Book - Comparative Embryology of the Vertebrates

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

   Comparative Vertebrate Embryology 1953: 1. The Period of Preparation | 2. The Period of Fertilization | 3. The Development of Primitive Embryonic Form | 4. Histogenesis and Morphogenesis of the Organ Systems | 5. The Care of the Developing Embryo | Figures
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This 1953 textbook by Nelsen compares development in a number of different species.

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

Comparative Embryology of the Vertebrates


Olin E. Nelsen, M.A., Ph.D.

Department of Zoology

University of Pennsylvania

With 2057 Drawings and Photographs, Grouped as 380 Illustrations


A study of the comparative embryology of a group of animals such as the vertebrates when followed to its logical conclusion leads to a consideration of the comparative anatomy of the group. Students claim, and justly so, that they learn best through the association of events, things, and concepts. As applied to the study of vertebrate embryology and anatomy, the principle of learning by association means this: observations upon the adult anatomy of the various organ-systems of a particular vertebrate species when correlated with the earlier stages of embryonic development of these systems lead to a more ready perception and understanding of structural principles and relationships involved. Furthermore, when the developmental anatomy and the adult anatomy of any one species is associated with similar phenomena in other species of the vertebrate group it naturally produces a clearer understanding of the development and morphology of the group as a whole. This broad, comprehensive approach is a fundamental one and it is a requirement for the furtherance of research in vertebrate biology, whether it be on the level of cellular chemistry or the physiology of organ-systems.

An endeavor to satisfy a demand for a comprehensive approach to vertebrate development by an extension of the descriptions of the earlier phases of the embryology of several representative vertebrate species into their final stages of development, and hence into the realm of comparative anatomy, is the main purpose of this book. This goal is the greatest defense which the author can give for his effort to assemble the material and data contained herein.

On the other hand, though the book correlates comparative vertebrate embryology with comparative vertebrate anatomy, its arrangement is such that the fundamental features of comparative vertebrate embryology readily can be divorced from the intricate phases of comparative anatomy. For example, Chaps. 1-11, 20, 21, and 22 are devoted to a consideration of basic embryological principles whereas Chaps. 12-20 treat particularly the relationships of comparative embryology and comparative anatomy. A proper selection of descriptive material in Chaps. 12-20 (which may be done readily by a survey of the outline heading each chapter) added to the basic embryological data affords a basis for a thorough course in comparative vertebrate embryology.

The selection of material suggested in the previous paragraph brings forth another motive for writing this text. It has been the author's habit — one common to many other teachers — never to give a course in exactly the same way two years in succession. This procedure enlivens a course and keeps successive groups of students out of the rut of looking forward to the same identical lectures and laboratory approach year after year. As a result, in reality this book is a compilation of the different aspects of embryology presented by the author over a period of years to classes in comparative vertebrate embryology. Consequently, by the use of certain chapters and the outlines at the headings of each chapter, various facets of embryology may be presented one year while other aspects are selected the following year, and so on. Moreover, a selective procedure allows the book to be used readily for short courses in embryology as well as longer courses. For example. Chaps. 3, 5-1 1 , and 20-22 may serve as the basis for a short course in vertebrate embryology.

Another feature of the text is the presentation of many illustrations well prepared. Illustrations are an important adjunct to the teaching of embryology. This is true especially where the teacher is burdened with the teaching of other courses and thus is handicapped by lack of time to make adequate blackboard drawings and illustrations of laboratory and lecture material. In Chaps. 3, 5-1 1, and 20-22, one finds illustrative material adequate to enable the student to gain an appreciation of the fundamental features of vertebrate development. Thus, this part of the book may be used extensively as a laboratory guide to the fundamental principles involved in vertebrate development.

A final aspect of the text may be mentioned, namely, the references given at the close of the chapters. References to literature are important especially in courses of embryology where small groups of students are assembled. Under these conditions the teacher often prefers to give the course on a seminar basis. With this approach, references are most valuable in the assignment of special reports and student lectures which the student later gives to the class as a whole.


The author expresses his great obligation to and appreciation for the superior artistic abilities, continual patience, and conscientious effort of Elisabeth R. Swain who executed the difficult task of preparing — with certain exceptions — the illustrations for this text. He also wishes to express his sincere thanks to Edna R. White and Julia A. Lloyd who contributed illustrations. These three artists were most exact in carrying out the author's instructions for illustrations, and also in transforming his preliminary sketches into finished drawings.

The author is indebted greatly to Wistar Institute of Anatomy and Biology, Philadelphia, for permission to redraw various illustrations from the journals published by the Wistar Institute. Appreciation similarly goes to the Carnegie Institution of Washington; The Marine Biological Laboratory, Woods Hole, Mass.; Williams and Wilkins Co., Baltimore; University of Chicago Press, Chicago; Yale University Press, New Haven; Academic Press, Inc., New York; Museum of Comparative Zoology at Harvard College; Oxford University Press, Inc., New York; Ginn and Co., Boston; W. B. Saunders Co., Philadelphia; McGraw-Hill Book Co., Inc., New York; Henry Holt and Co., Inc., New York; W. W. Norton and Co., Inc., New York; John Wiley and Sons, Inc., New York; J. B. Lippincott Co., Philadelphia; The Macmillan Co., New York and London; Knopf, Inc., New York; Appleton-Century Co., Inc., New York; Sidgewick and Jackson, Ltd., London; Cambridge University Press, England; and Columbia University Press, New York.

To his colleagues in the Department of Zoology of the University of Pennsylvania the author owes a debt of appreciation for encouragement during the writing of the manuscript, especially to Dr. J. Percy Moore, Dr. D. H. Wenrich, and Dr. L. V. Heilbrunn. Acknowledgments and appreciation go to Mrs. Anna R. Whiting, also of the Department of Zoology, and to Dr. Miles D. McCarthy of the Harrison Department of Surgical Research of the University of Pennsylvania Medical School and the Department of Zoology, Pomona College, Claremont, California, who read much of the manuscript and offered valuable suggestions. Frances R. Houston, Librarian of the University of Pennsylvania Medical School, and Elizabeth D. Thorp, Librarian of the Botany-Zoological Library of the University of Pennsylvania, deserve sincere thanks for cooperative understanding and help in securing and placing many periodicals at the author's disposal. Various students contributed clerical efforts toward the completion of this work, especially Barbara Neely Gilford, Carolyn Kerr, and Louise Mertz. Their endeavors are appreciated greatly.

Any attempt of the author to acknowledge obUgations would be incomplete, indeed, without mention of the extreme readiness to serve and cooperate on the part of Dr. James B. Lackey, then Science Editor of The Blakiston Co. (presently Research Professor, School of Engineering, University of Florida), and also to Irene Claire Moore, then Assistant Manuscript Editor (presently Book Editor, United Lutheran Publication House, Philadelphia), and to W. T. Shoener, Production Manager.

Olin E. Nelsen

Autumn 1952

Philadelphia, Pa.


Part I - The Period of Preparation

Part II - The Period of Fertilization

Part III - The Development of Primitive Embryonic Form

Part IV - Histogenesis and Morphogenesis of the Organ Systems

Part V - The Care of the Developing Embryo

  • 22. Care and Nourishment of the Developing Young


I. Some Definitions Relative to Embryology

The word embryo has various shades of meaning. In general, it is applied to the rudimentary or initial state of anything while it remains in an undeveloped or primitive condition. As used in zoology, it designates in one sense the earlier stages of the development of an animal before the definitive or adult form of the species is assumed; or, in a second sense, it signifies the entire period of prenatal existence.

The word development not only is used to denote the various changes evident in prenatal emergence, but also it applies to postnatal changes as well. Moreover, in the development of a particular animal it may be extended beyond the period of structural and physiological maturity to the changes involved in eventual senescence.

The developing young of viviparous animals while undergoing the later stages of development within the uterus is spoken of as a fetus. This term is used also, on occasion, to designate the later stages of development of oviparous species. The phrase mammary fetus is applied to the young of marsupial mammals such as the opossum while it remains attached to the nipple within the marsupial pouch of the mother.

The term descriptive embryology is applied to the method of embryological study concerned with the direct observation and description of embryological development. Up to the latter part of the last century embryology was concerned mainly with the direct observation of the changes going on in the intact embryo. However, beginning in the 1880's Wilhelm Roux and others initiated the expeirmental approach in embryological study and the school of experimental or causal embryology was formed. In experimental embryology various parts of the developing embryo are removed, transplanted, parts are exchanged, or the environmental conditions are altered. The end sought by this method is an analysis of the respective roles played during development by different parts of the developing organism and by different environmental factors, in an endeavor to give a mechanical and functional explanation of development. One of the outstanding results of the experimental method applied to embryological study is the great body of evidence which points to the fact that in the vertebrate group one of the main processes in development (morphogenesis) is the induction of organs and organ-systems by so-called organizer cellular areas present in certain parts of the developing embryo. Organization of the developing body, in other words, is dependent upon a series of changes mediated by cellular groups known as organizers which appear at the correct time and locus in development.

It soon became apparent, however, that the terminology employed in experimental embryology was vague because it substituted indefinite terms such as “inductors” or “organizers” as an explanation of developmental events. The use of the word organizer means little unless one is able to describe the manner of operation of the physical and chemical substances which effect the results produced by the organizer. Consequently, embryologists with physiological and biochemical training are concerned now with the effort of determining the specific chemical factors concerned with the various processes and steps involved in development. This type of embryological study is called biochemical or chemical embryology. Chemical embryology is divisible into two main lines of attack, namely, an investigation of the chemistry of cells and cellular parts or cytochemistry, and a study of the chemistry of groups of cells or histochemistry.

II. Free-living Versus Sheltered Embryological Forms; Periods of Development

The independence of a free-living existence on the part of developing young is assupied at different stages of development depending upon the species involved. For example, in the case of the frog, the developing embryo becomes free-living at an early stage and it experiences a free-living larval existence for an extended period before its metamorphosis into the adult or definitive form of the frog. In the chick, the young undertakes a kind of free-living existence at the time of hatching or about a week after it has assumed the definitive body form. The human young, on the other hand, experiences an extensive period of fetal development for about five months in utero after it has achieved definitive body form. Moreover, it is most helpless and dependent even after birth.

Regardless of the time during its development when an animal species assumes a free-living, independent existence, it is apparent that the development of the individual as a whole may be divided into two general periods, viz., embryonic and post-embryonic periods. The embryonic period of development begins at fertilization of the egg and continues for a time after definitive body form is achieved. The end of the embryonic period may be regarded as the time of birth in viviparous forms, hatching in oviparous species, and the end of metamorphosis in free-living larval species. This is an arbitrary and, for some forms, quke comprehensive definition. Nevertheless, for comparative purposes this definition is suitable. The post-embryonic period begins at the termination of the embryonic phase of development and continues through sexual maturity into later life.

The embryonic period of development in all vertebrate species may be resolved into three distinct phases:

a. An early embryonic period which begins at the time when the egg starts to develop and w^ich reaches its culmination when the embryo has attained the state of primitive, generalized body form (see Chaps. 10 and 11, and fig. 255).

b. A period of transition then follows during which the structural condi tions prevalent in primitive body form are transformed into the morphology present in definitive body form. Definitive body form is reached when the embryo assumes a general resemblance to the adult form, of the species. The changes described in Chaps. 12-20 are concerned to a considerable extent with this phase of development. '

c. The late embryonic period. This phase of development comprises the changes which the embryo experiences for a time after it has achieved definitive body form. In the human embryo, it includes several months of fetal growth in the uterus, and in the chick it is of about a week’s duration continuing from day 14 of incubation to the time of hatching around day 20. In the frog it is a brief period during the close, and possibly shortly after, metamorphosis.

The period of transition may be regarded as the larval period of development. If so conceived, two types of larval forms exist, namely ( 1 ) free-living larval forms such as the frog tadpole in which the body structures are adapted to a free-living existence outside of protective embryonic structures, and (2) non-free-living larval forms in which the larval or transitional period is passed within the confines of covering egg membranes or within the protective tissues of the female or male parent. Free-living larval forms include Amphioxus, most fishes, and amphibia, while some fishes and all reptiles, birds, and mammals may be regarded as having a protected larval existence.

III. Summary of Developmental Phenomena Associated with the Life of an Individual Vertebrate Animal

A. Period of Preparation

During this period the parents are prepared for reproduction and the reproductive cells or gametes are elaborated.

B. Embryonic Development

1. Early embryonic period

This period begins with fertilization of the egg and ends with the development of primitive embryonic body form with its basic conditions of the various systems. The basic or group condition of a particular vertebrate organ-system is that stage of development of the system when it possesses structural features common to all embryos of the vertebrate group. When the common or primitive embryonic conditions of the various systems are present, a common, basic, primitive embryonic body form also is present. Hence, all vertebrate embryos tend to pass through a stage of development in which the shape and form of the developing body resembles that of all other vertebrate species at this stage of development. This stage of body formation is known as the primitive, embryonic body-form stage,

2. The larval period or period of transition

During this phase of development, the basic conditions of the organ-systems, which are present at the end of primitive body formation, are transformed into the structural conditions present in definitive body form. At the end of this period of development the general form of the organ-systems, and of the embryo as a whole, resembles the adult morphology of the species. Hence the term: ** definitive body form”

3. The late embryonic period

This part of development intervenes between the time when definitive body form is established and the episode of hatching or birth. In free-living larval species it comprises a brief period at the end of metamorphosis.

C. Post-embryonic Development

Post-embryonic development may be divided into the following periods:

1. Prepuberal period

During this time the organ-systems grow and enlarge, and the reproductive mechanisms mature.

2. Puberal period and the adult

The organism now is capable of reproduction, and in size, activity, and appearance is recognized as an adult.

3. Period of senescence and decline

The sexual activities lessen and the organ-systems of the body may very slowly undergo regressive changes.

IV. A Classification of the Vertebrates and Related Species

A. Characteristics of the Phylum Chordata

The vertebrates belong to the phylum Chordata. This phylum is characterized by three main features which appear in the early embryo, viz., (1) a dorsally situated nerve cord which in most instances is hollow or tube-like; (2) a dorsally placed notochordal or median skeletal axis located always immediately ventral to the nerve cord, and (3) a complicated anterior portion of the digestive tract known as the pharynx. The pharyngeal area of the digestive tract is composed of a series of paired skeletogenous arches known as the visceral or branchial arches, between which are found the branchial pouches and branchial furrows or grooves.

B. Major Divisions of the Phylum

The entire phylum Chordata may be divided into the lower chordates and the higher chordates.

Lower Chordata (Acraniata)

Subphylum: Hemichordata

These are small, soft-bodied animals living along the shores of the sea, and in some instances to considerable depths into the sea. Dorsal and ventral nerve cords are present in the class Enteropneusta or the “tongue worms.” The notochord is a short structure confined to the anterior end. Gill slits are present.

Subphylum: Urochordata (Tunicata)

These forms inhabit the sea from the polar regions to the equator, and from the shores outward to considerable depths. It is in the larval form that this group lays most of its claim to a right to be placed among the Chordata, for the young hatches as a larva which resembles the amphibian tadpole superficially. In this tadpole a dorsal nerve cord is present, and in the tail region a well-formed notochord as well. Gill slits also are found. Later in life the larva settles down to a sessile existence and the tail with its notochord is lost. Examples: Styela partita; Molgula manhattensis; Ciona intestinalis.

Subphylum: Cephalochordata (Lancets)

To this group belong the familiar forms known as Amphioxus. Of all the lower chordates, the lancets possess characteristics closely resembling the higher chordate group. A dorsal tubular nerve cord is present, below which is an elongated notochord, and an extensive pharyngeal area is developed. The basic plan of the circulatory system resembles that of the vertebrate group, although many pulsating “hearts” are to be found, one in each of the numerous blood vessels coursing through the pharyngeal area. Examples: Branchiostoma virginiae; B. calif orniense; Asymmetron macricaudatum.

Higher Chordata (Craniata)

Subphylum: Vertebrata

Group I: Agnathostomata

To this group belong the cyclostomes or the vertebrates without jaws. The cyclostomes include the lampreys (Hyperoartia) and the hagfishes (Hyperotreta). They are parasitic on other fishes in the adult. The notochord and its surrounding sheaths serve as the main skeletal axis. True vertebral elements do not reinforce the notochord, although certain vertebral elements are present in some species. Examples: The California hagfish, Polistotrema (Bdellostoma) stouti, and the common sea lampreys, Petromyzon marinus, Okkelbergia lamotteni, Lampetra ayresii. The California hagfish has 12 pairs of gill slits whereas the sea lamprey, Petromyzon marinus, has 7 pairs.

Group II: Gnathostomata

The Gnathostomata are vertebrates which possess jaws. In a sense, they are the only true vertebrates in the chordate phylum, for the notochordal axis always is supplemented or displaced by vertebral elements.

1. Class: Pisces

Division 1: Chondrichthyes

To this group belong the selachian or elasmobranch fishes. The word chondrichthyes means cartilaginous fishes, i.e. the fishes with endoskeletons of cartilage. The adjective selachian has a similar meaning, whereas the term elasmobranch means plate-like gill.

The sharks, skates, rays, and chimaeras comprise the numerous species of cartilaginous fishes. The skin is covered with small placoid scales; median and paired fins are present; the sexes are separate, and elaborate reproductive ducts are developed. The heart, exclusive of the sinus venosus, is two chambered. Examples: Squalus acanthias, the dog fish; Rhineodon typus, the whale shark; Manta birostus, the “great devil ray.”

Division 2: Dipnoi

The dipnoan or lungfishes effect external respiration by means of gills and well-formed lungs. The heart, in harmony with its respiratory mechanisms, is practically three-chambered. Paired fins have a segmented, cartilaginous, central axis. Examples: The African lungfish, Protopterus annectens; the South American lungfish, Lepidosiren paradoxa; and the Australian lungfish, Neoceratodus forsteri.

Division 3: Teleostomi

In this group, the skeleton, in most species, is bony. A single opening for the gill-chamber is present on each side of the pharynx, the gills being covered by an operculum. An air bladder is found in most species. Paired fins are not supported by a median axis.

Series 1. The Ganoidei. The ganoid fishes, possessing ganoid or cycloid scales. An air bladder is to be found with an open duct united to the postpharyngeal area. A spiral valve is developed in the intestine. There are two groups of ganoid fishes, viz. the Chondrostei, which possess a cartilaginous skeleton and dermal bony plates, and the Holostei which have a bony skeleton.

Examples of Chondrostei are Acipenser julvescens, Scaphirhynchus plater hynchus, and Parascaphirhynchus albus. Lepisosteus osseus and Amia calva are representatives of the Holostei.

Series 2. Teleostei. In the bony fishes an air bladder is present but usually the pneumatic duct connecting the air bladder with the esophagus is rudimentary or absent. A spiral valve is absent in the intestine. The scales are cycloid or ctenoid, and in some instances are absent altogether. Examples: Oncorhynchus tschawytscha, the chinook or king salmon, the most important source of food fish in the country; Salmo salar, the Atlantic salmon; Trutta irideus, the rainbow trout; Salvelinus fontinalis, the speckled brook trout, and a host of other genera and species.

2. Class: Amphibia

The amphibians are cold-blooded vertebrates adapted to an existence in a watery or moist medium. Some species such as Necturus maculosus and the axolotl, Ambystoma mexicanum, spend their entire life within water, while others such as the frogs and salamanders are in and out of the water. The toads, on the other hand, are able to get along under fairly dry conditions. The skin is soft, moist, and glandular, and, with the exception of the Gymnophiona, it is devoid of scales. External respiration is carried on by means of gills in the larva, but in the adult the lungs and skin are the principal areas concerned with respiration. However, in those adults which live exclusively in the water, gills may be retained. Some species do not possess lungs and in these the skin and lining surfaces of the pharynx accommodate respiratory functions. In forms such as Necturus and the Axolotl, external gills function as the principal mechanism of external respiration in the adult. Excluding the sinus venosus, a three-chambered heart is typical of the group.

Order 1 : Caudata (Urodela)

The salamanders and newts form a large number of amphibian species. They have an elongate body with a conspicuous tail and the body muscles tend to retain a segmental condition. Many vertebrae are present. Examples: Cryptobranchus alleganiensis, Triturus viridescens, Ambystoma maculatum, Desmognathus fuse us, Plethodon cinereus, Amphiuma means, Necturus maculatus, Siren lacertina, Triton cristatus, etc.

Order 2: Anura (Salienta)

The frogs and toads. Short compact body; tail absent in adult; only nine vertebrae present; ribs ankylosed to vertebrae as short processes; hind legs long and muscular. Examples: Ascaphus truei, Scaphiopus holbrookii, Bufo americanus, Rana pipiens, R. sylvatica, R. catesbiana, Hyla crucifer, Discoglossus pictus, Xenopus laevis, Pipa pipa, Nectophrynoides vivipara.

Order 3: Gymnophiona

The caecilians are long-bodied, limbless amphibians resembling earthworms. They are inhabitants of the tropics with the exception of Madagascar. Scales are present in the dermal layer of the skin. Examples: Hypogeophis alternans, Scolecomorphus uluguruensis, Caecilia tentaculata.

3. Class: Reptilia

Scale-covered, cold-blooded, claw-digited vertebrates with a three- or fourchambered heart, and generally inhabitants of dry land or streams. External respiration carried on exclusively by means of lungs.

Order 1 : Crocodila

The crocodilians include the alligators and crocodiles. These are large greatly elongated reptiles covered with scales and bony plates. The eye has an upper and lower lid and a nictitating membrane. Teeth are thecodont. All species are oviparous. The anus is a longitudinal opening. Examples: Alligator mississippiensis and Crocodylus acutus.

Order 2: Lacertilia

The lizards are elongated reptiles of diverse sizes. Teeth are pleurodont or acrodont. The eye has an upper and lower eyelid and a nictitating membrane. The tympanum is not at the surface, and the ear opening may be covered by scales. A vestigial pineal or median eye is often present, and the tongue is well developed and protusile. Most species are oviparous, a few are ovoviparous, and some may be classed as viviparous. The anus is a transverse slit. Examples: Anolis carolinensis, the chameleon; Sphaerodactylus notatus, the reef gecko; Phyrynosoma cornutum, the horned toad; Heloderma suspectum, the Gila Monster; the Tuatera of New Zealand, and the dragon lizard of the Dutch East Indies.

Order 3: Serpentes

Snakes are crawling reptiles who have lost their legs. They form a large number of reptilian species. Acrodont teeth always are present. Functional eyelids are absent and they lack a tympanum or external ear opening. Some species are oviparous and others are ovoviviparous. Examples: Natrix sipedon, the common water snake; Thamnophis radix, the common garter snake; Crotalus horridus, the common rattler.

Order 4: Testudinata

Turtles possess short, compact bodies encased more or less completely in a box constructed of bony plates integrated to form a dorsal covering, the carapace, and a ventral shield, the plastron. The jaws are toothless and covered by a horny cutting edge. The tympanum is at the surface of the body and eyelids and nictitating membrane are present. All species are oviparous. Examples: Sternotherus odoratus, the musk turtle; Chelydra serpentina, the snapping turtle; Clemmys guttata, the spotted turtle; and Terrapene Carolina, the common box turtle.

4. Class: Aves

Birds are warm-blooded, lung-breathing vertebrates with feathers, without teeth, and with a horny beak. The body is built for flight and most species fly. All species are oviparous. Other than the extinct birds or Archaeornithes, all modern birds may be grouped together under the heading Neornithes. The Neornithes may be divided into two main groups:

Series 1: Ratitae (running birds)

The flightless running birds such as the recently extinct moas, and present living forms such as the kiwi. Apteryx; the cassowary, Casuarius sp., and the ostrich. Strut hio sp., belong in this group.

Series 2: Carinatae (flying birds)

This group contains many orders. The following orders are intimately associated with man: Anseriformes: Geese, ducks, swans Galliformes: The common fowl, turkey, pheasants, guinea hen, etc. Columbiformes: Doves, pigeons Passeriformes: Canary and other common song birds

5. Class: Mammalia

The mammals are warm-blooded, lung-breathing vertebrates with a coating of hair. They produce a nutritive substance for the young which is elaborated in glandular areas known as the mammae or breasts.

Division 1: Prototheria

These are highly specialized egg-laying mammals found only in Australia, Tasmania, and New Guinea. The spiny anteater, Echidna aculeata, is found in all of these localities and the Platypus or Ornithorynchus paradoxus, is an inhabitant of Australia. The urogenital ducts and intestine open posteriorly into a common chamber, the cloaca.

Division 2: Theria or true mammals

The Theria bring forth their young alive, possess true mammary glands with nipples, and all produce a small egg with, little stored food material. They also possess separate openings to the exterior for the urogenital ducts and the intestine, a cloaca being absent in the adult condition.

Series 1 : Metatheria. These are the marsupial or pouched mammals such as the Virginia opossum, Didelphys virginiana.

Series 2: Eutheria. The following orders are given:

Subseries 1. Unguiculata or mammals with claws

Order 1. Insectivora or insect-eating mammals Examples: Moles and shrews

Order 2. Chiroptera or flying mammals Example: The bats

Order 3. Carnivora or flesh-eating mammals Examples: Wolves, dogs, foxes, raccoons, otters, skunks, weasels, mink, hyenas, cats, lions, tigers

Order 4: Rodentia or gnawing mammals Examples: Rats, mice, rabbits, hares, guinea pigs, squirrels, beavers, gophers (ground squirrels), prairie dogs

Order 5. Edentata or mammals without teeth or with reduced condition of the teeth Examples: Armadillos, three-toed sloths, anteaters

Order 6. Pinnipedia or mammals with bilateral appendages adapted for swimming Examples: Seals, sea-lions, walruses Subseries 2. Ungulata or mammals with hoofs

Order 7. Artiodactyla or even-toed mammals Examples: Hippopotami, peccaries, swine, deer, moose, elk, pronghorn antelope, cows, sheep, goats, camels, giraffe, llamas, antelopes, gazelles

Order 8. Perissodactyla or odd-toed mammals Examples: Horses, zebras, asses, tapirs, rhinoceroses

Order 9. Sirenia or mammals with hind limbs absent and adapted to living in the water Example: The manatees or sea cows

Order 10. Proboscidea Examples: The elephants

Subseries 3. Cetacea or marine mammals

Order 11. Odontoceti or toothed whales Examples: Porpoises, sperm whales, killer whales, narwhals

Order 12. Mystacoceti or whalebone whales Examples: Sulphur-bottom whales, right whales, finback whales

Subseries 4. Primates or mammals with flattened, distal modifications of the digits known as nails

Order 13. Primates Examples: Man, monkeys, lemurs, apes


   Comparative Vertebrate Embryology 1953: 1. The Period of Preparation | 2. The Period of Fertilization | 3. The Development of Primitive Embryonic Form | 4. Histogenesis and Morphogenesis of the Organ Systems | 5. The Care of the Developing Embryo | Figures
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