the mass of the yolk, as a simple experiment shows: under careful pressure of the condensed yolk, it disintegrates into as many parts as the segments are delimited by the fissures" (p. 17). The first fissure divides the ovum into two unequal parts [Figure 36, c), so that the polar ring can be divided into halves, or from it a small part is separated, or the fissure occurs in direct proximity from the ring, not touching it completely. The second fissure divides the small segment approximately at a right angle to the first (Figure 36, d), and the third also divides the large segment (Figure 36, e] .

Thus, the ovum passes into the stage of four blastomeres (Grube noted that the large segment in many cases is divided earlier than the smaller one) . Of the four segments formed, one is larger than all the others, and it is divided by a fourth fissure which provides the beginning of the fifth segment. The following division is concerned with this last and takes place not along its length, but transversely, the result of which is that the divided fifth segment forms a region in the form of a polar field in the inactive pole of the ovum (Figure 36, f) . The sixth, seventh and eighth fissures again go from the active pole to the polar field of the inactive pole. After the sixth division the polar ring on the inactive pole usually disappears. Grube did not see more than eight meridional divisions, and said that in leeches the blackberry stage characteristic for many animals is absent; in this stage the meridional divisions are replaced by transverse ones.

At the same time as the division, accomplished by means of fissures, in the ovum of Clepsine the process of separation of the small globules from the large segments of the ovum first described by Grube takes place; these globules Grube called WANDUNGSBALLEN . In his opinion, the wall of the body of the embryo is built from them; "Segments of the yolk" and Wandungsballen are nothing more than the macromeres and micromeres of the terminology of today's embryology. The first small globule, as Grube noted, appears on the active pole after the formation of the first fissure and appears to be situated in this fissure. Judging by time and place of the appearance of this globule, here the discussion does not concern the formation of the micromeres, but the separation of the first polar body. The following small globules, forming on the active pole of the ovum, undoubtedly are micromeres. Grube described their accumulation, ascertaining during this that by the increase of the number of fissures the number of micromeres increases also; however, these phenomena do not stand in an indissoluble connection (Figure 36, e and g) . The formation of micromeres, according to Grube's description, is accompanied by the replacement of substances inside the ovum segments; he described these replacements in detail, alternating the statement of facts about his observations with theoretical conjecture about the forces of attraction, points of their application, and so on.

The micromeres, accumulated on the active pole, do not exhibit equal size; the smaller they are, there are relatively larger molecular substances (protoplasm) anc i smaller fatty globules (yolk) in them. The smallest consist only of protoplasm and a round transparent nuclear body. These micromeres, according to Grube, never arise from macromeres; therefore, he wrote, "I must conclude that the small globules originate from those large globules already present on the pole which earlier separated from segments of yolk" (p. 22). The accumulation of micromeres forms a plate in the form of an isosceles triangle, without sharp limits passing into the other surface of the ovum. This plate, which Grube called the embryonic field, is so situated that the summit of the triangle is turned toward the active pole and corresponds to the cephalic part of the future embryo.

It consists of "mosaic pieces" which arise as a result of the multiplication of micromeres; in addition, these pieces are smallest when they are nearest to the cephalic end. In the stage described, not only the anterior and posterior ends of the embryo, but also its upper and lower sides, as the surface of the embryo, on which the embryonic fluid is situated, correspond to its abdominal side. Using the descriptive phrase "mosaic pieces", Grube resolved consciously to avoid using the term cell, not being sure that it inherits its own features, which are characteristic of typical, in particular plant, cells.

The anterior end of the embryonic field quickly expands (Figure 36, c) , and by forming two summits it produces S-shaped curved shafts (Figure 36, j), which Grube called the abdominal shafts. This name is well-founded, because these shafts, appearing in the field of the active hemisphere of the ovum, i.e. the future dorsal side of the embryo, are displaced towards its ventral side, where they later accrete into a single ventral embryonic region. The ventral shafts, according to Grube, on the ventral side of the leech embryo play the same role which the dorsal shafts play in the vertebrate embryos, because from them the wall of the body, in particular its muscles, is formed. The ventral shafts are composed of extremely small, closely adjacent other globules. At the posterior end of the embryonic field remain large globules which form part of its molecular (protoplasmic) content; three of these terminal globules usually are present on each side (Figure 36, k) . The internal part of the yolk globule (the divided ovum) constitutes a mass containing little protoplasm. This rich yolk mass is used mainly for building organs which are present in the abdominal cavity, in particular the intestinal canal; above it a layer grows, composed from a molecular mass which takes its origin from the ventral shafts.

Figure 36. The development of Clepsine (by Grube) .

a — fertilized ovum, b— ovum on which the white disk with grey ring appeared, c— first fissure; the white disk is present near it. d — yolk globule, divided into four parts, e— yolk globule, divided by six transverse fissures; the polar field increases as the surrounded globular segments decrease in size due to the loss of albumen (molecular) mass used for the formation of WANDUNGSBALLEN on the active pole, f — -the same stage, globules of division from the lower side; on the inactive pole the polar field is seen, i.e. the surface of the seventh segment, separated from the other round fissures, g — somewhat later stage, view from, above; the number of WANDUNGSBALLEN increasing, they then form a small disk which can be regarded as a rudiment (KEIM). h — WANDUNGSBALLEN increase in number and occupy a great area, forming the embryonic field, i — the view of the abdominal shaft from behind; they move so far apart that they envelop the yolk globule by a ring . j— the abdominal shaft from the side; on the posterior end of each, three whitish globules are present, k — the abdominal shafts unite together.

The rudiment of the neural cord appears in the form of two white stripes, joined to the external side by the recently formed embryonic stripes. After their closure on the ventral sides, the halves of the paired rudiment of the nervous system are united.

In this period the surface of the embryonic body is already covered with epithelium, composed of flat cells of different sizes and forms. At the same time in the body cavity dissepiments are situated, their number gradually increasing.

In the third part of Grube f s work he included changes occurring after the hatching of Clepsine from the egg membrane. Contrary to Filippi, Grube found very essential differences between the just-hatched leech and the formed worms. In the short, cylindrical little worm hatching from the egg, there are still no posterior sucking discs, no eyes, no blood vessels, and the formation of the dissepiments is also unfinished. As the young larvae are also immediately fixed by the anterior end, Grube assumed the presence of the rudiment of the anterior sucker with longitudinal and circular muscles. Within one day after hatching this sucker is clearly noticeable in the form of a convex ring; after two days this ring elongates, as Grube thought, under the effect of the heaviness of the body of the leech hanging on it. Inside the elongated ring a canal appears representing, according to Grube, the rudiment of the sheath of the still unformed proboscis. The wall of the intestinal canal, or yolk sac, consists at this time of large cells. Later, by means of a circular twist of the most anterior part of the body, the caudal disc develops and the proboscis is formed, which the larva can let out and pull in. The digestive canal is formed in this way. On the intestinal canal extensions appear, and in addition to this, it is divided into three parts. The anterior and posterior parts are significantly thinner than the middle one which composes the future stomach. In the last the more enlarged lateral pouches appear. Later on the eyes develop; at first they have the form of circular red spots. The vascular system develops, and, on the dorsal side of the body, the pulsating heart first becomes noticeable.

Grube finished the description of his observations with the following words: "The development considered here is probably spread extensively in the class ANNULATA. This is established on the basis of detailed investigations on many representatives of Naidae and Lumbricinae namely in Saenuris variegata Hoffm., Euaxes acutirostris Gr. and Lwnbvioulus variegatus Gr., and if it can be judged by an analogy with the adult animals, it will be also correct for the genus Lumbricus, and also for many sea worms" Cp. 45). With this, Grube carefully observed that discussion of analogy without special investigations can lead to erroneous conclusions.

A discussion of Grube' s work leads to the conclusion that he promoted the study of embryology of the annulated worms (annelids) and discovered phenomena the detailed study of which was done significantly later. His most important achievement in terms of recent embryology can be summarized in the following way:

1. Grube established that in the eggs of Proboscidea leeches there are polar plasmas; especially conspicuous are protoplasmic rings on the animal pole.

2. He recognized the complete, unequal division of the ovum of Proboscidea leeches, during which the animal polar plasma can go in one of the blastomeres of the four-celled stage.

3. Grube clearly saw how in the stage of the four blastomeres, from the animal side, very small globules of division began to separate; i.e. he established the fact of the formation of micromeres, which are composed mainly of protoplasm, and macromeres, which are rich in yolk.

4. Later he established that the number of micromeres increased either by separation from macromeres, or by means of multiplication of the early formed micromeres.

5. By Grube' s observations, the "rudiment" which is formed on the animal pole, owing to the multiplication of micromeres, spreads over the surface of the ovum, as a result of which the macromeres appear inside the embryo. This phenomenon very nearly resembles epibolic gastrulation.

6. At the end, Grube described the embryonic stripes coming out from three pairs of terminal cells; i.e. he discovered in the annulated worms the phenomenon of teloblastic development.

7. The embryonic stripes, in Grube' s observations, are displaced towards each other and are united on the ventral side. From the material of the embryonic stripes ("the abdominal shafts") the wall of the worm's bodytegmens, muscles, and nervous system is formed.

Grube 's excellent work was forgotten, and superiority in the study of the embryology of annulated worms, in particular the leeches, was accorded to Rathke, whose work was published eighteen years later. Rathke 's work without doubt has merit, but also has many defects, so that it can be seen as a step backwards in comparison with the work of Grube. Thus in the maxillary leech Nephelis vulgaris Rathke clearly saw the formation of micromeres, which in Clepsine complanata he did not see, and instead of the micromeres only granularity is illustrated in his drawing in the animal parts of the blastomeres. At a later stage it is shown as if the unlacing of the micromeres has begun, but he did not see whether this led them to division. Together with this, Rathke reproached Grube for seeking the source of micromeres CWANDUNGSBALLEN) in the depths of the globules of the division, while Rathke himself in general did not see their separation. According to his description, a number of hillocks on the animal pole of the divided ovum directly turned into a thickening which is the rudiment of the embryonic stripes. The embryonic stripes of Nephelzs are illustrated very schematically by Rathke; he did not observe the characteristic cellular rows in these stripes, neither did he recognize the connection between the only three teloblasts illustrated in his drawing which are situated as if they lie at the extreme ends opposite to the embryonic stripes and the middle one as if it lies between these ends. Rathke also represented the embryonic stripes of Clepsine less distinctly than Grube.

All this is mentioned not to underestimate the significance of Rathke ! s investigations, but only to drawattention to the more perfect work, in many aspects, of Grube, to call to mind its priority and to show the importance of his work in the history of Russian and world embryology.

In the 1830' s and 1840' s, the development of invertebrates also interested A. D. Nordmann (1803 - 1866). Aleksandr Davidovich Nordmann was the son of a Russian officer born in Finland. In 1821 he entered the university in Abo, and after graduation he worked for some years in Berlin with Rudolphi and Ehrenberg. During his stay in Germany he accompanied Oken, Tiedemann, and Chamisso on a trip for the study of sea fauna. In Berlin Nordmann published his first work, MICROGRAPHICAL INFORMATION, dedicated to the structure and taxonomy of parasitic worms (describing many new forms, in particular "spainika" - Diplozoon paradoxwi) , and also the structure and development of parasitic copepods.16 In 1832 Nordmann was invited to join the department of zoology and botany of Lyceum Rishel in Odessa, and in the following year he took the post of director of Odessa Botanic Garden C128) . In 1833 Nordmann together with Rathke, S. S. Kutorgaya and Steven travelled to the Crimea. In the following years he travelled much in the south of Russia; in particular, he led excursions in the Crimea of students of the Odessa Lyceum. When he was sent with a scientific mission to Paris, Nordmann visited with Milne-Edwards the coast of Normandy. In 1849 Nordmann came to Helsingfors University, where from 1852 to the end of his life he headed the department of zoology. During the period of his scientific activity, Nordmann published fifty-seven works in Russian, Latin, German, French and Swedish on anatomy, embryology, taxonomy and zoography of different groups of vertebrates (mammals, birds, fish) and invertebrates (insects, spiders, crustaceans and worms, mainly the parasitic molluscs, bryozoans and Coelenteratae) ; he also studied botany and paleontology.


In the previously mentioned MICROGRAPHICAL INFORMATION, the results of Nordmann ! s observations on the development of parasitic copepoda Aahtheres peroanon appear, and the larval stages of other related forms (Ergasilus Sieboldi Nordm. , Traoheliastes polycolpus Nordm. and Lernaeoaera cyprinacea) are described. These investigations met the need to explain the systematic situation of parasites, which at that time were related either to molluscs or to the annulated worms, or even to coelenterates. If the relationship of some representatives of the mentioned groups Cf° r example, Caligus) to crustaceans was to some extent only probable, then these forms, such as Lemaeocera , which in the adult condition are completely unlike arthropods, remained in their systematic relations mysterious.

The embryonic development of Aahtheres was described by Nordmann rather incompletely. "On the upper surface of the yolk," he wrote, "is found at first a more transparent region, an4 a granular part of the yolk, having the significance of a rudiment (KEIM) , turned into round or spherical forms from which the more peripheral give material for the formation of the rudiment membrane (KEIMHAUT) ... The latter completely envelops the yolk and... forms later on the wall of the body of the embryo (p. 78). After referring to the separation of the head and the appearance of the rudiments of the extremities, Nordmann turned to the characteristics of the nauplius larva, whose structure becomes complicated after moulting.

Its central point was the description of the larval stage of Lemaeooera oyprinaoea . The adult animals of this genus were already known to Linnaeus; they are characterized by a sacculated unopened body, deprived of extremities and organs of sensations (Figure 37, A). "If naturalists," Nordmann wrote, "are astonished at the structure of the body of the mature animal of this kind, their astonishment will be more natural when an opportunity arises for them to observe the young animals. It can hardly be imagined that there is anything more striking than an offspring having absolutely nothing in common with its parent. Before my eyes the egg receptacle in the mature animal was ruptured, and the embryos set free. I saw young animals exactly the same as I represent them in the drawing (Figure 37, B) ; they have extremities, antennae, and even bright red eyes" (128).

Nordmann 1 s observations met, according to him, decided distrust from Berlin zoologists to whom he demonstrated the nauplius larvae of Lemaeooera oyprinaoea. Later the significance of this discovery was universally recognized (129) .

After some years Nordmann published a small embryological work concerning the Black Sea bryozoan Tenchca zosterioola A 7 In one zooid Nordmann found from four to seven eggs and saw the penetration of the spermatozoids in the female cells through an opening in their base. Later he saw the embryos hatched from the eggs swimming by means of cilia and finally settling in the seaweed Zostera. "As far as it is possible," Nordmann wrote, "I observed the transformation of the young animals and the development of polyps from them." Greater significance was possessed, however, by other embryological work of Nordmann' s on the development of molluscs.

17. A. v. Nordmann, "Recherches microscopiques sur l'anatomie et le developpement du Tendra zosterioola, espece de polype de la section des Bryozoaires , " ANN. SC . NAT. ZOOL., 11 (1838), pp. 185 - 191.

Figure 37. Lernaeooera cyprinacea (A) and its larva (B) (by Nordmann) .

The molluscs, especially the gastropods and lamellibranchs, are considered easily available material for the study of development processes; therefore they early attracted the attention of embryologists . The division of the ova of the gastropod molluscs is described in the investigations of Van Beneden and Windischmann, and the division of the ova of the lamellibranchs — in the work of Loven. In the ova of Modiolaria marmorata {Mytilus aisoors) Loven observed a maturation division and described it, as a process of ejection of nucleolus (embryonic spot) . The important result of his work was the establishment of the fact that during the division of the ovum of Modiolaria, the separation in the vegetative hemisphere of non-nucleated lobes takes place which soon merges with one of the blastomeres. This phenomenon is repeated many times during the time of the following divisions. Loven saw clearly the process of division itself, i.e. the deviation of the ovum into separate blastomeres, but he did not trace the changes in the nuclei, the equal and mutual distribution of blastomeres .

The separation of polar bodies in the ovum of the grey slug is sufficiently clearly described in the above-mentioned works of Van Beneden and Windischmann, although the division following the separation of the polar bodies was inaccurately described by them; they spoke about the formation of elevations on the surface of the yolk, divided by fissures. As a result of this the entire surface of the ovum becomes at the end as if embossed (BOSSELE) and resembles a raspberry, The Belgian authors did not connect the formation of "the yolk cells" with the appearance of the cuts on the surface of the yolk; according to their concept, the yolk cells originate under the superficial layer of the yolk. The drawings illustrating their work show round bulges on the surface of the developing ovum, and not its division as a whole into separate blastomeres. The internal processes in the divided ovum and blastomeres are neither described nor illustrated in the drawings. Erroneous ideas of the processes of division in the gastropodan molluscs were retained in the work of 0. Schmidt appearing ten years later. 18 "The description of the ovum and the processes of division inside it up to the formation of the embryo," Schmidt wrote, "was given so completely by van Beneden and Windischmann that I cannot add anything to it" (p. 278).

18. O. Schmidt, "Uber Entwickelung von Limax agvestis , " ARCH ANAT., PHYSIOL. (1851), pp. 278 - 290 .

The subject of Nordmann's above mentioned work on the development of the gastropodan molluscs was the nudibranchiate mollusc Tergipes Edwardsii . The description of the phenomena of its embryonic development constitutes part of the extensive monograph, 19 beginning with the presentation of anatomical data. Nordmann described in detail the development of the ovum in the ovary (5 28 - 31), characterized the yolk elements of the ovum (§ 32) , the structure of the egg ready for oviposition (5 33) and its membranes (5 34) . The process of division {§ 39) of the ovum of Tergipes was described by Nordmann as the following: the first fissure of division can take place in different directions and divide the ovum either into equal, or into very unequal parts. The fissure dividing the ovum into four globules of division takes place at right angles to the first fissure, after which the ovum is divided into eight globules, and so on. The generally uneven character of division is noted in the following expression. "Although the tendency to even progressive division cannot be disputed, it also shows that the yolk, especially up to its acquisition of the mulberry form, appears to divide very unevenly" (p. 573) (Figure 38, a-g) . Nordmann noted especially that the fissures cut all the mass of the yolk, i.e. that the division was complete.

19. Nordmann, "Versuch einer Natur- und Entwickelungsgeschichte des Tergipes E 'dwards ii , " MEM . , pres . a l'Acad. Sc. St.-Petersb. par divers savants, 4 (1845) , pp. 495 - 602. A brief translation of this work in French was published by K. Fogt ("Essai d'une monographie du Tergipes Edwardsii," ANN. SC. NAT., 3, ser. Zool., 5 (1846), pp. 109 - 160).

All the course of development of Tergipes Nordmann summarized in the following statements:

1 . The chorion extend to one-fifth the diameter of the ovum and becomes oval . 2 . At this time a light fluid, similar to albumin, discharges from the ovum. 3. The yolk loses its spherical form, its mass loosens, and the contours become wrinkled. 4. The embryonic vesicle and the embryonic spot disappear. 5. The upper layers of the yolk lose their reddish coloration. 6. In too many cases separate yolk cells are separated from the other mass of the yolk and give origin to the parasitic creatures. 20

7 . The ovum is divided by a fissure into two globules .

8. The division process continues regularly.

9. The yolk acquires the form of a mulberry.

10. A bubble of air leaves the yolk (?) . 11. The surface of the yolk becomes granular. 12. First establishment of the embryo. The yolk acquires an elongated form, and then the form of a roughly outlined triangle. 13. The distinct appearance of the animal system and the cutaneous system. The configuration of the embryo. 14. Twisting of the anterior part of the embryo is noted (future organs of movement) . 15. On the wide anterior area folds appear, from which two lateral round growths are gradually formed. The growths are transformed into lobes and between them a third growth, the foot, appears. 17. Beginning of the formation of mantle and concha. 18. On the lobes the cilia grow. 19. The first movement of the embryo. On the foot the vibrating cilia appear. 21. The lobes of the sail become disc-shaped. The rotatory movement of the embryo. 22. Cells sharing in the formation of mantle are dissolved and disappear. 23. The concha significantly enlarges. 24. The isolated cellular rows indicate the formation of the attached muscles .

25. The formation of the internal organs, among which the intestine is distinctly differentiated.

26 . The liver and other glandular bodies are clearly seen. The anus and ganglia.

27. The cells forming the attaching muscles disappear. 28. Deposition of the pigment in the eyes . 29 . Between the chorion and the embryo the astonishing parasites appear to rush out. 30. The complete formation of the embryo larva; the roof of the shell opens and closes. 31. The extension of the chorion. 32. The presence of the young in the common ovum chamber. 33. The larva cuts the chorion. 34. Hatching, (pp. 565 - 567)

20. Nordmann gave this parasite the name Cosmella hydrachnoides and suggested the possibility of its origin from particles of the yolk of Terg-ipes.

Figure 38. The development of Tergipes Edwardsii (by

Nordmann) a-g — division of the ovum; h — larva,


From the above it can be seen that Nordmann's main attention was to the relatively late stages of embryonic development, while he investigated the process of division of Tergipes only extremely incompletely.

The brief discussion of the works of van Beneden, Loven and Nordmann on the embryology of molluscs demonstrated the level of knowledge in this sphere at that time. When the development of molluscs was studied by the Russian embryologist N. A. Warnek, his work can be unconditionally called classical (131). Warnek's scientific activity, unfortunately, was brief and was cut short by external circumstances,

Nikolai Aleksandrovich Warnek was born in 1821. When he was eighteen years old he entered the faculty of law at Petersburg University, but in the same year he transferred to a department of the faculty of philosophy. During his years of study, Warnek received in 1843 the gold medal for his "Process of Moulting of External Tegmens and Formation of Millstone in the Ordinary River Crayfish." In 1846 he graduated from the university with a candidate's degree, and for three years he taught botany and zoology in Gorn Institute. Warnek received his master of science degree for his work on the structure and function of the crayfish liver^l and in 1849 he started reading lectures to naturalists and medical men in comparative anatomy and physiology in Moscow University, first as junior scientific assistant and then as extraordinary professor (from 1852) . In 1848 Warnek wrote the vast work "On the Formation and the Development of the Embryo in the Gastropodan Molluscs," which was published two years later in the BULLETIN OF THE MOSCOW SOCIETY OF NATURALISTS; 22 a summary of this work was published abroad under

21. N. A. Warnek, "On the Liver of River Crayfish in Anatomical and Physiological Relations" (SPb., 1847), 40 pp.

22. warnek, "Ober die Bildung und Entwickelung des Embryos bei Gasteropoden," BULL. SOC . NATUR . MOSCOW, 23

(1850) , pp. 90 - 194.

the title "On the Process of Division and Structure of the Ovum in Gastropods. "23

In 1860 Warnek retired and departed to Tver; where for three years he was director of a secondary school, and then director of the schools of Tver Province.

Until recently it was thought that Warnek 's scientific and literary activity ended in 1863 and that in 1867 he died. From recent work by T. P. Platovaya 2 ^ it is now known that in 1880 and 1881 Warnek read a report to the Moscow Society of Naturalists on the biology of agricultural pests, and in 1884 a report on the microscopic structure of the fish ovum and on the morphology and taxonomy of fish. The exact date of his death remains unknown.

Concerning his retirement from Moscow University evidence has been kept which is probably not completely objective and in any case explains only incompletely the cause of his retirement from teaching and from scientific work.

In the multi-volume apologetical biography of the reactionary historian Pogodin, its author N. Barsukov25 reminisced about I. A. Mitropolsky, who was in 1850 a student in the Faculty of Medicine in Moscow University. According to Mitropolsky, when Warnek was reading zoology and comparative anatomy he was noted for an extremely scornful nature; due to this he was disliked by the students and medical men. One of Warnek 's clashes with the students, Mitropolsky reported, ended in a noisy scandal, in which both students and medical men participated. In order to confirm to the reader the ultimate rightness of the students, Mitropolsky declared that "the students could endure the tricks of the professor only if his lectures gave them anything useful. But they got nothing from them."

23. Warnek, "Uber den Furchungsprozess und die Struktur des Eies der Gasteropoden," FRORIEP ' S TAGESBERICHT , No. 280 (1851) , pp. 43 - 44.

24. See (131) .

25. N. Barsukov, ZHIZN'I TRUDY M. P. POGODINA (Life and Works of M. P. Pogodin), Vol. 16 (1902), pp. 116 - 117

Another aspect of this incident is mentioned in the reminiscences of L. V. Lebedinsky, which were published in VOICE OF THE PAST in 1912.26 Lebedinsky characterized Warnek as talented, but very proud and sharp in his treatment of people, who "excited against himself the students and medical men by his tactless relations with them and an unusual strictness during examinations." Handbooks in Russian in zoology and comparative anatomy were absent at that time, and Warnek suggested foreign books, which the students, due to their ignorance of the languages, found difficult to use. He did not read a systematic course, but selected works of his own, which he considered more important and interesting. In answer to the protests of students on this occasion Warnek declared that "the students of a university are not secondary schoolboys, they must work independently. The professor in his lectures has only to point out a direction and a method by which the students must carry out their work." The dissatisfaction of the students and medical men was expressed in the organization of meetings in which the students of other faculties shared, students of law and philologists who did not attend the lectures of Warnek and could not judge their effects and defects. In these meetings it was decided to criticize the professor, and then oblige him to leave the department. Only some of the students and medical men, Lebedinsky continued, strongly defended the professor. "They said that Warnek was regarded differently and they loved him for his talented presentation of the subject." The planned obstruction was nonetheless carried out, mainly by students of other faculties who shared in the action, especially those of the faculty of law. This produced on Warnek a stunning effect. "Warnek went into an adjacent room," Lebedinsky wrote, "where he fainted away; it is even said that blood gushed from his throat." Soon after this incident Warnek retired, insisting on it despite intensive persuasion.

26. UZ ZHIZNI MOSKOVSKOGO UNIVERSITETA. VARNEKOVSKAYA ISTORIYA. "GOLOS MINUVSHEGO" (From the life of Moscow University. Warnek' s History. "Voice of the Past") (Otd. ottisk, pp. 210 - 218). For this source, as well as for the answer in HERZENOVSKY "K0L0K0LE" (see [132]), the author expresses deep gratitude to V. V. Sorokin .

Already this comparison of evidence shows the impossibility of placing complete responsibility on Warnek himself for what occurred. The objective evidence of Lebedinsky about Warnek f s talented lectures to the students and medical men and their good relation to him unmasks the tendentious assertion of Mitropolsky that Warnek' s lectures gave nothing to the students. The impression one has of Warnek is of a serious, exacting professor who loved his subject and taught it on a high level. Certain students and medical men understood his efforts and valued his services to them, while the majority of students were interested in the applied questions and not in understanding Warnek' s theoretical views, and saw him only as a strict examiner who shamed them, in addition to his ridiculing of their ignorance.

In all this sad "history of Warnek" there is still one significant side which can be read between the following lines in Lebedinsky 's memoirs: "Even among the professors there were people who sympathized with the students, the professor of theology Sergievsky among them. This handsome, somewhat fanciful, young orator was sometimes present in the department. He unnoticeably and cleverly approached the evils of the day, said a few words hinting at an excellent understanding of the students, and was zealously rewarded with applause. In the time of the aforementioned event, one of his lectures, which was full of these hints, ended with the following significant words: 'Yes, this darkness does not triumph over the world!"'

It is logical to ask why Warnek was not pleasing to the professor of theology. What world of darkness and damage spreading from Warnek 's department was frightening to Sergievsky? To answer this question is not difficult. Warnek was a convinced materialist, as is easy to ascertain from a study of his basic work. It is improbable that in his lectures Warnek did not touch upon questions of ideology, which, of course, was used against him by Sergievsky, who considered it his responsibility to protect the world of "religion" from the "darkness" of the natural-scientific materialism. Sergievsky's Jesuitical activity made the best use of the dissatisfaction of the protesting group of students, who played a role obviously of considerable importance in the organization of the obstruction, which took place with the obvious support of the administration of the university, usually so vigilant when the matter is concerned with student disturbances (132) .

The fault, or rather misfortune, of Warnek was his sharp and derisive character, his inability to adapt himself to his surroundings. Warnek did not want to reconcile himself with the manifestations of ignorance, even if its carriers were respectable scientists. This can be witnessed by Warnek 's review of a scholarly book by A. Bogdanov. 27 This review shows great and extensive knowledge and unquestionable educational talent of the reviewer and his understanding of the problem of the teaching of natural sciences. In addition, the review is written in an overly particular and caustic tone; Warnek did not let pass any mistake, any slip of the tongue, or any lame expression from the author of the book. It is easy to imagine what reaction Warnek' s article could produce in Bogdanov. If Warnek behaved similarly in the professional milieu of Moscow University, and used to deride the lectures of his colleagues, then he undoubtedly provoked against himself not only a certain group of students, but also some professors.

Valuable evidence of Warnek 's high character as a lecturer and scientific worker is contained in the words of I. M. Sechenov : 28

Junior Scientific Assistant Warnek taught us zoology. He read simply and clearly, dwelling mainly on general signs applied in zoology departments, and the description of protozoa was prefaced by a long treatise on the cell in general. This last study was built, however, on unprepared ground. Moscow still did not think much at that time of the microscope; among Warnek' s students it was not used successfully, and in mockery they nicknamed him "Cellular."

27. Warnek, "Zoologiya i zoologicheskaya khrestomatiya v obieme srednykh uchebnykh zavedenii , " Author Anatolii Bogdanov, OTD. PERVY ZH. MNP, ch. 118 (1863), pp. 47 - 73.

28. I. M. Sechenov, AVTOBIOGRAFICHESKIE ZAPISKI (Izd. AN SSSR, 1945) , 176 pp.

In a footnote Sechenov added: "Too late, I have learned that Warnek and the famous botanist Tsenkovsky were among the first Russian biologists who worked at that time with the microscope."

On Warnek' s socio-political opinions there is no information. His statement against Eogdanov's book, based on the principles of Darwin's evolutionary study, was published in a government journal; objectively, it could have played a reactionary role. The mutual relations of Warnek with the students creates the suspicion of the possibility that the rising conflict was of a political nature. Concerning Warnek 's "political orientation," it can be seen that he was later assigned responsible posts in the Department of Education. A search of the archival material may throw light on this subject.

For a characterization of Warnek' s scientific work and world view, it is necessary to gain acquaintance with the contents of his basic work, "On the Formation and Development of the F:mbryo in the Gastropodan Molluscs. "^ As an epigraph to this work Warnek selected the words of Reii: "The phenomena of individual life are the necessary result of formation and merging." This idea, as can be seen from the final chapter of this work, Warnek interpreted materialistically. Warnek himself also considered that only in the definite mixing of substances in definite spatial position [form) can the solution of vital phenomena be sought. Science, in his opinion, is not in need of additional idealistic assumptions.

As it is seen from the introduction of his work, Warnek considered his principal task to be the explanation of how the yolk, i.e. ^substances of the ovum, are transformed into tissue of the embryo and what conditions this transformation. The solution to this question, in Warnek' s opinion, is possible only through a thorough investigation of the processes of development, and that is why he also outlined the following vast program. First of all, Warnek suggested, it is necessary to study the reproduction of molluscs, either hermaphrodites or separate sexes. For this aim the following must be studied: 1) structure of male and female sexual organs; 2) origin of

29. See footnote 22 of this chapter

embryo, i.e. development of yolk (ovum) and semen; 3) processes occurring during copulation, i.e. the influence of sperm upon the yolk, and finally 4) formation of the additional parts of the ovum — albumin, membranes and ovum cocoons .

Only after this it is possible to begin the study of embryonic development, which Warnek divided into two periods. The first period includes the development of the fertilized ovum, i.e. the process of division and preparation of the development of organs, and the second, the period of development of all systems of organs of the developing animal.

In his investigations Warnek proceeded from the following situations established by the embryologists: 1) for the transformation of yolk into an embryo fertilization is absolutely necessary; 2) this last consists of the material influence of semen on the yolk; 3) this influence takes place only in an infinitely small space, therefore the spermatozoa of the semen must come in direct contact with the mass of yolk; 4) the result of fertilization is the formation of the elementary organs of the embryo (cells); 5] the cells acquire different forms, grouped in complicated organs of the embryo, and thus form its body.

Warnek expressed regret that embryologists could not completely solve the following important questions related to the development of animals: 1) how the yolk, i.e. substance of the ovum is transformed into tissue of the embryo; 2) how its transformation at the time of development occurs; 3) in what does the secret influence of the semen on the yolk consist.

The first question Warnek considered to be solvable, and he expressed the hope that in a short while the other two questions would also be explained.

The solution of the principal questions of embryology, in Warnek' s opinion, could take place on the basis of the following presentations:

The cause of the phenomena conditioning the beginning and the subsequent development of the embryo is ordinarily attributed to the vital power, which absolutely clearly shows that the beginning of life must be sought in the formation of the ovum in the ovary, and the beginning of its development in fertilization, namely in the influence of semen on the yolk. And, of course, the essence of the powers which condition all phenomena of nature remains unknown. If all this can reduce the different phenomena to one cause, this will make a great step toward the goal we have established, (pp. 94 - 95)

During the last four years, aspiration to a solution of the aforementioned general questions was my dearest hope. Devoting myself year after year to the study of the formation, development, and functions of cells, I have stuck to the idea that elucidation of the ideas of development and activity of the cells is the only way to select from the labyrinth of recent presentations about the organic world. The explanation of the causes of the vital activity in the cells leads to a clear presentation about life in general and about causes directing it throughout the organic world, (p. 95)

The following work of Warnek represents the first part of investigations of his planned program; it is concerned with ovum structure and processes of its division in the gastropodan molluscs.

Warnek began with the description of the form of oviposition of the different fresh-water snails; he detailed the periods of oviposition in the region of Petersburg and the structure of gelatinous mass surrounding the ovum. Later he described the structure of the laid ova, especially in the species used in this work: Lymnaeus stagnalis and the slug Limax agrestis . Transferring to the principal part of the work, throwing light on the study of division, Warnek paused at the characteristics of the yolk granules filling the fertilized ovum. Among these granules Warnek saw a light spot which was not delimited from the yolk membrane and was always situated in the center of the ovum. All the process of division of the ova of the molluscs which he studied Warnek divided into stages, described in succession.

FIRST STAGE: For this stage, characteristic phenomena are taking place in the above mentioned light spot, which, in Warnek's words, occupies the place of the rudimentary (embryonic) vesicle. This spot is in the beginning completely round (Figure 39, 3), then becomes elongated and subsequently takes the form of a biscuit and the shape of a figure eight (Figure 39, 4 and 5). After that when the spot (i.e. nucleus) is twisted by the means just described, it comes nearer to a certain region of the periphery of the yolk (ovum) . The end of the spot which is turned to the surface of the yolk widens, and it acquires the form of a blunt rounded cone, in addition between the outer area of this cone and the membrane of the yolk a transparent crescent-shaped region appears (Figure 39, 7) . From this crescent-shaped region two small vesicles become separated, which, being isolated from the ovum, remain near it throughout the following development. The place of deviation of these vesicles becomes the center of formation of fissures, later dividing the yolk into two, then into four parts.

The separation of the vesicles Warnek described as follows: on the external surface of the crescent-shaped region a small elevation appears under the yolk membrane. It gradually enlarges, acquiring the form of a spherical segment, a hemisphere, then a complete globule, which is set on a sufficiently thick stalk. Then this stalk becomes unlaced, and the globule becomes free (Figure 39, 9). After the formation of one vesicle the second one appears exactly as the first (Figure 39, 9 and 10). Thus, in Warnek's observations, the crescent-shaped region separates the forming vesicles from the light spot (i.e. from both nuclei of the ovum) . He concluded that the nucleus does not share in the formation, at least, of the external vesicles. He made this erroneous conclusion because the intravital observations which Warnek used did not suggest tracing the processes taking place in the nucleus. From this came the further erroneous claim that the separating vesicles could not be regarded as the vesicles of Purkinje or its remnants. In accordance with this assertion Warnek refused to recognize for the vesicles separated from the ovum that important role claimed for them by many authors, and he objected to the name "directing vesicle" {vesioula directrix') . Warnek did not like this name as it returned embryologists to the time when they believed in an Archean spirit directing vital phenomena.

The presence of vesicles where the formation of fissures of division begins did not prove, in Warnek's opinion, that the topographical position of the fissures was determined by the vesicles. Preferably, as he thought, the matter was the contrary: the vesicles are separated where the center of division is present. In later stages of division many fissures appear without preliminary separation of the vesicles

After the separation of both vesicles the transparent crescent-shaped region also disappears. In the ova of the slug there appear two nuclei distinctly separated from each other. At this time the nuclei each acquire distinct contour and a large nucleolus.

THE SECOND STAGE begins with the loss of the membranes of the nuclei and their merging into one common mass. This mass acquires an extended form, situated on the longitudinal diameter of the yolk, i.e. at right angle to the position which is characteristic for the first stage (Figure 40, 11) . Then the nucleus becomes biscuit-shaped (Figure 40, 12) and at the same time the division of the yolk begins. In the last description Warnek used topographical terms, and their significance is explained in the literal remark. The transverse diameter he called the diameter; passing through the vesicles from the yolk at right angle to it the longitudinal diameter is situated. The terminal points of these diameters he called poles: the dorsal pole in the place where the external vesicles were present; the ventral pole situated against the dorsal one; the poles of the longitudinal diameter designated as right and left.

The division of the yolk is preceded by a thickening of the dorsal pole, in its field; then a fissure in the form of a cut appears. Due to its deepening the ovum becomes in form more like the kidney (Figure 40, 13) . The direction of the fissure does not coincide with the transverse diameter; it is inclined to it at a 45-degree angle. The light spot decreases and becomes less noticeable even in the ova of the slug, and in (Lymnaeus stagnali-s) it is not seen in the majority of cases from the very beginning of the division.

Figure 39 . Warnek ' s drawings for his work on the development of gastropodan molluscs (development of Lymnaeus stagnalis)

3-5 — the division of the light spot in the fertilized ovum;

7 — at the external area, light spot coming near the surface; a light crescent-shaped formation appears; the beginning of the formation of elevation;

the elevation acquires the form of a club-head;

9 the second elevation begins to form;

10 — the second vesicle separates; the light spot becomes spherical; the crescent-shaped region disappears;

19 — completely separated yolk globules;

22— the beginning of secondary nearness of the yolk globules ; between them a light space ;

25— the greatest nearness of the yolk globules; the nuclei are not seen;

25a— no n- simultaneous division of nuclei into two primary yolk globules;

27— non-simultaneous division of yolk globules;

28-30 — gradual distortion of first fissure of division;

3 4 cross -shaped position of the globules of division;

36a — no n- simultaneous division of the nucleus into four yolk globules;

40— globules of division of the fourth stage (la- Id) are situated in the space between globules of the third stage CIA- ID) ;

43— extension of nuclei in the yolk globules of the third stage (1A-1D) ;

45— the yolk globules of the fifth stage (2a-2d) separated from the globules of the third stage (2A-2D) and situated between the latter;

46a — adjacent situation of nuclei, showing the origin of the yolk globules of the fifth stage from the globules of the third stage;

47 — the yolk globules of the sixth stage (la 2 -Id 2 ) separated from globules of the fourth stage (la 2 -Id 2 );

50 — the ninth stage of division.

Figure 40. War-nek* s drawings for his work on the development of gastropodan molluscs (development of a slug)

11 — in the place of the lost membrane of the nucleus, the oval light spot is seen;

12— the yolk has the form of a globule thickened from one side; the light spot is elongated;

13 — in the thickened side of the yolk the beginning of the formation of a fissure is seen; the light spot becomes more elongated;

14— the fissure twists the yolk diagonally; the light spot is extended;

15 — the fissure envelops half of the periphery of the yolk; at the ventral pole is the beginning of the formation of a fissure in the form of a deepening;

16 — deeper twisting of the yolk, viewed from the dorsal pole;

17 — the yolk globules are completely separated from each other; the light spot is hardly noticeable;

26 — the nuclei are deprived of membranes and begin to extend in a direction perpendicular to the longitudinal pole;

26a — still more distinct changes, the beginning of which is illustrated in Drawing 26;

29a— the stage of four yolk globules.

On the basis of experiments on ova which had been pressed in water, Warnek concluded that the structural changes in the nuclei depend upon the change of their chemical composition. The chemical changes that appear, in Warnek' s opinion, condition also the further transformations in the developing ova. After that when the fissure begins in the dorsal side (Figure 40, 14), it passes around half the periphery of the yolk; also in the ventral side, a deepening appears (Figure 40, 15) and the yolk is twisted by the meeting fissures. It acquires first the form of a biscuit, and then two united or even completely isolated globules (Figure 40, 17) . At the time of division of the yolk the light spot (nucleus) divides into two parts, each of which at first have caudiform processes, directed to the point of contact of the yolk globules (Figure 40, 16) . The processes quickly disappear, and the spot becomes spherical.

These phenomena, according to Warnek, characterize the first half of the second stage of division. At this time it is not possible to isolate the nuclei by (pressing) the yolk globules; from which it must be concluded that the nuclei are still deprived of membranes,

The second half of the second stage begins with the dividing globules moving nearer. Between them a noticeable cavity, formed from a transparent substance, emerges . Warnek considered this transparent substance the product of separation of the yolk globules. At the time the dividing globules are moving closer, the contours of the nuclei in them again become clear, i.e. the membrane appears (it is clearly noticeable in Li-max) by strongly refracting the color of the nucleolus. In Lymnaeus stagnalis the nuclei, at first, are situated near each other (Figure 39, 22), then separate, sink in the depth of the yolk globules, and at the end become invisible (Figure 39, 25) . Following the described phenomena each of the two yolk globules become pear-shaped. This outlines the passage to the third stage of division, during which the yolk is divided into four parts.

THE THIRD STAGE begins with changes in the nuclei, acquiring a biscuit or figure-eight shape (Figure 39, 25a); in the drawing it is seen that their division is not accomplished simultaneously: when one divides, the other keeps the form of a quadrant. The details of these changes can be traced only in the transparent ova of slugs. When the yolk takes the biscuit form, the nuclei are elongated (Figure 40, 26); following this the elongation and twisting of the yolk globules themselves takes place (Figure 40, 26a) . The twisting in Lymnaeus stagnalis and slugs begins in one globule earlier than in the other (Figure 39, 27), however, soon after this difference smoothes out. The boundary between the globules of division, corresponding to the first fissure, is at first straight (Figure 39, 27), and then becomes curved (Figure 39, 29, 30), and the yolk globules are situated crosswise in two planes.

The processes taking place in the nuclei were described by Warnek as follows: "The membranes of the nuclei disappear, the nuclei elongate, take an oval biscuit-shaped form, then bulge out, and finally each nucleus from the beginning of the division of the yolk globules is divided into three parts. From these parts of the nuclei only four are present in the globules of the division, and two gradually disappear in the fissures between the globules. The four nuclei at first have the form of a comet. When the division of the yolk globules is finished, the CAUDIFORM processes of the nuclei extend and the nuclei again acquire the rounded form" (pp. 146 - 147). The three parts into which, according to Warnek, each nucleus is divided correspond to the two daughter nuclei and to the achromatic figure of mitosis situated between them.

The final step of the third stage is the formation of the membranes around the nuclei and subsequent turning of the cross-shaped globules of division. Two of them are in contact with each other on the dorsal side, and the other two on the ventral (Figure 39, 34) . Between the yolk globules a rhomboid space appears during which this is especially clear in Limax.

THE FOURTH STAGE . At the beginning of the fourth stage the nuclei in the yolk globules of Lymnaeus stagnalis again become unnoticeable from outside. During the crushing out of ova it is possible, however, to see the changes occurring in the nuclei, which, as in the previous stage do not take place at the same time. In Figure 39, 36a it is seen that in the two globules of division which have the longitudinal form, one nucleus is biscuit-shaped, and the other consists of two isolated parts. After that, when the nuclei become elongated (in slugs this is seen also in the intact globules of division] , each yolk globule stretches and becomes pear-shaped. Then the twisting occurs in this form so that the newly forming globules are of unequal size, each separating a region of one third the size. The four smaller globules become displaced and are situated in the spaces between the larger two (Figure 39, 40) . "The remarkable reciprocal situation of the yolk globules," Warnek wrote, "is kept and is repeated in all the following stages; this allows one, without ever noticing the further formation of the yolk globules, to solve the question, what globules of division result from each present globule. During this it is necessary to keep in mind the position of the nuclei and the relative size of the yolk globules" (p. 153).

These accurate observations surpassed those investigations of nearly a quarter of a century afterwards. By the initiative of A. 0. Kovalevsky, the blastomeres of the dividing ova were given individual designations (in letters and numbers) , tracing during the process of development the fate of each blastomere and its derivatives. Warnek formally applied a less suitable and obvious method of designation of the globules of division and their descendants. He named the blastomeres arising in one or the other stage by the number of this stage, keeping for them the same designation also in the following stages of division. During this he mentioned that the yolk globules changed from stage to stage, so that, for example, during the transfer to the fifth stage the globules of the fourth stage were already unequal to the globules of the fourth stage at the moment of their formation. They decrease in size and are changed by chemical properties and internal structure.

THE FIFTH STAGE. In this stage the formation of the new globules of division follows the rules which also hold for the following stages of division. Instead of the sixteen yolk globules which must be present if each of the eight globules of the fourth stage is divided, here only twelve globules are found. This can be explained by the fact that during the fourth stage only four large globules are divided, and the other four small globules remain unchanged. Before the division itself the nucleus of the ventral (large) yolk globules become invisible, although on crushing out of the ova, it is seen that they elongate, i.e. they are present in a condition of division, while the nuclei or the dorsal (small) globules are not divided and remain round (Figure 39, 43) . The dividing yolk globules are stretched and twisted in the diagonal direction (Figure 39, 45). The newly arising yolk globules (again smaller in size than those which gave them the origin) are generally situated by the general rule, in the spaces between the large vegetative globules. The twelve globules of division present in the fifth stage are situated in three rows. The ventral row consists of four globules of the third stage, in the dorsal side four yolk globules of the fourth stage are present, and between the ventral and dorsal globules four newly arising yolk globules of the fifth stage are situated. The globules of the upper and lower rows stand against each other, and in the spaces between the globules of these rows the globules of the middle row are present. Concerning the origin of globules of the fifth stage from the globules of the third stage it is judged by the neighboring situation of their nuclei (Figure 39, 46a). In the second half of the fifth stage, as in the previous one, the smoothing of the surface of all globules of division and their nuclei becomes more distinct. Between the yolk globules a vesicular light space appears.

THE SIXTH STAGE. At the beginning of this stage four yolk globules of the fourth stage become more convex and the globules of the sixth stage separate from them. The yolk globules of the fifth stage remain the spaces between the globules of the third stage (Figure 39, 47). The total number of globules of division in this stage is sixteen.

THE SEVENTH STAGE is characterized by the three divisions of the globules of the third stage, giving rise to the four yolk globules of the seventh stage. The total number of globules of division is twenty.

THE EIGHTH STAGE. In this stage the four globules of the eighth stage are separated from the yolk globules of the fourth stage. The total number of yolk globules grows to twenty- four.

SUBSEQUENT DIVISION. In each of the following stages four yolk globules are formed. As an example the ninth stage can be employed. In the given drawing (Figure 39, 50) the dividing ovum is pressed, so it is possible to see a great number of globules of division. In the middle the yolk globules of the fourth stage are situated. To these last the oldest and large globules of the fourth stage are adjoined, moving far aside from each other by pressure. To the left of the globules of the third stage the globules of the fifth stage are present, obliquely from which the youngest globules of the ninth stage are twisted; they are situated in the spaces between the globules of the fifth and seventh stages. The globules of the seventh stage originate also from the globules of the third stage and are situated to the right of the last globules. Finally between the yolk globules of the fifth, seventh and fourth stages the globules of the eighth stage are present, arising as a result of a second division of the globules of the fourth stage.

Further Warnek carried out an analogical analysis of the fifteenth stage and established the origin of all globules present at this moment of division.

It is instructive to compare the genealogy of blastomeres, established by Warnek, in gastropodan molluscs with recent data. This comparison shows the complete agreement of Warnek 's results with recent data, as seen from the table. In it the designations applied by Warnek and the presently applied literal numerical designations are given.

The same comparison is given in Figure 41, where the contours of drawings 40, 45, 47 and 50 of Warnek are repeated and the data of the recent designations of blastomeres and their designations by Warnek are compared.

There is no doubt that Warnek completely and distinctly chose to follow the fate of the separated blastomeres ("yolk globules") and the participation of their descendants in the formation of organs of the developing animal. Selecting an irreproachable method by which this problem can be solved, Warnek in the first published investigations on the embryology of gastropodan molluscs described the first period of development up to the formation of the spherical multicellular stage, i.e. the blastula. Only at the end did he briefly mention the following period, when "some yolk globules share in the formation of first internal organ — the yolk sac." This first internal organ, the yolk sac, is of course nothing other than the endoderm of the embryo.

Concerning some details of division, Warnek noted that during the division of the nuclei of the yolk globules the nucleolus appears earlier than the nucleus when divided into two parts, therefore it is possible to find nuclei with two nucleoli. Warnek never saw the process of division of the nucleolus itself. The division of the nucleus in his experience was always accomplished by one plan, which in the early stages of divisions was the same as in the subsequent development of the embryo. This division in all conditions takes place after the stage of stretching of the nucleus, which then acquires the shape of a biscuit and a figure eight and is finally transformed into two separate nuclei.

The globules of division Warnek identified as the elementary organs, i.e. cells, and considered that their multiplication, beginning at the time of division, continued throughout the period of development and even through the entire life of the animal.

All the activity of the developing embryo and the animal forming from it is, in Warnek' s opinion, the result of that primary influence which the ovum ["yolk mass") is subjected to by the semen. "This influence," Warnek wrote, "has a purely chemical nature; therefore the explanation for this is still obscure for us; the vital phenomena must be given by physicists and chemists" (p . 168). "The effective element in the organism," Warnek continued,

is the material; this same material influences also outside the organism. If we explain this activity by chemical and physical powers, then there is no reason to deny the activity of these powers in the organism as well. Although these powers still cannot be completely explained, we do not possess the right to discard them and resort to the help of this power, which exists only in the imagination. Can we explain the phenomenon of crystallization? Why does sodium chloride always crystallize in the form of a cube, and pure carbon in the form of an octahedron? Is not the formation of globules of division, from the point of view of form, a kind of crystallization of organic matter? The successes of organic chemistry belong to us, because the processes accomplished during nutrition, respiration, and excretion are more satisfactorily explained by means of physics and chemistry than by means of a special vital power. This power has retreated into the dark field of our knowledge about the functions of brain and nerves and still dominates in the sphere of embryology. However, new histological directions make the study of the vital power even more unsteady in this sphere, so we are not far from the time when chemistry will completely exclude it from there as well. The concept of vital power must remain as a reminder of our previous ignorance. Only quite recently the influence of semen on the yolk was called dynamical; this expression shows only that the phenomena of fertilization could not be explained, (p. 170)

In these words Warnek exhibited the materialistic world view with complete clarity. He decisively objected against the dull idea of the vital power for the explanation of phenomena of organic life. The only way in which this explanation could be achieved Warnek considered to be the physico-chemical investigations of vital phenomena. Regarding fertilization as a chemical process, Warnek thought that the subsequent transformations of the dividing ovum have as their source continuous chemical changes. Of course, Warnek' s materialism has a mechanical character, but it is not excessively simplified vulgar materialism.

Warnek 's embryological opinions are expressed in his theses, the most important being:

The yolk mass after fertilization undergoes chemical changes, therefore the fertilization itself must be regarded as a chemical process. It causes changes in the unfertilized ovum which are necessary for further development of the embryo

Table The Genealogy of Blastomeres

Comparison of Warnek's designations— stage numbers (in parentheses! in comparison with the recent literal-numerical. Cln the square brackets the designation of the resulting blastomeres of the previous stage are repeated.)


1 . Stage of division by Warnek

5. The sixth

2. The third

6 . The seventh

3. The fourth

7. The eighth.

4. The fifth

Figure 41. Comparison of Warnek's designations of blastcraeres (stage numbers) with the recent literal-numerical designations of blastomeres . These and the other designations are put in the contours of Warnek's drawings .

During the development of the embryo, further changes of chemical processes take place.

The gastropodan molluscs are characterized by complete division.

The globules of division may be considered true cells.

In each stage of a division process four yolk globules are formed, i.e. the division proceeds not in geometrical, but in arithmetical progression.

Beginning with the third stage, the globules of division have unlike sizes.

Warnek's work produced a new page in embryology, directing the investigations of the history of individual development towards the study of subsequent changes of the fertilized ovum and the forming from it of blastomeres, and towards a study of the fate of the separate blastomeres and their descendants during the subsequent formation of the embryo. In this sense Warnek's investigation foreshadowed the works of A. 0. Kovalevsky and his countless followers who were studying either descriptively or experimentally the transformation of the elementary organs of the dividing ovum the blastomeres into systems of organs of the forming organism

N. A. Warnek was for a long time undeservedly forgotten. His classical work is rarely cited and not always mentioned even in the embryological summaries and textbooks, although he unquestionably deserved a place of honor in the history of Russian and world embryology.

The investigations of Grube, Nordmann and Warnek were monographical descriptions of the embryological development of certain representatives of the invertebrates. These works, with all their significance, did not answer, however, the requirements of comparison of the phenomena of development in different types of animals. The first attempt to include a wide number of invertebrates in embryological investigations was done by A. Krohn, whose services in this sphere are much undervalued. Krohn was so thoroughly forgotten that his name was not mentioned either in the encyclopaedias or in the biographical reference books. 30 The following circumstance is sufficient to attract the attention of historians of Russian science to Krohn.

30. For help given in researching biographical and bibliographical data about Krohn, the author thanks the biological section of Saltykov-Sedrin Gos. Publichnaya library in Leningrad, especially librarian V. L. Levin

During the first committee discussion of Baer's prize of the Russian Academy of Science in 1867, considering possible candidates for the prize, the following was stated: "If the matter concerned the crowning of previous scientific works, then the committee does not doubt that the prize belongs to one of our compatriots, Krohn, who was born in Petersburg. For many years from the fertile shores of the southern seas he collected a rich material which he investigated for the development of different animal forms. His investigation resulted in many excellent works which deserve respect from the scientists of all countries. However, the competition was to take under consideration only the works of the last three years. "31

The absence of biographical information about Krohn is compensated for by some bibliographical data. It is established that Krohn published no less than eighty works, 32 including some small monographs. Many of his publications were accompanied by indications of the time and place of performance of the corresponding work. With these indications one can form opinions about the life of Krohn, who spent no less than thirty years in travel with the aim of scientific investigations, zoological and embryological .

31. "Extract from the report of the committee on the discussion of the prize of secret adviser K. M. Baer, read in public meeting of the Academy of Science on February 17, 1867 by Academician Ovsyannikov, " NATURALIST (1867), Vol. 4, No. 7-9, pp. 98 - 104; No. 10 - 12, pp. 146 - 148. The extract cited is on p. 99.

32. The list of publications by A. Krohn is presented in the following :

1. CATALOGUE OF SCIENTIFIC PAPERS, compiled by the Royal Society of London (Vol. Ill, 1869- Vol. VIII, 1879) ;


33. Information on the dates of Krohn' s birth and death are taken from the brief bibliographical catalogue published by the Library of Congress in Washington.

August David Krohn was born in Petersburg in 1803.33 Concerning his birth and student years of study of Krohn we could not discoyer any information. One of his early works was produced in Vienna (1836); of the next work there is a memorandum, Petersburg (1 83 7) , Later on Krohn was in Heidelberg (18:59], and from 1840 he worked nearly continuously on the shores of the Mediterranean Sea and the islands of the Atlantic Ocean. In 1840 and later he was in Naples; from the autumn of 1844 to the spring of 1845 in Messina; 1848 in Nice; 1850 in Naples. In the beginning of 1835 and the winters of 1853/54 and 1856/57 he again was in Messina; in the winter 1855/56 and the spring and summer of 1865 he worked in Funchal (in Madeira) and in Santa Cruz (Tenerif e) . In December 1860, in May 1861 and in 1867 Krohn was in Nice, and in the first half of the year 1869 he was in Naples. In the intervals between travelling he lived in Paris (winter of 1851/52, spring of 1857) and in Bonn (summer months of 1851, 1853, 1855, 1857 and 1859, winters of 1859/60, 1864/65 and 1856/66, and also the second half of the year 1869) .

Concerning the last twenty years of Krohn 1 s long life of eighty-eight years there is again no information.

During his travels Krohn maintained contact with his country, as seen by the report of the conference of the Academy of Science in Petersburg, which presents the following records:

"Mr. August Krohn is a doctor who is famous for his works on anatomy and physiology. He has sent the Academy a significant collection of invertebrate sea animals which was collected by him near Naples, which, in quantity and quality, deserve the thanks of the Academy" (Report from December 16, 1842) .

"Dr. Krohn sent again, as a gift to the Academy Museum, two new collections, about one hundred species of fish, crustaceans and others" (Report of September 13, 1844).

On the 7th of November 1855 Krohn was recommended as candidate for corresponding member of the Academy of Science in Petersburg, hit was not elected. The biographical data given during this presentation shows only that he was born in Petersburg, lived abroad, and wrote about thirty valuable works dedicated to molluscs (IPaludina, Phyllirhoe and oephalpoda) ,

worms (SipunculuSj Syllisj Aloiopa) , and tunicates {Doliolum) , 34

During his travels Krohn entered into friendly relations with many great zoologists of the time, such as Johannes Muller, M. Sars, and Delle-Kyaie, and also the young investigators A. Kolliker, 35 k. Gegenbaur, 36 an( j a. Schneider. Johannes Muller (1801 - 1851) was a great German zoologist, embryologist and physiologist. For a long time he was the editor of the widely distributed journal ARCHIV FUR ANATOMIE, PHYSIOLOGIE UND WISSENSCHAFTLICHE MEDIZIN, in which Krohn published about thirty articles. Part of this information Krohn sent to the editor of the journal in the form of letters, containing information about his last works, and Muller published them in his ARCHIV which he sometimes accompanied by remarks and additions, always with a friendly and positive tone. In those remarks of Mill Her' s, discussions can frequently be found revealing his high regard of Krohn' s scientific activity.

Krohn willingly related his observations to the zoologists who were working at the same time with him along the sea coast C133) . His objective was to verify his data and to confirm their authenticity, and equally to help also the beginning investigators. His description of the planula hydromedusa Cladonema, Krohn accompanied with the remark that he showed them to Sars and Gegenbaur, thus certifying the accuracy of his observations.

34. Archives AN SSSR, fund 2, inventory 17, No. 6. The author is deeply grateful to B. E. Raikov. On a commission from him, extracts of reports of conferences of the Academy

of Science were carried out here and an Archives Certificate received.

35. "For accurate information about the structure of marginal bodies in medusa," Krohn wrote, "I am grateful to my young friend Kolliker from Zurich" (A. Krohn, "Einige Bemerkungen und Beobachtungen uber die Geschlechtsverhaltnisse bei den Sertularinen," ARCH. ANAT . , PHYSIOL. (1843), pp. 174 181} . Albert Kolliker was a well-known histologist and embryologist .

36. Karl Gegenbaur (1826 - 1903) later became a famous comparative anatomist and embryologist.

Anton Schneider (1831 - 1890) was a famous German zoologist, who in a work on the development of the mollusc Phyllirhoe bucephalumS? warmly mentioned "the repeated friendly directions" which he received from Krohn not only during the observations taken for that work, hit also throughout the time of their presence together in Messina in the spring of 1858. Significantly later, in 1867, Krohn met with Schneider in Nice; Schneider was interested there in larvae of any polychaetes which were covered with peculiar porous membrane. Schneider found that these larvae were well known to Krohn. In addition, as Schneider wrote, "Krohn, with his characteristic generosity, gave me the relevant pages of his journal that I might use the information contained in it at my discretion, expressing the hope that I could trace the further development of these larvae" (p. 498, footnote) .

Other works reveal that Krohn did not intend to publish his materials on the development of this polychaete, and agreed that Schneider would do this himself. Schneider wrote the work, its first part (description of the early stages) containing his own materials, and the second the results of Krohn 's observations on subsequent development. The work was published under the names of both authors, Krohn 's name in first place. 38

Helping to increase his material, Krohn showed at the same time extreme punctiliousness in relation to the strange data. This is illustrated by Krohn' s following remark on one of the early works about the structure of the nervous system in the echinoderms,39 as he sought to eliminating all shades of suspicion of incorrectness in relation to the published data of other investigators:

37. A. Schneider, "Uber die Entwickelung der PHYLLIRHOE BUCEPHALUM," ARCH. ANAT . , PHYSIOL. (1859), pp. 35 - 37.

38. A. Krohn und A. Schneider, "Uber Annelidlarven mit porosen Hullen," ARCH. ANAT., PHYSIOL. (1867), pp. 498 - 508.

39. A. Krohn, "Uber die Anordnung des Nervensystems der Echiniden und Holothurien im Allgemeinen," ARCH. ANAT . , PHYSIOL. (1841), pp. 1 - 13.

After I finished my observations on the nervous system of echinoderras and reported all the existed to Mr. Delle-Kyaie, I learned from this scientist that Mr. Van Beneden, a year before, had already discovered traces of the nervous system in echinus, information about which had appeared in L'INSTITUT. Because I could never get the proper issue of this journal, I should not be birred for not mentioning Van Beneden' s observations . (p. 7)

In all his works Krohn, with exceptional honesty and modesty, mentioned the results of the work of his predecessors, not fearing to recognize the superiority of foreign observations over his own. 40

No information was kept on personal events in Krohn 's life. He was not connected in his work either with scientific institutions or with universities. His life as a travellernaturalist hardly assisted the acquisition of a family of his own. To his relatives Krohn superficially referred in a letter to Johannes Muller:41 "After eight months absence, during which I spent April and May in Santa Cruz in Tenerife, I returned to Europe. The immediate cause for this was a forthcoming meeting with close relatives, whom I had not seen for some years" (p. 515).

In the first period of scientific activity (up to 1846) Krohn' s scientific interests were concentrated in the anatomy of vertebrates (fish, amphibia, birds) and invertebrates (coelenterates, annelids, arachnids, chaetognatha, molluscs, bryozoa, crustaceans, echinoderms, tunicates) . Incidentally to his morphological investigation, Krohn found parasites in the venous sinuses of cuttlefish (apparently diciemid) and described new species of pteropod and cephalopod molluscs.

Zootomical and zoological investigations were continued by Krohn, investigating the structure of protozoa, siphonophora,

40. Concerning the budding of the complex ascidian, Krohn wrote: "Mechnikov, with greater success than I, has traced the gradual development of buds" (Krohn, "Uber die Fortpflanzungsverhaltnisse bei den Botrylliden, " ARCH. NATURGESCH., 35 (1869), pp. 190 - 196).

41. ARCH. ANAT., PHYSIOL. (1856), pp. 515 - 522.

annelids, sea spiders, and arachnids, and describing new species of annelids, chaetopods, and gastropodan molluscs.

There is much authoritative evidence on the accuracy of Krohn's observations and morphological descriptions. These are sufficient to justify the opinion of Kovalevsky, 42 who was shared in the discussion of the nature of the so-called "ventral saddle" of sagitta. Krohn in 184443 considered this formation to be due to the nervous ganglion. Later, W. Busch44 corrected Krohn's opinion, and, in spite of the latter 1 s objection, Busch shared this point of view with Keferstein, R. Leuckart, Pagenstecher, and K. Gegenbaur. Keferstein did not agree with Krohn, but he gave credit to his anatomical investigations: "Krohn, as it is known, related this saddle's very great size to the nervous ganglion. I, together with Busch, do not doubt in that this excellent investigator was in the present question mistaken. "45

Kovalevsky, again investigating the anatomical structure of sagitta, strongly supported Krohn against the above mentioned authoritative zoologists. "I am against the new investigators," Kovalevsky wrote, "in considering Krohn correct concerning the ventral ganglion, and I hope to convince my readers of this also" (p. 135). "The ventral ganglion," he continued, "has the form of a long oval or quadrangular body with four large nervous trunks, from which two on the anterior end continue to the brain or the cephalic ganglion, connecting, as Krohn showed correctly, with the lateral nerves of the cephalic ganglion" (p. 136) .

42. A. O. Kovalevsky, "Embryologicheskie issledovaniya chervei i chlenistonogikh" (Embryological investigations of worms and arthropods) (1871), IZBRANNYE RABOTY (Izd. AN SSSR, 1951) , pp. 123 - 266.

43. A. Krohn, ANATOMISCH-PHYSIOLOGISCHE BEOBACHTUNGEN OBER DIE SAGITTA B I PUNCTATA (Hamburg, 1844), 16 pp. This work was published a year later in French (ANN. SC . NAT., 3 Ser., Zool., 3 (1845), pp. 102 - 116) and in English (ANN. NAT. HIST. 16 (1845), pp. 289 - 304).


45. Cited in the article by A. O. Kovalevsky.

From 1846, Krohn frequently turned to the study of the phenomena of reproduction and development of different invertebrates - coelenterates, worms, molluscs, crustaceans, and mainly the echinoderms and tunicates.

The development of coelenterates is described in the following words — first, concerning the hydromedusa Cladonema and its\ development from the polyp Stauridium^ : the polyp forms buds from which the medusae are formed. Similar to oceanids in these medusae, as in Ooeanidae , in the walls of the stomach the sexual products develop. If mature males and females are situated in separate vessels, then after a short time on the bottom and walls of the latter ova can be seen, covered by a closely adjacent yolk membrane. That these ova are fertilized, Krohn judged by the absence of any embryonic vesicle and embryonic spot (nucleus and nucleolus) . Krohn mentioned I later the process of ovum division, though not describing it in detail, and referring to the fact that this process was observed already by Dujardin, who did not, however, evaluate/ its significance. Within two days after fertilization the formed larva is seen in the egg, which later on leaves the ovum membrane and swims with the help of cilia. The larva was characterized by a light superficial layer and included a dark, probably hollow nucleus (Figure 42, A). By its structure the larvae of Cladonema are similar with the young of higher organisms such as planulae (Aurelia, Cyanea, Cephea) . After two to five days the planula Cladonema becomes rounded, situated on the bottom, loses its cilia, and is transformed into a disk, not changing its internal structure. In the middle of the disk appears a round, hollow hillock, which grows later into a cylindrical process, composed of two layers present in planulae. On the upper end of the cylinder (rudiment of the polyp) four hillocks form, corresponding to the external ends of the future antennae. Already at this stage the first stinging capsules are seen (Figure 42, B) . Thus, Krohn concluded that "Stauridium resulted from the budding of the medusa Cladonema, which reproduced by ova; the young developing from the ovum is again transformed into the form of a polyp. The subsequent change of heteromorphic generation, from which more highly organized medusa develop must be regarded as a generic form and is considered, consequently, factually proved" (pp. 425 - 426).

46. A. Krohn, "Uber die Brut des Cladonema radium und deren Entwickelung zum Stauridium," ARCH. ANAT., PHYSIOL. 1853, pp. 420 - 426.

A preliminary report on these observations Krohn included in a letter addressed to Miiller.47

Figure 42. Planula hydromedusa Cladonema (A) and the polyp developed from it Stauridium (B) by Krohn .

Two years later Krohn published a report about the structure of the early stages of development of the medusa Pelagic. noctitucaA^ At first he found near Messina invertebrate medusae similar to ephyra scyphomedusa, separating from "polypform helminths" (scyphistoma) , and he found also earlier larvae.

47. A. Krohn, "Uber einige niedere. Thiere. Brief liche Mitteilung a. d. Herausgeber, " ARCH . ANAT . , PHYSIOL C1853) , pp. 137 - 141.

48. A. Krohn, "Uber die friihesten Entwickelungsstufen der Pelagia nootiluca," pp. 491 - 497. ARCH. ANAT., PHYSIOL. (1855),

The supposition that they were stages of development of Pelagia noctiluca was completely confirmed. After many unsuccessful attempts, Krohn could carry out artificial insemination. As a result of division ,larvae formed having a cylindrical, usually stretched form (Tigure 43, A). The end of the larvae (a) which is directed forward while swimming is rounded, and the other (b) is chipped off. The surface of the larva is covered by short cilia. In the blunt end occurs a depression with an extremely small, round orifice is seen. This orifice is the mouth, which leads to the round, clearly outlined cavity of the stomach (c) , occupying the posterior third of the body. Tne mouth and stomach, in Krohn 's words, are already clearly differentiated already in the natural forms, but it is still a non-hatched embryo. However in the present stage the stomach is shorter and more rounded than in the free larvae.

Mechnikov rated highly this discovery by Krohn. In his monograph EMBRYOLOGICAL INVESTIGATIONS ON MEDUSA (1866), 49 he noticed the weak interest in embryology by the zoologists of the mid-nineteenth century. He wrote:

Even important generalizations, such as the similarity between the two layers of coelenterates and the embryonic layers of the higher animals, emphasized by Huxley, and significant facts, such as Krohn' s discovery of the formation of a stomach in pelagia by a stretching of the blastomeres, remained without attention and in a lower plane, (p. 284)

The correctness of these observations by Krohn was later confirmed by Kovalevsky and Mechnikov. In this work also, Krohn reported one important discovery: "On the contrary to Medusa aurita and other above-named medusae, "50 he wrote, "Pelagia noctiluoa develops without the generation of helminths" (p. 469). Krohn could trace how the swimming planula Velagia, while not settling on the bottom and not transformed into scyphistomae, forms on the edges of the mouth orifice processes, later becoming part of Ephyra (Figure 43, B and C) with marginal sensory bodies. The citation of this discovery can be found either in later investigators of the embryology of medusae (for example, KovalevskySl and Mechnikov (134)], or in textbooks. 52

49. Cited in I. I. Mechnikov, IZBRANNYE BIOLOGICHESKIE PROIZVEDENIYA (1950), pp. 271 - 472.

50. Krohn compared the development of Pelagia with the development of Medusa, Cyanea, Chrysaora, Cephea, and Cassiopea .

In 1861, during his residence in Nice, Krohn observed the reproduction and development of hydromedusa Eleutheria , 53 The ova arise between ecto- and endoderm and there they develop into the larval stage of larva. Ectoderm, covering the embryonic chambers, swells into hillocks, which subsequently break and release young. The larvae are considered typical planulae and are subjected to the same transformation as in the planula Cladonema.

The budding occurs not only in asexual, but also in completely differentiated bisexually related individuals. Krohn described the process of budding, and noticed that the budding begins in very young individuals, which are still not completely separated from the maternal individual.

51. A. O. Kovalevsky, "Observations on the development

of Coelenterata," IZV. OBSHCH . LYUBIT. ESTESTV. ANTROP. I ETNOGRAFII, 10 (1874), vyp. 2, pp. 1 pp. 1 - 36. To the work of Krohn there is reference on p. 7. ^

52. K. N. Davydov, TRAITE D 'EMBRYOLOGIE COMPARES DES INVERTEBRES (1928), p. 78. The drawings given by Davydov (Figure 36) illustrating the development of Velagia were taken by him from the work of Delap, published more than fifty years after Krohn' s. They are not a bit better than Krohn ' s drawings .

53. A. Krohn, "Beobachtungen iiber den Bau und die Fortpf lanzung der Eleutheria Quatref. , " ARCH . NATURG., 27 (1861), 1, pp. 157 - 170.

Figure 43. Later stages of development of medusa Pelagia noot-iluca (by Krohn) .

a — anterior; b— -posterior end of the body; stomach.

The development of worms54 Krohn described in many separate reports. In 1851 he wrote an article on the reproduction and larval stages of "gefirei," 55 He established the fact of dioecious Phasoolosoma and described the structure of mature ova of Sipunculus nudus . Attempts at artificial insemination of these ova were unsuccessful, and Krohn had to be satisfied with the study of the larvae of Sipunculus caught in plankton, whose description constituted the final part of the work. Two small remarks by Krohn are concerned with the vegetative reproduction of the annulated worm Syllis and Autolytus . 56 He found in them this change of sexual and asexual reproduction, which permitted comparison with true alternation of generations. Many years later 57 Krohn again turned to the study of reproduction in sillids, describing the new viviparous species of polychaeta of this genus.

54 . Of the types of worms known in the mid- nineteenth century, many were distinguished later on in independent groups of forms, including phoronids and chaetognaths , and are so listed.

Other of Krohn 's reports separate information about the development of nemertineans, phronids, and chaetognaths . The description of larvae and partial transformation of the first two forms constitute the contents of a special article^ (135) . On the question of the development of nemertineans inside pilidium, Krohn inclined to the opinion that pilidium is considered a helminth, giving origin to worm-shaped sexual generation. Actinotrocha, in Krohn' s opinion, is a larval stage of any worm, tentatively relating to echiuroids. The process itself of the transformation of actinotrocha he did not observe and noted only the disappearance of the larval organs and the concentration of antennae in the circumoral corona.

In his excellent investigations on the structure of chaetognaths, Krohn added the study of their development.

55. A. Krohn, "Uber die Larve des Sipunoulus nudus nebst vorausgeschikten Bemerkungen liber die Sexualverhaltnisse der Sipunculiden," ARCH. ANAT . , PHYSIOL. (1851), pp. 368 - 379.

56. A. Krohn, "liber die Erscheinungen bei der Fortpflanzung von Syllis prolifera und Autolytus proKfer," ARCH. NATURG., 18, 1 (1852), pp. 66 - 76; "Uber die Sprossling von Autolytus prolifer Gr.," ARCH. ANAT., PHYSIOL.

(1855) , pp. 489 - 490.

57. A. Krohn, "Uber eine lebendiggebarende Syllisart," ARCH. NATURG., 35 (1869), pp. 197 - 200.

58. A. Krohn, "Uber Pilidium und Actinotrocha," ARCH. ANAT., PHYSIOL. (1858), pp. 289 - 301.

In a letter to Johannes Miiller sent on February 2, 1853 from Messina, 59 he reported: "I have at the same time studied the development of sagitta. What Darwin has said about it relates to the development of any fish" (p. 141) (136). Krohn did not publish any further special work on the development of sagitta.

Of Krohn' s two articles on Cirripedia, one concerns the structure of the cement gland of Lepas anatifera and Conohoderma virgata and the anatomy of the female genital system of Lepas and Balanus trintennabulwn . The other^O includes some data about larval development. Krohn described the intermediate stage between the young larva which is similar to the nauplius of the copepods and the late cirripesshaped larva. The work is illustrated with graphs of very young larva of cirripeds, and also larvae of Balanus species and Lepas antifera.

The development of gastropodan molluscs (pteropods and heteropods) was the subject of four reports, the last with the character of a detailed monograph. ^1 in these investigations his main attention is given to a detailed description of larvae of pteropods (subclass opisthobranchia) : Cymbulia Peronii, Tiedemannia neapolitana, Gastropteron Meokelii 3 and also the larvae of carinate molluscs (subclass prosobranchia) Pterotraohea (two species) , Carinaria mediterranean and Fir-iolides (Figure 44) .

59. ARCH. ANAT., PHYSIOL. (1853), p. 137.

60. A. Krohn, "Beobachtungen iiber die Entwickelung der Cirripedien," ARCH. NATURG . , 26, 1 (1860), pp. 1 - 8.

61. A. Krohn, "Beobachtungen aus der Entwickelungsgeschichte der Pteropoden, Heteropoden, und Echinodermen. Brief 1. Mitt. a. d. Herausgeb.," ARCH. ANAT., PHYSIOL. (1856), pp. 515 - 522; "Beitrage zur Entwickelungsgeschichte der Pteropoden und Heteropoden," ibid . (1857), pp. 459 - 468; "Uber die Schale und die Larven des Gastropteron MeckelM," ARCH. NATURG., 26, 1 (1860), pp. 64 - 68; BEITRAGE ZUR ENTWICKELUNGSGESCHICHTE DER PTEROPODEN UND HETEROPODEN (Lepizig, 1860),

46 pp.

Figure 44. A-Larva Ctio from the dorsal side with not completely straightened fans of sailing; B^— extracted from ovum larva F-ivioZides , view from above (by Krohn) .

The accuracy of Krohn's observations concerning the structure of the larvae of gastropodan molluscs is noted in recent books on comparative embryology, For example, K. N. Davydov wrote that "Already, long ago, zoologists turned their attention to the development of gastropodan molluscs, and Krohn and Nordmann left for us memoirs which even at present do not lose their significance" (p. 625) .62 ^ n( j elsewhere: "It is known that the classical case of heteropods ( was described by Krohn in 1860" (p. 651).

A significant place in Krohn's scientific heritage is occupied by his investigations on the development of echinoderms; they were described in no less than ten special reports and transitional notes in works describing other subjects. His systematic study of the embryology of echinoderms Krohn began in 1848 at the time of his three-month stay in Nice. In February-April he experimented with the artificial insemination of the ova of the echinoid Echinus lividus . His observations were compared with data published shortly before by Derbes, 63 w ho also studied the development of ichinus brevispinosus .

The mature ovum of Echinus lividus, according to Krohn, is covered by membrane (chorion) and composed of yolk (this term Krohn called the ooplasm with nutritional inclusions), with an embryonic vesicle (nucleus) and embryonic spot (nucleolus) . Derbes assertion that the embryonic vesicle disappears in the mature ova up to fertilization, Krohn considered the result of Derbes' insufficiently thorough investigation. Within approximately half an hour after fertilization of the ova, the, membrane is separated from the yolk, a phenomenon, which Krohn explained by processes of endosmosis and exosmosis: "the ovum membrane absorbs the fluid from the surroundings and again gives it, to its internal surface" (p. 6). After fertilization the embryonic vesicle and the embryonic spot are no longer seen. In the place of the ovum nucleus, not far from the surface of the ovum, Krohn saw an empty vesicle. With Baer and Derbes, he considered this vesicle the nucleus of the fertilized ovum and suggested that the process of yolk division can begin only after the appearance of this nucleus. Within three to four hours after fertilization the division of the nucleus begins. Referring to Baer's excellent observations, Krohn left out the description of the initial processes of the division. In the conclusion of the monograph he noted that the result of the yolk division is the formation of cells, from which the body of the embryo is formed, because the globules of division are unnoticeably transformed into cells of the developing larva. The evidence of this transformation is found in the contents of the cells; "numerous molecules inside the last substance are nothing but yolk granules, from which the division globules are previously formed" (p. 29). "In the same genetic relation, the nuclei of the cells are related to the vesicular nuclei of the division globules" (p. 30).


63. Derbes, "Observations sur le mecanisme et les phenomenes qui accompagnent la formation de l'embryon chez l'Oursin comestible," ANN. SC . NAT., 3 Ser.,

8 (1847) , pp. 80 - 98.


Krohn prefaced the characteristic of the fully formed echinus larva with the development of the larva, suggesting that knowledge about the final development must help the understanding of the phenomena leading to it. The external form of the pluteus, the structure of its skeleton and the digestive system are represented in the drawings and are described as follows. The formed larva, within eleven days after the fertilization, is pear-shaped (Figure 45, F) ; it possesses two pairs of limbs: the short ee and the long dd. On the convex side, facing the mouth £, and situated between the limbs, the anal opening c^ is present, although frequently closed, which is why Johannes Miiller missed it in the larvae of ophiuroids and echinus. The bilateral symmetry of the larva is absolutely clear and is expressed in the pairing of limbs and the calciferous skeleton carrying their branches, in the situation of the mouth and the anus in the plane of symmetry. Inside the larva is a cavity extending to the end of the limbs; the digestive tract lies in this cavity, surrounded by loose fibrous tissue. The larval surface is covered with skin; the last is formed from twinkling cells, in each of which there is a nucleus with nucleolus. The calciferous skeleton consists of four pairs of toothed branches (Figure 45, G) , the longest branches gg_ are club-shaped, the next pair is found in the long limbs hh, another pair in the short limbs kk, and the last pair H_ is situated across the longitudinal axis of the larva.

The digestive canal consists of three parts, the anterior (the pharynx) , the middle (stomach) and the posterior (intestines), all covered by cells with cilia, similar to the cells of the skin. The digestive canal is attached to the body cavity by fibrous tissue; it forms a network united with the internal surface of the skin. In the junctions of the network, nucleus-like formations are situated; they are numerous also under the skin, in the neighborhood of the calciferous skeleton. Krohn noted Johannes Miiller's error when he assumed them to be strong fibers instead of nerves, and the nuclei of fibrous tissue instead of nerve ganglia. The larva swims with its limbs, mouth opening forward; its movement is carried out by the activity of the cutaneous twinkling cilia, which also drive into the mouth food particles suspended in the water.

The formation of the larva described here takes place as follows. After the division is completed, the young spherical larva rotates in the ovum membranes with the help of the long cilia which cover all the surface of its body. In the larva at this time can be observed a closed central cavity and cover, which can be differentiated, forming the wall of the latter. Krohn erroneously considered that the wall of the body consists of many layers of cells. Within a day after fertilization, when the number of the cover cells becomes sufficiently large, the ovum membrane is torn and the larva begin to swim. Soon after hatching it acquires its ovoid form (Figure 45, A); during swimming its narrow end is directed forward.

In the body cavity of the larva, from the side of the blunt end, an accumulation of closely situated dark bodies, similar to the nuclei, is seen. The number of these nuclei at first is small, but it quickly increases so that they are found filling half of the body cavity of the larva, toward its blunt end (Figure 45, A,e) . Later, nuclei are separated from each other and distributed evenly in the body cavity, and then partially accumulate not far from the rudiments of the calciferous skeleton. After this, the nuclei become angular or fusiform and begin their transformation into the fiber of the reticular tissue which strings the body cavity and holds the digestive tract.

Figure 45. Development of echinus, Echinus liwidus (by Krohn) .

A — larva shortly after hatching: a — blunt pole; b — sharp pole; c, c — cover; d — central cavity; e — accumulation of "nucleuslike formations" from which the fibrous tissue is later formed.

B — "ideal sketch" of the larva, already having the form of the body with three surfaces: a, b, c — anterior, posterior, and upper surfaces; d — anterior angle; e, e — posterior angles; f — lower angle ; g — anus .

C— — cross-section of this same stage of development, in which for elucidation of the passage the previous apple-shaped form was drawn: a, b, c, d, e, f, g — as in figure B; h — rudiment of the digestive tract in immature form; k — its rudiment in later form.

D — larva of the following form, back view: a, b — posterior and upper surfaces; c — anus (back passage) ; gg, 11, kk — rudiments of club-shaped, arch-shaped, and transverse branches of calcareous skeleton.

E — later larva: a, b : c, g, k, 1 — as in figure D; dd — rudiment of posterior limbs; hh — first traces of hydrants in it.

F — larva of pear-shaped form, back view: dd — posterior limbs; ee — anterior limbs; f— -mouth; q — -anus; ssss — cilia apparatus.

G — calcareous skeleton of the formed larva, represented in

an isolated view: gg — club-shaped hydrants; hh — hydrants of anterior limbs; kk — arch-shaped hydrants; 11 — transverse hydrants .

From Krohn's description it is clear that he saw, for the first time, in the cavity of echinus division that accumulation of cells, which was called by the latest embryologists the primary mesenchyma. He correctly determined the fate of these cells in larval development. This discovery of Krohn's was so thoroughly forgotten that Mechnikov, who was well acquainted with the old embryological literature, attributed it to Selenka, whose work was published exactly thirty years later after Krohn's monograph. 65

"The development of Echinus micro tub eraulatus ," Mechnicov wrote, "was studied by Selenka. The first differentiation of the embryonic layers begins with the bulging of the lower cells of the blastoderm. Until the beginning of their protrusion they form a number of wandering cells, which later develop into the cutis and are considered mesodermal or mesenchymal cells. "66

Krohn observed that after nearly thirty hours after fertilization the following important changes take place in the larva of Echinus lividus . In the center of the blunt pole a small hole-like deepening appears. The blunt pole widens and thickens; the hole in its center becomes deeper and wider, so that the body of the larva becomes similar in form of an apple. The more enlarged hole "formed as a result of that," Krohn wrote , caused the skin in this area gradually to protrude (EINSACKT ODER EINWARTSSTULPT) into the cavity of the body. The protrusion (EINSACKUNG) submerged deeper in the body cavity and extended into a canal which finally reached the walls of the body cavity toward the blunt pole. The sac which appeared by this means stretched through the body cavity (Figure 45, C, h, k3 and is the rudiment of the digestive cavity. The edge or circumference of the primary hole becomes a leading opening in the canal, which is the anus. In this view's favor — on the manner of appearance of the digestive tract — is the fact that it also indicates that the wall of the canal is absolutely equivalent to the skin in thickness and structure. (p. 18)

65. E. Selenka, "Keimblatter und Organanlage bei Echinicen," ZTS. WISS. ZOOL.,, 33 (1879) .

66. I. I. Mechnikov, "Vergleichend-embryologische Studien.

3: Uber die Gastrula einiger Metazoen," ZTS. WISS. ZOOL., 37 (1882) , pp. 286 - 313.

Krohn's discovery of the nature of the formation of the larval intestine of echinus undoubtedly possesses outstanding significance. This, unquestionably, is the first description of the invaginated gastrulation phenomenon. Its study subsequently played a great role in the progress of comparative embryology. Comparative analysis of methods of separation of the endoderm is included in the basis of Kovalevsky f s evolutionary concept about the formation of the embryonic layers, and also in the basis of Haeckel's gastrula theory. Krohn's priority in this question was not definitively underlined. Mechnikov, in the article just cited, wrote the following.

Derbes (1847) described the formation of the larva of Echinus esoulentus and mentioned a stage in the form of a double sac with skin layer in which the caecum opened to the outside. Twenty-five years later, Haeckel gave this the name gastrula, which was accepted by scientists all over the world. Derbes thought that the opening of the rudiment of the intestine was the mouth, but August Krohn (1849) showed that it corresponded to the anus of the pluteus. He described the process of protrusion itself.

Krohn referred to Derbes' observations: "The reader can form an excellent concept of the gradual formation of the digestive tract just described by looking at Figures 13 and 14 in Derbes' article; however, the author apparently did not pay attention to the process of formation itself" (p. 19). The following is written by Derbes: "The spherical form of the larva is changed by pressure at one point of the surface. Gradually this pressure becomes more pronounced, and its center is penetrated by an opening which leads to a rudiment of the intestinal cavity. Beginning from this moment, the movement of this opening is always directed forward and, later, upwards. . . that is, the mouth looks towards the zenith" (pp. 91 - 92). It is clear that Derbes did not put the deepening on the surface of the spherical larva in genetic relation with the formed intestine. According to his opinion, the opening formed in the center of the deepening united in an unknown way the developed rudiment of the digestive tract with the external world. Krohn pointed also to DerbSs' mistaken assumption that this opening was the mouth. He established that the opening in the area of protrusion is the anal opening, and the mouth is formed in another place significantly later (on the fourth day after fertilization] .

Further observations of Krohn are concerned with the changes in the larval form" as it becomes bilaterally symmetrical (Figure 45, B and C) . He described the formation of limbs and the calciferous skeleton (Tigure 45, D and E) , which gradually acquires a different configuration.

In the following year, Krohn repeatedly returned to the study of development of the different echinoderms and published many reports on this subject, One of these reports considered the development of the holothurian and echinus. 67 The larvae of holothurians (Holothuria tubulosa} were obtained from plankton, because attempts at artificial insemination proved unsuccessful. The youngest larva observed by Krohn "is similar to an elongated egg (Figure 46, A); in its sharp pole there is an opening leading to a sac-like protrusion in the body and an ampul la- shaped canal widening at the end Ik This sac is the rudiment of the digestive tract, the opening doubtlessly is the anus" (p. 345). Krohn noticed that the larva of Holothuria tubulosa, at this stage, is very similar to the larva of Echinus lividus described earlier by him (1849) . Similar to the latter, the larva of Holothuria tubulosa is covered by cilia, and with their help it swims with the imperforate pole forward. The surface of the body and the digestive tract consist of cells which, with their nuclei, become noticeable with the addition of fresh water. In the body cavity, as in the larvae of Echinus lividus, fibrous tissue with fusiform cells is found. Later (Figure 46, B) on the abdominal side of the larva a depression appears — which is the future transverse fissure of ausicularia; in this fissure the mouth opening later appears. Later the body acquires a kidney-shaped configuration (Figure 46, C) . The digestive tract, at this time, forms the rudiments of the three parts, pharynx e, stomach d and intestines c. The appearance of the mouth opening coincides with the beginning formation of the ciliary strings in the auricularia. At this stage there is some data about the transformation of the Echinus lividus .

67. A. Krohn, "Beobachtungen aus der Entwickelungsgeschichte der Holothurien und Seeigel," ARCH. ANAT . , PHYSIOL. (1851) , pp. 344 - 352.

Krohn's other reports on the development of echinoderms briefly designate some representatives of the type68 or species. 69

Special attention must be given to Krohn's investigations on the development of tunicates. Originally his interest was attracted by Salpa, in which the wonderful phenomenon of the alternation of sexual and asexual generations had already been recognized. This discovery belongs to the poet Adalbert Chamisso (1781 - 1838) . With the Dorpat-born zoologist J. F. Eschscholtz, he traveled around the world on the Russian ship RURICK. Chamisso published the results of his investigations in "On Some Animals of the Linnean Class of Worms, noted during a world tour, performed by Count N. Romanzoff, under the command of Otto von Kotzebue, from 1815 to 1818, Part I: On Salpa. "70

68. A. Krohn, "Bemerkungen uber einige Echinodermenlarven, " ARCH. ANAT., PHYSIOL. (1851), pp. 353 - 357; "Uber die Entwickelung der Seesterne und Holothurien (Brief 1. Mitt. a. d. Herausgeb . ) , " ibid . (1853), pp. 317 - 321; "Beobachtungen iiber Echinodermenlarven (Brief 1. Mitt, a. d. Herausgeb.)," ibid . (1854), pp. 208 - 213; "Uber neuen Entwickelungsmodus der Ophiuren," ibid . (1857), pp. 369 - 375.

69. A Krohn, "Uber die Entwickelung einer lebendiggebarenden Ophiure (Brief 1. Mitt. a. d. Herausgeb.)," ARCH. ANAT. PHYSIOL. (1851), pp. 338 - 343; "Uber die Larve von Spatangus purpureus (Brief 1. Mitt. a. d. Herausgeb.)," ibid . (1853) , pp. 255 - 259; "Uber die Larve des Echinus brevispinosus ," ibid . (1853), pp. 361 - 364.


Figure 46. Three stages of the development of the sea cucumber Holothuria tubulosa (by Krohn) .

A and B: a — anus; b — digestive cavity, Id — anus; c, d, e — rudiments of the

estine, stomach and pharynx.

C: inte

The alternation of sexual and asexual generations in Salpa was described by Chamisso as follows:

The species of Salpa is found in double form: each generation of the species is dissimilar to its parents, but through birth posterity is similar to the last, so that any Salpa differs from its parent but is identical with its grandparents. Both forms are similar to headless molluscs, hermaphrodites or the female sex. Both of them are viviparous, but one of them is a solitary animal, the originator of many descendants . The other represents a complicated branch consisting of animals, each united with the others by the necessary connection which gives birth to one descendant. These changed forms of the unchanged species are called solitary (Proles solitaria) and aggregated or colonial (Proles gregata) generations, (p. 2)

After the discovery by M. Sars^l of the analogical change of generations in scyphomedusa, Steenstrup united these facts to produce one biological regularity. Steenstrup 's general conclusion was highly regarded by his contemporaries, in particular by Baer.72 Krohn also gave great significance to Chamisso's discovery and Steenstrup 's ideas. He made it his task to study in detail the reproduction and development of Salpa. To do this, he settled for many months on the coast of Sicily, where the sea provided him the necessary material. Krohn put the results of his observations into a special work, 73 in which, first of all, he completely confirmed Chamisso's observations, by distributing them over seven species of Salpa which were, for the first time, partly described by him. The comparison of the solitary and colonial forms (Proles solitaria and Proles gregata, in Chamisso's terminology) allowed Krohn to regulate the taxonomy of this group of tunicates. He showed that salpae described under different names frequently proved to be different stages of the development of one and the same species (137) . Later, Krohn gave the characteristics of the structure of the heteromorphic generation, and also described the ovum, the seminal glands, and the process of fertilization in the sexual generation.

Within the present book, the greatest significance is placed on the section in Krohn' s article (111) in which he discussed development of the embryo in the maternal organism. After fertilization, the embryonic vesicle and the embryonic spot disappear, after which the ovum enlarges in size and acquires a regular spherical form. This was according to Krohn, who was not completely convinced of its authenticity. Sometimes the ovum is not seen like that, and in its place a round body appears, raising a region of tunica of the mother and jutting into its cavity in the form of papilla. This body, Krohn wrote, is nothing other than the rudiment of the placenta which, by deepening in the cavity of the body of the maternal organism, enters in connection with two of its blood vessels. Only after the formation of the placenta does the embryo begin to develop, at first in the form of a very small body appearing on the summit of the placenta under its cover (138) . In this rudiment of the embryo all organs of the last are developed; "however," Krohn wrote, "all that concerned its development during the first period remained for me almost completely unknown" (p. 123). He could only establish that one of the first organs of the embryo by the time of appearance is "the respiratory cavity." The embryo changes from compact to hollow, following which the rudiments of the branchae and nervous ganglion are already seen, while the organs "visceral nucleus" and heart become noticeable only later. Only after that does the embryo acquire a definite form; the anterior and posterior openings appear in it. At the end the embryo becomes more voluminous than the placenta, and all its organs intensively enlarge, especially the nervous ganglion, from which numerous nervous branches grow. At the same time muscular strips and blood vessels appear, which are not completely formed.

71. M. Sars, "Uber die Entwickelung der Medusa aurita und Cyanea caprtlata," ARCH. NATURG . , 7 (1841), pp. 9 - 34.

72. See Chapter 23.

73. A. Krohn, "Observations sur la generation et le developperaent des Biphores (Salpa)," ANN. SC . NAT., 3S6r., Zool., 6 (1846), pp. 110 - 131.

Krohn' s concluding paragraph of the work is dedicated to the processes of budding in the asexual regeneration of Salpa, and to the formation of colonies; the character of the last varies in different species. Here the description of the stolon and the embryos is given, situated along it so that their axes cross the stolon at a right angle. These embryos develop in definite succession, depending on their situation on the stolon.

The development of salpa, especially the formation of their embryos from fertilized ova, represents one of the most difficult principles of embryology. Krohn' s investigations began this study and recent opinions are credited to many Russian embryologists at the end of the nineteenth and the beginning of the twentieth century, including A. 0. Kovalevsky, M. M. Davydov, A. A. Korotnev, and V. V. Zalensky. 74

Six years after the publication of the above-mentioned work, Krohn published an article presenting the results of his investigations of the little-studied group of tunicates, the doliolum.75 Q U oy and Gaimard made a voyage on the

74. "The first investigations of the development of Salpa go back to. Krohn," K. N. Davydov wrote in his handbook CTRAITE, p. 867) .

75. A. Krohn, "Uber die Gattung Doliolum und ihre Arten," ARCHIV. NATURG., 18 (1852), pp. 53 - 65.

ASTROLABE 7 ^ to Road Island, Ambon (in the Moluccas), and to the coast of the Vanikoro Islands (between the New Hebrides and the Solomon Islands) . During this time they discovered this small transparent tunicate, which they described and presented only incompletely, and for which they suggested the generic name Doliolwn.

Later T. Huxley, on a tour around the world aboard the ship RATTLESNAKE, saw doliola in the southern part of the Pacific Ocean and described their structure more exactly and in more detail than Quoy and Gaimard,77 but in his notes the male genital glands were given for individuals of sexual regeneration.

Krohn, for the first time, discovered the presence of doliola in the Mediterranean Sea (near Messina and Naples) , studied the structure and described three new species. According to Krohn, doliola "are free living ascidians, but in many respects are similar to salpa and form, therefore, an interesting intermediate link between both these orders of tunicates" (p. 53). From Krohn's description, the structure and reproduction of doliolum, according to the majority of the features, must be related to salpa; this applies also in the present taxonomy.

Krohn observed that from the ova of doliola cercarialike larvae develop which later undergo metamorphosis . The peculiarities of the larvae are connected, according to Krohn, with the mode of life of the tunicates more than to ascidians, which are in their adult condition fastened motionless to the substrate. In accordance with this, in the ascidian larvae the tail disappears early, but in the larvae of doliola it remains throughout metamorphosis and serves as an organ of movement. The process of reduction of the tail in the larvae of doliola takes place as in the larvae of ascidians . In the latter the disappearance of the tail was noticed for the first time by Milne-Edwards in an example of the colonial ascidian Amouruoium proliferum, and was described in more detail by Krohn for Phallus-La mammillata .


77. T. Huxley, "Remarks upon Appendicularia and Doliolum, Two Genera of Tunicates," PHIL. TRANS. ROY. SOC . LONDON (1851), pp. 599 - 602.

The metamorphosis of doliola was again studied by Krohn on the species Doliolum Nordmanni. Krohn represented two stages of transformation; Figure 47, A represents the stage when the tail of the larva has not begun reduction; and Figure 47, B, the larva with shortened tail. Until metamorphosis the tail of the larva is tapered, with the two ends covered with gelatinous membrane a_; its axis d_ is composed of cuboidal cells situated in one row, so that it seems to be segmented. The root of this axial shaft penetrates into "the vesicular appendix" c_, lying under the digestive tract of the larva. This vesicle decreased simultaneously with the reduction of the tail and at the end it disappears (139) . In the stage represented in Figure 47, B, all organs in the larva are already differentiated; on its dorsal side there is the placing of the stolon £.

Krohn ! s description of the structure of the larva of doliolum and its transformation distinctly shows that he saw in it all the principal features of organization, on the basis of which the doliolum was later counted as a type of chordate (subtype tunicate, class salpa) : the presence of the reduced cord at the time of metamorphosis ("axis of tail"), the situation of the nervous ganglion o_ on the dorsal side and the heart m on the ventral side.

Correct evaluation of Krohn ! s investigations was given by V. N. Ulianin in the classic monograph on doliola, first published in Russian, and two years later in German.' 7 ** Ulianin wrote the following: "Soon after the appearance of Huxley's article the doliolum was found in the Mediterranean by Krohn.


The article in which this discovery was published possesses important significance in the history of doliolum, as it contains the first observations on the development of this animal. Krohn not only described the tailed larva of doliolum, tut he also pointed out the alternation of generations in the reproduction of this animal.... On the basis of his observations Krohn concluded that from the ovum of doliolum is formed the tailed, freely swimming larva, which after the loss of the tail is transformed into an asexual doliolum" (p. 2). Later on Ulianin noticed Krohn's mistake in dividing doliolum according to the number of muscular strips. In all species of sexual generation there are eight, but in asexual species there are nine strips. Therefore the species described by Krohn, Doliolum Trosoheli , is in fact an asexual individual D. dentioulatum Q. and G., and D. Nordmannz Krohn is the asexual generation of D. Mulleri Krohn. Particular significance was given by Ulianin to Krohn's embryological observations. "All that is known presently about the embryological development of Doliolum," Ulianin wrote, "comes exclusively from Krohn, who for the first time described the free larva. All later authors.... only redescribed it, not adding anything essential to Krohn's description (p. 47).

The central place among Krohn's investigations of the development of tunicates is occupied by his work concerning the solitary ascidians.? 9 The artificial insemination in ascidians which was used successfully for the first time by Baer, Krohn also used, observing the development of Phallusia mammillata step by step for three months. He described the mature ovum of this ascidian in the following way. The ovum present in the ovum-fluid is supplied by papillae and covered by a cover membrane under which the proper ovum membrane is present. Somewhat deeper lies a hyaline membrane containing inclusions which is green in color. The yolk itself is colorless, the embryonic vesicle and the embryonic spot in the mature ova are unnoticeable . The above mentioned green hyaline membrane was considered by Krohn, following MilneEdwards, a source of formation of tunica. The error of this view was later established, but it was repeated in many

79. A. Krohn, "fiber die Entwickelung der Ascidien," ARCH.

ANAT., PHYSIOL. (1852), pp. 312 - 333. In the following year this work was published in English as "On the Development of the Ascidians," SC. MEM. NAT. HIST. (1853) , pp. 312 - 329.

subsequent embryological works . The division of the ovum begins two to three hours after contact with the sperm. Krohn considered that the division, at least in the first stages, follows the rule of progress. The vesicular nuclei of the globules of division disappear before every division and then again become visible. "Instead of nuclei," Krohn wrote,

in every divided globule an absolutely peculiar distribution of yolk molecules is noticed. Namely, they are distributed in the form of strands, which are directed from the depth, from the medial point by radius in all directions to the lighter periphery of the ovum and, apparently, come out from two centers of irradiation. After the end of division, inside the new globules the nuclei again become noticeable, then these radiant figures disappear and the yolk granules are found to be situated close to each other, (p. 315)

These observations show that Krohn exactly described many details of mitotic division in blastomeres.

Embryonic development goes quickly, and a day after fertilization a cercaria-like embryo with a more or less developed tail is already present in the ovum membrane. The body and the tail of the embryo are composed of cells which are especially noticeable on the surface. The cells have a polygonal form, and contain granules and nuclei in the center. The axis of the tail, according to Krohn 1 s description, is composed of larger rectangular cells with nuclei situated in a row, one following the other, and therefore they have a striated or a disjointed form (Figure 48, A, b) .

Shortly before the final formation of the larva the tail undergoes remarkable transformations. According to Krohn' s observations, they amount to the following. The axis is transformed into a canal, as its cellular structure gradually disappears due to the destruction of the partitions between the neighboring cells and the liquefaction of their contents. The small cells surrounding the central tail strand are transformed into longitudinal muscular fibers. On the dorsal side of the larva at first appears one, and then behind it another, pigmented spot of granular origin (Figure 48, A, d, e and B, e, f ) . At the time of transformation this formation is destroyed, and the pigment passes into the blood channel. The formed larva is set free from the membrane by the tail movement. The body of the larva CFigure 48, B) in the anterior end is supplied by three similar processes on the sucker. The larva is soon attached at the anterior end and undergoes transformation, one of its marks being the disappearance of the tail. Milne-Edwards saw only that the axial part of the tail is set free from its covered sheath and extends into the body of the larva, but he did not elucidate the subsequent fate of this formation. "By my observations," Krohn noted,

the setting free and the extension of the tail axis , the deep immersion by the tail in the body of the larva only precedes the processes of reduction which it soon undergoes. Directly after the extension, the tail axis remains undamaged at the posterior part of the body. It is situated here convoluted into a spiral coil . . . . With the beginning of the development of the young ascidian, this coil first disintegrates into a large number of strips situated close to each other, which then are gradually destroyed; the number and size of the strips decrease, but the insignificant remnant does not disappear entirely. (Figure 48, C) (pp. 318 319)

Krohn himself considered his observations on the development of ascidians incomplete, and he acknowledged only the most essential changes. He described in particular the formation of the vessels of the tunica and the development of the respiratory cavity, or gill cleft, and behind it the rudiment of the digestive canal in the form of a loop-shaped canal . Somewhat later three openings on the spinal side of the body appear: the most anterior, the inlet into the respiratory cavity and digestive canal, and two posterior which later merge together in a common excretory opening. Simultaneously, the nerve ganglia develop in an elongated formation in the middle of the back near both pigmentation spots. Near the nerve ganglia the rudiment of the muscular strands form and the dorsal fissure appears. The digestive canal is differentiated into three parts: a canal which opens into the respiratory cavity, stomach, and intestine. In the walls of the respiratory cavity there develop near the stomach the first branchial clefts with cilia at the edges, and at the ventral fissure the heart develops, possessing the form of a short duct. The metamorphosis is completed by the specialization of the gill-clefts and the development of siphons .

Figure 47. Larvae of Doliolum Nordmanni .

A — Larva up to transformation: a — larval membrane; b— young doliolum; e, f — posterior and anterior opening; d — axis of the tail; B — larva of doliolum in the beginning of transformation: a, d, e, f — as in A; c — "vesicular appendix; g— wall of the respiratory cavity; h — -digestive tract; k — stomach; 1 — intestine; m — heart; n— ventral fissure; o — nervous ganglion with outgoing nerves; p—— the third from back muscular strand, penetrating the rudiment of the stolon (q) (by Krohn) .

Krohn's work represents the first systematic description of ascidian development in world literature; it remains incomplete and not free from mistakes, which, of course, does not reduce the historical significance of this undoubtedlyremarkable investigation. But the exact and detailed study of the embryology of ascidians belongs to A. 0. Kovalevsky. In his work, their relationship to the vertebrates was proved, delivering a fatal blow to the metaphysical theory of types in the animal kingdom. It formed the basis of comparative evolutionary embryology, first advanced by Krohn. In the work dedicated to the development of ascidians, 80 Kovalevsky wrote the following:

Leaving aside the investigations of earlier authors, whose results either are already completely reworked by present scientists, or, to a lesser extent, are partially expanded, we must mention Milne-Edwards, Van Beneden, Kolliker, and, in particular, Krohn. Of all these investigations the results of Krohn's investigations are in closest agreement with our own results. Although he described the accumulation of pigments, which completely coincides with our observations , he did not discover the walls of the saccule in which these organs of sensation are situated, and generally he traced the development step by step. The formation of the axial strand in the tail of the ascidian larva was observed by Krohn, although he explained it as a formation of emptiness in the cells . Although the transformation of the larva into the sessile form was described by him in detail, he had only a slight understanding of the anatomy of the larva and therefore he could not observe the particular features, (p. 41)

80. A. O. Kovalevsky, "Istoriya razvitiga prostykh astsidii"

(The history of development of the simple ascidia) (1886) , SELECTED WORKS (Izd. AN SSSR, 1951) , pp. 41 - 78.

81. A. Krohn, "Uber die fruheste Bildung der Botryllusstocke," ARCH. NATURG., 35 (1869), pp. 190 - 196; Uber die Fortpflanzungsverhaltnisse bei den Botrylliden, " ibid ., pp. 326 - 333.

At the end of the 1860s, in Naples, Krohn studied budding in the complex ascidian Botryllus , and he presented the results of his observations in two reports. 81 In the first of these articles Krohn disproved the erroneous data of Milne-Edwards and Sars and confirmed the observations of Mechnikov that the larva of Botryllus possesses the same simple structure as that of the solitary ascidian and undergoes analogical metamorphosis. After settling on the bottom, the young Botryllus , already in the process of transformation, produces a bud from which a second individual originates, which in turn begins to bud. As a result a stellate colony is obtained. The budding of the colonial ascidian is represented in the second article, in which Krohn compared its details with the corresponding phenomena in salpa.

With these fragmentary investigations of vegetative multiplication, Krohn' s scientific activity apparently came to an end. In the following years (1870 to 1880) his reports were regularly placed in journals, but no works appeared after this time.

For thirty-five years he collected facts from the field of anatomy and embryology, mainly of invertebrates, covering a very great number of systematic groups CI 40) . His embryological works (including descriptions of larvae and means of reproduction) concerned coelenterates, nemerteans, annelids, molluscs, crustaceans, echinoderms, and tunicates.

Krohn did not belong to those investigators paving new roads in science. All his comparative-anatomical and comparativeerabryological remarks are concerned with the comparison of closely related forms. Comparing, for example, representatives of different classes of echinoderms and establishing the features of similarity and difference between salpa and ascidians, Krohn did not offer sympathy either to the theory of types, nor to the idea of unity of planes, nor to evolutionary study. Krohn cited Darwin only in his works on the structure and development of cirripedes, highly rating his monograph dedicated to this order of crustacean.

Besides this, evidence exists about Krohn' s deep interest in the investigations of A. 0. Kovalevsky, who established the similarity of the embryonic development of ascidians and vertebrates. The first report by Kovalevsky dedicated to the development of ascidians was published in 1866 in ZAPISKAKH PETERBURGSKOI AKADEMII NAUK (Notes of the Petersburg

Figure 48. The development of the ascidian Phallusia mammillata (by Krohn) .

A — late embryo with two pigmentation spots: a — body;

b — beginning of the tail; c — rudiment of upper sucking processes? d, e — anterior and posterior pigmentation spots .

B — larva, side view: a— tunica with green bodies; bb— the axis of the tail; cc — its canal; d — horizontal half; e, f — anterior and posterior pigmentation spots; g — right anterior; h — posterior sucking processes.

C — Phallusia in the process of metamorphosis: a— widely opened respiratory siphon; bb— posterior (constrictor) siphons; c — nervous ganglion with the nervous branches; d: — digestive tract; e — stomach; f — intestine; g — situating coil of the larval tail; hhhh — first two pairs of gill openings of the respiratory sac; i — pigmentation mass over the nervous ganglion; k — ventral fissure; 1, 1 — tunica.

Academy of Science); two years later an article followed in NACHRICHTEN VON DER GESELLSCHAFT ZU GOTTINGEN. The data mentioned there, apparently, did not convince Krohn. In 1871 in ARCHIV FUR MIKROSKOPISCHE ANATOMIE, edited by Max Schultze, a new work by Kovalevsky was published, 82 a t which time Schultze added the following postscript to a letter directed to Kovalevsky on January 18,1871: "Krohn, who read your article in proof and made minor corrections, sends you heartfelt regards. At first he was, as you can imagine, very much against the relationship with vertebrates; 83 b u t then he began to hesitate." 84

Krohn was one of the pioneers of zoological investigations on the Mediterranean coast, which later became a place of pilgrimage for naturalists from different countries of Europe.

It is highly probable that Kovalevsky and Mechnikov were well acquainted with the works of Krohn. His remarkable comparative embryological investigations, of which they became aware in the mid-1860s, revealed the nature of Krohn 1 s scientific activity and produced an impression on them. Like Krohn, they spent many years of their lives as travelling naturalists, more than once following him to those localities

82. A. O. Kovalevsky, "Entwickelungsgeschichte der einfachen Ascidien," MEM. AC. SC . ST. PETERSB . , VII Ser., 10, No. 15 (1866), 16 pp.? "Beitrag zur Entwickelungsgeschichte der Tunicaten," NACHRICHTEN VON DER GESELLSCHAFT ZU GOTTINGEN, No. 19 (1868), pp. 401 - 415? "weitere Studien iiber die Entwickelung der einfachen Ascidien," ARCH. MIKR. ANAT., 7 (1871), pp. 101 - 130.

The Russian translation of the first and third articles are to be found in A. O. Kovalevsky, SELECTED WORKS, editor and commentator A. D. Nekrasov and N. M. Artemov (Izd. AN SSSR, 1951), pp. 41 - 78 and 79 - 122.

83. The paper was on the relationship of ascidians to vertebrates .

84. Schultze' s letter is included in the book PEREPISKA A. 0. I V. 0. KOVALEVSKY (Postscripts of A. O. and V. O. Kovalevsky) , edited by A. A. Borisiak and

S. Ya. Streich. The extract of this letter is published here with the permission of S. Ya. Streich.

rich in the zoological material of the sea — Naples, Messina, Nice, Madeira — and using, in particular, those subjects on which Krohn made many important observations leading to serious theoretical meditation. Kovalevsky later cited with respect the works of Krohn on the structure of sagitta and the development of tunicates, and Mechnikov cited his works on the development of coelenterates and echinoderms .

In the preparation of that revolution in embryology which was accomplished by Kovalevsky and Mechnikov, converting the comparative-descriptive embryology into comparative evolutionary embryology, Krohn 's modest investigations played their role, and therefore his name must not be forgotten in the history of Russian science.

   Historic Russian Embryology 1955: 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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