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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
Online Editor 
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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 20. Third Part of Uber Entw I C Klungsgesch I Chte - Development of the Bird Egg and Embryo

Turning to the structure and formation of the bird's egg, that is, its history prior to hatching, Baer considered the structure of the laid but not yet hatched egg (§ 2) . He successively described the parts of the laid egg, starting with the shell and the underlying two- layered shell membrane. These layers are only separated at the blunt end of the egg, with the air chamber between them. The external layer carries papillae on the external surface, which penetrate into the shell. In the albumen, Baer distinguished three layers under the shell membrane — an external, middle, and internal or third albumen. Concerning what was called the middle membrane of albumen, Baer referred to it as a solitary layer on the surface of the middle albumen, and noted that it is not seen in live eggs and appears only with the effect of water. The same thing apparently also occurs with another formation, the albumen ligament, which Tredern had described in detail (see Chapter 11). Next, Baer briefly referred to the chemical composition of the albumen. In the center of the albumen mass sits the yolk ball, ellipsoid in form; its longitudinal axis corresponds to the longitudinal axis of the shell.

On the surface of the yolk there is a yolk membrane, composed of a single layer. Of the two layers of the yolk membrane which Wolff had thought existed, the internal one actually represents the embryonic pollicle (blastoderm) . The yolk itself is composed of granules of equal size, irregularly shaped whitish masses and bright light fat droplets. The central cavity in the yolk communicates with the surface by a canal. 1 In front of its external end on the yolk surface is found the most important part of the egg, the cocks' trace or cover (cicatricula) . It consists of a thin surface disk, which Pander had called "the rudiment pellicle" (Keimhaut, blastoderma) , but which Baer preferred to label the rudiment (Keim, bLastos) . The rudiment is composed of firmly laid small whitish granules, under which lies the rudiment layer, whose whitish-yellow mass Pander called the nucleus of the cover. Baer considered all this to be only layers of the yolk, which connect with the rudiment and merge with the remaining yolk. Only in the middle is the nucleus of the cover separated from the yolk by fluid to form the hillock of the rudiment layer.

1. Actually the center of the yolk ball is occupied with what is called the white yolk.

The formation of the yolk ball (§ 3) Baer traced from its presence in the nonsexual ly mature hen; such a hen's ovary contains vesicles with transparent fluid. Reaching the size of a millet grain or seed, the vesicles sharply increase in size and become filled with a milky white, then yellowish content at copulation. The yolk is connected to the ovarian stem and is covered with a capsule composed of firmly adjoined layers. These layers have openings for vessels which do not, however, penetrate the yolk; thus the yolk membrane remains intact. At the emerging part of the yolk, which is still in the ovary, an arch-shaped white zone-— cicatrice — appears. When the yolk is ready for separation, the ovarian membrane ruptures in the region of the cicatrice. After the yolk exits from the capsule, its remains and stem form a deepening, called the cup. The membrane directly covering the yolk appears on its surface before maturation. Also long before maturation, near a small yolk equator (beside the cup stem, sometimes at the cicatrice), a white spot appears, corresponding to the cover of the laid egg. But in the ovarian egg it does not acquire such clear outlines. This part is called the rudiment layer. In its middle there is a light spot, an extremely delicate vesicle filled with a transparent fluid, called the embryonic vesicle or Purkinje vesicle.

The term "embryonic vesicle" (for the egg nucleus) was widely employed in embryological literature until recently, and even in current works. Baer noticed that in hens the embryonic vesicle is revealed very early, while the corresponding transference of the nucleus takes place at a much later stage of egg development in other animals. With the transference of the egg nucleus comes the formation of the central cavity of the yolk and its canal. In the frog egg, where the transference of the embryonic vesicle occurs late, Baer observed it with greater distinctness than in the bird's egg.

After copulation, the cicatrice is ruptured and the yolk falls from the cavity of the capsule. For the release of the yolk, copulation is not necessary, but it stimulates this process. Many authors after Baer described the disappearance of the embryonic vesicle upon maturation of eggs of many types of animals. But only after the investigation of N. A. Warnek (see Chapter 25) did this phenomenon become connected with maturation division and the formation of the polar bodies. Baer considered the appearance of the rudiment (KEIM) from the rudiment layer a direct consequence of fertilization, and without fertilization the rudiment is not formed .

Describing the further formation of the egg, which is already occurring in the oviduct (§ 4) , Baer first considered the organization of the sexual conducting routes and described the structure of the funnel, especially the oviduct and uterus, from which a narrow passage leads to the cloaca. The pressing of the yolk ball by the funnel and its passage along the oviduct is accomplished by active movement of the latter; hence the yolk, passing by the oviduct, turns around the longitudinal axis so that the rudiment layer always remains on one side. After passage of the egg, the walls of the oviduct produce albumen, which gradually envelops the yolk ball. The shell membrane forms, in Baer's opinion, from the surface layer of the albumen, because the latter rolls up when the egg reaches the uterus. In this division of the oviduct the "hail-stones" (HAGELSCHNURE) and the shell are formed. In the uterus the egg remains nearly a day. Concerning the formation of the rudiment, Baer assumed that it develops from the content of the embryonic vesicle, indicating a contradiction because there is no rudiment in the unfertilized eggs even though the embryonic vesicle may have ruptured even without fertilization.

Not deciding beforehand the question of whether the embryo is produced only from the egg nucleus, Baer accorded the latter great significance and strived to explain its origin and subsequent fate. His comparative investigations led Baer to believe that the embryonic vesicle exists at the earliest stages of egg formation in the ovary. Thus, in hens he saw the embryonic vesicle in ovarian eggs with a diameter of not more than half an inch (about 1.3 mm); the remaining egg parts, including the yolk, are apparently formed later. (Baer had already stated in his DE OVI his belief in the primary existence of the embryonic vesicle.)

G. Rathke, studying salmon eggs, 2 objected against Baer and confirmed that the "Purkinje vesicle arises . . . considerably later than the yolk." To resolve this dispute, Baer referred to the other objects investigated by Rathke, the river crayfish and fishes. He reported in a later section of his main work (§ 11, footnote pp. 392-394 (11 Bw, 296)) that in the autumn, when the eggs increase in size and acquire a color, it is easy to extract the embryonic vesicles included in the voluminous yolk mass. In immature eggs, containing considerably less and still uncolored yolk, the embryonic vesicles are also seen, but they are smaller in size. Even in the smallest eggs with few granules, the embryonic vesicles already exist. In such eggs, there is also a substance dissimilar to yolk, and a few fluid vesicles. Baer concluded that the yolk of eggs which acquire embryonic vesicles is formed only in the period of egg maturation. In the fish Baer also observed that such egg has a nucleus; however, in younger eggs the nucleus is larger and is surrounded with less substance.

In the process of incubation, the egg loses weight as a result of evaporation, but during the same period unincubated eggs lose less weight than incubated eggs. Simultaneously with the development of the embryo, the volume of the air chamber increases, and the air present in it contains oxygen which the embryo uses. (99) Due to the loss of water, the egg albumen thickens. Changes in the yolk are especially distinct: to the fifth day of incubation the yolk increases in volume, arises to the shell, becomes thin, and its granules become more obvious or distinct. These changes were observed by the Kazan professor Eikhval'd. In the process of development not only morphological, but also chemical transformations occur, and new chemical substances appear.

2. Rathke, "Uber das Ei einiger Lachsarten," ARCH. ANAT. PHYSIOL (Martin Heinrich Rathke, "Darstellung der spatern Umbildung," in NEUESTE SCHRIFTEN DER NATURFORSCHENDEN GESELLSCHAFT IN DANZIG, VI, Heft 4.)

Structural changes in the egg lead to the disappearance of the yolk membrane and to the development of the rudiment. The latter enlarges in size and begins gradually to cover the yolk; the central part of the rudiment becomes the embryo, and the large peripheral region remains thin and has the shape of a pellicle. Baer called it the rudiment pellicle or blastoderm. It represents a continuation of the embryo, with which it is directly connected, and eventually most of it becomes part of the embryo. The rudiment pellicle contains the bloodcarrying vessels which receive the nutritional materials from the yolk and transfer them to the embryo. And thus, as the rudiment becomes enlarged, it divides into two parts distinguished by the external shape but connected by a common vital process: its center becomes the embryo, and the periphery becomes the blastoderm. At the beginning the rudiment lies on the surface of the yolk in the form of a plate, and then, on growing, it gradually covers the yolk and acquires a sac form. Already on the fourth day between the embryo and the underlying sac a narrow communication remains. The vesicle containing the yolk is called the yolk sac, or the intestinal or yolk vesicle.

In the process of development, the rudiment is divided into two incompletely separated layers. From the surface layer the animal parts of the embryo form, and from the internal layer the vegetative or plastic parts. Therefore Baer named them the animal and vegetative layers. At the same time Baer recalled that this animal layer is nothing other than Pander's serous layer, and the vegetative layer corresponds to Pander's vascular and mucous layers. Pander's terms had been worked out during his time in Wiirzburg and had spread since that time. Baer considered the names vascular and mucous layers appropriate but the designation serous layer inappropriate, because the covers of the embryo form only from its peripheral parts, while the middle gives rise to the most important internal organs. Besides that, it represents a division from both other layers, and the animal part of the embryo develops from it.

This division of the animal and plastic layers Baer distinctly implemented in his first volume (§ 1) and in the fourth scholium of the second part of UBER ENTWICKLUNGSGESCHICHTE. Here he noted the importance of such opposition for the comparison of embryonic development of different vertebrates and for the comparison of development of the vertebrates and the lower animals. "The vegetative layer," Baer wrote, "contains the layer of the mucous membrane and the vascular layer, but the animal layer at first corresponds with Pander's serous layer. Later it divides in the middle into two layers, the lower of which I call the fleshy layer and the upper the skin layer" (II, 51, pp. 46 - 47 fn. (64)).

The embryo, on separating from the yolk, connects with the remaining parts of the egg by means of the umbilicus. The external umbilicus represents the border between the embryonic and extra- embryonic parts, which previously represented the periphery of the animal layer. The internal umbilicus, the yolk duct, represents the transition of the internal vegetative parts into the vegetative layer of the embryonic sac. In the vegetative layer of the yolk sac there are two subordinated layers, a vascular and a mucous layer. The blood- carrying vessels, present only in the former, are transferred into the embryonic vessels. In mammals they are called the umbilical-mesenteric vessels because they go through the umbilicus from the mesentery. The animal layer of the rudiment undergoes transformation upon the appearance of the amnion on the third or fourth day, when the blastoderm is divided into two main layers.

Baer's description of amnion development is very unclear. He proposed to withdraw Wolff's idea of the "false amnion" (see Chapter 5) from use, especially since Pander did not employ it in the same sense (see Chapter 12) ; this produced some confusion.

Baer designated the cephalic fold of the animal layer the cephalic cap; it gradually enlarges and forms the cephalic vagina. Slightly later, this same process takes place at the caudal or tail end (the caudal cap becomes the caudal vagina) 3 and at the sides. By this means develops the circular fold, whose top fuses to form the amnion which thus grows from the animal layer.

When the opening of the amnion closes, the lower layer of its forming fold contributes to the construction of the amnion, while the upper is not connected with amnion formation For this part, Baer suggested keeping Pander's name serous vesicle or serous membrane. The serous vesicle includes the amnion with the embryonic and yolk sac. Between the serous membrane, amnion and the yolk sac there remains an interspace connected with a gap between the dermal and intestinal umbilicus. This ring-shaped gap leads into the embryonic abdominal cavity and could be named the abdominal umbilicus. Therefore, the cavity of the serous vesicle is an extraembryonic part of the abdominal cavity, as the yolk sac is the continuation of the digestive canal situated outside the body.

Turning to the development of the allantois, Baer described how on the third day from the most posterior end of the digestive canal, i.e. from the future cloaca, a small rounded sac protrudes. Upon elongating, it passes through the abdominal umbilicus and appears in the space between the amnion, yolk sac and serous membrane. Later it spreads, covers the amnion, penetrating between it and the serous membrane, and then also grows over the yolk sac. Baer called this formation the urinary sac, not only because it develops from the cloaca, into which the urinary tracts flows, but also because urine collects in it the second half of incubation. (100) The stem of the urinary sac is called the urinary duct (urachus) . Because the intestine consists of two layers (internal -mucous and external -vascular) , these same layers should be present also in the urinary sac, and they actually can be distinguished there very early. In the external layer of the urinary sac a network of vessels develop to receive the blood from the two branches of the aorta, the umbilical arteries. Through the single umbilical vein, the blood goes from the urinary sac backwards into the body of the embryo. These vessels had been named "umbilical arteries" because they go through the umbilicus, but in Baer's opinion they would better be called the vessels of the urinary sac. The external half of the urinary sac represents the embryonic organ of respiration. It firmly adjoins the shell emembrane, then gradually separates from the shell and forms the chorion. The internal half of the urinary sac becomes thin and fits close to the amnion and yolk sac.

3 . Its cavity represents the extra-embryonic part of the complete circular cavity.

Baer summarized changes in the incubated egg as follows. In the process of incubation the quantity of albumen decreases partly from evaporation and partly as a result of its use by the embryo; thus the volume of the air chamber enlarges at the blunt end of the egg. The yolk mass at first increases, then decreases as the embryo uses it; the membranes of the yolk and chalazae disappear. The middle part of the rudiment is transformed into the embryo, and the periphery into the blastoderm covering the yolk. Upon separation of the embryo from the blastoderm, the umbilicus forms, and the extraembryonic blastoderm forms the yolk sac hanging from the embryo. In the latter, the blood-carrying vessels of the yolk sac branch and become the vessels of the mesentery along the yolk duct. The animal layer of the rudiment membrane forms two vesicles (or membranes) , the amnion and the serous vesicle, of which only the amnion remains to the end of development. From the vegetative region of the embryo, the urinary sac, rich in vessels, protrudes and is gradually overgrown by the embryo with its appendages, the yolk sac and the amnion. The external part of the urinary sac forms the chorion adjoining the shell membrane. The parts of the extra- embryonic formations (albumen, yolk membranes, chalazae, the yolk itself, the peripheral zone of the blastoderm, the internal half of the urinary sac) are reduced at different times. Only the embryo develops and grows continuously.

At the end of incubation, the yolk sac enters through the umbilicus to the embryonic abdominal cavity, where the remains of the yolk are used for some weeks after hatching. During the nineteenth and twentieth days of incubation, the umbilicus narrows, which leads to blockage of blood circulation in the umbilical vessels. The chicken tries to breathe with its lungs by penetrating its beak into the air chamber or by breaking the shell. Then the movement of the blood in the umbilical vessels stops completely; the umbilicus closes and isolates the chick from its embryonic appendages. On hatching, the chick leaves its membranes, amnion, and chorion with the shell membrane and the shell.

The next division of Baer's work, entitled "General Method of Formation of the Bird Embryo," (§ 6) concerns the previously established forms of differentiation: the primary, morphological, and histological. In the organization of all vertebrates, Baer considered essential, not the specialized organs of blood-carrying, nervous or digestive system, but the parts similar for all vertebrates. According to Baer, a generalized understanding such as that of the body layers spreading along the entire extension of the body could be useful. These layers lie one above the other, as if they develop each other. They become noticeable in the earliest developmental stages, but can also be recognized in the adult. (101)

Baer prefaced his scheme of the structure of all vertebrates with the characteristics of primary differentiation. Along the longitudinal axis extends the stem (vertebral column) , above which is the spinal part of the animal and below which the abdominal part of the animal. The spinal part consists of the neural tube, the vascular layer, the muscular layer and skin. All these layers have the form of a tube, which forms the primary organs of the vertebrate animal. If we do not take into consideration the extremities, it is possible to consider that the body of the vertebrate animal forms the following parts or layers:

1. The firm or solid part, which never extends beyond the surface of the trunk.

2. The spinal part, composed of:

1) the closed neural tube;

2) the muscular tube surrounding it;

3) half of the dermal tube, covering the muscular tube.

3. The abdominal part, composed of:

1) the closed mucous tube, which forms the internal surface of the abdominal part;

2) the vascular tube which surrounds the mucous tubes, goes above the muscular tube to the trunk, and adjoins to its lower surface;

3) the muscular tube which is closed upwards by the trunk ;

4) the second half of the dermal or skin tube which covers the muscular tube.

Therefore, with a transverse section, such a figure is obtained: the two opposing tubes of the muscular layer form a figure eight, in the middle of which is the trunk; in the upper circle of the figure eight is a tube of neural material, and in the lower circle is the tube of the mucous membrane, surrounded by the vascular layer which continues to the trunk. The muscular layer forms two tubes— the spinal and abdominal— both of which are covered by the common skin cover. The spinal and abdominal tubes represent primary organs and are initially strictly symmetrical. Deviations from symmetry, observed in some vertebrates, have a secondary character.

The source of symmetry of the primary, tube-shaped organs is their formation from curved and accreted paired plates. All the paired plates (those of the spinal cord, spinal, abdominal, mesenteric and intestinal) which give rise to the aforementioned tubes, could be converted into two pairs of the main plates, the spinal (dorsal) and the abdominal (ventral). Previously, all these plates formed one general plate composed of heterogenous layers. Earlier the different layers were not recognized.

Thus gradually the grounds of development occur, "but only in reverse succession" (II, 6 (Ah) p. 90. (63)) With these words, Baer stated his method of embryological investigations. He considered that in the beginning it is necessary to study the general organization of the formed animal, that end to which the long chain of developmental processes leads. Later it is possible to pass gradually along the links of this chain to its beginning, to trace development retrospectively, and then it is possible to establish the succession of events in actual chronological order.

The beginning phase of primary differentiation Baer described in the following words: "Initially there are no separable differentiated layers, but only the surfaces of the rudiment, in which one can see differences, as in polyps, indicating the opposition between external and internal surfaces. The space between these surfaces, as in the polyps, is occupied by an indifferent mass" (II, 6(4)i, p. 91 (67-68) ) c The given quotation indicates with complete clarity that the idea of comparing the embryonic layers in vertebrate embryos with the body layers of the coelenterates belongs to Baer, and not to Thomas Huxley, 4 as is frequently claimed. 5 Baer was ahead of the English zoologist by not less than twenty years, if we take into consideration that the printing of his second volume began in 1829.

In the embryo, Baer continued, the opposition of external and internal surfaces results in differentiation of the upper skin and lower mucous layers. Simultaneously, a differentiation occurs along the surface: the middle of the rudiment becomes thin, giving rise to the transparent zone (Wolff's area pellucida) , and the periphery forms the dark zone (Pander's area opaca) . The vascular layer reaches only the middle of the dark zone. Because the vessels are present only in the vascular layer, the periphery is broken up into the internal, vascular zone (Wolff's area vasculosa) which is separated from the yolk zone (area vitellaria) by the broad terminal vein (vena terminalis, which Baer called the sinus terminal) . The transparent zone is also divided: the middle is raised in the form of a shield (the future embryo) , and the periphery forms what is called the fetal zone. The shield elongates at straight angle to the egg axis, outlining the axis of the embryo along which forms the primary zone.

Thomas Huxley, "On the anatomy and the affinities of the family of the Medusae," PHIL. TRANS. ROY. SOC, 2 (1849) , pp. 413-434.

See, as an example: I. I. (Ilia or Elias) Mechnikov, EMRYOLOGISCHE STUDIEN AN MEDUSEN. EIN BEITRAG ZUR GENEALOGIE DER PRIMITIV-ORGANE (Wien: A. HcSlder, 1886? Moscow: Akademii Nauk, 1950), pp. 284, 418.

As a result of the fusion or closure of the embryonic layers, they are converted into tubes. The beginning of this process Baer described as follows. Along both sides of the primary zone two unremarkable thickenings arise which appear as a line of dark small balls. This line, the spinal cord, forms the middle part of the trunk. The lateral thickenings, the spinal plates (Laminae dorsalis) , contain only the skin and muscular layers. They correspond to what Pander had called primary folds (plicae primitivae) and what Burdach had called "mirror plates." The crests of the spinal plates deviate from each other and accrete, forming the back of the embryo. The internally folded part of the skin layer is separated from the muscular layer, and quickly thickens to form the central part of the nervous system as a somewhat laterally compressed neural tube. Soon after the formation of the spinal folds, curving of the wide abdominal folds or the abdominal plates (Laminae ventrales) begins downwards. Wolff had called them Fasciae abdominales, because he assumed that they do not reach the posterior part of the body.

This process is concluded along both sides of the spinal cord where, in the vascular layer, two thickened bands form; their external borders incline to each other and accrete. The mucous layer in the region of the cord separates from it, so that the bands of the vascular layer, called the mesenteric plates (laminae mesentericae) , appear in that free space between the cord and mucous layer. The place of fusion of the mesenteric plates Baer, like Wolff, called the suture of the false amnion. Soon after the formation of the suture, along the sides two other bands separate from it, composed from vascular and mucous layers. They thicken, acquire the form of plates, incline to each other and accrete; hence each intestinal plate (lamina intestinalis) represents a half -canal, and they together form the intestinal tube. The fusion of the intestinal plates downwards represents a simultaneous separation of the embryo from the blastoderm.

A double conversion of the plates into tubes leads to the formation of the primary organs of the embryo. In all the primary tube-shaped organs it is possible to distinguish, first, a central line and, second, a fusion line by which accretion takes place. The fusion lines of each primary organ correspond to the peripheral borders of these plates, from which the organ is formed. The central line of each primary organ had occupied the center while in the stage of the plates. All the central lines lie in the middle plane, one above the other and close to each other. Only the skin, which has two fusing lines, has no central line. At an early stage of development, all the central lines of the future primary organs are concentrated in the primary zone.

Turning to the development of the extremities, Baer suggested that both pairs of vertebrate extremities are connected with broad muscular belts and that the basic segments of the extremities and the adjoining muscles form the external muscular layer, including both muscular tubes of the trunk. The formation of the first fold of the extremity, in Baer's opinion, confirmed that suggestion, because it appears from each side as a long common fold which constitutes part of the external muscular tube; this in turn represents a primary organ.

The described relations Baer illustrated with a schematic drawing (Figure 28) . If we lay flat the plane of the layer of the spinal plates ab" and that of the abdominal plates ac", they will look like ab and ac. When the plates of the extremities b"c M are located in the plane ab be, then the transverse section shows that they extend from the closing line of the back (b" or b) to the closing line of the abdomen (c" or c) . From the scheme it appears that in the zone of the rudiment, through the rise of the spinal plates, the plates of the extremities will be the continuation of the back plates. (II 6A, 77-78)

Figure 28. Baer's scheme of vertebrate development and structure (explanation is in the text)

Further breakdown of the primary organs leads byindirect morphological differentiation to the formation of definitive organs. Thus, in the cephalic part of the mucous tube, the entrance is differentiated into the respiratorysystem and the entrance into the anterior part of the digestive canal. In the latter, the esophagus is narrowed and serves only for transfering food; the second (stomach) is wide and provides digestion for the food Individual areas of the mucous tube protrude and branch away, forming glands (salivary, liver, and the pancreas) . The same principle is implemented also in the development of other tubes such as the neural tube, in which the anterior and thickens in the form of the brain, and the posterior end narrows to form the spinal cord; individual parts of the brain in turn develop into subordinated parts.

The processes or morphological differentiation, which occur after primary organ formation, conform to some general regulations. Individualization is implemented gradually by irregular growth, such as narrowing, branching and so on. Morphological differentiation spreads from the inert region of the central line toward the fusing line; this route, recognized also in the development of the most primary organs, Baer called the generating arch.

The form- generating process does not proceed directly from the central lines to the fusing or closing lines; it differentiates the similar morphological elements one after the other. The entire vertebrate body consists of a combination of such morphological elements. Thus, the vertebra with its upper and lower arch is a morphological element of the bone system; the double ring nerves, with part of the central nervous system, represent a morphological element of the nervous system; the blood- carrying system is also composed of morphological elements.

Along the longitudinal axis, the morphological elements, such as vertebrae of different regions, are not identical. The group of morphological elements with similar features, such as the neck vertebrae, Baer called a morphological segment. By the principle of dividing it into morphological segments, the whole vertebrate body is divided into the head and trunk, the latter of which is further divided into the thoracic and abdominal parts. The morphological elements are established very early in the embryo, and the differences between them develop late; much later the morphological segments develop. The morphological elements and segments stand in a different relationship to the particular organs.

Assuming that understanding of organs is devoid of morphological content, Baer found it necessary to introduce a more complete understanding of morphological elements and segments. Thus he held that the eyeball belongs to one morphological element, while the brain occupies a whole segment, which in turn consists of elements. The liver, regardless of its size, is a product of one morphological element, and the small thyroid gland belongs to two elements. In adults, the breakdown of vegetative organs into morphological elements is not noticeable. The younger the animal, the more this division is obvious; thus, the branchiate slits with their five vascular arches undoubtedly relates to the division of the throat cavity into five parts. In the arthropods there is an obvious breakdown of the whole intestine.

Baer considered this morphological analysis a very important task which had not previously attracted the attention of embryologists. He expressed his belief that it would be possible to explain the factors upon which all the particular properties of animal structure depend. But this has not yet been accomplished. Nonetheless, interest in problems of animal structure, in particular the phenomena of metamerism which is characteristic of contemporary morphology (investigations of A. N. Severtsov, B. S. Matveev, D. P. Filatov, P. P. Ivanov, N. A. Livanov, V. N. Bekhemishev, and others), bears witness to Baer's insight.

The actual properties of each primary organ are determined by the character of its morphological division. In particular, cored growths or protrusions form from the neural or intestinal tubes, but the other primary organs only form compact growths. The cored or hollow organs such as the heart and blood vessels can develop not only by means of protrusion (morphological differentiation) , but also by means of histological differentiation, particularly due to the develppment of hollow passages in the vascular layer. By these observations, Baer established the beginning of a theory of development of the vascular system. This theory gained embryological use much later, and Baer's role in its establishment is not always evaluated fairly.

Turning to histological differentiation, Baer introduced histological elements. At first the embryo is composed of a nearly homogenous mass, partially consisting of dark or light small globules or vesicles, and partially of a transparent formless mass. Individual organs at first are also almost entirely homogenous, and only later do fibres, plates and hollow passages appear in them. Baer stated that "modern anatomists called the study of tissue histology, in contrast to anatomy, or the study of the external form. Therefore, in the embryo, the development of separations into multiformed tissues is called histological differentiation, represents not a new formation, but a change in what already exists, particularly by separation of the homogenous into variable histological elements. Histological differentiation usually develops later than morphological differentiation; however they are not completely distinct temporally (II 6(c) 11, pp. 122-123 (92)).

Blood forms, according to Baer, by a thinning of certain parts of the organism, but the walls of the blood-vessels appear with the movement of the blood. All this occurs at first in the vascular layer and then throughout the embryo.

The processes of histological differentiation are very distinct in muscle formation. At first they look extremely soft, like unclear ly formulated, fairly thick fibers with alternating widened and narrowed portions. These fibers do not grow from other muscles and do not connect one bone with others, but develop in a formless mass located between the bones. Baer objected to Ham's view, that muscles form from small blood globules in a row. Muscle bundles develop by splitting of the initially developing fibers, as a result of which the latter become thinner.

Concerning nerve formation, Baer believed that nerves do not represent growths from the neural tube. Only the nerves going to the sensory organs represent a growth from the brain, while the other nerves develop by means of histological differentiation in other primary organs.

From his comparison of the three types of differentiation Baer generated the following aphorisms. "Primary, morphological and histological differentiations repeat the same distinctions, the first above the others, the second behind the others, and the third in the others" (II 6D, p. 126 (94)). Therefore, these distinctions are not absolute, but only relative, because the distinctions, which are essential in the primary organs, are repeated as subordinated distinctions in individual parts of the body. C102)

The divisions concluding the description of bird development (§ 7) considers the formation of individual systems and organs. Baer first addressed the histological differentiation of the skeletal parts of the bony or osseous system. According to his description, all the bones are composed of cartilage, which comes from closely arranged small dark globules. The mass of these globules becomes light and forms a soft cartilaginous material; the periphery of the cartilage becomes a cartilaginous membrane, and the middle becomes a firmer cartilage. The cartilaginous parts at first remain formless and only later develop defined features, acquire appendages, and so on. In other words, for the skeletal elements, morphological differentiation is preceded by histological. Ossification in individual cartilage proceeds from the middle to the surface, frequently beginning in several spots at the same. time. The joints appear simultaneously with the cartilage by similar histological differentiation, as distinctly observed by Tredern on the fingers of the anterior and posterior extremities. From the parts of the body system, the axial skeleton is established earliest. Along the embryonic axis, dark granules form a thin string, the vertebral cord. Baer reminded his readers that in De ovi he gave this formation the name spinal cord and later concluded that it should have been named the vertebral cord. It must be noted that in embryological literature after Baer his first choice gained distribution.

By means of histological differentiation, the cord is separated from its lighter membrane, the cord sheath.

Ossification of the vertebrae proceeds from the anterior backwards, as described in detail in the first part (I 1Z, 2c, 2d and llf) . The body of each vertebra has. an independent point of ossification. The upper arches of the vertebrae form from two halves, the opposing aggregations of dark granules in both the spinal plates.

Baer's ideas of the development of the osseous skeleton and the skeleton of the extremities were perceptive. Thus, the first steps of skull formation in connection with the developing brain constitute the form-producing interactions of parts in embryonic development. Talking about the order of ossification of the skeletal elements, Baer noted that this histogenetic process begins earliest in the fastest growing parts of the skeleton.

The transverse appendages of the vertebrae and ribs are initially established as an entity and then separated by joints. In respect to the development of the skull, Baer stuck to "the vertebral theory" formulated by Oken and Goethe. The skull, in Baer's opinion, is "the sum of the most anterior vertebral arches." It develops like the other vertebrae, only the development here is "modified by the strong extension of the brain" (II 7d, p. 132 (99)). The bones, of the facial part of the skull "are formed from the most anterior end of the ventral plates and represent, in this way, the lower arches of the cephalic vertebrae" (II 7c, p. 133 (100)) .

The extremities are formed by the expansion of the layer lying above the spinal and abdominal plates and which becomes observable only at the third day of incubation. The fold from which the extremities form spreads upwards, downwards and externally; development upwards and downwards produces the trunk part of the extremities, shoulder girdle and pelvis. In Drawing 28 the original point of development of the extremities is shown by the spot d". The growth externally raises the crest of the fold in the form of a layer, after which the foundation of the extremities is divided into a stem and plate, the middle and the terminal segments of the extremities.

Concerning the development of the jaws, Baer leaned towards the nature-philosophical analogy identifying the jaws with the extremities.

As already noted, the central part of the nervous system develops, in Baer's opinion, by primary differentiation through exfoliation from the internal surface of the spinal plates, while the peripheral part is formed through histological differentiation of the muscular layer. At the beginning, the tissue differentiating into the neural tube is characterized by histological homogeneity, but soon differences in the structure of the surface and in deeper layers appear; particularly in the latter which connects with brain tissue. The division of the neural tube into rudiments of the brain and spinal cord takes place before the fusion of the spinal plates, insofar as the anterior part of the tube appears wider than the posterior.

The spinal cord and its extension maintains a regular thickness, with the exception of the places of formation of the extremities, where there are thickenings of the spinal-brain tube. The internal structure of the spinal cord becomes visible gradually. In it there appear four main main structures, which are particularly obvious at the internal surface. The number of structures increases, and still later fibers become recognizable in it. The internal part becomes grey, and the external white, while the grey matter at the transverse section acquires the form of a cross.

The cephalic brain at the early stages of development is little different from the spinal cord. Yet, it must not be thought, Baer said, that the cephalic brain represents an anterior extension of the spinal cord into the skull cavity, or the reverse. They both represent a modification of one primary organ, the brain tube, and are formed from it by means of morphological differentiation. The earliest part of the cephalic brain soon divides into separate portions, each of which expands to form brain vesicles; between them interceptors form. At first the anterior vesicle separates from the most elongated posterior one, then the latter subdivides into two and produces the stage of the three vesicles; the anterior, middle and posterior. The anterior vesicle represents the future large brain, the posterior becomes the cerebellum and the medulla oblongata, and the middle becomes the four-hil locked mass. The anterior vesicle soon divides into two, and its anterior part becomes paired. The posterior vesicle also is converted into two, so the number of brain vesicles increases to five. The cavity of the anterior vesicle is the beginning of the lateral ventricles of the brain, and its walls become the hemispheres. Inside the second vesicle, the cavity of the third ventricle appears. The third vesicle is the rudiment of the four-hillocked mass. The fourth vesicle becomes the cerebellum, and the fifth is the medulla oblongata. All the brain vesicles which communicate among themselves at first lie in one line, where the curving of the cephalic brain and the reciprocal displacement of its division begins. The subsequent development of the brain is described in the first part (I 2m, 5aa, 9v, lOt, lip, 12g) . All sensory organs are formed from the anterior part of the cephalic brain.

Prior to other sensory organs, the eyes develop. They are already detectable on the second day as two prominences on the sides of the intermediate brain. Their connection with the brain is narrowed, and at that time the eye rudiments have the shape of vesicles which are situated on cored stems; from the latter the optic nerves form, and from the vesicles the eye balls. The vascular and hard eye membranes develop with the splitting into layers of the initial single cover of the eye, the same that takes place in the brain, the cornea represents a part of the hard membrane; later the anterior chamber of the eye develops under it (see v. I, 2n, 5bb, 6v, 7w, 9w, lOu, llq, 12h) .

The ear is founded at the end of the second day; its primary rudiment, according to Baer, is a protrusion of the posterior part of the cephalic brain. 6 By what means the auditory vesicle turns into a labyrinth remained unknown to Baer; undoubtedly the auditory nerve forms like the optic nerve. From the throat cavity to the ear, a protrusion covered with mucous membrane grows ,. forming the eustachian tube and the drum cavity.

It is difficult to imagine how such a sharp-sighted and careful observer as Baer could allow such a mistake. The auditory vesicle (rudiment of the internal ear) is formed from the increasing unlacing of the ectoderm.

For the formation of the organ of small, the anterior brain forms protrusions, against which olfactory depressions appear on the surface; the nasal passages develop later, after the formation of the palate and upper jaw. Thus, Baer wrote, "the eye is a protrusion of the brain tube through the muscular layer, and the nose is a protrusion from the brain to that bony region" (II 7q, p. 156 (117)). The organ of taste stands by itself. "I could not recognize," Baer wrote, "whether for the formation of the tongue a part of the brain protruded" (II 7r, p. 157 (118)).

Recalling that the abdominal plates of the mucous layer approach each other along the length of the embryo, forming an internal tube at the same time as the unlacing of the embryo, Baer described the subsequent changes. In agreement with Rathke, in the anteriormost part of the tube, which Baer called the mouth part of the intestine, the mouth slit bursts open. In the posterior, the mouth part of the intestine opens by an orifice, which Wolff called the fossa cardiaca, facing the yolk. Because it is not connected with the heart and does not correspond with the future stomach, Baer suggested calling it "the anterior entrance into the intestinal canal." At the posterior end of the embryo, upon initiation of unlacing a blind hole is formed; this is converted afterwards into a tube whose end later opens. Instead of Wolff's name, "lower hole" (foveola inferior), Baer suggested the term "posterior entrance into the digestive canal." The posterior portion of the intestine, he called the posteriorcommunicating intestine, in agreement with Rathke. The middle portion of the intestine, located between the anterior and posterior entrances, at first remains flat. Upon development of the mesentery, it is converted into a gutter, the borders of which are formed by the intestinal plates. By continuation of the unlacing, the anterior and posterior portions of the intestine pull up to each other, their entrances become nearer and form a general passage from the intestine into the yolk sac (the intestinal umbilicus) ; at the fifth day this passage draws up in a narrow canal, the yolk duct (see v. 1, 5e, f, r; 6g, 7h) .

Baer noted the historical study of digestive canal development, writing that Wolff was the first who understood this method of development and explained it in the greatest work which we know in the field of description in the natural sciences, in his treatise DE FORMATIONE INTESTINARUM. It was published in the twelfth and thirteenth volumes of the NOVI COMMENT ARI I ACADEMIAE PETROPOLITANAE . Meckel performed this work and issued it in the form of an individual book under the name of C. F. Wolff, 0BER..DIE BILDUNG DES DARMKANALS IM BEBRUTETEN HUHNCHEN (Halle, 1812), accompanied by an introduction, in which he discussed the concurrence of mammalian and avian development. I refer to Wolff's book not only for the study of the development of the intestine, but also for the early history of development in general .... This book had an unfortunate fate, because its main content, the discovery of the method of intestine formation and physiology, in great part was incorrectly understood. To Wolff an opinion was added, that the intestinal plates grow out from the vertebral column and are put together with each other. However, Wolff, in many places, says definitely that the intestinal plates represents parts of his false amnion, and the false amnion of Wolff is our "title cap," i.e. part of the vegetative layer of the blastoderm. 7 (II 7s, fn. 161 (121))

Baer's words testify to his high evaluation of his predecessor's classical work, as well as in the work of the Petersburg Academy of Science.

7. About this work of Wolff see Chapter 5.

It is known that Oken, not understanding Wolff's work, considered intestine formation as if both ends of the intestine grow into anterior and posterior regions of the embryo. Arguing with Oken, Baer affirmed that the embryo develops not from parts isolated at the beginning, but from a common rudiment; the anterior and posterior intestines from the beginning occupy fixed places in the embryo. Before the connection of the mouth and posterior-communicating portions of the intestine, the digestive canal remains straight and does not show a structural difference along its length. For this stage of development of the intestine, Baer considered acceptable Wolff's term "primitive intestine." Along its length, it consists of two layers, the internal mucous and the external vascular; above the intestine, the vascular layer extends towards the vertebral column. Between the mesentery plates, there remains a space which Baer named the mesenteric aperture. "This aperture," Baer remarked "represents what Wolff named the fistula intestinalis, because he did not distinguish it from the aperture of the intestine. There is his main mistake" (II, 7s fn.p. 164 (123))

The intestine at the described stage of development represents a primitive organ not only for all the digestive apparatus, but also for the respiratory and some urinary and sexual organs. The most anterior part of the digestive canal, the gullet, after jaw development, is directed downwards to form the mouth cavity, from which the nose cavities are separated by the palate. In the posterior part of the gullet cavity at the third day, three pairs of slits appear which, as in fish, should be named the branchiate slits. On the third or fourth day the first slit usually becomes covered, and posteriorly the fourth is formed. On the fifth or sixth day, as a rule, the other slits are closed. The portions of the gullet walls between the slits are called branchiate arches .

The intestinal tube develops somewhat later than the gullet, and the crop significantly later. The stomach at the beginning is not distinguished by width from the other parts of the digestive tube, but later it widens. The intestine gradually elongates. The small intestine, forms many loops, some of which pass through the umbilicus from the abdominal cavity and then extend internally with the remnants of the yolk sac, which in some birds such as nightingales never disappear. The posterior part of the intestine (cloaca) continues .

The liver is a protrusion of the intestinal walls, which forms externally and downwards two blunt hollow appendages enveloping a venous stem. The bases of the protrusion gradually narrow. The protrusions of the mucous layer branch into the vascular layer, which is raised in the form of a mound. On the subsequent formation of the liver, its ducts and gall bladder appear. The enlarging mass of the vascular layer forms the parenchyma of the liver, and the vein which is jammed between the liver rudiments branches in the parenchyma as the portal vessels. Similarly, but without that much close contact with the vessels, the pancreas develops Baer wrongly considered that it develops as a single structure.

All the respiratory apparatus develops by protrusion from the digestive tube. Directly behind the last branchiate slit, on the eighth day a pair of small hollow elevations appear; these are converted into sacs along a common narrow base. These sacs represent the lung rudiments from whose stem-like base the respiratory tube develops. Subsequently the lungs form a system of branching tubes, and the air sacs develop to penetrate the entire cavity of the body and bones.

Histological differentiation of the vascular system Baer described as follows: Blood forms earlier than the vessels as a result of the dilution or thinning out of the previously compact or firm parts. This occurs only in the vascular layer. The fluid is initially colorless, then it becomes yellow and finally it becomes red blood. Under the effect of the movement of blood, the permanent passages form and soon acquire firmer walls.

The process of morphological differentiation in the vascular system Baer divided into four periods, each involving small changes which prepare for the subsequent period. The first period, in which the vascular system appears, continues during the first two days of incubation. On the second day, between the anterior ends of the ventral plates, two elongated granular masses appear forming a figure called the cardiac canal. Proceeding from it anteriorly are two vascular arches, which Baer correctly noted are only like veins at first; Wolff was incorrect when he considered that all the vessels of the first arch are veins. These arches soon become less detectable, then the second pair of arches is formed, later the third, and finally, between the second and third days the fourth pair is formed. The arches of each side pour into canals (roots of the aorta) above the gullet, which are fused into the impaired aorta passing under the vertebral column; its branches give rise to the arteries of the yellow sac.

Simultaneously, vein formation occurs, and the veins join into two main stems in the posterior and anterior halves of the vascular field; both stems enter the left bend of the cardiac canal. The posterior part of the heart forms a horseshoe- shaped elevation directed to the right. At that time the heart does not yet appear in the region of the future chest cavity, but in the region of the neck.

The second period of vascular development is characterized by a circular blood movement, which is connected with the blastoderm. The blood moves primarily anteriorly and in two great stems is transferred into one or two anterior veins of the blastoderm. From the posterior part of the blastoderm, blood collects initially in one (left) posterior vein of the blastoderm, and only later the right posterior vein appears. All venous blood enters both bends of the heart, but mainly the left. From the two stems of the aorta go the posterior vertebral arteries and the blastoderm arteries. The latter, branching, reach the border sinus. In the second period the vessels of the blastoderm become vessels of the yolk sac.

The blood-carrying system at this stage has been demonstrated in a distinct and refined schematic drawing, with letter designations as mentioned below in the text. 8 (Drawing 29) The vascular system of the embryonic body is composed of the following vessels. The vein of the yolk sac N begins in the mesentery; both arteries of the blastoderms join to make a common stem, the artery of the yolk sac p. The embryonic veins pour into a vessel which passes between the liver and heart and produces the liver branchings (the portal system) . The veins going from the head along the lateral sides of the neck collect blood from the brain and neck region (the anterior vertebral veins g) and pour into a common venous stem o. From the posterior end of the body, the blood flows in the vessels, passing at the upper border of the mesenteric plates. In these vessels (posterior vertebral veins hi) veins pour in from the tail h and posterior extremities, cloaca, pelvic region, the posterior end of the kidneys and small branches of each intervertebral space. The general stems, in which the anterior and posterior vertebral veins join, are named the umbilical venous stems K; both these stems go to the heart ab. Only later is the posterior hollow vein m separated. From the union of the vessels going along the lower border of each abdominal plate, in the third period the umbilical vein forms .

8. The relationship of Baer's idea by the vascular system in the amniotes to the contemporary data given by P. G. Svetlov in the note to the Russian translation of the second volume of HISTORY OF ANIMAL DEVELOPMENT (see note 85 on pp. 468 - 469) .

The changes in the heart in the second period are as follows. From the posterior part of the cardiac canal the common venous stem forms, from the anterior part, the arterial stem, and from the middle, the whole heart chamber. Behind the passage from the venous stem, two sacs protrude into the heart chamber; these are the nondivided auricles. In the cavity of the middle part of the cardiac canal, from the convex side a lower fold grows inside and separates the single blood current into two. The arterial division of the heart widens into the trunk of the aorta. The first branchiate vascular arch disappears on the fourth day, and in replacement the fifth arch develops. Simultaneously, in the place of the union of the first arterial arch with the spinal aorta, the rudiment of the vertebral artery d is formed; and from the place of union of the first arch with the stem going from the root of the aorta, the cephalic artery c forms. The spinal aorta at the posterior end of the kidney divides into two vertebral arteries. "For blood circulation of the second period, it is characteristic that the blood in the whole route does not pass through the differentiated organs of respiration," Baer asserted (II 7gg, p. 188 (132)) .

The third period is characterized by blood circulation through the external respiratory organ, whose role is responding to the quickly growing urinary sac. The branches of the aorta going into it, the umbilical arteries, enlarge. Consequently, the umbilical veins also enlarge, especially the left vein and the posterior hollow vein. The posterior vertebral veins are reduced into an unpaired vein. The anterior end of the venous stem is increasingly converting into the auricle. The route of the umbilical vein from its branching in the liver to its union with the posterior hollow vein Baer called the venous passage {ductus venosus) ; the latter disappears in the fourth period. The common auricle is further divided into two by a septum. The cardiac chambers (ventricles) are already separated by the beginning of the third period. The vessel going from the right ventricle is the stem of the pulmonary artery, and that going from the left ventricle is the stem of the aorta. The passages from the vascular arch into the root of the aorta (Botallo's ducts) narrow. Both anterior arches of this period remain in connection with the cephalic and vertebral arteries, and also with the arteries of the anterior extremities. In the place of the middle arch only the Botallo's ducts remain; from the external half of the last vascular arch the stem of the aorta forms, and from the internal half, the right pulmonary artery. The left root of the aorta soon is converted into a thin vessel, the direct continuation of the Botallo's duct of this side. "In this period," Baer concluded,

the blood, which shares in respiration, passes to the body through the umbilical vein, . . . is mixed with the blood from the rest of the body, and goes into the heart together with blood returning from the liver. It (the blood — L. B.) is divided into two streams, one of which goes into the pulmonary artery, and the other, the stranger, into the aorta .... Respiration takes place in the urinary sac .... The physiologists call such blood circulation an incomplete double circle (II 7hh, pp. 194-195 (147))

The fourth period after hatching is characterized by the formation of a complete double circle of blood circulation. The auricles are completely separated, and all the blood from the body goes through the right half of the heart into the lungs for gas exchange, and from them through the left half of the heart, then into the whole body for nutrition. Respiration through the urinary sac stops. The umbilical arteries and veins become empty. The yolk artery becomes a branch of the portal vein, and finally disappears.

(Figure 29 , Caption)

10. A representation of the vascular system of birds: ab — the heart, from which five pairs of arterial arches go? c — the cephalic artery; d — the vertebral artery, for the formation of which a part of the arterial root (e) is used; f — division into branches in the umbilical artery; g — the anterior vertebral vein; hi — the posterior vertebral vein; h — the caudal vein; k — the transverse venous system; 11 — the umbilical vein (the lower vein of the stomach) ; m — the hollow vein; n — the vein of the yolk sac; o — the common venous stem; p — the yolk artery.

14. The conversion of the branchiate vascular system into constant arteries in mammals: a — arterial stem; b — the aortic roots; c - the aortic arch; d, c - the carotid arteries.

19. A section of the chicken egg: a - the embryo;

b — the amnion; c — the yolk duct; d — the yolk sac; h — the serous membrane; i — the condensed albumen; k — the shell membrane.

20. Ovum of a rabbit.

21. Ovum of a bitch.

22. Ovum of swine.

23. Human ovum .

24 . Scheme of the formation of the amnion and the serous membrane in mammals.

The predecessors of the permanent kidneys, which exist at a later age, are temporary organs carrying the name of primary or false kidneys; in birds they are named the Wolffian bodies. The primary kidneys form from the mesenteric plates; however, the method of their initial development Baer considered insufficiently clear. These organs have a glandular character. Along them extends a duct, the false ureter, which opens in the cloaca. This duct, in Baer's opinion, is formed by histological differentiation of an intact structure, which is transferred later into a tube.

In the development of primary kidneys, Baer discovered a regularity other than that of the digestive glands. For the latter, he described the determining role of the mucous membrane, which initially forms an excretory duct, and then all the branchings of the gland which only later receives a network providing it with the blood vessels. The primary kidneys, in his opinion, develop in a different manner, particularly in the first changes in the blood vessels. Already under the effect of the blood vessels, secretory canals are formed. This combination of Baer's wrong ideas has great historical interest, because they show his great attention to the interrelations of parts of the developing organism and his striving to explain, by these interrelations, the processes of organ formation.

On the sixth day of incubation, the mesenteric plates form extensions; these are the foundations of the permanent kidneys, in the border zone of which Miiller had seen vesicles with their tubules extending inward. These small urinary canals later become thin, branch, and through smaller stems pour into the ureter. The genital system (divided into reproductive organs and conducting passages) is formed from other systems of organs which are detectable later. The reproductive organs, according to Baer, are formed by the expansion of the abdominal part, particularly the mesenteric plates. They have the shape of elongated flat bodies, located on the internal surface of the primary kidneys and devoid of any defined structure. The sexual organs are initially paired and identical in both sexes; then the right ovary in chickens diminishes in size fairly early, though in wild birds both remain the same size. The right oviduct in chickens also develops less. The testicles change the rounded form into a bean-shaped form. Histological differentiation occurs there also, but not in the form of vesicles as in the ovaries but in the form of small canals, whose terminal parts go through the external layer of the primary kidneys and reach the excretory duct.

The essay on bird development Baer terminated with a short summary. There he gave the differences between the two aspects of the individual development, the development for oneself and development for the species. The first begins early but quickly progresses, while the second begins only near the end of development of the individual, and then is renewed annually. At the time of embryonic development in the egg, vital functions differentiate by the contents of the egg taking their origin from the maternal body. But after release from the egg membranes, the organism enters into an interaction with the external world. In development of the methods of nourishment, it is possible to distinguish three periods: the use of the yolk, then the fetal fluid and finally nourishment from the external world. The four periods of respiratory and circulatory development Baer designated by the terms: blood formation, simple circle, incomplete double and complete double circle of blood-circulation. These periods of development follow each other, and in each period one can observe "preparation for what is produced in the subsequent periods" 9 (II, III, 7u, 153).

9. To Baer belongs, in this way, the first attempt to divide chick development into periods depending upon the different interrelations with the conditions of the surrounding medium, in particular depending on the character and sources of nutrition.

   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

Cite this page: Hill, M.A. (2024, April 13) Embryology Book - Russian Embryology (1750 - 1850) 20. Retrieved from

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