Book - Developmental Anatomy 1924

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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg
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Developmental Anatomy - A Text-Book And Laboratory Manual Of Embryology


Leslie Brainerd Arey (1891-1988)

Professor Of Anatomy At The Northwestern University Medical School, Chicago.

With 419 Illustrations Many In Color

Philadelphia And London, W. B. Saunders Company


Copyright., 1924, by W. B. Saunders Company, Made in US.A .


This book has been prepared for the use of medical students and others whose interests center primarily on man and mammals. The emphasizing of structural rather than functional aspects of Embryology is reflected in the title; such presentation is consistent both with the practical demands of modern courses and with the meagre information existant as to the physiological factors in developmcnt.

The volume contains three sections. In the first part the early stages are treated comparatively and the full course of prenatal and postnatal development is outlined. The second section traces the origin and differentiation of the human organ-systems, grouped according to their germlayer derivations. The third division comprises a laboratory manual for the study of chick and pig embryos.

Many illustrations are from the earlier Prentiss-Arey text and discontinuous fragments of description have likewise been retained. Yet, in plan and content the work is essentially new. It is hoped that the developmental story has been told in an orderly and clear, but concise fashion, and that it records accurately the present state of the subject.

L. B. Arey.

Chicago, ill., September, 1924.

Leslie Brainerd Arey (1891-1988)



Entodermal Derivatives

Mesodermal Derivatives

  • Chapter VII. - The Mesenteries and Coelom
    • The Mesenteries
    • The Primitive Mesentery
    • Differentiation of the Dorsal Mesentery
    • Differentiation of the Ventral Mesentery
    • The Ccelom
    • The Primitive Coelom
    • The Septum TransveRsum
    • The Pleuro-pcricardial and Pleuro-peritoncal Membranes
    • The Pericardium and Diaphragm
  • Chapter VIII. - The Urogenital System
    • The Urinary Organs
    • The Pronephros
    • The Mesonephros
    • The Metanephros
    • Differentiation of the Cloaca
    • The Genital Organs
    • Indifferent Stage
    • Internal Sexual Transformations
    • The External Genitalia
    • Homologies of Internal and External Genitalia
  • Chapter IX. - The Vascular System
    • Origin of the Blood Vessels and Blood Cells
    • Hemopoiesis
    • Development of the Heart
    • The Primitive Vascular System
    • Development of the Arteries
    • Development of the Veins
    • Fetal Circulation and the Changes at Birth
    • The Lymphatic System
  • Chapter XVI. - The Study of Chick Embryos
    • The Unincubated Ovum and Embryos of the First Day
    • Embryo of Five Segments (Twenty-Three Hours)
    • Embryo of Seven Segments (Twenty-five Hours)
    • Embryo of Seventeen Segments (Thirty-eight Hours)
    • Embryo of Twenty-seven Segments (Two Days)
    • Embryos of Three to Four Days
  • Chapter XVII. - The Study of Pig Embryos
    • The Anatomy of a Six Mm. Pig Embryo
    • The Anatomy of Ten to Twelve Mm, Pig Embryos
    • The Anatomy of an Eighteen Mm. Pig Embryo
    • The Anatomy of a Thirty-five Mm. Pig Embryo
    • Methods for the Dissection of Pig Embryos

Part I. General Development


The Scope of Embryology

Developmental anatomy, or embryology, traces the formative history of the individual from the origin of the germ cells to the adult condition. Although the most striking changes in human development occur while the young (called an embryo or fetus) is still inside its mother's womb, yet development by no means ceases at birth. Birth is a mere incident which occurs when the new individual is sufficiently advanced to allow its transference from a protected riterine environment to one in the external world. Some vertebrates, like fishes and amphibia, are capable of an active and independent existence at very immature stages; these free-living larvae, as they are termed, then gradually progress to adults. The human newborn, although far more complete anatomically, is still utterly dependent for food and care: many years of infancy and childhood must elapse before it becomes self-maintaining in human society. During all this period, postnatal development continues. Birth, itself, initiates anatomical changes of profound influence on the body. Throughout the entire growth period, with its uneven but steadily slowing growth rate, come the completion of some organs and a gradual remoulding of the shape of the body and its parts. Only at the age of twenty-five are these progressive changes complete.

All vertebrate, or backboned, animals are organized upon a common anatomical plan, and even many of their structural details are comparable, though superficially disguised. Similarly, their fundamental mode of development is essentially identical. The minor variations that do occur are caused by such secondary modifying factors as the crowding yolk content of the egg or adaptations to development inside or outside the mother's body. While the comparative viewpoint is indispensable for gaining a broad understanding of embryology, it has been of especial importance in supplying missing parts of the human developmental story and in interpreting many perplexing conditions. For, the earliest human embryos known are about two weeks old and have the three primary germ layers already formed. Even invertebrate material is highly useful for demonstrating such early stages as maturation, fertilization, cleavage, and the formation of blastula and gastrula.

The Value of Embryology

A general conception of how man and other animals develop from a single cell by orderly and logical processes should share in the cultural background of every educated mind. To the medical student, embryology is of primary importance because it affords a comprehensive understanding of the intricacies and variations of human anatomy, and thus is essential to sound surgical training. It also explains many anomalies and 'monstrous - conditions, and the origin of certain tumors and other pathological changes in the tissues. Obstetrics is essentially applied embryology. From the theoretical side, it is the key with which we may unlock the secrets of heredity, the determination of sex, and, in part, of organic evolution.


Fig. 1. - Human sperm cell containing a miniature organism, according to Hartsoeker (1694).

The science of modern embryology is comparatively new, originating with the use of the compound microscope and advancing with the improvement of microscopical technique. Aristotle (384-322 B. c.), however, centuries before the introduction of magnifying lenses had followed the general development of the chick, day by day. The popular belief that slime and decaying matter is capable of giving rise to living animals, as also asserted by Aristotle, was disproved by Redi (1668).

A few years after Harvey and Malpighi had published their fundamental studies on the chick embryo, Leeuwenhoek reported the discovery of the human spermatozoon by Ham in 1677 - At this period, it was believed either that fully formed animals existed in miniature in the egg, needing only the stimulus of the spermatozoon to initiate development, or that similarly preformed bodies, male and female, constituted the spermatozoa and that these merely enlarged within the ovum. According to this doctrine of preformation, all future generations were likewise encased, one inside the sex cells of the other, and serious computations were made as to the probable number of progeny (200 millon) thus present in the ovary of Mother Eve, at the exhaustion of which the human race would end! Dalenpatius (1699) and others even believed they had observed a minute human form in the spermatozoon (Fig. 1).

The preformation theory was strongly combated by Wolff (1759), who saw that the organs of the early chick embryo were differentiated gradually from unspecialized living substance. This theory, known as epigenesis, was proved correct when von Baer discovered the mammalian ovum in 1827, and later demonstrated the germ-layer composition of all embryos.

About twenty years after Schleiden and Schwann (1839) had shown the cell to be the structural unit of the organism, the ovum and spermatozoon were recognized as true cells. O. Hertwig, in was the first to observe and appreciate the events of fertilization. Henceforth, all multicellular organisms were believed to develop each from a single fertilized ovum. This conception is expressed in the famous aphorism: ornne vivum ex ovo .

Modern Embryology

As an organized and definite science, began with Balfour (1874), who reviewed, digested, and made accessible the earlier scattered facts. Throughout this period, the experimental method of investigation has been used increasingly; without it many structural and physiological aspects of development would remain unsolved.

General Features Of Development

A multicellular embryo results from the division of the fertilized ovum to form daughter cells. These are at first quite similar in structure, and, if separated, in some animals each may become a complete embryo (sea urchin; certain vertebrates). In general, the development of an embryo depends: (i) upon the multiplication of its cells by division; (2) upon the growth in size of the individual cells; (3) upon changes in their form and structure.

Cell Division - All cells arise from pre-existing cells by division. There are two methods of cell division - amitosis and mitosis.

Amitosis - Cells may divide directly by the simple fission of their nuclei and cytoplasm. This rather infrequent process is called amitosis. Amitosis is said by many to occur only in specialized or moribund cells. It is the type of cell division demonstrable in the epithelium of the bladder.

Fig. 2. Diagrams of the phases of mitosis (Schafer).


In the reproduction of typically active somatic cells and in all germ cells, complicated changes take place in the nucleus. These changes give rise to thread-like structures, hence the process is termed mitosis (thread) in distinction to amitosis (no thread). Mitosis is divided for convenience into four phases (Fig. 2) :


  1. The centrosome divides and the two minute bodies resulting from the division move apart, ultimately occupying positions at opposite poles of the nucleus (I-III).
  2. Astral rays appear in the cytoplasm about each centriole. They radiate from it, and the threads of the central or achromatic spindle are formed between the two asters, thus constituting the amphiaster (II).
  3. The nuclear membrane and nucleolus disappear, the karyoplasm and cytoplasm becoming confluent.
  4. During the above changes the chromatic network of the resting nucleus resolves itself into a skein, or spireme, which soon shortens and breaks up into distinct, heavily-staining bodies, the chromosomes (II, III). The definite number of chromosomes is always found in the cells of a given species, the chromosomes may be block-shaped, rod- shaped, or bent in the form of a II or V.
  5. The chromosomes arrange themselves in the equatorial plane of the central spindle (IV). If U- or V-shaped, the angle of each is directed toward a common center. The amphiaster and the chromosomes together constitute a mitotic figure, and at the end of the prophase this is called a monaster.

Metaphase - The longitudinal splitting of the chromosomes into exactly similar halves constitutes the metaphasc (IV). The aim of mitosis is thus accomplished, an accurate division of the chromatin between the nuclei of the daughter cells.

Anaphase - The two groups of daughter chromosomes separate and move up along the central spindle fibers, each toward one of the two asters. Hence this is called the diaster stage (V, VI). Each centriole may divide in preparation for the next diviSion of the daughter cells.

Telophase - i. The daughter chromosomes resolve themselves into a reticulum and daughter nuclei are formed (Vu, VuI).

2. The cytoplasm divides in a plane perpendicular to the axis of the mitotic spindle (VIII). Two complete daughter cells have thus arisen from the mother cell.

The number of chromosomes is constant in the cells of a given species. The smallest assortment, two, occurs in Ascaris megalocephala univaleus, a round worm parasitic in the intestine of the horse. The largest number known is found in the brine shrimp, Artemia, where 168 have been counted. The chromosome enumeration for the human cell has been variously stated but the results of Winiwarter (1912), Grosser (1921), and Painter (1923) now agree on a relatively high number, which Painter establishes as 48 for whites and negroes of both sexes.

The Germ Layers

Fig. 3. Mesenchyme from a chick embryo (Prentiss). X 495.

The first changes in the form and arrangement of the cells establish three definite plates, the primary germ layers, which are termed from their positions the ectoderm (outer skin), mesoderm (middle skin) and entoderm (inner skin) (Fig. 4). Since the ectoderm covers the body, it is primarily protective in function, but it also gives origin to the nervous system, through which sensations are received from the outer world. The entoderm, on the other hand, lines the digestive canal and is from the first nutritive. The mesoderm, lying between the other two layers, naturally performs the functions of circulation, of muscular movement, and of excretion; it also gives rise to the skeletal structures which support the body. While all three germ layers form definite sheets of cells known as epithelia, the mesoderm takes also the form of a diffuse meshwork of cells, the mesenchyme (Fig. 3).

The cells of these layers are modified in turn to form tissues, such as muscle and nerve, of which the various organs are composed. The organs, associated as organ systenis, constitute the organism, or body, that of adult man containing 2 5 million million red blood cells alone. In every organ, one tissue, like the epithelial lining of the stomach, is predominately important; the others are accessory.

Histogenesis. -The cells of the germ layers are at first alike in structure. Thus, the evagination which forms the primordial arm is composed of a single layer of similar ectodermal cells, surrounding a central mass of diffuse mesenchyme (Fig. 406). Gradually the ectodermal cells multiply, change their form and structure, and give rise to the layers of the epidermis. By more profound structural changes the mesenchymal cells ahso are transformed into the elements of connective tissue, tendon, cartilage, bone, and muscle - aggregations of modified cells which are termed tissues. The development of modified tissue cells from the undifferentiated cells of the germ layers is known as histogenesis.

During histogenesis, the structure and form of each tissue cell are adapted to the ])erformance of some special function or functions. Cells which have once taken on the structure and functions of a given tissue cannot give rise to cells of any other type. In tissues like the epidermis, certain cells retain their ])rimitive embryonic characters throughout life, and, by continued cell division produce new layers of cells which are later specialized. In other tissues all of the cells are differentiated into the adult type, after which no new cells are formed: this takes place in the nervous elements of the central nervous system. Contrariwise, most tissue cells are undergoing retrogressive changes throughout life. In this way, the cells of certain organs like the thymus gland and mesonephros degenerate and largely disappear. The cells of the hairs and the surface layer of the epidermis become cornified and eventually are shed. Thus, normally, many tissue cells are continually being destroyed and replaced by new cells.

This series of changes - an embryonic (undifferentiated) stage; progressive functional s])ecialization ; gradual degeneration; death and removal - which tissue cells experience is designated by the term cytomorphosis.

Derivatives of the Germ Layers

The tissues of the adult are derived from the primary germ layers as follows:

Ectoderm Mesoderm Entoderm
I. Epidermis and derivatives.

Hair; nails; glands.

Lens of eye.

2. Epithelium of:

Organs of special sense. Cornea.

Mouth; enamel organ.

Oral glands; hypophysis.


Amnion; chorion.

3. Nervous tissue.


Chromaffin tissue.

4. Smooth muscle of; Iris.

Sweat glands.

A. Mesothelium.

1. Pericardium.

2. Pleura.

3. Peritoneum.

4. Urogenital epithelia.

5. Striated muscle.

B. Mesenchyme.

1 . Smooth muscle.

2. Notochord.

3. Connective tissue; cartilage; bone.

4. Blood; bone marrow.

5. Endothelium of blood vessels and lymphatics.

6. Lymphoid organs.

7. Suprarenal cortex.

Epithelium of:

I. Pharynx and derivatives. Auditory tube.





2. Respiratory tract.

Larynx; trachea.


3. Digestive tract.

Liver; pancreas.

Yolk sac; allantois.

4. Bladder (except trigone).

5. Urethra (except prostatic).

6. Prostate.

Primitive Segments - Metamerism

Fig. 4. Diagrammatic transverse section of a vertebrate embryo (Minot-Prentiss).

A prominent feature of vertebrate embryos are the primitive segments, or metameres (Fig. 59). These segments are homologous to the serial divisions of an adult earth-worm's body, divisions which, in the earth worm, are identical in structure, each containing a ganglion of the nerve cord, a muscle segment, or myotome, and pairs of blood vessels and nerves. In vertebrate embryos, the block like primitive segments lie next the neural tube and are known as mesodermal segments, or somites (Fig. 4). Each pair gives rise to a vertebra, to two myotomes, or muscle segments, and to paired vessels; each set of mesodermal segments is supplied by a pair of spinal nerves: consequently, the adult vertebrate body is segmented like that of the earth worm. As a worm grows by the formation of new segments at its tail-end, so the metameres of the vertebrate embryo begin to form in the head and are added tailward. There is this difference between the segments of the worm and the vertebrate embryo; the segmentation of the worm is complete, while that of the vertebrate is incomplete ventrally.

Somatopleure and Splanchnopleure

In early embryos the mesoderm splits into two layers, the somatic (dorsal) and splanchnic (ventral) mesoderm (Fig. 4). The ectoderm and somatic mesoderm constitute the body wall, which is termed the somatopleure. In the same way, the entoderm and splanchnic mesoderm combine as the splanchnopleure; it forms the mesenteries and the walls of the gut, heart, and lungs.


The space between the somatopleure and splanchnopleure is the coelom, or body cavity. At the first splitting of the mesoderm, isolated clefts are produced. These unite on each side and eventually form one cavity - the coelom. With the extension of the mesoderm, the coelom surrounds the heart and gut ventrally (Fig. 4). Later, it is subdivided into the pericardial cavity about the heart, the pleural cavity of the thorax, and the peritoneal cavity of the abdominal region. The epithelia lining the several body cavities are termed mesothelia.

The Nephrotome

The bridge of cells connecting the primitive segment with the unsegmented somatic and splanchnic layers is the nephrotome, or intermediate cell mass (Fig. 4). From these will develop the urogenital glands and ducts.

Developmental Processes

The developing embryo exhibits a progressively complex structure, the various steps in the production of which occur in orderly sequence. There may be recognized in development a number of component mechanical processes which are used repeatedly by the embryo. The general and fundamental process conditioning ilifferentiation is cell multiplication, and the subsequent growth of the daughter cells. The more important of the specific developmental processes are the following: ( 1) cell migration; (2) localized growth, resulting in eidargements and constrictions; (3) cell aggregation, forming (a) cords, (b) sheets, [c] masses; (4) delamination, that is, the splitting of single sheets into separate layers; (5) folds, including circumscribed folds which produce (a) evaginations, or out-pocketings, (b) invaginations, or in-pocketings.

The production of folds, including evaginations and invaginations, due to unequal rapidity of growth, is the chief factor in moulding the organs and hence the general form of the embryo.

Fundamental Conceptions

The Anlage

This German word, which lacks an entirely satisfactory English equivalent, is a term applied to the first discernible cell, or aggregation of cells, which is destined to form any distinct jiart or organ of the embryo. In the broad sense, the fertilized ovum is the anlage of the entire adult organism; furthermore, in the early cleavage stages of certain embryos it is possible to recognize single cells or cell groups from which definite structures will indubitably arise. The term anlage, however, is more commonly applied to the primordia that differentiate from the various germ layers. Thus the epithelial thickening over the optic vesicle is the anlage of the lens.

The Law of Genetic Restriction

As development advances, there is a constantly increasing restriction in the kind of differentiation open to the various parts. Each emerging tissue or organ is more rigidly bound to its particular type of differentiation than was the generalized material from which it came. A line of specialization, once begun, cannot be abandoned for another type. The parent tissue, likewise, is limited by losing the capacity for duplicating anlages already formed. Thus, the primitive thyroid can never become anything but a thyroid, whereas the gut that formed it also buds off, at other levels, the lungs, liver, and pancreas. Yet if the embryonic thyroid were destroyed, the pharynx would never replace it. From mesenchyme arise connective tissue, blood cells, and smooth muscle; when once the specialization begins, there can be no retraction or transformation to another type.

Continuity of the Germ Plasm

According to this important conception of Weismann, the body-protoplasm, or soma, and the reproductive-protoplasm differ fundamentally. The germinal material is a legacy that has existed since the beginning of life, from which representative portions are passed on intact from one generation to the next. Around this germ plasm there develops in each successive generation a shortlived body, or soma, which serves as a vehicle for insuring its transmission and perpetuation. The reason, therefore, why offspring resembles parent is because each develops from portions of the same stuff.

The Law of Biogenesis

Of great theoretical interest is the fact, constantly observed in studying, embryos, that the individual in its development repeats hastily and incompletely the evolutionary history of its own species. This law of recapitulation was first stated clearly by Muller in 1863, and was termed by Haeckel the law of biogenesis. In accordance with it, the fertilized ovum is compared to a unicellular organism like the Ameba: the blastula is supposed to represent an adult Volvox type; the gastrula, a simple sponge; the segmented embryo, a worm-like stage ; and the embryo with gill slits may be regarded as a fishlike stage. Moreover, the blood of the human embryo in development passes through stages in which its corpuscles resemble in structure those of the fish and reptile; the heart is at first tubular, like that of the fish, and the arrangement of blood vessels is equally primitive; the kidney of the embryo is like that of the amphibian, as are also the genital ducts. Many other examples of this law may readily be observed.

Some apparently useless structures appear during development, perfunctorily reminiscent of ancestral conditions; certain other parts, of use to the embryo alone, are later replaced by better-adapted, permanent organs. Representatives of either type may eventually disappear or they may persist throughout life as rudimentary organs; more than a hundred of the latter have been listed for man. Still other ancestral organs abandon their provisional embryonic function, yet are retained in the adult and utilized for new purposes.

The Vertebrate Groups

There are five vertebrate classes, the higher characterized by the possession of an enveloping embryonic membrane, called the amnion, and another embryonic appendage, known as the allantois:

(A) Anamniota (amnion absent).

1. Fishes - lamprey; sturgeon; shark; bony fishes; lung fish.
2. Amphibia - salamander; frog; toad; etc.

(B) Amniota (amnion present).

3. Reptiles - lizard; crocodile; snake; turtle.
4. Birds.
5. Mammals. Characterized by hair and mammary glands.
(a) Monotremes - duck-bill; primitive mammals that have a cloaca and lay eggs with shells.
(b) Marsupials - oppossum; kangaroo; etc. The young are born immature and are sheltered in an integumentary pouch.
(c) Placentalia. All other mammals whose young are nourished in the uterus by a placenta.
Ungulate series. Hoofed mammals (cattle; sheep; pig; deer; horse; etc.).
Unguiculate series. Clawed mammals (mole; bat; rat; rabbit; cat; dog; etc.). The highest order is the Primates (lemur; monkey; ape, man).

The Vertebrate Body Plan

All vertebrate animals are constructed in accordance with a common body plan. The distinctive characteristics of the vertebrate type include: .

  1. A tubular central nervous system, dorsally placed (Fig. 4).
  2. A notochord, between the neural tube and gut (Fig. 4). This cellular |3rimitive-axis is replaced, wholly or in part, by the vertebral column.
  3. A pharynx, which develops paired pouches and clefts that determine the positions of important nerves, muscles and blood vessels (Fig. 91).
  4. The position of the mouth. Unlike the condition in many invertebrates, it is not surrounded by a circumoral ring of nervous tissue which connects a dorsal - brain - with a ventral chain of ganglia.
  5. The limbs, Two pairs, with an internal skeleton (Fig. 227).
  6. A coelom, which is divided into a dorsal, segmental part (cavities of the somites), and a ventral, unsegmented part, partitioned by the septum transversum (diaphragm) into thoracic and abdominal portions (Fig. 4).

Titles for Collateral Reading and Reference

Broman. Normale und abnorme Entwicklung des Menschen.

Corning. Entwicklungsgeschichte des Menschen.

Duval. Atlas D - Embryologie.

Hertwig. Handbuch der Entwicklungslehre der Wirbeltiere.

Keibel and Mall. Human Embryology.

Kellicott. A Textbook of General Embryology.

Kollmann. Handatlas der Entwicklungsgeschichte des Menschen.

Lillie. The Development of the Chick.

Minot. A Laboratory Text-book of Embryology.

McMurrich. The Development of the Human Body.

Patten. The Early Embryology of the Chick.

Wilson. The Cell in Development and Inheritance.

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg


Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

Cite this page: Hill, M.A. (2024, February 28) Embryology Book - Developmental Anatomy 1924. Retrieved from

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