Book - Russian Embryology (1750 - 1850) 21
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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 21. Third Part of Flber Entwi Cklungsgesch I Chte Continued - Development of Reptiles, Mammals, and Animals Deprived of Amnion and Yolk Sac
Baer based his presentation of reptile development on data concerning turtles, monotremes, and viviparous snakes and lizards CÂ§ 8). The aim of his detailed study, according to Baer, was comparison of the peculiarities of development characteristic of the separate classes with features characteristic of all vertebrates.
Baer's information about turtle development was limited. His attempts to receive more or less late stages of development were unsuccessful, as described in detail in his specialized study. 1 (1Â°3) The peculiarities of early turtle development, which Baer observed up to the tenth day of development, showed that interpretation of the embryo was an error of observation CFigure 30). In UBER ENTWICKLUNGSGESCHICHTE, Baer referred to the observations of Tiedemann,2 who saw a turtle embryo surrounded by amnion. The urinary sac is connected with the urinary bladder and by the right side rather than in the body of the embryo. The yolk sac, by the help of the yolk duct, is united with the intestine. As in birds, towards the end of embryonic life, the yolk sac passes through the umbilicus to the abdominal cavity. Bending of the embryo and its internal organization is the same as that of birds.
1. K. E. v. Baer. "Beitrag zur Entwickelungsgeschichte der Schildkroten." ARCH. ANAT . PHYSIOL. U. WISS. MED. C1834) , pp. 544-550 (104).
2. (Ed.: Friedrich Tiedemann, ZU SAMUEL V. SOMMERINGS JUBELFEIER. Heidelberg, 1828, pp. 23 ff.)
Concerning the monotremes, snakes and lizards, Baer enumerated in detail the similarities of their embryonic development with the development of birds. The ova of reptiles are supported by a small amount of albumen and deprived of chalazae. Their development elapses more slowly than in birds, so that the heart, for example, although generally similar to the heart of birds, is delayed in early stages of development; the same holds for the development of extremities. The ova are laid when development is sufficiently advanced. The development of the ova outside the maternal body corresponds to the third period of avain development. The difference between the embryos of reptiles and birds are these: the vascular arches, coming out from the aorta, are longer in lizards and snakes than in birds, and their aorta in the post-embryonic period retains two roots, from which the right is much larger than the left. The cardiac chamber remains without septum. Thus, Baer concluded, "with respect to circulation, the reptiles remain in an embryonic condition, so the circulation system remains in an incomplete double form. In contrast, most birds do not acquire external genital organs. In this regard, the birds thus remain in a more embryonic condition in comparison with reptiles" (II, 8b. 213 (141)). In the development of viviparous snakes and lizards Baer did not notice any essential peculiarity.
Turning to the development of mammals (Â§ 9 (164) and 10 (233)), Baer first noticed that in different representatives of his class, the moment of birth coincides not with one or another stage of embryonic development, but in accordance with the fact that there can be both early and late-born mammals. Immediately after birth in the former, the young are incapable of independent movement, while in the latter the young are actively moving. "The early-born mammals," Baer wrote, "are, thus, transitory forms, and the late-born mammals constitute a proper branch of this class" (II, 9a, p. 218 (164)). The development of early-born mammals Baer did not study himself. Rather he referred to the limited literature of his time on monotremes and marsupials.
Concerning the late-born mammals, Baer first noticed great differences in the external form of the embryonic ova^ and the embryos themselves. The existence of the umbilical cord, amnion, chorion, allantois and placenta in mammals had been known for a long time. In the human embryo, according to Baer, until recently the importance of the small vesicle between the chorion and amnion and connected with the umbilicus by a long stalk could not be interpreted.
3. Embryonic vesicles, by recent terminology.
Figure 30. Baer's illustration of the development of turtles.
17 - embroyonic corselet "Emys europea," view from above ; a- entrance in the spinal canal ; 18- the same also, in transverse section (enlarged 10 times); ab- spinal plates; be- abdominal plates; d- boundary vessel.
In the first and second decades of the nineteenth century, however, it was learned that in the mammalian embryo the yolk sac is actually present. Thus the umbilical vesicle of man and other mammals and similar to the yolk sac of birds, is connected with the intestine. In a special report, Baer carried out on different mammals studies that showed that the formation of allantois and chorion changes with changes in the human ovum, the entire uterus is covered by a thick layer of coagulated matter, the caudal membrane. Baer, following Hunter, 5 repeated that this membrane forms a pocket in which the ovum lies in an open sac, a peculiarity which distinguishes humans from other mammals.
1 In the initial period of development of
4. K. E. v. Baer, "Untersuchungen iiber die Gef assverbind.ung zwischen Mutter und Frucht in den Saugethieren, " EIN CLUCKWUNS'CH ZUR JUHELFEIER S. T. SOMNERINC, Leipzig, 1828, pp. 30 ff.
Returning to the question about the origin of the ovum, Baer recalled the unsuccessful searches for mammalian ova by Haller and his student Kuhlemann, the observations of Cruikshank, and his own investigations, leading to the discovery of the ovum in the Graafian follicles. The history of this discovery is considered above (in Chapter 14).
After a description of the female genital system in mammals, Baer addressed the characteristics of the ovum in the ovary. On this question, Baer could not reach complete clarity. In his opinion, the follicle content is a fluid rich in protein, surrounded by a granular membrane, which he erroneously compared with the vitelline membrane of bird's egg. In the albuminous fluid Baer saw vitelline globules and therefore considered it possible to call the follicle content an ovum, noticing, however, that it represents something more than the yolk ball of a bird's egg, because not all the mass covered with the granular membrane is converted into the fetus. The embryo of mammals is developed from a small vitelline globule situated at the inner surface of the follicle and ejected after the latter bursts â€ž This globule Baer compared with the embryonic vesicle in the eggs of birds and reptiles. He recognized, however, some consequences of this comparison, noticing that in birds and reptiles the embryonic vesicle in the fertilized egg disappears, while in mammals it is the beginning of the embryo. 7 Further Baer spoke about the movement of the ovum in the tubes and the formation of the corpus luteum (11, 9 g-k (176-183)). He supposed that "in the ovum of mammals following entry into the uterus or shortly after that, the embryo is isolated" (II, 91; p. 244 (184)). The ovum, which is composed of vitelline globules soon becomes diluted, quickly grows at the expense of the liquid composing parts of the surrounding albumen, and hence becomes covered by a membrane closely adjacent to the ovum reservoir. This membrane, which Baer studied in swine and sheep, was called the outer ovum membrane. Under it was present that membrane directly covering the yolk (vitelline membrane), which later dissolved.
Baer referred to a treatise by (William) Hunter, ANATOMIA UTERI HUMAN I G RAVI DAE TABULIS ILLUSTRATA, Birmingham, 1774. See also Baer, NACHRICHTEN, p. 331(323). There Baer stated his polemics with Plagga, attempting to ascribe to himself the priority of the discovery of the ovum of mammals.
Detailed analysis of Baer's errors connected with the study of the mammalian ovum is given by P. G. Svetlov in his remarks on the translation of the second volume of UBER ENTWICKLUNGSGESCHICHTE (remark 139, pp. 476-479) .
The primary form of the embryo Baer described as follows: When the yolk becomes liquid and transparent, it is clear, that the sac-like rudiment is composed of two unequal parts: the smaller is the embryo, in the middle; the larger part surrounding it is the embryonic membrane. The part that is the origin of the embryo first has a rounded form, then it changes to the form of a shield, becoming transparent and deprived of all organization" (II 9p, p. 252 (189-190)). Later the shield elongates, and along the edge appears a strip similar to the primary strips in the bird's egg.
Just after embryonic formation begins, it is untwisted from the rest of the blastoderm by formation of the umbilicus, As a result of this, the embryo itself and the yolk sac are formed. During the study of development of dogs, rabbits, swines and sheep, Baer noticed that the yolk duct connects the yolk sac with the intestine. In mammals, as in birds, the yolk sac and intestine represent two parts of the vegetative part of embryo, which are only untwisted from each other. Their connection by means of the yolk duct in different families of mammals is kept for different lengths of time: the longer the time, the greater the size of the yolk sac. Not in any mammal does the yolk sac enter the body of the embryo. It is torn away with the ovum membranes or it disappears earlier. The yolk sac of mammals and of birds is composed on the outside of a vascular layer and inside of a mucous layer, which are never completely separated. The form and size of yolk sacs of different mammals are very different. For example, in carnivora it is large, the form changes from spherical to ellipsoidal and then to fusiform, and for a long time it remains connected with the intestine. In human beings the yolk sac, called here the umbilical sac, remains spherical. In rodents it is very large and rounded, winding under the serous membrane around the amnion.
Before the beginning of the untwisting of the embryo from the yolk sac, the embryo is split into two layers â€” animal and vegetative â€” connected in the region of the primary stripe. The abdominal layers, after separation, turn upwards. Because of this the formation of the amnion quickens to the extent that the moment of its formation is difficult to recognize. "However," Baer wrote, "I was lucky enough to see in dogs, and more frequently in swine and sheep, embryos completely uncovered by the amnion. Therefore I can assure absolutely that the amniotic sac is formed as that of birds. Following amnion formation, all other parts of the outer layer of the blastoderm, as in birds, form the serous membrane, in which the amnion and yolk sac are included" (II 9r, 255-256 (192)) (Figure 29, 24). Baer noticed that attention was not given to the serous membrane, and thus it was impossible to understand the development of membranes.
The urinary (allantoic) sac of mammals, Baer observed, grows when body of the embryo is opened nearly the whole length. It represents the growth of cloaca and is composed of two layers: an internal layer which is the continuation of the serous layer, and an external or vascular layer. Into the latter enter two branches and ends of veins, passing by the lower edge of abdominal layers. These vessels are the future umbilical arteries and veins, which share in the formation of chorion and placenta. The subsequent development of the allantoic sac of carnivora (similar to birds) leads to its growth, so that it extends through the back of the embryo from right to left and meets with its opposite part, leaving free only the place for the vitelline vesicle. In the internal half of the allantoic sac, adjacent to the amnion, the blood vessels are few, and in the internal half which intergrows with the superficial membrane, forming the chorion, the vessels grow and form the vascular network of the placenta. The case is different in hooved animals. The allantoic sac widens a little and lies near the amnion. Because it is more elongated, it digs through the external layer and goes beyond its limits. The layers of the allantoic sac of hooved animals are situated far apart, so the two inserted sacs are in each other, the internal one deprived of vessels and assumed to be the urinary outlet. Even before Baer, it was called specifically the allantois. The allantoic sac of rodents does not extend through the amnion, situated against it on the ventral side of the embryo. Finally, the allantoic sac of human beings is very small (Figure 29, 23) .
Baer called the outer vascular layer of the mammalian embryonic vesicle the chorion. It is formed originally from that external embryonic layer deprived of vessels. The placenta, according to Baer, was considered growths of the chorion, which served to create conditions for the maternal blood to influence the embryonic blood. On the basis of observations Baer suggested broadening understanding of the placenta to include all the blood -containing villi present on the surface of the ovum. When the flow of blood to the uterus is strengthened, a structureless material is distinguished on its internal surface; later vessels penetrate it and there arises the caducous membrane, which Baer favored calling the uterus cover or uterine placenta.
Concerning embryonic respiration and nutrition, Baer noticed that the maternal blood vessels nowhere communicate with the fetal vessels. In the blood of both mother and fetus, which pass in close proximity, "changes must take place, and these changes must be called respiration" (II 9x, p. 275 (205)). The feeding of the ovum at the beginning, when there are still no vessels, takes place by absorption at its surface. Later, with the appearance of blood vessels, this absorption occurs through them.
Turning to the details of mammalian development, Baer first established the great similarity to avian development in organ formation. For example, in the skeletal system, the spinal column and tail form as in birds. At early stages, the limbs are similar to those of birds, and at first a long fold forms, from which the basic segments of limbs, articulated joints, and digits are formed. The last is at first undifferentiated, so the four hooves of sheep are basically indistinguishable from the digits of dogs and the hooves of swine. Jaw development takes place from the same rudiments as in birds. The lower jaw is a derivation of the first and second branchial arches. Peculiarities of formation of upper jaws in mammals result because the middle does not extend into a long neck like the bird's beak. Every upper jaw develops an inside crest, which gives rise to the corresponding half of the hard palate and nasal septum. Only after this do the jaws of the mammalian embryo begin to extend. In connection with brain development in the previous stages, the facial parts of sheep and swine are very similar to the human face.
For the digestive canal, the abdominal cavity of mammals closes as in birds, but somewhat later. At first the intestine is connected with the yolk vesicle by a very wide space, the construction and extension of which forms the vitelline duct of the cutaneous umbilicus. The short and non-differentiated digestive canal divides into parts, quickly elongates and moves aside from the backbone, especially in the middle part as a result of the growth of the mesentery. In this place the intestine forms a projection, into which the vitelline duct falls. As in birds, from the digestive tract develop the salivary glands, organs of respiration, liver, pancreas and urinary bladder. A part of the intestinal loop for some time lies in the umbilical branch.
Differences in the structure of the digestive organs which are inherent to different families of mammals, appear gradually. Furthermore Meckel saw that in the earlier period of development of ruminants, the stomach is single. Notches appear in it, and then the parts of the stomach become separated. Baer noticed Meckel's mistake here,, pointing out that Meckel, in support of his opinion that embryos of higher animals pass through the stages corresponding to organization of the lower organisms, confirmed that the stomach of man passes, as in ruminants, the stage of the sac divided into parts.
In the vascular system, the heart is initially similar to birds and has the form of a double-branched canal . Becoming shorter, it gains the beginning of five vascular arches, which are transformed into two aortae. Further features appear which are characteristic of mammals. The ventricle turns more to the right side, which is why both ventricles from the beginning appear close together. The current of blood from the right ventricle moves into the fifth left arch more than in the fourth. From the fourth left ventricle, it passes into the fourth left arch more than in the fourth right. Therefore, in mammals the movement of blood in the left side is more intensive, and the arch of aorta forms from the left root, while in birds it is from the right.
In a footnote Baer objected to Allen Thomson's scheme (II cc, pp. 212-213). The latter had taken Baer's drawing of the blood vessels in a chick embryo, transferred the right parts to left and vice versa, and proposed that it is thereby possible to represent the vessels of the mammalian embryo. To prevent such erroneous interpretations, Baer made a new drawing (Figure 29, 14) of the mammalian transformation of the branchial vascular system into constant arteries. This drawing makes clear that from the anterior bifurcated end of a primarily single arterial trunk, five pairs of vascular arches proceed to the two roots of aorta b. Early disappearing arches are represented by a dotted line, and those remaining are represented by a thin contour, while the final vessels are black. In the venous system of the mammalian embryo, the same vessels which appear in birds are present. Vessels of the yolk sac and the boundary vein are also present. The coincidence between birds and mammals is especially great with the venous vessels of the embryo itself. In those and others, there are anterior and posterior vertebral veins. The posterior vein trunk in mammals, as in birds, unites with the posterior veins of the body; after that the diameter of the vertebral veins decreases. The jugular veins only at first are connected with the anterior vertebral veins and then are separated from it.
The formation of the neural tube and differentiation of its anterior part into five cerebral sacs takes place in mammals as in birds. However, in mammals the middle cerebrum is not so high and sac-formed, while in birds it is more elongated and curved. Unlike that in birds, the predominance of the anterior cerebrum over the other parts appears very clearly in mammals. But the head cerebrum, is more bent in mammals than in any other class of vertebrates. The transformation of the spinal cord to the medulla oblongata takes place nearly at right angles. Owing to this, a clear occipital projection is seen from the outside. Baer considered the bends of the brain to be the result of its intensive growth, which the skull cavity has no time to follow. The organs of sensation develop as in birds.
The primary kidneys, similar to those in birds, appear and disappear. Toward the outside the constant kidneys originate; at first they are stretched in length, then they become rounded and move away from the vertebra than in birds.
The transformations of the genital system are very complicated. The genital glands have the form of spindles in the internal sides of the primary kidneys; in the external convex edge of the primary kidneys, simultaneous with the genital glands, a thread-like conducting canal appears. This is the future seminal duct or oviduct. In the cloaca, two lateral folds form, which then unite. They separate the rectum from the part which gives rise to the allantoic sac. Besides this, the external orifice divides into two parts for the formation of perineum. Further, the genital glands become rounded, and by histological separation in males the canalicules are developed, and in females the Graafian follicles later develop. The conducting genital ducts in females are widened and supplied by an orifice in the abdominal cavity; the uterus and vagina develop later on.
Baer described in detail the development of the external genital parts, cavernous bodies, urethra, uterus and vagina, and also the descending of testicles from the abdominal cavity to the scrotum.
Baer mentioned little about the development of the diaphragm, noting that he could not give it special attention, He did, however, establish the facts of gradual backwards displacement of the diaphragm, of elongation of the embryonic thoracic cavity, and of the growth of muscles from the wall of the body. Baer described the serous membranes briefly, whereas he described development of the mesentery in more detail. He described its displacement during the formation of intestinal loops and mentioned the turning of the stomach and the origin of omentum.
The formation of the umbilicus of the mammalian embryo takes place as in birds. Concerning the umbilical cord, in the majority of mammals the yolk duct dies off early and the vessels of the yolk sac disappear. The length of the umbilical cord is not equal in different mammals; man's is the longest, it is slightly shorter in monkeys, followed by that of hooved animals, and with the rodent's shortest.
Description of mammalian development he concluded by counting the embryonic parts lost during birth, the chorion, amnion, yolk sac and placenta with the umbilical cord. These parts in mammals are correspondingly larger than those in birds.
In a special section, Baer described the structure and development of the embryonic sac in different mammals, especially in humans. The preliminary remarks which open this paragraph stress the importance of comparative embryological investigation of the embryonic sac and the history of development of embryonic membranes. Thus "exact knowledge of the different mammalian forms of ovarian membranes can help the understanding development of human embryonic membranes. The first steps of their formation are unavailable for investigation" (II 10 a, p. 313 (235)). For explanation of the text, Baer offered diagrammatic illustrations. The first of these (Figure 29, 19) represents a transverse section of a bird's egg about the eighth day of incubation, with which the embryonic ova of different mammals are compared. In this drawing, a is a section of the embryo, b the amnion. In the body of the embryo are seen the primary kidneys, mesentery and intestine, with the yolk duct leaving at c, which passes to the yolk sac d. On the latter the distribution of vessels and the boundary vein are shown. The allantoic sac still occupies only part of the ovum; its external half /, adjacent to the shell membrane, marks the beginning of the chorion, and the internal half g surrounds the amnion. The allantoic sac already extends over the back of the embryo on the opposite side. Further, the allantoic duct e, remnants of the serous membrane h and the albumen of the egg i are also represented.
The embryonic sac of carnivores (Figure 29, 21) is shown in the stage of the already-formed chorion and developing placenta. The yolk sac, with an elongated form, is surrounded by the remnant of the serous membrane h t separated from the yolk sac. The fate of the serous membrane becomes clear upon comparison with the earlier stage when the amnion is in contact with the serous membrane, and when the allantoic sac remains very small. The allantoic sac of a dog's embryo, developing from the cloaca at the age of about three weeks, covers the embryonic body touching the external surface of the amnion and internal surface of the chorion. The vessels of the external layer of the allantoic sac form the villi. Later, the embryonic sac stretches into the form of cylinder with rounded ends, and the villi forms the belt-like placenta. The yolk sac of carnivores is rich in vessels, and its duct remains open for a long time.
The embryonic sac of swine (Figure 29, 22) is represented at the moment when the formation of chorion continues (up to the end of the second week) ; the formed embryo is seen with the allantoic sac coming out from it. The latter appears earlier in thick-skinned animals than in carnivores and grows very quickly. Umbilical arteries enter the allantoic sac from the two sides of the embryo, and the umbilical veins go out into ramifications in the abdominal wall. After the formation of anastomosis between the umbilical veins, the right vein gradually disappears. In the external membrane of the embryonic sac the blood vessels appear only when the chorion is formed, or after the accretion of the membrane with allantoic sac. In swine embryos, Baer followed in detail the development of villi of the chorion. At first the beginning skin transverse fold lines appear with a height of 0.1; the free edges of folds are covered by cuts that lead to the formation of many villi. The same process takes place in the internal surface of the placenta, while between its villi arise the villi of the chorion. Also, Baer followed up the process of the union of the allantoic sac with the external membrane of the embryonic vesicle and the intergrowth of blood vessels in the embryonic vesicle with the formation of vascular networks in the villi. After the end of the fourth week, the network of the chorion's blood vessels increases, filling all the villi and spaces between them; a denser network of vessels develops in the internal wall of the uterus.
The embryonic sac of ruminants is in the essential features similar to the ovum of the thick-skinned animals, with the difference that the placentae of the ruminants are multiple and differ in form in different species. The embryonic sac of rodents (Figure 29, 20) is characterized by a single placenta, confined to part of the ovum surface.
In sloths (Baer mentioned the data of Carus and Rudolphi) the placenta is elongated and rounded, consisting of multiple placentae adjacent to each other. This forms a remarkable transition between extremely unlike forms, namely between ruminants and monkeys.
The embryonic sac of the monkey is very similar to the human sac, but more elongated in accordance to the form of the uterus. The monkey umbilical sac is larger than that of man, is also kept until birth, and possesses a similar long stalk. The length of the umbilicus, which in monkeys is larger than in all other mammals, also brings the monkey nearer to man.
Considering that he had to rely on the observations of others for questions of human development more than for other parts of his work, Baer at first refused to enter into arguments concerning the nature of human beings. "We shall," he maintained, "regard man only as a member of the great animal kingdom" (II 10k, pp. 351-352 (264)). Thus Baer determined his role as a naturalist with respect to opinions on human nature. "Comparison with the development of animals, and namely with the mammals," Baer continued, "is considered the most reliable guiding star .... without the flame of the comparative history of development we cannot clarify the significance of the separate parts of early embryonic sac of man" (II 10. k, p. 352 (264)). And further: "You know the history of development of the mammalian egg and can imagine that the history of the human egg represents only a special case of the general history of the mammalian egg" (p. 353 (II 10Z 265)) .
The cover of the uterus is developed in human beings earlier than in other animals; Baer claimed that he saw the dropping off of the membrane on the eighth day of pregnancy. This cover is supplied by fossae, in which the villi of embryonic vesicle grow. The investigation of two-week-old human embryos showed that the embryo begins to form in the internal sac, as all other mammals do. Then the embryo untwists, so that the remaining part of the sac produces the beginning of the yolk sac, which is called in human beings the umbilical sac. It quickly moves away from the embryo, while connected with it by the yolk duct. The umbilical sac usually disappears in the third month of uterine life. It may be inferred by analogy with birds and mammals that the amnion of the human embryo grows extremely quickly, even though the formation of the human amnion was not observed. Baer asserted that in human beings its development "must not take place differently" (II 10 q, p. 363 (233)). Direct observations, showing the ingrowth of blood vessels from the wall of the allantoic sac to the external membrane of embryonic vesicle, forced Baer to consider the comparative embryological suggestion that the human chorion is formed as in other mammals. Baer considered the question about the structure of the human allantoic sac controversial, but he did not doubt that, here also, the principal stages of its development are the same as those of other vertebrates. The villi of the human chorion are longer, thinner and more ramified than those of other mammals. They are entwined by a network of vessels to form placenta. The development of the umbilicus and the human embryo itself was not described by Baer, who referred this aspect of humans to other mammals (105) .
The concluding section of his third part, Baer entitled "Development of animals which have no amnion and no yolk sac" (II Â§ 11, p. 280). Mentioning the similarity characterizing development of reptiles, birds and mammals, Baer indicated that fish and amphibia possess essential differences in the structure of the ovum and membranes. This forced him to recognize that the lineal group of amphibia must be divided into two classes, amphibia and reptiles.
The main peculiarity of fish and amphibia is that they are always deprived of the amnion and allantoic sac. Instead of the latter, fish and reptiles develop other organs of respiration, the external gills.
In spite of this difference of fish and amphibia from the other vertebrates, the formation of their embryo follows the general scheme for all vertebrates: from the embryo two spinal arches arise, two abdominal layers form, and by the closure of these and others the dorsal and ventral sides of the animal form.
Further, Baer described the ova and their development in amphibia and fish, and devoted separate published works to the embryology of these two classes of vertebrates.^
In the ovum of amphibia, which is present in the ovary, the embryonic vesicle appears early. From the beginning it is large and is present in the middle of the ovum. At the time of maturation it ascends to the surface at the place where the embryonic layer is present from outside, not clearly demarcated from the yolk. The embryonic layer with the vesicle present inside it ascends slightly in the form of an embryonic hillock. In a footnote Baer said that he represented this stage in his Epistota de ovi mccmmaliwn genesi (see Figure 26, XXIV-XXVI) . Covered by a thin layer after maturation, the ova fall into the abdominal cavity, and from there into the funnel of the oviduct. Embryonic vesicles are not found in the ova, which are present in the abdominal cavity. During the passage through the oviduct, the ovum is covered by gelatinous material secreted by the walls of the oviduct, which Baer equated to the albumen of a hen's egg.
8. Baer, "Die Metamorphose des Eies der Batrachier vor der Erscheinung des Embryo und Folgerungen aus ihr fur die Theorie der Erzeugung," MULLER'S ARCHIV ANAT. PHYSIOL. (1834), pp. 481-509; "Entwickelungsgeschichte der ungeschwanzten Batrachier," BULL. SCIENT., publie par l'Acad. Imp. d. Sciences de St. Petersb., No. 1 (1835), pp. 4-6, No. 2, pp. 9-10 j "Untersuchungen uber die Entwickelung der Fische nebst einem Anhange iiber die Schwimmblase , " Leipzig (1835) , iv + 42 pp. in quarto; "Beobachtungen uber die Entstehungswei der Schwimmblasen ohne Ausfuhrungsgang," BULL. SCIENT., publie par l'Acad. Imp. d. Sciences de St. Petersb., 1, No. 2 (1836), pp. 15-16; "Uber Entwickelungsweise der Schwimmblase der Fische," FRORIEP NEUE NOTIZEN, 39, No. 12 (1846), pp. 177-180.
Fertilization in the tailless amphibia is external; the sperm cover the ova falling into water. In salamanders the sperms also are ejected into water before they penetrate the genital apparatus of the female.
The structure of the deposited ova and the early stages of development Baer described in Miiller's ARCHIV in 1834 and below. 9 The freshly deposited ova closely adjoin each other and are covered by compact albumen. In water the latter is swollen and becomes transparent. Meanwhile the ovum becomes spherical and begins to turn in its membrane, directing the dark side upwards. Baer called the upper and lower regions of the frog ovum the dark and light surfaces; their centers he called the dark and light poles and the line joining the poles the axis of the ovum.
Concerning the first stages of development of the fertilized frog ovum, Prevost and DumaslO reported their observations. According to them, "astonishing division" takes place on the surface of ovum as Baer had said. These observations, Baer reported, "caused a lively interest in all directions, partly due to the unexpectedness of the fact that the yolk sphere, which will become a frog, is preliminarily covered by a network of geometrically and regularly situated fissures. It was due, also, to the apparent improbability that such a noticeable phenomenon had escaped the attention of many observers of the development of ^the frog ovum, including Swammerdam" (106) . Although Prevost and Dumas did not understand the essence of the division phenomenon, Baer accurately stated that they "remained on the surface of the phenomenon" (MULLER'S ARCHIV, 1834, pp. 482-483). Rusconi 11 also saw the same picture of fissures on the ovum surface; also Baumgartner, who incompletely described the phenomenon. The merit of final discovery of the secret of ovum division undoubtedly belongs to Baer (107) . The study of the developed ova of brown and green frogs led him to the conclusion that fissures, which are seen on the surface of the yolk, represent nothing more than the boundaries of the divisions which are manifested by the whole embryonic sphere. Baer gave to embryological terminology names for the fissures of division. He named the fissures connecting the poles of the ovum, the planes of which pass through the axis of the ovum, the meridional fissures; the fissure crossing the axis of the ovum at a right angle, he called equatorial if it divided the axis approximately in half and parallel if it was situated near one of the poles. In addition to the accurate description of division, Baer thought he saw non-existent phenomena in the frog ovum. Thus he referred to an orifice which seemed to be present in the region of the dark pole and which he thought led through a canal into the deeper cavity and remained there even after the disappearance of the embryonic vesicle. This description came, evidently, from the penetration by the spermatozoa, which leave behind the dark trace that Baer erroneously took for a canal .
9. Baer, "Die Metamorphose des Eies der Batrachier vor der Erscheinung des Embryo und Folgerungen aus ihr fur die Theorie der Erzeugung."
10. J. L. Prevost et J. A. Dumas, "Nouvelle theorie de la generation," ANN.^SC. NATUR. , 2 (1824), pp. 100-121, 129-149
11. M(auro) Rusconi, DEVELOPPEMENT DE LA GRENOUILLE COMMUNE DEPUIS LE M3MENT DE SA NAISSANCE JUSQUE A SON ETAT PARFAIT (Milan: Giusti, 1826) .
The phenomenon which Baer called transformation is externally detected by the development of the first meridional fissure. He described it in extreme detail, since he clearly recognized its importance. Five hours after oviposition, the first meridional fissure forms beginning from the dark pole and it gradually moves from there along a spherical arch in the direction of the light pole, where its ends are united. This process of fissure formation, Baer asserted is not continuous, but takes place in separate stages as if the movement of the fissure were overcoming some resistance. The essence of this first transformation in the developing ovum is that the "yolk sphere is divided into two hemispheres or, more accurately, into two spheres, which become attached to each other" (p. 487).
After the closure of the first fissure, according to Baer, "apparent quiescence begins. However it only seems to be, because the cleavage is imperceptibly distributed from the inside surface" (pp. 487-488).
The following stage in transformation consists of the appearance of a second meridional fissure, appearing six to seven hours after fertilization. It goes also from the dark region to the light, and its plane is situated under and at right angle to the first. The process leads to "cleavage of the sphere into four quarters," as can be confirmed by cutting the compact ovum.
In the third stage of transformation, the equatorial fissure appears and division into eight spherical parts attached to each other begins, followed by two parallel fissures (above and below the equator) producing sixteen yolk masses. Further division continues until there are sixty- four. All previous divisions were performed in vertical (meridional) , or horizontal (equational or parallel) planes.
Baer hinted at the difference in the rhythms of division of the upper and lower halves of the ovum. "It must also be noticed," he pointed out, "that the first and also the second meridional fissures in the light region move slower and fit in less deeply" (p. 489). However, in his drawings the inequality of division of the animal and vegetative halves of the ovum is clearly represented. Later, the division of yolk regions into central and peripheral regions takes place. At this time there are already so many fissure separated regions, that the surface seems shagreen (leathery), and after a little time when the total number of the separated yolk masses is approximately 3,000, the surface is very finely granulated, resembling grains of sand. Finally, the divided parts of the ovum become microscopic, their surface seems to be smooth, and a great number (many thousands) of yolk aggregations can be found in one section. In the following stage of development, the formation of the embryo (KEIM) takes place, according to Baer, and the demarcation of the embryo begins.
His description shows that Baer could clearly follow the process of complete division of the ovum and establish its various stages. In the following pages of his outstanding work, "General Observations of the Mechanism of Division," he attempted to reveal the regularity of division processes and to understand their purpose. First, he objected to formal geometrical interpretation, because the geometrical regularity of distribution of the fissures is frequently misrepresented without violation of the following notion of development. Further Baer established the following general rule: "... If in the separated yolk mass (or blastomers, as they are now called â€” L.B.) one side is significantly longer than the other, then it also undergoes division. According to this rule, the equatorial regions must be divided by vertical fissures, and the circumpolar regions by horizontal fissures. However, deviations from this rule are possible" (p. 499) .
After more than forty years, the established regularity of the axis of division was identified as Hertwig's law, 22 while, in fact, it must be called Baer's law.
The succession of the planes of division in the frog ovum Baer explained in the following way. The first fissure, beginning from the dark pole, is meridional. It divides the ovum into two parts with identical vertical and horizontal diameters; that is, at these parts, according to Baer's law, the second division in vertical and horizontal planes is equally possible. However, it always takes place only in the vertical plane and it, as the first, also begins from the dark pole. In the four parts of the ovum, the vertical diameter is twice greater than the horizontal; they are divided in accordance with Baer's law, in the equational plane. Then again eight blastomeres ("yolk masses") appear. Every one of them is bound on three sides by equal planes, and on the fourth side by an identical part at all the spherical surfaces. Here again it would not seem to matter in which plane the division of these parts of the divided ovum takes place. The division regularly begins from the dark pole, however, and the fissures of division appear again meriodionally. Later, strict regularity of division is lost, but the tendency to distribute the division from the dark pole towards the light pole is retained; therefore in the region of the latter the blastomeres are always larger and distributed less regularly. "All this indicates," Baer concluded, "that the determination of division must originate from the rudimentary orificel3 and its canal. The exact geometrical characteristics of the first divisions depend on the rudimentary orifices and forms the initial point of division. The canal marks the axis. This confirmation, apparently, is based on those cases where the rudimentary orifice is sufficiently distant from the center of the region. All fissures of division preserve the usual position to the rudimentary orifice and its canal, and therefore the equatorial fissure on one side advances deeply towards the light region (p. 501) .
12. It is known under this name everywhere, including the Russian handbooks (see for example A. Maksimov, OSNOVY GISTOLOGII (THE BASIS OF HISTORY), Part 1:UCHENIE KLETKE (Study of the cell) (1917) , p. 334.
Interpreting the "determinations of division originating from the rudimentary orifice and its canal," Baer considered it unquestionable that "divisions are performed under direct influence of a producing substance" (he means the substance of sperms) . It is true that Baer erroneously indicated that in fertilization "not the seminal animals, but the fluid or even the more delicate parts of sperms play the role" (p. 503), because he could not discover the penetration of spermatozoa in the ovum.
Baer's extremely interesting analyses of development begin from the division of the ovum into its separate parts. An initial individual ovum is divided into countless numbers of individualities, each of them with negligible importance which proves to be only an elementary component of the new individual. "A vital process (the process of development) which dissolves the initial individuality is not quite destroyed, because fron; its fragments the new individual originates. In the latter, when the process of division has gone sufficiently far, the rudiment separates from the yolk, and in the rudiment the embryo of the future frog is set apart" (p. 504) .
13. For example, from place of entry of the spermatozoan and its way into ooplasm.
Figure 31. Baer's illustration of "Transformation of the ovum of amphibia"
1- First division; 2- second division; 3- third division; 4- typical fourth division; 5- atypical fourth division; 6- stage of blackberry; 7- stage of raspberry; 8- stage of leathery surface; 9- stage of sandy surface; 10- section in first meridional fissure, the rudimentary orifice with its canal is seen; x- the remaining place of union of the hemisphere of the ovum; 11 and 12- formation of second meridional fissure; 13- vertical section after third division;
14- vertical section through ovum in blackberry stage;
15- section through ovum in stage of leathery surface;
16- the same also in stage of the sandy surface.
Baer understood that the discovery of ovum division, from which the development of amphibia begins, represented a fundamental discovery in embryology. Against the importance of this phenomenon, he admitted that an objection might be raised "if in other animals there is nothing which is similar. We, however, suspect that they have these phenomena" (p. 505) . In fish, in Baer's opinion, it is impossible to see the division clearly because of the transparency of their eggs, and in birds he assumed that division into many small grains takes place, but in another form. "I consider," he concluded, "that the division of the yolk mass is a prototype of all histological isolation" (108) .
Returning in his later memoirs^ to the discovery of ovum division in amphibia, Baer said that the study
of the preparatory stage of development which is the auto trans formation of material by continuous division (led him) to the innermost sanctuary of the history of development, as was subsequently shown through the corroboration of countless investigations .... A similar process of division was observed in different animals as a consequence of fertilization, in the form of division of all yolk mass, or in the form of division of small layers of the latter, which I called the rudiment.
Further in his autobiography, Baer discussed the behavior of nuclei during division. At first the nucleus has a rounded form, then is enlarged and stretched. Its center becomes narrower and the nucleus takes a biscuit form; then the substance of the nucleus disperses into different directions, forming two small bodies. The division of the ovum itself follows the division of the nucleus. Baer wrote that the situation is like that of two rulers, originating by division, each of whom collects around itself part of the kingdom in order that after a short period of quiescence it may again separate, but only in another direction. Is the nucleus causing the division? Baer confirmed that "this I could not establish conclusively in the frog, because its nuclei are excessively dark and large. Later this process was clarified for me in the ova of the sea-urchin, which I investigated in Trieste during a journey from Petersburg" (see Chapter 23) .
14. Baer, NACHRICHTEN, pp. 381-383 (377).
The phenomena of division of the frog ovum gave Baer the grounds to speak decisively about preformation. "The history of metamorphosis of the yolk sphere in amphibia," Baer wrote, "had clearly solves this important question, which was for me an unexpected joy. "15 He did not consider it appropriate in the special works of his volume to discuss the old moot point about the preexistence, or epigenesis of a new individual . Baer noted that although studies about preformation, assuming the preexistence of a new organism up to fertilization, had long been considered an unfounded fantasy, all questions cannot be solved conclusively by direct observation. In truth, the new embryological investigations supported the status that all separated parts of the new individual are formed as a result of transformation of earlier formed more general parts. Hence, the animal section of the vertebrate body is formed from the outer or animal layer of the rudiment, and the vegetative parts are formed from the lower (internal) layer. Thus the embryo is considered the result of isolation of a part of the rudiment It seems that the rudiment is, without question, the undeveloped animal. However, the formation of the rudiment up to fertilization precedes the formation of the less determined organized mass, which clearly differs from the proper yolk. Baer called this mass the rudimentary layer; in the nonfertilized frog ovum, the dark cover in one of the sides of the ovum is considered this rudimentary layer. The rudimentary layer is used for the formation of rudiment as a single unity, and not as a substance. The rudimentary layer in the frog ovum up to fertilization is continuous, but from the beginning of development its continuity is disturbed with each division of the ovum.
15. Baer, "Die Metamorphose des Eies der Batrachier, " p. 506
If one turns to the history of theoretical ideas about development, Baer said, then it can be confirmed that some used observations on frog development for the idea of preexistence. Specifically, on the ova of the frog Swammerdam had tried to show that the dark cover of the ovum is directly transformed into the tadpole. Swammerdam was so imbued with this certainty that he declared that the formed frog embryo which he discovered had preexisted before fertilization. Referring to his statement about the disturbance of continuity of the rudimentary layer during division, Baer confirmed that the frog ova provides a basis for refutation of the theory of preformation.
For examination of Baer's data concerning the subsequent development of amphibia, it is necessary to return to the concluding section (Volume II 11) of his UBER ENTWICKLUNGSGESCHICHTE. Following the scheme of development he had presented for birds and mammals, Baer assumed that in amphibia, division of the rudiment into two vegetative layers takes place. In this primary stripe which forms from the vegetative layer (if we speak according to the language of recent embryology, then we speak about the roof of the primary intestine) , the spinal cord forms, which is so thick that from embryos condensed in nitric acid it can be removed by the fingers. Next Baer described the formation of spinal shafts, which are widely spaced at first, and narrowly spaced later; they ascend as high edges and bend towards each other. At the time of closure of the spinal shafts their internal layer is separated, so that the cerebrospinal canal is formed from two intergrowing layers. In the anterior part of this canal, until its accretion, are seen dilations which represent th* future cerebral cavities. All these processes, Baer noticed, can~be seen more clearly in amphibia than in birds and mammals.
Baer undoubtedly saw the phenomenon of epiboly of the ectoderm, since he comments on how "the rudiment till the formation of the embryo (that is to say up to neuralization) covers nearly all the yolk sphere." In connection with this, he questioned the comparison with the meroblastic ova, asking "Is the entire rudiment becoming the embryo, or is the rudiment divided into two parts, the embryo and the blastoderm?"
The answer to this question is: "since the umbilicus is not formed, then gradually all the rudiment becomes the embryo, so that for the following stage of life nothing remains excessive, contrary to mammals, birds and reptiles. 16 Therefore the entire rudiment must be considered as the embryo" (II 11 f; p. 380 (286)) .
At the time of closure of the back, the embryo changes from a spherical to an elongated form. In the anterior part of the trunk, in the region of abdominal layers of both sides, the branchial protuberances appear stretched downwards; there the parallel fissures form. Towards them from the inside, deeper fissures grow, and the branchial slits originate in this way. Earlier observers saw only three branchial slits, but in his monograph published in Bardach's "Physiology," Baer proved the existence of four slits. 17 Rusconi saw the same, and also found a questionable fifth slit. The surface of the branchial arches has nodules, which are transformed into delicate branched protuberances supplied by blood vessels and then transformed into external gills on three branchial arches.
The development of brain and organs of sensation takes place as in higher vertebrates, only the bends are less pronounced.
When the tail reaches the length of the trunk and the external gills are already well branched, the embryo ruptures the yolk and gelatinous membranes. Hatching occurs at a very early stage of development in comparison with birds.
The larvae coming out from the membrane are attached to the jelly covering the spawn, or to other objects in the water by special suckers, which disappear shortly afterwards. Initially, the larvae eat the jelly and frequently the dead bodies collected by it, and with the appearance of extremities they move to plant food. At this time the internal gills are formed, and the opercular fold covering them grows with an orifice to the outside.
16. In Baer it stands â€” "amphibia." This is either a misprint or an application of Linneaus* designation of a group which includes amphibia and reptiles.
17. (Ed.: Karl Friedrich Burdach, DIE PHYSIOLOGIE ALS ERFAHRUNGSWISSENSCHAFT, Bd. I (1826), Bd. II (1828).)
In amphibia, during the process of development a greater part of the primary vascular system remains than in mammals and birds. Both roots aortae are kept; the ventricle of the heart remains undivided.
Concerning the development of extremities, Baer mentioned that the anterior pair suddenly appears outwards as a result of breaking the intact branchial skin fold. Later he noticed the disappearance of the tail as a result of absorption of the mass filling its skin. The development of the central nervous system of amphibia takes place as in higher animals with specified differences, which depend upon the peculiarities of the brain structure. With nerves, Baer repeated the earlier mistake that their appearance is caused by histological separation from the surrounding parts. Baer stated the development of the digestive system very briefly, noticing that the digestive canal of amphibia is formed without the untwisting of the yolk sac. The development of the urinogenital system he also awarded a small place. The primary kidneys, described by Muller, are situated in the innermost part of the trunk; they are kept until the disappearance of the tail, when the permanent kidneys appear. The genital organs develop later on, while the genital glands' appearance precedes the formation of the fatty body.
Turning to fish, 18 Baer noted that their development is similar to the development of amphibia, because there is no amnion and no allantoic sac in these two classes. Incidentally, the history of development of different fish is not completely equal; the differences partially depend on the greater or smaller quantity of yolk ("and on the peculiarity of albumen," Baer added), and also on peculiarities of organization. The ova of fish are formed in the ovary; in its rudimentary layer can be found yolk spheres containing embryonic vesicles and surrounded by a membrane. In connection with this, Baer entered a long controversy with Rathke and other authors about the question of which forms earlier in the ovum - the embryonic vesicle or the yolk. Baer indicated that in young ovarian ova the embryonic vesicles are very large, and the bigger they become the less the nourishing materials in the ovum. The details of this controversy have not been established.
18. The development of fish was described earlier in a separate edition of a very well known work (see footnotes) . In future when referring to this work, the pages of its text will be shown. Illustrations are taken from it and mentioned here (Figure 32).
Fertilization in fish takes place in the same way as in the frog, at the moment of depositing the egg in the water; in the viviparous fish, for example (Blennius viviparous) , the four-eyed (anableps) and some silurids, the fertilizing substance penetrates from the water into the genital orifice of the female, and in some fish (Selachians), the fertilizing substance is introduced there by the male, as occurs in mamma 1 s .
The structure of the ovum membrane of fish is variable; it is a fine-grained or cartilaginous membrane (for example in perch) or even shelled with four edges (egg-laying selachians) . The rudimentary layer occupies a smaller space on the surface of yolk in the fish which Baer investigated than in amphibia, but relatively larger than in birds. Thus, in Cyprinus blicoa.19 and pike the rudimentary layer occupies a quarter of the yolk surface. In the center it is thicker than at the edges, denser than the yolk and very transparent. The embryonic vesicles in the deposited ova are not detected. The rudimentary layer, according to Baer, after fertilization is transformed into a rudiment without the preliminary division of the yolk sphere, which he described in the amphibia. In a footnote Baer referred to Baumgartner, who apparently saw something similar in trout20. Although Baer could not directly observe segmentation in the ova of fish, this did not prevent him, as mentioned above, from recognizing that the process of segmentation is a principal and universal phenomenon, with which development begins. Later, referring to his work on fish development, Baer remarked that he had not noticed divisions taking place after fertilization. These divisions, he thought, took place at night, and the protuberances on the surface of the rudiment appearing after artificial insemination he assumed to be a sign of death and the beginning of destruction.
19. Cyprinus blicca , an old classification used by Baer, which corresponds to the present name ABRAMIS (BLICCA) BJORKNA, or ABRAMIS BRAMA.
20. (Ed.: Baumgartner, "Beobachtungen viber die Nerven und des Blut," p. 13.) .
The rudiment gradually begins to be covered by the yolk; in three to four hours after oviposition it occupies nearly one third of the surface of the ovum; in seven hours, half the surface (Figure 32, 3); and within nine hours, three quarters (Figure 32, 4). After that, as in the rudiment, the division takes place in the thin animal and vegetative layers, the separation of the embryo begins, which at the beginning is not distinguished on the surface of the ovum. Shortly after formation of the embryo, the spinal fissure becomes noticeable (Figure 32, 6) . The shafts ascend over the surface and move nearer to each other, at the same time the very delicate vertebral cord can be distinguished between them. At nineteen hours after oviposition the spinal shafts are high and the fissures between them become deeper (Figure 32, 7 and 8). By the end of the first day the back is closed and separation of the primary vertebrae begins (Figure 32, 9 and 10), and the spinal cord becomes noticeably thicker. The head at this time is equal in length to the trunk, rounded projections appear, formed by the cerebral vesicles. An examination from the side shows formation of the transparent projection of the eye in the middle of the cerebral vesicle. Somewhat later the ear rudiment is seen; the eyes at this time are beginning to bulge and the first rudiments of abdominal layers are distinguished. At the end of the second day the embryo becomes pear-shaped (Figure 32, 13); its ventral part consists of two divisions: the anterior is rounded and the posterior is in the form of a curved tube (Figure 32, 14 and 15) . Then the embryo begins to straighten (Figure 32, 16) . The separation from the yolk in different fish takes place differently. In those cases where the mass of yolk is not large, the abdominal layers envelop it; in fish with voluminous yolk matter, the embryo from which the yolk sac hangs disconnects (for example in selachians and batrachidae) . In those cases, where the yolk mass is not large, the abdominal layers envelop it; while in fish with voluminous yolk mass, an unfastening of the embryo takes place and the yolk sac is attached to it (for example in selachians and batrachidae) .
The branchial protuberances are expressed less clearly than in amphibiae, but they are divided also by four fissures Because these five pairs of arches are separated, the anterior pair provides the beginning of the lower jaw and hypoglossal bone, and the other four become the branchial arches. On the surface of the latter the branchial platelets develop, situated in two rows.
The development of the vascular system of the fish embryo can be very easily observed due to its transparency. The rudiment of the heart at first is very similar to the cardiac canal of birds and all other vertebrates. Its two branches are united, and a curved canal is formed to the right from which the vascular arches are subsequently separated. They meet above, as usual in two-root aortae. In the posterior end of the heart two venous branches flow. The rudiments of anterior extremities have the form of small triangular elevations (Figure 32, 18).
In the moment of hatching the length of the tail constitutes about two-fifths of the total length. The eyes are pigmented, but the iris membranes have no metallic brightness, which appears in the second or third day after hatching. The ear is large and transparent, with the auditory drums visible in it. Parts of the brain are clearly differentiated, especially the cerebellum, the cavity of the middle and intermediate brain. The structure of the vascular system one day after hatching is illustrated in Figure 32, 20. The auricle of the heart is curved to the left, formed by the accretion of the two venous branches. The first vascular arch goes to the eye and branches in the slit of the iris membrane, while its other branch is the vertebral artery. The aorta develops intravertebral branches, passing into branches. The caudal vein passes through the kidneys, forming ramifications in them. The posterior vertebral veins (Baer equated the right posterior vertebral vein to the posterior hollow vein) flow together with anterior veins (from brain, ear, and occipital region in a transverse branch) ; both transverse branches run around the yolk sac and flow into the auricle.
In a special work on the development of fish of 1835, Baer described in detail the changes of the arterial and venous systems of Cyprinus bl-iooa in the days after hatching, and carried out comparisons with cyclostomes and skates (adults and embryos) , and also with the embryos of birds and mammals (pp. 24-31). In a section about UBER ENTWICKLUNGSGESCHICHT (II, 11 bb) , also in detail, he reviewed the transformation of cerebral vesicles of the embryo into the part of the definitive brain as in Cyprinus blioca, and as also in other carps (Cyprinui erythrophthalmus) and cartilaginous fishes.
Baer gave limited information on the digestive system. The oral orifice opens at the end of the first day after hatching. The intestinal canal up to the fifth day is very wide, completely direct and easily distinguished from the yolk sac, which is diminished. The mesentery appears later on (at the beginning, the intestine is adjacent to the spinal column); however, its development undoubtedly takes place exactly as in birds and mammals.
More briefly, Baer mentioned the development of the excretory system, noticing that kidneys of fish do not undergo that transformation which is characteristic of the higher vertebrates. In connection with this, the corresponding reconstruction of the circulatory system does not take place.
Later Baer gave a fluent description of the development of the paired extremities and unpaired fins. He noted that at the beginning, the unpaired fins in all fish, regardless of the future structure of fins with dense edgings, are situated from the back through the tail to the ventral side. Then the fringes are broken into as many parts as there are separated unpaired fins in the given fish. Between these regions the fringes disappear, and in the remaining fins cartilaginous or bony arms develop.
The development of the swimming sac was given special detailed attention by Baer in separate works (see footnote 8) . The posterior swimming vesicle of carp fish, according to Baer, is analogous to the underdeveloped lung because, like the latter, it arises from the protruding anterior region of the digestive canal . In hatching embryos there are no traces of the swimming vesicle and only near the end of the first day in the material, condensed in nitric acid can two protrusions be seen, one from the back and the other from the ventral side, each of them about 0.1 inch in size (Figure 32, 24) . On the second day the dorsal protrusion acquires a finger- like form and is elongated backwards (Figure 32, 25) Later it becomes longer, the ventral becomes rounded and its internal cavity is situated on a lobule (Figure 32, 27) . The first protrusion represents the rudiment of the swimming vesicle, and the second the rudiment of the liver. By the fourth to fifth day the swimming vesicle consists of two parts, an elongated sac and a hollow stalk similar to the branchus of the simple lung. The vesicle, when seen under the microscope or in live fish, still does not contain air. After the fifth day the swimming vesicle fills with air and thereby becomes significantly larger and distinguishable by the naked eye.
The anterior swimming vesicle in adults is connected with the posterior one and with the auditory organ. It is formed later than the anterior, after four weeks, when the body of the fish is already opaque and its formation is thus very difficult to follow. Nevertheless, on the basis of his direct observations Baer presented an astute discussion of development of the anterior swimming vesicle with the auditory apparatus. The last question is discussed in a special addition to the above-mentioned work of 1835 on fish development, in which he classified the swimming vesicles of the different fish in connection with the history of development of those organs.
Two earlier works, especially illustrating the study of branchial slits and branchial vessels in different vertebrates, serve as an addition to the third part of ttBER ENTWICKLUNGSGESCHICHTE. They represent a continued essay of the comparative embryology of vertebrates. 21 The first of these works he began with "Not long ago my dear friend Dr. Rathke (109) wrote me. Finally I found in the human embryo hints of gills, in particular in one six or seven-week-old aborted embryo. From each side there were two gills, the anterior large and the posterior significantly smaller. They were clearly seen, because between them there were slits penetrating up to the pharynx; thus there was no doubt of their existence. This information reminded me of the investigations which I had carried out the previous winter on human embryos. In the smallest of them, I did not discover branchial slits, which were also absent in the embryos of other vertebrates, although I frequently saw them in birds, frogs and snakes. In those human embryos I investigated, I saw them more clearly in that five weeks old embryo, than in the embryos which I knew to be six weeks old" (p. 556).
21. Baer, "Uber die Kiemen und Kiemengefasse in den Embryonen der Wirbelthiere," ARCH. FUR ANAT . U. PHYSIOL. (1827) , pp. 556-568? "Uber die Kiemenspalten des Saugethier-Embryonen," IBID. (1828), pp. 143-148.
In the first specimen Baer saw three pairs of branchial slits; the posterior pair was significantly shorter than the others. The branchial slits were especially well seen from inside the pharyngeal cavity. Based on comparison of available data Baer concluded that in humans and maybe in all land vertebrates, there are primarily four pairs of branchial slits, but they appear and disappear at separate times. In his work illustrating the branchial slits of vertebrates, Baer referred to investigations of Huschke,22 who discovered in bird embryos that a vascular arch passes in every branchial arch. This vascular arch begins from a general stem and comes out from the heart; it then flows into the aorta which consists of two roots. Thus every root of the aorta receives the vascular arch of its side. Huschke, however, did not see all the vascular branchial arches of the embryo. Baer himself clearly saw in the three-week-old embryo of a dog four arches filled with vascular blood, and he assumed that there was also a fifth pair of very delicate arches not filled with blood. He also distinguished four pairs of vascular arches in chickens.
Baer stated with certainty that the transformation of the vascular system of mammals and birds is very similar because the four pairs of vascular arches, which he saw in the dog embryo, had great similarity to the four pairs of arches of the bird embryos in the first half of the fourth day after hatching. The comparison of the vascular system of adult lizards and snakes, on one hand, and of embryos of birds before hatching, on the other hand, shows that in both the aorta begins with two roots. But only in birds is this condition transitory (kept only to hatching) , whereas in reptiles it is permanent. Baer found an interesting comparison in the embryo of a lizard which had five functioning branchial vascular arches. In LACERTA AG I LIS a similar condition is found before oviposition (110) .
22. E. Huschke, "Uber die Kiemenbogen im Vogelembryo, " ISIS (1828), pp. 160-164.
In the embryo of a lizard, according to Baer, blood circulation can sometimes be observed under a microscope, and he confirmed the presence of all mentioned vessels. From his data Baer concluded that in all embryos of vertebrates developing outside water there are five pairs of vascular branchial arches. In addition, they are present at the same time in lower forms, but in higher organized forms they appear and disappear in a known sequence. Amphibian larvae have four pairs of vascular arches, which remain for a longer time than in higher animals. Baer considered it necessary to clarify whether there was a fifth (outermost) arch in an earlier period under the formation of the lower jaw. The means of formation and the situation of vascular arches in amphibia are the same as in birds and mammals, but the distance between the anterior branchial slit and oral orifice of amphibia is more significant. In bony fish, throughout life four vascular arches remain in the gills. In plagiostomes there are five pairs of vascular branchial arches, and in cyclostomes more, but the cyclostomes, Baer noticed, generally strongly deviate in structure from other vertebrates. The last note is interesting, because in it, on the basis of comparative embryology, Baer presented a well-founded discussion of the systematic situation of cyclostomes. The separate situation of cyclostomes was recognized later, and they were distinguished from the class of fish in the dependent class of vertebrates.
Figure 32. Baer's illustration from "Investigations on the development of fish" (Caption on the following page) .
3 - The shape of the ovum from the side; embryo covers half of the yolk sphere. 4 â€” The same; embryo covers three-quarters of the yolk sphere. 6 â€” The embryo with wide spinal fissure, the shape from above. 7 and 8 â€” The embryo with well-defined ascending spinal shafts, shape from the side and from above. 9 and 10 â€” The embryo with closed back in the beginning of the formation of vertebrae, lateral and upper view. 13 â€” Pearshaped embryo with closed spine at the beginning of vertebrae formation, lateral and upper views. 14 and 15 â€” Twisted embryo, lateral and upper view. 16 â€” The beginning of straightening of the embryo, 18 â€” The embryo before hatching. 20 â€” Second day after hatching. 23-27 â€” The anterior part of the digestive tract, the first to fourth day after hatching.
Cite this page: Hill, M.A. (2021, April 15) Embryology Book - Russian Embryology (1750 - 1850) 21. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Russian_Embryology_(1750_-_1850)_21
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