Book - Russian Embryology (1750 - 1850) 17
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Blyakher L. History of embryology in Russia from the middle of the eighteenth to the middle of the nineteenth century (istoryia embriologii v Rossii s serediny XVIII do serediny XIX veka) (1955) Academy of Sciences USSR. Institute of the History of Science and Technology. Translation Smithsonian Institution (1982).
Publishing House of the Academy of Science USSR
Translated from Russian
Translated and Edited by:
Dr. Hosni Ibrahim Youssef # Faculty of Veterinary Medicine Cairo University
Dr. Boulos Abdel Malek
Head of Veterinary Research Division
Arab Republic of Egypt
The Smithsonian Institution and the National Science Foundation, Washington, D.C, by The Al Ahram Center for Scientific Translations 1982
The Smithsonian Institution and the National Science Foundation, Washington, D.C by The Al Ahram Center for Scientific Translations (1982)
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Chapter 17. First Part of Uber Entw I Cklungsgesch I Chte - Development Of The Chicken in the Egg
In discussing the history of chick development, Baer began with the first period of development of the incubated egg. To this description he prefaced a short introduction. Following Pander, Baer indicated that the temperature necessary for chick embryo development ranges from approximately 28Â° to 32Â° C. The temperature conditions distinctly affect the speed of development. Apparently contradictions in the data of different authors about the periods of development result from that difference; for even Wolff, who in Baer's words was an extremely accurate observer, had such contradictions.
Baer differentiated two types of deviations from typical development: irregularity of the general course of development in different eggs, and irregularity in the realization of different features in the same egg. Greater or less speed of development may depend upon the time of the year as well as the temperature of incubation, and especially upon how long the egg is kept before incubation.
Depending on how long the egg is kept, slowing of development may be fairly significant, especially during the first five days of incubation. Taking into consideration the existing variations, Baer divided development into three periods. The first period lasts approximately two days and ends with the complete formation of blood circulation. The second period has a duration of three days and involves circulation of the blood through the vessels of the yolk sac. The third period continues through the appearance of blood circulation in the lungs, that is, until hatching.
First Period of Chick Development
Baer's description of the first period began with the events of the first day of incubation (Â§ 1) (Figure 27) . He knew that development of the chick embryo begins even before incubation. Therefore he described the developmental process as a continuation of the separation, already begun, of the embryo from the yolk and the primordial membrane. (92) The embryo at that time becomes more compact and thin, and its center is raised in the form of an overgrowth. At about the twelfth hour of incubation one can detect the division of the primordial membrane into two layers: the surface layer, which is thin and dense, and a deeper layer, thicker and looser. Baer, in agreement with Pander, named the first layer "the serous layer" and the second "the mucous layer." Simultaneous with that separation, separation is also observed along the surface, so that the center becomes lighter than the borders. The light part (the area pellucida) changes form from rounded to oval, and then into a pear shape.
With the continuing separation of the rudiment from the underlying yolk, as a result of which the sub-embryonic cavity forms, 1 further separation in the rudiment membrane occurs: the dark border, surrounding the transparent embryonic area, is divided into concentric rings. At the same time, between the serous and mucous layers, a third "vascular layer" develops; therefore, Baer gave it the name vascular area (area vasculosa) . The external ring adjoining the yolk he designed "the yolk area" (now called the "area opaca") .
After the fourteenth or fifteenth hour of incubation, the first sign of embryonic organization appears, which leads to the appearance of the dark zone. Baer gave this formation the
1. It is possible not to stop at Baer's wrong idea about nutrition of the embryo by the fluid which, in his opinion, collects in the cavity of the white yolk, but which actually does not exist.
2 name it still has, the "primary zone"; it represents, in his opinion, the structure of the vertebral column and defines, by its location. the longitudinal axis of the embryo. (93)
From the primary zone, two longitudinal cylinders develop, called Pander primary folds. In Baer's opinion, these require another name, however, because they do not represent primary formations of the embryo and they do not represent true folds. A transverse section shows that these cylinders at first have internal and external slopes of similar form. Then the internal slope becomes sheer, while the external remains gentle. The tops of the cylinders become sharp, like a blade, and incline to each other above their division by a fissure; they merge so that the fissure becomes a closed tube. Baer called these cylinders "spinal plates or disks," from which develop the axial parts of the embryo, namely the central nervous system, the axial skeleton and the muscles. At the same time, the spinal plates develop; at the bottom of their dividing groove the spinal string or cord develops in the form of a central dark zone, situated along the axis of the embryo. This detection of the spinal cord is considered one of Baer's most important embryological discoveries, as he recognized. He likened the identity of the embryonic cord to the rods which a cartilaginous fish has throughout its entire life. The spinal cord, Baer said, "is not only an axis around which the first parts of the embryo develop, but also a true scale for the whole body and all its main system" (I, 1 1; p. 44 (15)) \ With a preparation method using the thinnest dissecting needles, Baer could separate the spinal cord from its transparent but firm case and pull it out like a lace.
2. Baer especially stressed that particularly the control zone, and not two parallel folds as Pander thought, represents the first organization of the developing embryo .
3. Here and next; Lhe Roman number means the volume, the arabic number and latin letter refer to the paragraph and its subdivision.; the pages are related to the Russian issues of UBER ENTWICKELUNGSCESCHICHTE . (Ed.: references to German edition (p. ^).)
Figure 27. Baer/.s illustration from the first part of UBER ENTWICKLUNGSGESCHICHIE.
1. Transverse section of the embryo in the middle of the first day.
2. Transverse section of the embryo in the second half of the first day.
3. Transverse section of the embryo in the beginning of the second day.
4. Transverse section of the embryo at the end of the second day.
5 and 6. Transverse section of the embryos in the fourth day (earliest and latest) .
7. The same as in 6, longitudinal section. The designations on all the figures of the transverse sections (1-6) : a â€” spinal cord: b, b' â€” external border of the spinal plate; c â€” upper border of it; be â€” spinal plate; d â€” external, later lower border of the abdominal plate; bd â€” abdominal plate: e â€” the bend of the serous layer; de â€” skin part of the abdominal wall; f â€” border of the lateral folds; g â€” lateral part of the amnion folds; deg â€” amnion; h, i â€” upper and lower angles of the mesenteric plates; k â€” vascular layer of the intestine; 1 â€” mucous layer of the intestine; If â€” the false amnion of Wolff; m â€” Wolff's body; o â€” aorta; p â€” urinary sac; n â€” opening the mesentery. The designations on the figure of the longitudinal section (7) : f â€” stomach; g â€” anterior entrance into the digestive canal; gk â€” the unclosed part of the intestine; e â€” respiratory apparatus; m â€” urinary sac; p, p' â€” the band of the vascular, mucous, and serous layers; q â€” the transfer of the embryo into the caudal fold; pr â€” the head or cephalic fold; qs â€” caudal fold; tu â€” the closed anterior and posterior fold of the amnion; r, t, u f s â€” serous membrane, or the false amnion of Pander; v â€” auricle; y â€” stem of the body vein.
On the first day of incubation, the growth of different parts of the embryo is already regular. Baer noted that the curving of the anterior end of the embryo does not represent a direct consequence of the intensified growth of the spinal plates. Instead, he wrote that "this change depends upon the deeper general foundation which is observed at every moment of embryonic formation to separate it from the surrounding parts of the rudiment and from the remaining egg."
"The (separation) thus depends: 1) on the growth of the embryo, which grows faster than its base (i.e. the region of the union of the embryo with the rest of the blastoderm â€” L.B.), as well as 2) on the regions of the initial narrowing, connecting the embryo with the primordial membrane; this narrowing becomes noticeable only in the second day" (I, 10; p. 47 (17)).
Further development of the spinal plates concludes with the final closure of the slits called by Baer the closure of the back, and their breaking up into separate straight-angle parts, described by Baer as the foundation of the vertebrae. This process, it is now known, does not represent the formation of the vertebral column, but the segmentation of the axial mesoderm into parts called somites. From the somites, or the primary vertebrae as they were named not too long ago, the vertebrae and the hard brain membrane develop, and also the muscles, the connective tissue layer of the skin, the kidneys and the sexual glands.
Summarizing the processes occurring during the first day of incubation, Baer then described the condition of the embryo at that time. The embryo has the shape of an inverted boat. Only the spinal part is distinct, with some pairs of the primary vertebrae; the abdominal side is still entirely unseparated from the primordial membrane. The separation of the embryo from the neighboring parts of the blastoderm takes place only in the anterior part. From all sides the embryo, which has no defined borders, is joined with the primordial membrane and contains the same layers as the latter. The mucous layer lies freely under the primary vertebrae, the region of which represents the most compact part of the embryo. The serous layer of the blastoderm continues into the smooth surface of the spinal plates. The loose layer of the vascular layer continues from the vascular area outside the embryo into the embryo between the serous and mucous layers, indistinctly delineated from both. "All the rudiment," Baer wrote, "can be considered as the unformulated body of the whole animal, which appears as nothing other than a large unclosed intestinal sac" (I It, p. 51 (20)).
The spinal plates Baer considered the expansion of the serous layer; in a footnote (I It (20), he discussed Pander's terms "serous layer" and "vascular layer" and approached a modern subdivision of the embryonic layers. Baer remarked that Pander's experiment "distinguishing the layers of the rudiment membrane was a turning point in the study of the history of development and was a true light for the latest investigation." Baer, however, considered that the early separation of the layers only serves as a preparation for future development.
Running somewhat ahead, he stated that at the end of the second day he could already clearly distinguish in both the embryo and the rudiment membrane the animal and its plastic or vegetative parts. The first consists of two layers, the future skin and the animal part of the body, and the second is composed of the vascular and mucous layers. And thus, the most characteristic picture for the first day of incubation, besides the separation of the layers, is the "growth of the embryo from the primordium which is yet on the anterior end, as a result of which the primordium is divided into the embryo and the primordial membrane" (I lu, p. 52 (21)).
The extensive second section Baer devoted to the events of the second day of incubation which completes the first period of development. During the second day, the separation of the embryo from the yolk continues, which leads to the lifting of the anterior half of the body.
The previously described accretion of the spinal plates does not occur at the same time along the entire length; rather, it starts in the region behind the head and continues to the region of the future sacrum. The number of primary vertebrae gradually increases so that by the end of the second day the number ranges from 10 to 12. In the head or cephalic region, the knitting spinal plates leave a wider canal, which indicates the location of skull cavity formation. At about the thirtieth hour, the separation of three divisions of this cephalic expansion corresponds to the frontal brain, the four-hillock ("VIERHUGEL") , 4 and the medulla oblongata. On the sides of the posterior part of the frontal expansion, projections form which are the first rudiments of the eyes; at about the thirty- third hour, the frontal end of the embryo assumes a similarity to the head of a fly.
Baer concluded that the rudiments of the cephalic brain and eyes are hollowed vesicles, formed from an initially homogeneous tube. He asked, "What is the stimulus in that process of development?" The answer required an explanation of the further question, "What preexists in the canal prior to the appearance of the cephalic and spinal brain, and when and how do the central parts of the nervous system appear?" (I 2e, p. 56 (24)). Making sure that the spinal plates and even the brain vesicles do not contain "a compact primary mass," Baer concluded that "originally in the place of the cephalic and spinal brain, there is only fluid in the cavities. Under its effect, the initial bulging out of the eyes occurs" (I 2e, p. 57 (25)).
The abdominal plates begin closing under the vertebral column; this process takes place slowly and continues during the whole period of incubation. In the anterior part of the embryo, where its body is already closed, the abdominal plates form the lateral walls of the body. Wolff named this belt the abdominal plates, more appropriate than Pander's term of abdominal folds.
4. (Ed.: It is not quite clear what Baer meant by these parts of the early developmental stage. Today embryologists refer to the 33-hour chick as having a prosencephalon (probably Baer's forebrain) , a mesencephalon, rhombencephalon (perhaps together forming the "Vierhugel"?) , and no medulla as yet.)
In connection with the bending of the anterior end of the embryo, under the head part an increasing deepening of the blastoderm appears in the form of folds. Baer called this formation the "head cap" (Kopfkappe) (I 2h (26)). The displacement of the top of the cap backwards denotes the separation of the head end of the embryo. The deepening of the mucous layer extends, forming the first rudiment of the digestive canal in the form of a blind sac. This sac opens downwards and backwards with a wide opening into the stomach. Baer called this opening the "anterior entrance into the alimentary canal, "^ suggesting his rejection of Wolff's inappropriate term Fovea cavdiaca.
Between the close anterior ends of the abdominal plates, the vascular layer becomes thick and a granular mass appears in it, forming two lateral projections whose ends are directed backwards. This is the material for the formation of the heart. In the middle of the second day, the central part of the described rudiment becomes transparent; internally its contents become blood, and the external layer becomes the heart wall. At the same time, the central nervous system becomes independent. On the second half of the second day, at the internal surface of the spinal plates, a layer forms a hollow cylinder, the lateral walls of which are thickened. This is the fold of the cephalic and spinal brain. In the head division the future parts of the brain are originally one unclear mass. They then become separated by weak interceptors, which begin dividing into two hemispheres. The canal from the brain to the eye, which is the future optic nerve, remains hollow at this stage. The primary foundation of the ear begins to occur, according to Baer, in the same way as the formation of the eye rudiment, i.e. as a hollow outgrowth from the medulla oblongata. 6
Turning to the foundation of the vascular system, Baer referred to Wolff's and Pander's observations on vascular growth. Confirming their observations, he remarked that the fluid moving in the fetal area is colorless at first, even when the heart is already contracting. Only after pulsation begins is it possible to see the movement of red blood from the vascular area. Baer, however, was somewhat doubtful as to the accuracy of his observations. He was unsure whether he could talk about the movement of blood in canals which are still devoid of walls and therefore provide no clearly outlined route (I 2r, p. 69 (34)).
5. The term is kept also in the modern embryology.
6. Actually, the rudiment of the internal ear (auditory vesicle) is formed from the unlaced deepening of the ectoderm.
In the rudiment of the heart, the anterior canal grows into two thin branches, which disappear without reaching the roof of the digestive sac. The pulsation of the heart still pushes a colorless blood anteriorly, as in the heart of insects. In this period the heart is located directly under the future head and from both sides is enclosed by the anterior parts of the abdominal plates. Later on, it stretches forward from these plates and protrudes downwards. The canals coming out from the anterior end of the heart, Baer indicated, joint later, forming the beginning of the aorta. The heart becomes curved by the middle of the second day; by the end of this day it curves downwards and anteriorly even more, so that its ends come close together. Three pairs of arterial arches form, one after the other. Nearly in the middle of the embryonic body at that time, the entry point of the veins appears.
The initial condition of the other parts of the vascular system Baer described as follows. At the border of the vascular region, a blood receptacle forms as two semi-circles, at first without defined walls. This is the blood circle, which embryologists later called the border vein. Blood enters the middle of each semi-circular sinus and moves anteriorly and posteriorly. Anteriorly from the sinus go many vessels, flowing into a common stem which reaches the embryo (frequently there are two such stems) . Vessels from the posterior part of the sinus merge into an ascending vein, which with the anterior vein pours into the left heart protrusion. The blood goes into the heart anteriorly and, passing through the stem, it goes out through three pairs of arterial arches which curve upwards and pour into two stems. These merge together at first, then they divide again along the way to the posterior end of the embryo. In the middle part of the body, they form two thick branches which reach to the border sinus.
Because of its expansion, the serous layer of the head cap completely covers the heart at the end of the second day, At the same time as the increase of the head cap, a caudal cap appears as a fold of the blastoderm; hence, in the posterior part of the embryo the mucous layer forms a pit which is the rudiment of the posterior part of the digestive canal.
The dropping of the abdominal plates leads to the subsequent separation of the embryo from the blastoderm. By the end of the second day, the place of the future mouth opening is seen; the head end of the embryo still bends downwards.
The general cnaracteristic of the first period of development, Baer stated in Â§ 3: "(The history of the first period shows that) the embryo is a part of the primordium which arises to a higher independence; that as this independence becomes expressed, the vertebrate type and development above and below that family stem emerges; and that there appears in the animal part the articulation which is characteristic of the . . . type" (I 3, p. 38).
The Second Period of Chick Development
Baer considered the second period to begin with the appearance of blood circulation through the yolk vessels and to end with the development of the urinary sac, as the organ of respiration. This period extends from the third to the fifth day of incubation, and during this period the separation of the embryo and the formation of the temporary embryonic membranes also occurs (Â§ 4).
The events of the third day of incubation are stated in Â§5. In this period only the separation of the embryo occurs, leading to the development of the chest and the lower part of the body, and at the same time the formation of the intestine and mesentery. The chest as well as the abdominal cavities form by the changing situation of the abdominal plates; the external borders of the abdominal plates become inclined increasingly downwards. At the same time the abdominal
plates split into two layers, the external and internal. Wolff described the corresponding changes completely and correctly but not perfectly clearly. Thus misunderstandings later occurred, caused mainly by Wolff's term "intestinal fissure." Actually this fissure represents the slit between the layers of the mesentery which later close up. Even Pander understood Wolff incorrectly, and did not know the fact. 7 The results of Baer's specific investigations led him to conclude that Wolff's description was unclear only because he did not distinguish the mucous from the vascular layer. If Pander's discovery of the division of the embryonic layers was applied to Wolff's description, then everything would have become clear.
After the division of the abdominal plates into two layers, it is possible to see that the internal layer in its turn is composed of two layers â€” an external vascular, and an internal mucous (endoderm and visceral layer of the mesoderm) . Subsequent to that in the external layer, two layers also become prominent: the serous sheet and the "generating tissue," forming, in Baer's opinion, the fibrous tissue, bones, muscles, nerves, and walls of the body (the ectoderm and the parietal layer of the mesoderm) . This external layer of the abdominal plates, together with the spinal plates, form the animal part of the body. The internal layer of the abdominal plates forms the beginning of the vegetative part. Protrusion of the internal layer toward the yolk along the middle line where the vascular is not separated from the mucous layer, produce a longitudinal gutter, whose borders bend from the sides under the body of the embryo. These lateral plates Wolff had named the "false amnion," and the gutter itself "the mouth of the false amnion." Because the lateral plates join together anteriorly and move into the head cap, and posteriorly they join in the caudal cap, these lateral folds Baer called the lateral caps, or folds, which represent a part of the united common fold.
The parts of the internal layer of the abdominal plates, descending vertically to the yolk, form the layers of the mesentery which comes together shortly below (Fig. 27, 5).
Baer refers to this on page 22 of Pander's German dissertation.
The line of this union Wolff named a "junction." Wolff erred only in his indication that before the formation of the junction the space between the layers of the mesentery opens downwards, whereas actually it is covered by a mucous layer. Gradually the layers united below the mesentery begin to accrete with each other. This process proceeds along the long axis of the embryo, so that its variable stages can be seen on different levels of the same individual. In the anterior part of the body, where the digestive canal is already present, the layers of the mesentery envelop it, so that it is composed of the internal mucous canal and the external canal formed by the vascular sheet.
After the closure of the mesentery junction, the mucous and the vascular layers protrude upwards along the middle line of the body and form a gutter, the walls of which Baer called "the intestinal plates" (Fig. 27, 5) and the gutter itself the "intestinal gutter." It closes below but, as Baer wrote, "not by means of the middle junction, but in such a way that from both ends a lengthening of the already closed beginning and end parts of the food canal takes place to the middle" (I 5e, p. 84 (45)). By the end of the third day, only the third part of the length of the digestive canal remains, in the form of unclosed gutter.
Baer warned against the simplification of the process of intestinal formation which could be concluded from Wolff's description and his specific data. "It is logical by itself," he wrote, "that this protrusion must not represent a pure mechanical process through which the layers of the rudiment membrane, which were located earlier on one plane, must be later made up into folds; soon this protrusion is accompanied by organic growth . . ." (I 5f, p. 85 (46)). This warning is important because a half century later in embryology, the simple mechanical interpretation of formative processes was still widely distributed. For example, gastric invagination was understood as a simple protrusion of the vegetative hemisphere of the blastula, and the formation of the embryonic folds as a mechanical protrusion of the previously planar parts of the embryonic layers.
Baer considered the formation of the intestine, and the formation of the mesentery which precedes it, and at the same time the approach of the abdominal plates to each other as a process of gradual separation of the embryo from the yolk, "as if an unseen hand outbalances the places of communication among the embryo, embryonic membrane and yolk" (I 5f, p. 86 (47)). In this case the protrusion of the mucous and serous layers takes place, and also the growth of the ends of the digestive canal.
At the same time as the described separation, the external edges of the lateral plates form the fold extending above the surface of the blastoderm, which spreads into the peripheral part of the blastoderm at first under a blunt, then under a straight, and finally under an acute angle. At the cephalic and caudal ends, this fold appears earlier and gradually becomes sharper. Baer considered that Wolff's term "false amnion" referred especially to this ring of the common fold.
The true amnion forms, according to Baer, from the serous layer (he gave it the still current name "amniotic fold"**). The ring of this fold becomes narrow above the back of the embryo, then by the end of the third day it closes, and the embryo appears included in a sac which is the true amnion.
At the time of the formation of the amnion, the head end of the embryo bends increasingly downwards. At the same time the head turns to the right, and gradually the whole embryo lies on its left side. The lateral projections of the heart, described previously (I 2q) , are now transformed into lateral veins of the vascular area, and the common stem into which they pour forms the posterior end of the heart. The heart still has the form of a canal, which grows wide and bends a bit to the left, then strongly to the right and down. This canal continues to narrow, then moves left and upwards, where it divides into four pairs of arches. Between the vascular arches, the substance of the abdominal plates becomes thin, and three pairs of slits appear, leading from the outside into the digestive cavity or the future gullet. The crescent-shaped space between the slits Baer called "the branchial arches" because "of their correspondence with the branchial arches of fish" (I 51, p. 94 (53)) (94). The arches of each side join at the dorsal side of the gullet in one vessel, which Baer called "the root of the aorta."
8. In fact the amniotic fold is composed from the ectoderm and the parietal layer of the mesoderm.
At this stage one can already give names to the individual parts of the vascular system. The veins going from the vascular areas in the abdominal cavity of the embryo are the umbilical-mesenteric veins; they represent the general system of the portal vein. In the embryo itself, at this time, the veins are not yet distinguishable. The heart is yet not divided into chambers. The arteries going out from the embryo are the umbilical-mesenteric arteries. The aorta branches first, beginning with the carotid arteries, and its terminal branches enter the urinary sac developing at that time. In the embryo veins appear, the jugulars appearing first.
The most important changes occur in the heart: 1) it gets displaced backwards; 2) as a result of its ends coming close together, the heart protrudes downwards more, moving between the abdominal plates; 3) it bends increasingly to the right; and 4) it begins dividing into chambers. In the venous part of the heart, two lateral widenings of the anterior part appear. These are the primordia of the auricles, actually ears, but the cavity of the auricles is not yet divided. Where the common venous stem contacts the digestive canal on the sides of the vein, pyramid -shaped hollow projections grow and acquire a leaf -shaped form enveloping the vein.
Turning to the development of the digestive canal, Baer objected to Wolff's and Pander's idea that "each part of the digestive canal acquires its individuality in the formative process and ... the stomach forms, the duodenum and so on." In contrast, he considered that the intestine "in the beginning becomes separated from all the other body by its common individuality; however, it still remains homogeneous in all its extension, and only later are the differences in its individual parts observed" (I 5r, p. 103 (60)).
Baer further described how the vascular layer of the digestive canal becomes swollen and gives rise during the third day to the lungs, liver, the pancreas, caecum and urinary sac, in whose formation the protrusions of the mucous sheets also take part. Thus, the rudiments of the lungs appear in the form of two hollow cone shapes. They develop by the same method as the liver and the pancreas. The protrusions of the caecum do not appear earlier than the end of the third day; somewhat earlier, from the posterior end of the digestive canal a single projection, the future urinary sac (allantois), grows out. It is formed of two layers, an external vascular and an internal mucous layer.
In the second half of the third day, Baer detected the appearance of "Wolff's bodies," as they were called. They appear in the form of knotted thread-like cylinders in the corner between the mesenteric and the abdominal layers (Drawing 27, 6m). At the same time, on the abdominal layer, the primary rudiments of the extremities appear with the initial form of narrow cylinders.
The spinal brain or cord during this period remains compressed from the sides, but the walls become thicker and subdivided into upper and lower layers. The divisions of the cephalic brain are still poorly differentiated. Backwards toward the marked hemispheres, the outlet of the optic nerves appears. At the lower surface of the hemisphere arises the basis of the olfactory nerve.. In the eye, one can distinguish the retina and the lens.
The separation of the embryo becomes more and more distinct, because the ring-shaped interceptor between it and the rest of the egg becomes narrow, forming an opening which Baer called the umbilicus. A comparison of the successive stages shows that the umbilicus was previously the wide opening of the body. Even earlier, the umbilicus formed the whole circumference of the opened body. The external layer of the umbilicus is formed by the serous layer (Drawing 27, 7 P'> q f )Â» which from one side of the embryonic skin represents a continuous extension and from the other side a surrounding fold, i.e. the amnion. Inside the serous canal, which Baer called "the skin umbilicus" or "abdominal umbilicus," there is a tube composed of vascular and mucous layers, "the intestinal umbilicus." The mucous and serous walls of this tube form the passage of the vascular and mucous layers of the blastoderm into the corresponding layer of the digestive canal .
The cavity of the intestinal umbilicus connects with the intestine, and the gap of the skin umbilicus connects with the cavity of the embryonic body. Baer explained the origin of the abdominal cavity of the embryo with unusual perspicacity, stating that "it is nothing other than the union of both the gaps which were formed in the abdominal plates in the third day" (II 6e, p. 112 (67)).
Directly after the splitting of the abdominal plates, two abdominal cavities form as narrow slits (Fig. 27, 5). Then these slits become wide and between them a communicatiion develops in the anterior part of the embryo near the heart. The separation into layers spreads from the abdominal plates through the umbilicus to the fold surrounding the body of the embryo. Therefore, the abdominal cavity at first was, to a considerable degree, outside the embryo and contained only the heart; later it included the intestine also.
On the fourth day, the embryo already has distinct rudiments of the extremities and tail. Its neck curves so strongly that the most anterior part of the head represents the transition of the spinal cord into the medulla oblongata. The heart and even the liver still are found in the neck region.
The digestive tube remains almost completely straight. In its anterior part, the throat cavity is defined, after which comes a narrowed short part, the esophagus, transferring into an elongated widening, the stomach. The duodenum ends at the "anterior passage." In the posterior part, the large intestine still cannot be distinguished from the small one, which ends anteriorly at the "posterior passage"; the posterior-entrance opening in this period has not yet appeared
The lungs still connect with the digestive canal by means of a short respiratory tube. The liver plates increase in size, then envelope the portal vein. Inside the liver, the liver ducts branch and the veins undergo ramification. The urinary sac grows quickly in the second half of the fourth day and enters between the serous and vascular layers of the caudal and then of the lateral folds. Now it is transparent, and in its vascular layer the arterial network is distinctly seen. Inside each Wolff's body passes a longitudinal vessel with branchings.
In the vascular system during the fourth day, the following changes occur. The system of the portal vein clearly separates from the system of the hollow vein. The ears (auricles) of the heart increase in size, open into the common venous sac, whose walls are thickened and can now be called the auricle. Inside the ventricular chamber of the heart, a strongly marked fold appears.
Two primary arterial arches in the branchial arches become reduced, and the third and fourth ones are enlarged. In addition, the fifth arch appears. The first branchial slit closes, but posteriorly to the fourth arch a new slit appears, so there again appear to be three. The aorta branches laterally into the intervertebral spaces.
The extremities acquire the form of plates, situated in the space between the spinal and abdominal plates.
The changes in the nervous system during the fourth day are as follows. The lateral walls of the spinal cord still thicken; the upper and lower layers are divided by a groove. The cerebellum is already distinctly marked. The largest bladder in the cephalic brain corresponds to the four-hillock parts, in which a cavity appears. The third brain cavity goes down to the base of the skull and forms the cone of the brain. In the frontal and sincipital regions, one can distinguish, although still not completely, the lateral ventricles from each other by an upward hanging fold. The lower layers of the spinal cord clearly extends also into the brain at the bottom of the fourth ventricle and the cavity there, forming the cone of the third ventricle. The sensory nerves still have the shape of hollow tubes. In the entrances into the corresponding brain ventricles are seen the auditory nerve (in the fourth ventricle) , the optic nerve Gin the third) , and the olfactory (in the lower surface of the lateral ventricle) .
Summarizing his observations on sensory nerve development, Baer concluded, that they represent protrusions of the brain into the body mass, and the sensory organs represent a modification of the ends of the sensory nerves. This conclusion he presented most clearly, in eye development. The retina on the fourth day represents a thick -walled rounded structure, connected by a canal to the third brain cavity. Under the eye, there is a thickening, which represents the rudiment of the upper jaw.
The marked thickening of the first branchial arch indicates the beginning of its conversion into the lower jaw. The vascular area occupies somewhat more than half of the yolk circle by the end of the fourth day.
During the fifth day (Â§ 7) , the separation of the embryo reaches the greatest degree, and the urinary sac becomes the organ of respiration. The intestinal umbilicus becomes narrow and is transferred into the yolk duct. The anterior and posterior entrances into the digestive canal join together so that the gutter-shaped part of the intestine disappears. The urinary sac, very rich in blood vessels and on the right side of the embryo between the amnion and the serous membrane lying above it, extends to the surface of the yolk in the form of the serous layer of the ectoderm. At this point, the embryo is so distorted that the head and tail come in contact with each other. The body cavity still forms somewhat at the neck, but. the liver is already placed in the body.
One can easily distinguish the esophagus, stomach and intestines, and the respiratory tube from the esophagus. In the region of the developing pancreas, the primary intestinal loop corresponding to the duodenum forms. The caecum and large intestines are still poorly developed, but the posterior passage opening is already present in the form of an umbilical slit. The Wolff's bodies are strongly enlarged and extremely rich in blood. On their internal surface, the rudiment of the sexual glands appears as a longitudinal strip.
In the heart, at the "central venous sac" (auricle), the beginning of the extension is marked. A partition divides the "heart chamber" (ventricle) into two parts connected by an elongated opening. In the stem of the aorta are two orifices.
The extremities lose their leaf-shaped form and become chisel -shaped. In the shoulder and thigh, dark patches form the rudiments of skeletal elements; correspondingly, in the forearm and shank two dark strips appear. The upper jaw has the form of a shield, which becomes elongated opposite the frontal projection; thus it appears doubly split. 9
A special membrane covers the spinal cord. The cerebellum in the embryo at that age projects considerably farther. The canal between the cerebellum and the four-hillock structure extends more than before; this corresponds with the posterior part of the water passage (urethra) . The vesicle of the four-hillock structure is strongly enlarged. The cavity, corresponding to the third brain vesicle, grows insignificantly, but its bottom becomes elongated. The brain stalks become more prominent than before, and become the brain stems. The external membrane of the eye splits into two layers: a hard membrane whose continuation is the cornea, and a vascular membrane .
Section 8 concludes the second period of development. Here, Baer distinguished three types of processes: a continuing separation of the embryo and its envelopment by membranes; the separation of the vegetative from the animal part; the turning of the embryo to the left and the displacement of the blood supply to the left side. The final process represents the progressive process of internal differentiations, which are entirely characteristic of the vertebrate type.
The separation (unlacing) of the embryo from the blastoderm outside the embryo takes place by the appearance of folds â€” anterior, caudal and lateral, all representing parts of one common fold, which is transformed at the final end into the umbilicus .
The turning of the embryo to the left side, and the corresponding displacement of the vessels, Baer compared to the particularities of development in molluscs. Observing the similarity, Baer made a reservation: "It is impossible, however, to say that the chick embryo is now present at the level of development of molluscs. Against that, the presence of the vertebral column, the spinal cord, and the brain are a definite proof" (I 8 c, p. 142 (89)).
9. This fits the description of the development of the face skeleton of the chicken embryo given by Tredern and his drawings (see pp. 125 - 126 and drawing 18) .
At the beginning of the third day, the chick embryo contains all the existing features inherent to the vertebrate animal in general. By the formation of the urinary sac, the embryo already shows the features of the land vertebrates.
The Third Period of Chick Development
Baer divided the third period of chick development into six stages. The first, described in Â§ 9, continues through the sixth and seventh days. At this time the size of the air cavity increases further. The allantois passes the embryo in growth, penetrating between the serosa and the amnion.
The beginning of the third period corresponds to the first movements of individual embryonic parts. Its form changes somewhat, the curvature in the neck region particularly decreasing. The trunk grows wide as the liver increases and the heart drops, but the volume of the head remains not less than the size of the trunk. The umbilicus now represents not a single opening or ring, but a short canal, which gave Baer the basis to discuss the presence of an umbilical cord in birds.
The vascular area of the yolk occupies more than half its surface; its vessels (border, ascending and descending veins) begin to disappear. The part of the umbilical vein, which goes to the hollow vein after branching into the liver is comparable, according to Baer, to the ductus venosus Arantii in mammals.
In the abdominal plates, covering not more than half the walls of the cavity, the rudiments of the ribs assume the form of dark strips. At the upper arches of the vertebrae, bony protuberances form.
The extremities increase in dimension and distinctly break into four divisions. The elbow and knee joints are directed externally ("as in the majority of amphibia"). With the exception of the terminal divisions, the anterior and posterior extremities are similar, as are the terminal parts, except for differences in the wing, where there are three fingers, and the leg, where there are four or five fingers.
The facial parts undergo the following changes. The frontal projection between the olfactory depressions elongates. The projections of the upper jaws grow, reaching the frontal projection by the seventh day. Since the upper jaw projections do not reach the end of the front projection, wide slits form and connect from both sides with the mouth opening. The lower jaw forms from the primary branchial arch, then becomes enlarged and pointed at the end. The foundation of the tongue appears .
The esophagus becomes elongated; the gizzard is displaced to the left; the glandular stomach is already marked separately, but not sharply limited from the gizzard. The intestinal loop sits behind the stomach (the duodenum). The next loop, the beginning of the small intestine, enters the umbilicus. Part of the small and the large intestine forms the last curve of the intestine, or the whole remaining intestine. The respiratory tube elongates; the lungs are completely distinguished from the esophagus and are divided into anterior (larger) and posterior divisions. The larynx appears in the place of the passage of the respiratory tube into the throat .
From each Wolff's body, Baer observed a sudden appearance of a thickly walled canal, which passes along the Wolff's body and parallels that canal in many fishes which extends from the abdominal cavity to the sexual orifice. This canal, thin at the anterior part, becomes lost somewhere near the auricle. Its posterior end can be traced to the point where it falls into the cloaca. Baer concluded from an error that the Wolff's body forms the blood-carrying vessels. Observing on the seventh day the presence of many ducts going from the Wolff's body into the canal mentioned earlier, Baer stated the possibility that this canal also represents a modified blood vessel. Since the walls of this canal are very thick and its diameter is large, Baer came to the conclusion that the described canal represents a sexual duct. Baer did not resolve the contradictions of the vascular origin of the Wolff's body with other peculiarities. Thus he stated that "the method of forming the Wolff's body remains so fer unclear"( I 9 q; p. 154 (98)).
In this place, Baer's investigations reveal with great clarity his thoroughness of observation, his carefulness in conclusion, and his readiness to withdraw inaccurate conclusions.
In the vascular system at the start of the third period, Baer noted the change in the heart as well as its location and structure. Further development of the partition extends to the stem of the aorta; at the end of the seventh day, the stem widens and reveals two canals coming from the right and left chambers of the heart.
The nervous system has the following features. The hard and soft brain membranes remain distinctly separated from each other. The trunk part of the spinal cord has thickenings where nerves exist from the extremities; however, the thickenings which correspond to the anterior and posterior extremities directly cross each other. The spinal cord nerves, regardless of their insignificant thickness, could extend a fairly great distance. In the brain, the predominant part is the fourhillock structure, connected with the cerebellum. The deepening of the fissures between the divisions of the brain leads to their great separation. Inside the brain, the lateral ventricles appear. The funnel of the brain is well developed, the brain appendage still not clearly separated from it. Anterior to the funnel, there is a projection from which the optical nerves go out, which do not yet form the actual crossing. Baer investigated the condition of the visual organs in detail. He noted a cored entrance in the optical nerve, the nerve itself already compact. The retina remains very thick, thicker than the cover of the brain hemisphere. The vascular membrane of the eye is completely separated from the still very thin hard membrane, whose continuation is the cornea. The openings of the external ear lie above the mouth slit. The openings of the Eustachian tubes lie close together. The olfactory depressions become deep; the nostril passages go to the outside from the space between the upper jaws and frontal projections into the mouth cavity.
The second stage of the third period of development is described in Â§ 10, continued throughout the eighth, ninth and tenth days of incubation. At this stage, the vascular area covers about three fourths the surface of the yolk. The border vein disappears entirely; the veins and especially the arteries of the vascular area narrow considerably.
The urinary sac, in the form of a closed vesicle, covers a great part of the yolk sac. One surface of the allantois lies adjacent to the amnion and -yolk sac; the other surface, richer in blood, is adjacent to the serous and shell membrane.
At the ninth and tenth day, the feather appears on the skin cover,- at first on the middle of the back and thighs; the rudiments of the rudder feathers are especially marked. The differences between the anterior and the posterior extremities become more distinct. The elbow now is directed backwards, and the knee anteriorly; after the eighth day, the fingers are differentiated; on the tenth day, the terminal divisions of the extremities completely acquire the features of the wings and legs .
The abdominal plates in the anterior part of the trunk become closed. At the place of the closure or junction, the chest becomes established. Baer followed the trunk nerves throughout their extension during this period. In connection with the study of the development of nerves, he raised the question, do they grow out of the spinal cord or grow into it? The centrifugal direction of development of the head nerves ("nerves of the sensory organs"), Baer presumed, does not suggest that the same method of development occurs in the spinal cord nerves. He suggested that the development of the nerves does not take place in either the central nervous system or in the muscles. He wrongly concluded that the nerve develops along all its extension at once by its separation from the producing tissue.
The muscle fibers become noticeable after formation of the cartilage. At the beginning, they appear in the region of the thighs and shoulders, and then in the forearm and shanks. Ossification begins in the extremities, first in the large tibial bone, then somewhat later in the femur and fingers of the feet .
The internal structures change in this stage more than in others as a result of the backward displacement of the heart, liver and stomach.
The glandular stomach is still not clearly distinct from the gizzard, which in general is characteristic of wild birds. The distinct separation of these parts, characteristic of grain-eating birds, appears later. Starting with the eighth day, the crop appears clearly as a widening in the esophagus. The liver becomes less red in color than before as a result of the growth of the parenchyma and the narrowing of the blood vessels. The gall bladder then appears. As a result of the intensive growth and branching of the respiratory tube, the lungs develop quickly. "All their dispersion, Baer wrote, "represent on the tenth day an extensive picture" (I lOn, p. 173 (112)). Next he observed the foundation of the air sacs, the lower and upper larynxes.
The kidneys become shorter, as a result of which the ureters are clearly seen. Simultaneously, the Wolff's bodies decrease in size. In male embryos the Wolff's bodies are symmetrical, and in female the right one is somewhat smaller than the left. The testicles acquire a bean-shaped form, and the ovaries the shape of a triangular plate.
The vascular system does not undergo major changes. From the five pairs of arches existing from the beginning, the first disappears. Then the second disappears. Later, the fifth left arch disappears; the aorta still has two roots which are somewhat shorter than before.
Among the parts of the brain, one sees the most intensive growth in the region of the large hemispheres, which increase in size in all directions, especially in the direction of the four-hillock body. Behind the latter, the cerebellum has a clearly seen central lobe (the little worm) . The brain takes the appearance of a fibrous structure, in the form of separated thick bundles. The brain base also has crossing fibers. This study of the optic nerves led Baer to the conclusion that sensory nerves grow from the brain.
The skull maintains the skin consistency. The regions of the wedge -shaped and occipital bones appear more compact, and so does the auditory capsule.
The eyes, whose size Baer considered "almost monstrous," occupy more than half of the whole head. By the end of the tenth day, a skin edge surrounds them with a little fold, which later provides the beginning for the nictitating membrane. The hard membrane of the eye is very thin. The retina forms a growth immersed in the hyaline body; the iris remains colorless
The third stage of the third period of development CÂ§ 11) covers days 11 through 13. The yolk sac starts to diminish as a result of yolk consumption. The vascular area occupies nearly the entire surface of the yolk and forms wrinkled folds immersed in the yolk mass. Baer considered these folds analogous to the intestinal folds of the lower vertebrates; they play the role of intestinal fibers. The urinary sac surrounds the yolk and the amnion. Hence, its opposing borders coincide and accrete together. The internal wall of the allantoides protrude into its cavity in the form of a fold-structure rich in vessels, sometimes called the chorion. The fluid of the allantoides contain flake-shaped lungs of the urinary precipitates. In the walls of the amnion, narrow vessels are distinguishable. The embryo moves more actively. The beak and the claws become horny.
Through the umbilical orifice, a twisted loop of the intestine hangs over, its length at that time significantly increasing. The abdominal plates have almost grown to the umbilicus, leaving around it an elliptical area covered only by the peritoneal membrane. From the substance of the abdominal plates themselves, cartilage, muscles and nerves are already formed. Ossification of the skeleton progresses quickly; however, Baer could not determine regularity of this process. The points of ossification in the vertebrae lie inside their bodies. The narrowing of the spinal cord relates to the ossification of the spinal column, which occurs backwards from the anterior end.
The organs of digestion intensively show the processes of differentiation. In the wall of the esophagus, longitudinal folds appear; the wall of the crop thickens and is accompanied with mucous glands. From the right of the muscular stomach (gizzard) the duodenum proceeds, and the loop envelops the pancreas. The growing liver pushes the remaining part of the intestine backwards and downwards, as a result of which the small intestine moves into the umbilical orifice. The gall bladder assumes a gray color, while the bile penetrates the stomach and the duodenum. The cloaca acquires a folded internal surface and is joined by the stem of the urinary sac.
The lungs firmly join the chest cavity, with deep marks of the ribs noticeable. The surface of the lungs begins as broom-shaped or velvety, because the terminal tubules hang above it. At the thirteenth day, the ends of the tubules become welded together, while the surface of the lungs becomes smooth again.
Further differentiation of the vascular system leads to the anterior part of the body, which becomes supplied with arterial blood from the left ventricle; both ventricles supply the posterior part.
In the brain, the four-hillock body has the shape of two widely separated follicles. Anterior to them lies the big brain, and the cerebellum is situated posteriorly; as a result, the whole brain, according to Baer, resembles the outlines of the ace of clubs.
The iris of the eye begins pigmentation from the side of its pupil border.
The auditory canal is situated in the open furrow of the wedge-shaped bone and is covered in obliquely located drum membranes .
During the fourth stage, from the fourteenth through the sixteenth day of incubation (Â§ 12) , the urinary sac covers practically the entire egg contents: the greatly wrinkled yolk sac, the embryo and most of the albumen. It represents an intact membrane adjoining the shell membrane and carries the name "chorion."
Since it will soon move out of the shell, the embryo undergoes active respiratory movements. Some intestinal loops hang from the umbilicus, which soon begin to be gradually drawn backwards into the abdominal cavity. The feather rudiments elongate but still do not appear clearly.
The organs of respiration, the kidneys, the central nervous system and the eyes continue developing, not undergoing remarkable quantitative changes. Differences in the sexual apparatus become more distinct. At the entrance to the nostrils, the scales characteristic of the chicken family appear.
The stage before the last stage (fifth) of the period described (17 - 19th day) is characterized by the following features (Â§ 13) . The yolk sac becomes empty and divided by one or several deep folds. The quantity of the urinary precipitates in the cavity of the chorion (allantoic cavity) increases. The albumen is almost completely absent, while the amniotic fluid also diminishes. The loops of the intestine situated outside the abdominal cavity continue retraction backwards, attracting the yolk surrounding the mucous and vascular sacs. The feather rudiments are very long, but the feathers do not yet appear.
The last stage (twentieth and twenty-first day of incubation; Â§ 14) is considered the preparatory stage for hatching. The fluid from the cavity of the amnion and chorion has almost disappeared. The embryo occupies nearly all of the egg cavity with the exception of the air sac. The yolk sac is gradually pulled into the abdominal cavity. It is possible to observe the stage when the yolk sac is present half in the abdominal cavity and half outside it, and they are connected by a narrow neck, situated in the umbilicus At that time, the umbilical orifice becomes tightened and begins healing.
The arteries which connect the aorta with the pulmonary arteries contract, forming a boat- like duct.
The movement of the chick's head ruptures the chorion. In this case, the end of the beak penetrates the air chamber, which makes breathing possible. A first squeak accompanies the breathing. Baer sometimes heard the cheep of the chick still in the unbroken egg shell two days before hatching. Sometimes the movement of the head breaks the shell, after which breathing and the first squeak occur. After that, breathing begins with atmospheric air; the vessels of the chorion, which were previously filled with blood, then become abated. The chorion becomes separated from the umbilicus, and the chick breaks out of the egg.
The general characteristics of the third period of development of the chick embryo Baer gave in his concluding section (Â§ 16) of the first part. His first concluding point leads to the assumption that during the third period of embryonic development the embryo gains predominance over the remaining parts of the egg. If at the beginning the embryo represented a part of the rudiment, then the latter now becomes a part of the embryo. The parts of the egg from which the embryo became separated in the second period now gradually become embedded in it. Baer identified the same three stages of separation: the separation of the embryo from parts outside itself, their retraction inside, and, finally, life outside the egg, in which the animal is no longer part of the egg, but becomes dependent on the outside world.
Another important point of the concluding paragraph established that development represents a transition from the general to the special. While in the second period the embryo acquires features which are general to all vertebrates, in the third period it reveals the particularities of birds ("the chicken becomes a bird") in the features of its respiratory system, extremities (wings) and covers (feather rudiments) . "However, from the beginning it is a bird in general, and not a bird from the family of chickens." Gradually, the features of land birds appear (shortening of the interdigital web or membrane); then, the features of chickens can be distinguished (form of the head, the separation of the glandular stomach from the gizzard, blunt claws and scales above the nostrils) . At the time of hatching, the generic feature appears (the comb on the head) while the individual particularities develop outside the egg as the chick reaches its maturity. The general conclusions given here by Baer are very briefly illustrated in the second part of the work.
Cite this page: Hill, M.A. (2021, April 11) Embryology Book - Russian Embryology (1750 - 1850) 17. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Russian_Embryology_(1750_-_1850)_17
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