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| {{Header}} | | {{Header}} |
| {{Ref-Nelsen1953}} | | {{Ref-Nelsen1953}} |
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| | {{Nelsen1953 TOC}} |
| {{Historic Disclaimer}} | | {{Historic Disclaimer}} |
| =Part III The Development of Primitive Embryonic Form= | | =Part III The Development of Primitive Embryonic Form= |
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| ==Basic Features of Vertebrate Morphogenesis== | | ==Basic Features of Vertebrate Morphogenesis== |
| | [[Book - Comparative Embryology of the Vertebrates 3-11|11. Basic Features of Vertebrate Morphogenesis]] |
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| A. Introduction | | A. Introduction |
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| 1. Purpose of This Chapter | | 1. Purpose of This Chapter |
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| In this chapter, the basic morphogenetic features which give origin to the
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| later organ systems are emphasized. These features arise from the stream of
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| morphogenetic phenomena which come down from the fertilized egg through
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| the periods of cleavage, blastulation, gastrulation, and tabulation. This chapter
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| thus serves to connect the developmental processes, outlined in Chapters 6
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| to 10, with those which follow in Chapters 12 to 21. As such, it emphasizes
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| certain definitions and basic structural features involved in the later morphogenetic activities which mold the adult body form.
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| 2. Definitions | | 2. Definitions |
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| a. Morphogenesis and Related Terms | | a. Morphogenesis and Related Terms |
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| The word morphogenesis means the development of form or shape. It involves the elaboration of structural relationships. The morphogenesis of a
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| particular shape and structure of a cell is called cytomorphosis or cytogenesis
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| and is synonymous with the term cellular differentiation, considered from the
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| structural aspect. In the Metazoa, the body is composed of groups of cells,
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| each cellular group possessing cells of similar form and function. That is, each
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| cell group is similarly differentiated and specialized. A cellular group, composed of cells similar in form (structure) and function, is called a tissue.
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| Histology is the study of tissues, and the word histogenesis relates to that
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| phase of developmental morphology which deals with the genesis or development of tissues. An organ is an anatomical structure, produced by an association of different tissues which fulfills one or several specialized functions.
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| For example, the esophagus, stomach, liver, etc., are organs of the body.
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| During development, each of the major organ-forming areas, delineated in
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| 518
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| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
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| Chapters 6, 7, 9 and 10, produce several specific organs. Organogenesis is
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| concerned with the formation of these specific organs. A group of organs
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| which are associated together to execute one general function form an organ
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| system. The digestive system, for example, has for its general function that
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| of obtaining nourishment for the body. It is composed of a series of o’-gans
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| integrated toward this end. The nervous system, similarly, is an assemblage
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| of specific organs devoted to the discharge of nervous functions. So it is with
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| the other systems of the organism. System development is concerned with
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| the genesis of such systems. The association of various systems, integrated
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| together for the maintenance of the body within a particular habitat, constitutes the organism. Finally, the organism acquires a particular body form
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| because of the form, structure, and activities assumed by its organ systems
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| as a result of their adaptation to the functional necessities of the particular
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| habitat in which the organism lives. It should be urged further that this nice
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| relationship between form and structure, on the one hand, and functional
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| requirements, on the other, is a fundamental principle of development from
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| the egg to the adult. It is a principle intimately associated with the morphogenesis of the organ systems described in Chapters 12 to 21.
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| During development from the egg to the adult form, three major types of
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| body form are evolved in the majority of vertebrate species.
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| b. Primitive, Larval, and Definitive Body Forms (see fig. 255) | | b. Primitive, Larval, and Definitive Body Forms (see fig. 255) |
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| 1) Primitive Body Form. The condition of primitive or generalized, embryonic body form is attained when the embryo reaches a state in which its | | 1) Primitive Body Form. |
| developing organ systems resemble the respective developing organ systems
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| in other vertebrate embryos at the same general period of development. (See p.
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| 520.) Superficially, therefore, the general structure of the primitive embryonic
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| body of one species resembles that of the primitive embryonic bodies of other
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| vertebrate species. Such comparable conditions of primitive, body-form development are reached in the 10 to 15-mm. embryo of the shark, Squalus
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| acanthias, of the frog embryo at about 5 to 7 mm., the chick at about 55 to
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| 96 hrs. of incubation, the pig at 6 to 10 mm., and the human at 6 to 10 mm.
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| 2) Larval Body Form. Following primitive body form, the embryo gradually transforms into a larval form. The larval form is present in the period
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| between primitive body form and definitive body form. The larval period is
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| that period during which the basic conditions of the various organ systems,
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| present in primitive body form, undergo a metamorphosis in assuming the
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| form and structure of the adult or definitive body form. In other words, during
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| the larval period, the basic or generalized conditions of the various organ
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| systems are changed into the adult form of the systems, and the larval period
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| thus represents a period of transition. Embryos which develop in the water
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| (most fishes, amphibia) tend to accentuate the larval condition, whereas
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| those which develop within the body of the mother (viviparous teleosts,
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| INTRODUCTION
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| 519
| | 2) Larval Body Form. |
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| | | 3) Definitive Body Form. |
| sharks, mammals) or within well-protected egg membranes (turtle, chick)
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| slur over the larval condition.
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| The larval stage in non-viviparous fishes (see Kyle, ’26, pp. 74-82) and in
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| the majority of amphibia is a highly differentiated condition in which the
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| organs of the body are adapted to a free-living, watery existence. The tadpole
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| of the frog, Rana pipiens, from the 6-mm. stage to the 11 -mm. stage, presents
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| a period during which the primitive embryonic condition, present at the time
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| of hatching (i.e., about 5 mm.), is transformed into a well-developed larval
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| stage capable of coping with the external environment. From this time on to
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| metamorphosis, the little tadpole possesses free-living larval features. Another
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| example of a well-developed, free-living, larval stage among vertebrates is
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| that of the eel, Anguilla rostrata. Spawning occurs in the ocean depths around
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| the West Indies and Bermuda. Following the early embryonic stage in which
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| primitive body form is attained, the young transforms into a form very unlike
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| the adult. This form is called the Leptocephalus. The Leptocephalus was formerly classified as a distinct species of pelagic fishes. After many months in
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| the larval stage, it transforms into the adult form of the eel. The latter migrates
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| into fresh-water streams, the American eel into streams east of the Rockies
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| and the European eel into the European streams (Kyle, ’26, pp. 54-58). The
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| larval stages in most fishes conform more nearly to the adult form of the fish.
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| The embryo of Squalus acanthias at 20 to 35 mm. in length, the chick
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| embryo at 5 to 8 days of incubation, the pig embryo of 12- to 18-mm. length,
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| and the human embryo of 12 to 20-mm. length may be regarded as being
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| in the stage of larval transition. The young opossum, when it is born, is in a
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| late larval state. It gradually metamorphoses into the adult body form within
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| the marsupium of the mother (Chap. 22).
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| 3) Definitive Body Form. The general form and appearance of the adult | |
| constitute definitive body form. The young embryo of Squalus acanthias, at
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| about 40 mm. in length, assumes the general appearance of the adult shark;
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| the frog young, after metamorphosis, resembles the adult frog (Chap. 21),
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| the chick of 8 to 13 days of incubation begins to simulate the form of the
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| adult bird; the pig embryo of 20 to 35 mm. gradually takes on the body features of a pig, and the human fetus, during the third month of pregnancy,
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| assumes the appearance of a human being. The transformation of the larval
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| form into the body form of the adult is discussed further in Chapter 21 in
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| relation to the endocrine system.
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| 3. Basic or Fundamental Tissues | | 3. Basic or Fundamental Tissues |
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| Through the stages of development to the period when the primitive or
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| generalized, embryonic body form is attained, most of the cells which take
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| part in development are closely associated. In the primitive embryonic body,
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| this condition is found in all the five primitive body tubes and in the notochord. These closely arranged cells form the primitive epithelium. In the de
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| 520
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| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
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| veloping head and tail regions, however, mesoderm is present in the form of
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| loosely aggregated cells, known as mesenchyme. While the cells of the epithelial variety are rounded or cuboidal in shape with little intercellular substance or space between the cells, mesenchymal cells tend to assume stellate
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| forms and to have a large amount of intercellular substance between them.
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| The primitive vascular or blood tubes are composed of epithelium in the sense
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| that the cells are closely arranged. However, as these cells are flattened and
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| show specific peculiarities of structure, this tissue is referred to as endothelium.
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| Also, while the cells of the early neural tube show the typical epithelial features, they soon undergo marked changes characteristic of developing neural
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| tissue. The primitive or generalized, embryonic body thus is composed of
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| four fundamental tissues, viz., epithelial, mesenchymal, endothelial, and neural
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| tissues.
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| B. Transformation of the Primitive Body Tubes into the Fundamental | | B. Transformation of the Primitive Body Tubes into the Fundamental |
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| 1. Processes Involved in Basic System Formation | | 1. Processes Involved in Basic System Formation |
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| As the primitive body tubes (epidermal, neural, enteric, and mesodermal)
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| are established, they are modified gradually to form the basis for the various
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| organ systems. While the notochordal axis is not in the form of a tube, it also
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| undergoes changes during this period. The morphological alterations, which
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| transform the primitive body tubes into the basic or fundamental structural
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| conditions of the systems, consist of the following:
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| (a) extension and growth of the body tubes, | | (a) extension and growth of the body tubes, |
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| (d) unequal growth of different areas along the tubes. | | (d) unequal growth of different areas along the tubes. |
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| As a result of these changes, the primitive neural, epidermal, enteric, and
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| mesodermal tubes, together with the capillaries or blood tubes and the notochord, experience a state of gradual differentiation which is directed toward
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| the production of the particular adult system to be derived from these respective basic structures. The primitive body tubes, the primitive blood capillaries, and the notochord thus come to form the basic morphological conditions of the future or gut ^ systems. The basic structural conditions of the various
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| systems are described in Chapters 12 to 21.
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| 2. Fundamental Similarity of Early Organ Systems | | 2. Fundamental Similarity of Early Organ Systems |
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| The general form and structure of each primitive embryonic system, as it
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| begins to develop in one vertebrate species, exhibits a striking resemblance
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| LAWS OF VON BAER
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| 521
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| to the same system in other vertebrate species. This statement is particularly
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| true of the gnathostomous vertebrates (i.e., vertebrates with jaws). Consequently, we may regard the initial generalized stages of the embryonic or rudimentary systems as fundamental or basic plans of the systems, morphologically
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| if not physiologically. The problem which confronts the embryo of each
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| species, once the basic conditions of the various systems have been established,
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| is to convert the generalized basic condition of each system into an adult
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| form which will enable that system to function to the advantage of the particular animal in the particular habitat in which it lives. The conversion of
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| the basic or primitive condition of the various systems into the adult form of
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| the systems constitutes the subject matter of Chapters 12 to 21.
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| The basic conditions of the various organ systems are shown in the structure
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| of shark embryos from 10 to 20 mm. in length, frog embryos of 5 to 10 mm.,
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| chick embryos from 55 to 96 hrs., pig embryos from 6 to 10 mm., crownrump length, and human embryos of lengths corresponding to 6 to 10 mm.
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| That is to say, the basic or generalized conditions of the organ systems are
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| present when primitive or generalized embryonic body form is developed.
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| It is impossible to segregate any particular length of embryo in the abovementioned series as the ideal or exact condition showing the basic condition
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| of the systems, as certain systems in one species progress faster than those
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| same systems in other species. However, a study of embryos of these designations serves to provide an understanding of the basic or fundamental conditions of the various systems (figs. 257-262; also fig. 347A).
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| C. Laws of von Baer | | C. Laws of von Baer |
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| As indicated above, the species of the vertebrate group as a whole tend to
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| follow strikingly similar (although not identical) plans of development during
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| blastulation, gastrulation, tubulation, the development of the basic plan of the
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| various systems and primitive body form. As observed in the chapters which
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| follow, the fundamental or basic plan of any particular, organ-forming system,
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| in the early embryo of one species, is comparable to the basic plan of that
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| system in other species throughout the vertebrate group. However, after these
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| basic parallelisms in early development are completed, divergences from the
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| basic plan begin to appear during the formation of the various organ systems
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| of a particular species.
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| The classical statements or laws of Karl Ernst von Baer (1792-1876) describe a tendency which appears to be inherent in the developmental procedure
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| of any large group of animals. This developmental tendency is for generalized
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| structural features to arise first, to be remodeled later and supplanted by features specific for each individual species. To interpret these laws in terms of
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| the procedure principle mentioned in Chapter 7, it may be assumed that
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| general, or common, developmental procedures first are utilized, followed by
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| 522
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| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
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| specific developmental procedures which change the generalized conditions
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| into specific conditions.
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| The laws of von Baer ( 1 828-1837, Part I, p. 224) may be stated as follows:
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| (a) The general features of a large group of animals appear earlier in development than do the special features;
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| (b) after the more general structures are established, less general structures
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| arise, and so on until the most special feature appears;
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| (c) each embryo of a given adult form of animal, instead of passing
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| through or resembling the adult forms of lower members of the group,
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| diverges from the adult forms, because
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| (d) the embryo of a higher animal species resembles only the embryo of
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| the lower animal species, not the adult form of the lower species.
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| D. Contributions of the Mesoderm to Primitive Body Formation and | | D. Contributions of the Mesoderm to Primitive Body Formation and |
| Later Development | | Later Development |
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| The mesoderm is most important to the developing architecture of the
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| body. Because the mesoderm enters so extensively into the structure of the
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| many organs of the developing embryo, it is well to point out further the
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| sources of mesoderm and to delineate the structures and parts arising from
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| this tissue.
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| 1. Types of Mesodermal Cells | | 1. Types of Mesodermal Cells |
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| Most of the mesoderm of the early embryo exists in the form of epithelium
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| (see p. 519). As development proceeds, much of the mesoderm loses the close
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| arrangement characteristic of epithelium. In doing so, the cells separate and
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| assume a loose connection. They also may change their shapes, appearing
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| stellate, oval, or irregular, and may wander to distant parts of the body. This
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| loosely aggregated condition of mesoderm forms the primitive mesenchyme.
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| Though most of the mesoderm becomes transformed into mesenchyme, the
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| inner layer of cells of the original hypomeric portion of the mesodermal tubes
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| retains a flattened, cohesive pattern, described as mesothelium. Mesothelium
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| comes to line the various body cavities, for these cavities are derived directly
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| from the hypomeric areas of the mesodermal tubes (Chap. 20).
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| 2. Origin of the Mesoderm of the Head Region | | 2. Origin of the Mesoderm of the Head Region |
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| The primary cephalic outgrowth (Chap. 10), which later forms the head
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| structures, contains two basic regions, namely, the head proper and the
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| pharyngeal or branchial region. During its early development, the heart lies
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| at the ventro-caudal extremity of the general head region; it recedes gradually
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| backward as the head and branchial structures develop. The exact origin of
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| the mesoderm which comes to occupy the head proper and pharyngeal areas
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| varies in different gnathostomous vertebrates. The general sources of the head
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| mesoderm may be described in the following manner.
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| CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
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| 523
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| a. Head Mesoderm Derived from the Anterior Region of the Trunk | | a. Head Mesoderm Derived from the Anterior Region of the Trunk |
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| The mesoderm of the branchial area in lower vertebrates, such as the snarks
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| and, to some degree, the amphibia, represents a direct anterior extension of
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| the mesoderm of the trunk (figs. 217D, E; 230D; 252E) . It is divisible into two
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| parts: (1) a ventro-lateral region, the hypomeric or lateral plate mesoderm,
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| and (2) a dorsal or somitic portion. The latter represents a continuation into
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| the head region of the epimeric (somitic) mesoderm of the trunk. That portion of the mesoderm of the branchial area which may be regarded specifically
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| as part of the mesoderm of the head proper is the mesoderm associated with
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| the mandibular and hyoid visceral arches, together with the hyoid and mandibular somites located at the upper or dorsal ends of the hyoid and mandibular visceral arches (fig. 217D, E).
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| In the higher vertebrates (reptiles, birds, and mammals), the mesoderm
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| of the branchial region appears early, not as a continuous epithelium, as in
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| the shark and amphibian embryo, but as a mass of mesenchyme which wanders into the branchial area from the anterior portion of the developing trunk
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| region (figs. 217F; 23 3B; 234B). This mesenchyme assumes branchial region
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| characteristics, for it later condenses to form the mandibular, hyoid, and more
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| posteriorly located, visceral arches. Also, mesenchymal condensations appear
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| which correspond to the pre-otic head somites formed in the early shark
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| embryo. For example, in the chick, there is an abducent condensation, which
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| corresponds to the hyoid somite of the shark embryo, and a superior oblique
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| condensation corresponding probably to the mandibular somite of the shark
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| embryo (cf. fig. 217D, F). (See also Adelmann, ’27, p. 42.) Both of these
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| condensations give origin to eye muscles (Chap. 16). Somewhat similar condensations of mesenchyme which form the rudiments of eye muscles occur in
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| other members of the higher vertebrate group.
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| b. Head Mesoderm Derived from the Pre-chordal Plate | | b. Head Mesoderm Derived from the Pre-chordal Plate |
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| The term pre-chordal plate mesoderm signifies that portion of the head
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| mesoderm which derives from the pre-chordal plate area located at the anterior end of the foregut. The pre-chordal plate mesoderm is associated closely
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| with the foregut entoderm and anterior extremity of the notochord in the late
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| blastula and gastrula in the fishes and amphibia. However, in reptiles, birds,
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| and mammals, this association is established secondarily with the foregut entoderm by means of the notochordal canal and primitive-pit invaginations during
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| gastrulation. (See Chap. 9 and also Hill and Tribe, ’24.)
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|
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| (Note: It is advisable to state that Adelmann, ’32, relative to the 19-somite
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| embryo of the urodele Ambystoma pimctatum, distinguishes between a prechordal mesoderm, which forms the core of the mandibular visceral arch, and
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| the pre-chordal plate mesoderm, which earlier in development is associated
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| with the dorsal anterior portion of the foregut entoderm. See figure 252E.)
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|
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| During the period when the major organ-forming areas are being tubulated.
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| NEURAL ECTODERM
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| .RANCHIAL POUCHBRANCHIAL GROOVE
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| OR gill- SL 1 T AREA
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| HYPOMERICl
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| MESOOERMAI^
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| CONTRIBUTION ^
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| TO LATERAL
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| BODY WALL
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| MESENCHYMAL
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| CONTRIBUTION
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| FROM SPLANCHNIC
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| LAYER OF HYPOMERE
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|
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|
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| Fig. 252. Mesodermal contributions to developing body. (A-D) Sections through
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| developing chick of 48-52 hrs. of incubation. (A) Section through somites of caudal
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| trunk area showing primitive area of mesoderm and coelomic spaces. (B) Section
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| through anterior trunk area depicting early differentiation of somite. (C) Section
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| through trunk area posterior to heart revealing later stage of somite differentiation than
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| that shown in B. (D) Section through developing heart area. Observe dermomyotome,
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| sclerotomic mesenchyme, and mesenchymal contributions of hypomere to forming body
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| substance. (E) Mesodermal contributions to anterior end of developing embryo of
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| Ambystoma of about 19 somites. (Redrawn and modified from Adelmann: 1932, J.
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| Morphol. 54.) (F) Frontal section of early post-hatching larva of Rana pipiens show
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| ing mass of mesoderm lying between gut, epidermal and neural tubes, together with the
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| contributions of the mesoderm to the visceral arches.
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|
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| 524
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| CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
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| 525
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|
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| the pre-chordal plate mesoderm separates as a mass of mesenchyme from the
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| antero-dorsal aspect of the foregut, anterior to the cephalic terminus of the
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| notochord (fig. 232G, H). It migrates forward as two groups of mesenchyme
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| connected at first by an interconnecting bridge of mesenchyme. Eventually
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| these two mesenchymal masses become separated and each forms a dense
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| aggregation of mesodermal cells over the mandibular visceral arch and just
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| caudal to the eye (fig. 252E). In the shark embryo and in the chick it gives
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| origin to the pre-mandibular somites (condensations) which probably give
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| origin to the eye muscles innervated by the oculomotor or third cranial nerves.
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| In Ambystoma, Adelmann (’32, p. 52) describes the pre-chordal plate mesoderm as giving origin to “the eye muscles†and “probably much of the head
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| mesenchyme ahead of the level of the first (gill) pouch, but its caudal limit
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| cannot be exactly determined.†Thus it appears that a portion of the head
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| mesoderm in the region anterior to the notochordal termination is derived
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| from the pre-chordal plate mesoderm in all vertebrates.
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|
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| c. Head Mesoderm Contributed by Neural Crest Material | | c. Head Mesoderm Contributed by Neural Crest Material |
|
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| A conspicuous phase of the development of the head region in vertebrate
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| embryos is the extensive migration of neural crest cells which arise in the middorsal area as the neural tube is formed (Chap. 10; fig. 222C-F). Aside
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| from contributing to the nervous system (Chap. 19), a portion of the neural
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| crest material migrates extensively lateroventrally and comes to lie within the
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| forming visceral (branchial) arches, contributing to the mesoderm in these
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| areas (figs. 222C-F; 230D, F). Also, consult Landacre (’21); Stone (’22,
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| ’26, and ’29); and Raven (’33a and b). On the other hand, Adelmann (’25)
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| in the rat and Newth (’51 ) in the lamprey, Lampetra planeri, were not able
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| to find evidence substantiating this view. However, pigment cells (melanophores) of the skin probably arise from neural crest cells in the head region
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| of all vertebrate groups.
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| d. Head Mesoderm Originating from Post-otic Somites | | d. Head Mesoderm Originating from Post-otic Somites |
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| There is good evidence that the musculature of the tongue takes its origin
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| in the shark embryo and lower vertebrates from cells which arise from the
| |
| somites of the trunk area, immediately posterior to the otic (ear) vesicle, from
| |
| whence they migrate ventrad to the hypobranchial region and forward to
| |
| the area of the developing tongue (fig. 253). In the human embryo, Kingsbury
| |
| (’15) suggests this origin for the tongue and other hypobranchial musculature.
| |
| However, Lewis (’10) maintains that, in the human, the tongue musculature
| |
| arises from mesenchyme in situ.
| |
|
| |
|
| 3. Origin of the Mesoderm of the Tail | | 3. Origin of the Mesoderm of the Tail |
|
| |
|
| In the Amphibia, the tail mesoderm has been traced by means of the Vogt
| | 4. Contributions of the Trunk Mesoderm to the Developing Body |
| staining method to tail mesoderm in the late blastular and early gastrular
| |
| | |
| | |
| | |
| 526
| |
| | |
| | |
| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
| |
| | |
| | |
| stages. At the time of tail-rudiment formation, this mesoderm forms two bilateral masses of cells located within the “tail bud†or “end bud.†These cellular
| |
| masses proliferate extensively as the tail bud grows caudally and give origin
| |
| to the mesoderm of the tail. Similarly, in other vertebrates, the mesoderm of
| |
| the future tail is present as mesenchyme in the terminal portion of the tail
| |
| bud. These mesenchymal cells proliferate, as the tail grows caudalward, and
| |
| leave behind the mesoderm, which gradually condenses into the epithelial
| |
| masses or segments (myotomes) along either side of the notochord and
| |
| neural tube.
| |
| | |
| 4. Contributions of the Trunk Mesoderm to the | |
| Developing Body | |
| | |
| The mesoderm of the trunk area contributes greatly to the development of
| |
| the many body organs and systems in the trunk region. Details of this contribution will be described in the chapters which follow, but, at this point, it
| |
| is well to survey the initial activities of the mesodermal tubes of the trunk
| |
| area in producing the vertebrate body.
| |
|
| |
|
| a. Early Differentiation of the Somites or Epimere | | a. Early Differentiation of the Somites or Epimere |
|
| |
| The somites (figs. 217, 237, 252) contribute much to the developing structure of the vertebrate body. This fact is indicated by their early growth and
| |
| differentiation. For example, the ventro-mesial wall of the fully developed
| |
| somite gradually separates from the rest of the somite and forms a mass of
| |
| mesenchymal cells which migrates mesad around the notochord and also
| |
| dorsad around the neural tube (fig. 252A-C). The mesenchyme which thus
| |
| arises from the somite is known as the sclerotome. In the somite of the higher
| |
| vertebrates just previous to the origin of the sclerotome, a small epithelial
| |
| core of cells becomes evident in the myocoel; this core contributes to the
| |
| sclerotomic material (fig. 252B). As a result of the segregation of the sclerotomic tissue and its migration mesad to occupy the areas around the notochord
| |
| and nerve cord, the latter structures become enmeshed by a primitive skeletogenous mesenchyme. The notochord and sclerotomic mesenchyme are the
| |
| foundation for the future axial skeleton of the adult, including the vertebral
| |
| elements and the caudal part of the cranium as described in Chapter 15.
| |
|
| |
| After the departure of sclerotomic material, myotomic and dermatomic
| |
| portions of the somite soon rearrange themselves into a hollow structure (fig.
| |
| 252C, D), in which the myotome forms the inner wall and the dermatome
| |
| the outer aspect. This composite structure is the dermomyotome, and the
| |
| cavity within, the secondary myocoei. In many vertebrates (fishes, amphibia,
| |
| reptiles, and birds), the dermatome gives origin to cells which migrate into
| |
| the region of the developing dermis (Chap. 12) and contributes to the formation of this layer of the skin.
| |
|
| |
|
| |
|
| |
| CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
| |
|
| |
|
| |
| 527
| |
|
| |
|
| |
|
| b. Early Differentiation of the Mesomere (Nephrotome) | | b. Early Differentiation of the Mesomere (Nephrotome) |
|
| |
| The differentiation of the nephrotome or intermediate mesoderm will be
| |
| considered later (Chap. 18) in connection with the urogenital system.
| |
|
| |
|
| c. Early Differentiation and Derivatives of the Hypomere | | c. Early Differentiation and Derivatives of the Hypomere |
|
| |
| The lateral-plate mesoderm (hypomere), figure 252A, performs an extremely important function in embryological development. The cavity of the
| |
| hypomere (splanchnocoel) and the cellular offspring from the hypomeric mesoderm, which forms the wall of this cavity, give origin to much of the structural material and arrangement of the adult body.
| |
|
| |
| 1) Contributions of the Hypomere (I^ateral Plate Mesoderm) to the Developing Pharyngeal Area of the Gut Tube. The developing foregut (Chap.
| |
| 13) may be divided into four main areas, namely, (1) head gut, (2) pharyngeal, (3) esophageal, and (4) stomach areas. The head gut is small and
| |
| represents a pre-oral extension of the gut; the pharyngeal area is large and
| |
| expansive and forms about half of the forming foregut in the early embryo;
| |
| the esophageal segment is small and constricted; and the forming stomach
| |
| region is enlarged. At this point, however, concern is given specifically to the
| |
| developing foregut in relation to the early development of the pharyngeal
| |
| region.
| |
|
| |
| In the pharyngeal area the foregut expands laterally. Beginning at its anterior end, it sends outward a series of paired, pouch-like diverticula, known
| |
| as the branchial (pharyngeal or visceral) pouches. These pouches push outward toward the ectodermal (epidermal) layer. In doing so, they separate
| |
| the lateral plate mesoderm which synchronously has divided into columnar
| |
| masses or cells (fig. 252E, F). Normally, about four to six pairs of branchial
| |
| (pharyngeal) pouches are formed in gnathostomous vertebrates, although in
| |
| the cyclostomatous fish, Petromyzon, eight pairs appear. In the embryo of the
| |
| shark, Squalus acanthias, six pairs are formed, while in the amphibia, four
| |
| to six pairs of pouches may appear (fig. 252F). In the chick, pig, and human,
| |
| four pairs of pouches normally occur (figs. 259, 261). Also, invaginations or
| |
| inpushings of the epidermal layer occur, the branchial grooves (visceral furrows); the latter meet the entodermal outpocketings (figs. 252F; 262B).
| |
|
| |
| The end result of all these developmental movements in the branchial area
| |
| is to produce elongated, dorso-ventral, paired columns of mesodermal cells
| |
| (figs. 252E; 253), the visceral or branchial arches, which alternate with the
| |
| branchial-groove-pouch or gill-slit areas (figs. 252F; 253). The most anterior
| |
| pair of visceral arches forms the mandibular visceral arches; the second pair
| |
| forms the hyoid visceral arches; and the succeeding pairs form the branchial
| |
| (gill) arches (figs. 239C, D; 240; 244; 246; 252E; 253). The branchial arches
| |
| with their mesodermal columns of cells will, together with the contributions
| |
| from the neural crest cells referred to above, give origin to the connective,
| |
| muscle, and blood-vessel-forming tissues in this area.
| |
|
| |
|
| |
|
| |
| 528
| |
|
| |
|
| |
| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
| |
|
| |
|
| |
| 2) Contributions of the Hypomere (Lateral Plate Mesoderm) to the Formation of the Gut Tube and Heart Structures. Throughout the length of the
| |
| forming gut tube, from the oral area to the anal region, the lateral plate mesoderm (mesoblast) contributes much to the forming gut tube. This is occasioned to a great extent posterior to the pharyngeal area by the fact that the
| |
| inner or mesial walls of the two hypomeres enswathe the forming gut tube
| |
| as they fuse in the median plane (fig. 241), forming the dorsal and ventral
| |
| mesenteries of the gut. However, in the heart area, due to the dorsal displacement of the foregut, the dorsal mesentery is vestigial or absent while
| |
| the ventral mesentery is increased in extent. Each mesial wall of the hypomeric
| |
| mesoderm, forming the ventral mesentery in the region of the developing
| |
| heart, becomes cupped around the primitive blood capillaries, coursing anteriad in this area to form the rudiments of the developing heart. The ventral
| |
| mesentery in the heart area thus gives origin to the dorsal mesocardium, the
| |
| ventral mesocardium, and the rudimentary, cup-shaped, cpimyocardial structures around the fusing blood capillaries (figs. 236C-D; 254A). The primitive
| |
| blood capillaries soon unite to form the rudiment of the future endocardium
| |
| of the heart, while the enveloping epimyocardium establishes the rudiment of
| |
| the future muscle and connective tissues of the heart (Chap. 17).
| |
|
| |
| On the other hand, in the region of the stomach and continuing posteriorly
| |
| to the anal area of the gut, the movement mediad of the mesial walls of the
| |
| two lateral plate (hypomeric) mesodermal areas occurs in such a way as to
| |
|
| |
|
| |
|
| |
| Fig. 253. Diagram illustrating the basic plan of the vertebrate head based upon the
| |
| shark, Scy Ilium canicula. (Modified from Goodrich: 1918, Quart. Jour. Micros. Science, 63.)
| |
|
| |
|
| |
|
| |
| CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
| |
|
| |
|
| |
| 529
| |
|
| |
|
| |
|
| |
| the hypomeres to the developing heart and gut structures in reptiles, birds, and mammals.
| |
| Sections are drawn through the following regions: (A) Through primitive tubular heart
| |
| anterior to sinus venosus. (B) Through caudal end of sinus venosus and lateral meso*
| |
| cardia. (C) Through liver region. (D) Through region posterior to liver. (E)
| |
| Through posterior trunk in region of urinary bladder.
| |
|
| |
| envelop or enclose the gut tube. This enclosure readily occurs because in this
| |
| region of the trunk, the gut tube lies closer to the ventral aspect of the embryo
| |
| than in the heart area. Consequently, a dorsal mesentery above and a ventral
| |
| mesentery below the primitive gut tube are formed (fig. 25 4C). The dorsal
| |
| and ventral mesenteries may not persist everywhere along the gut (fig. 254D).
| |
| The degree of persistence varies in different vertebrates; these variations will
| |
| be mentioned later (Chap. 20) when the coelomic cavities are discussed.
| |
| However, there is a persistence of the ventral mesentery below the stomach
| |
| and anterior intestinal area of all vertebrates, for here the ventral mesentery
| |
| (i.e., the two medial walls of the lateral plate mesoderm below the gut) contributes to the development of the liver and the pancreas. These matters are
| |
| discussed in Chapter 13.
| |
|
| |
| Aside from the formation of the dorsal and ventral mesenteries by the inward movement and fusion of the medial walls of the lateral plate mesoderm
| |
| above and below the primitive enteron or gut tube, that part of the medial
| |
| walls of the lateral plate mesoderm which envelops the primitive gut itself is
| |
| of great importance. This importance arises from the fact that the entoderm
| |
| of the gut only forms the lining tissue of the future digestive tract and its
| |
| various glands, such as the liver, pancreas, etc., whereas mesenchymal contributions from the medial wall of the lateral plate mesoderm around the
| |
|
| |
|
| |
|
| |
| 530
| |
|
| |
|
| |
| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
| |
|
| |
|
| |
| entodermal lining give origin to smooth muscle tissue, connective tissue, etc.
| |
| (figs. 254C, D; 258; 260; 262; 278C). It is apparent, therefore, that the gut
| |
| throughout its length is formed from two embryonic contributions, namely,
| |
| one from the entoderm and the other from the mesenchyme given off by the
| |
| medial walls of the lateral plate or hypomeric mesoderm.
| |
|
| |
| {Note: The word splanchnic is an adjective and is derived from a Greek
| |
| word meaning entrails or bowels. That is, it pertains to the soft structures
| |
| within the body wall. The plural noun viscera (singular, viscus) is derived
| |
| from the Latin and signifies the same structures, namely, the heart, liver,
| |
| stomach, intestine, etc., which lie within the cavities of the body. It is fitting,
| |
| therefore, to apply the adjective splanchnic to the medial portion of the hypomere because it has an intimate relationship with, and is contributory to, the
| |
| development of the viscera. The somatic mesoderm, on the other hand, is the
| |
| mesoderm of the lateral or body-wall portion of the hypomere. The word
| |
| splanchnopleure is a noun and it designates the composite tissue of primitive
| |
| entoderm and splanchnic mesoderm, while the word somatopleure is applied
| |
| to the compound tissue formed by the primitive lateral wall of the hypomere
| |
| (somatic mesoderm) plus the primitive ectoderm overlying it. The coelom
| |
| proper or spianchnocoel is the space or cavity which lies between the splanchnic
| |
| and somatic layers of the lateral plate or hypomeric mesoderm. During later
| |
| development, it is the cavity in which the entrails lie.
| |
|
| |
| 3) Contributions of the Hypomere (Lateral Plate Mesoderm) to the External (Ectodermal or Epidermal) Body Tube. The somatopleural mesoderm
| |
| gives origin to a mass of cellular material which migrates outward to lie along
| |
| the inner aspect of the epidermal tube in the lateral and ventral portions of
| |
| the developing body (fig. 252A, D). In the dorsal and dorso-lateral regions of
| |
| the body, contributions from the sclerotome and dermatome apparently aid
| |
| in forming this tissue layer. The layer immediately below the epidermis constitutes the embryonic rudiment of the dermis. (See Chap. 12.)
| |
|
| |
| 4) Contributions of the Hypomere or Lateral Plate Mesoderm to the Dorsal
| |
| Body Areas. Many cells are given off both from splanchnic and somatic layers
| |
| of the hypomeric mesoderm to the dorsal body areas above and along either
| |
| side of the dorsal aorta (fig. 254), contributing to the mesenchymal “packing tissue†in the area between the notochord and differentiating somite, extending outward to the dermis.
| |
|
| |
| 5) Contributions of the Lateral Plate Mesoderm to the Walls of the Coelomic Cavities. The pericardial, pleural, and peritoneal cavities are lined, as
| |
| stated above, by an epithelial type of tissue called mesothelium (fig. 254A-E).
| |
| These coelomic spaces (see Chap. 20) are derived from the fusion of the
| |
| two primitive splanchnocoels or cavities of the two hypomeres. External to
| |
| the mesothelial lining of the coelomic spaces, there ultimately is developed a
| |
| fibrous, connective tissue layer. Thus, mesothelium and connective tissue form.
| |
|
| |
|
| |
|
| |
|
| |
| Fig. 255. This figure illustrates different types of body form in various vertebrates
| |
| during embryonic development. A, D, H, M, and Q show primitive embryonic body
| |
| form in the developing shark, rock fish, frog, chick, and human. B, larval form of
| |
| shark; E and F, larval forms of rock fish; I and J, larval forms of frog; N and O, larval
| |
| forms of chick; R, larval form of human. C, G, K, L, P, and S represent definitive
| |
| body form in the above species. (Figures on rockfish development (Roccus saxatilis) redrawn from Pearson: 1938, Bull. Bureau of Fisheries, L). S. Dept, of Commerce, vol.
| |
| 49; figures on chick redrawn from Hamburger and Hamilton: 1951, J. Morphol., vol.
| |
| 88; figure Q, of developing human embryo, redrawn and modified from model based
| |
| upon Normentafeln of Keibel and Elze: 1908, vol. 8, G. Fischer, Jena; Dimensions of
| |
| human embryos in R and S, from Mall: Chap. 8, vol. 1, Human Embryology, by
| |
| F. Keibel and F. P. Mall, 1910, Lippincott, Philadelphia.)
| |
|
| |
|
| |
| 531
| |
|
| |
|
| |
|
| |
| 532
| |
|
| |
|
| |
| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
| |
|
| |
|
| |
| in general, the walls of the coelomic spaces. These two tissues arise directly
| |
| from the hypomeric mesoderm.
| |
|
| |
|
| 5. Embryonic Mesenchyme and Its Derivatives | | 5. Embryonic Mesenchyme and Its Derivatives |
|
| |
|
| The mesenchymal cells given off from the mesodermal tubes of the trunk
| |
| area, namely, (1) sclerotomic mesenchyme, (2) dermatomic mesenchyme,
| |
| (3) mesenchymal contributions from the lateral plate mesoblast (hypomere)
| |
| to the gut, skin, heart, and (4) the mesenchyme contributed to the general
| |
| regions of the body lying between the epidermal tube, coelom, notochord,
| |
| and neural tube, form, together with the head and tail mesoderm, the general
| |
| packing tissue which lies between and surrounding the internal tubular structures of the embryo (fig. 254). Its cells may at times assume polymorphous
| |
| or stellate shapes. This loose packing tissue of the embryo constitutes the
| |
| embryonic mesenchyme. (See Chap. 15.)
| |
|
| |
| This mesenchyme ultimately will contribute to the following structures of
| |
| the body:
| |
|
| |
| (a) Myocardium (cardiac musculature, etc.) and the epicardium or covering coelomic layer of the heart (Chap. 17),
| |
|
| |
| (b) endothelium of blood vessels, blood cells (Chap. 17),
| |
|
| |
| (c) smooth musculature and connective tissues of blood vessels (Chaps.
| |
| 16 and 17),
| |
|
| |
| (d) spleen, lymph glands, and lymph vessels (Chap. 17),
| |
|
| |
| (e) connective tissues of voluntary and involuntary muscles (Chap. 16),
| |
|
| |
| (f) connective tissues of soft organs, exclusive of the nerve system (Chap.
| |
| 15),
| |
|
| |
| (g) connective tissues in general, including bones and cartilage (Chap. 15),
| |
|
| |
| (h) smooth musculature of the gut tissues and gut derivatives (Chap. 16),
| |
|
| |
| (i) voluntary or striated muscles of the tail from tail-bud mesenchyme
| |
| (Chap. 16),
| |
|
| |
| (j) striated (voluntary) musculature of face, jaws, and throat, derived
| |
| from the lateral plate mesoderm in the anterior pharyngeal region
| |
| (Chap. 16),
| |
|
| |
| (k) striated (voluntary) extrinsic musculature of the eye (Chap. 16),
| |
|
| |
| (l) intrinsic, smooth musculature of the eye (Chap. 16),
| |
|
| |
| (m) tongue and musculature of bilateral appendages, derived from somitic
| |
| muscle buds (sharks) or from mesenchyme possibly of somitic origin
| |
| (higher vertebrates) (Chap. 16), and
| |
|
| |
| (n) chromatophores or pigment cells of the body from neural crest mesenchyme (Chap. 12).
| |
|
| |
|
| |
|
| |
| SUMMARY OF DERIVATIVES OF ORGAN-FORMING AREAS
| |
|
| |
|
| |
| 533
| |
|
| |
|
| |
| £. Summary of Later Derivatives of the Major Presumptive Organforming Areas of the Late Blastula and Gastrula
| |
|
| |
|
| 1. Neural Plate Area (Ectoderm) | | 1. Neural Plate Area (Ectoderm) |
|
| |
| This area gives origin to the following:
| |
|
| |
| (a) Neural tube,
| |
|
| |
| (b) optic nerves and retinae of eyes,
| |
|
| |
| (c) peripheral nerves and ganglia,
| |
|
| |
| (d) chromatophores and chromaffin tissue (i.e., various pigment cells of
| |
| the skin, peritoneal cavity, etc., chromaffin cells of supra-renal gland),
| |
|
| |
| (e) mesenchyme of the head, neuroglia, and
| |
|
| |
| (f) smooth muscles of iris.
| |
|
| |
|
| 2. Epidermal Area (Ectoderm) | | 2. Epidermal Area (Ectoderm) |
|
| |
|
| This area gives origin to:
| | 3. Entodermal Area |
| | |
| (a) Epidermal tube and derived structures, such as scales, hair, nails,
| |
| feathers, claws, etc.,
| |
| | |
| (b) lens of the eye, inner ear vesicles, olfactory sense area, general, cutaneous, sense organs of the peripheral area of the body,
| |
| | |
| (c) stomodaeum and its derivatives, oral cavity, anterior lobe of pituitary,
| |
| enamel organs, and oral glands, and
| |
| | |
| (d) proctodaeum from which arises the lining tissue of the anal canal.
| |
| | |
| 3. Entoderm AL Area | |
| | |
| From this area the following arise:
| |
| | |
| (a) Epithelial lining of the primitive gut tube or metenteron, including:
| |
| (1) epithelium of pharynx; epithelium pharyngeal pouches and their
| |
| derivatives, such as auditory tube, middle-ear cavity, parathyroids, and
| |
| thymus; (2) epithelium of thyroid gland; (3) epithelial lining tissue
| |
| of larynx, trachea, and lungs, and (4) epithelium of gut tube and gut
| |
| glands, including liver and pancreas,
| |
| | |
| (b) most of the lining tissue of the urinary bladder, vagina, urethra, and
| |
| associated glands,
| |
| | |
| (c) Seessel’s pocket or head gut, and
| |
| | |
| (d) tail gut.
| |
|
| |
|
| 4. Notochordal Area | | 4. Notochordal Area |
|
| |
| This area:
| |
|
| |
| (a) Forms primitive antero-posterior skeletal axis of all chordate forms,
| |
|
| |
| (b) aids in induction of central nerve tube.
| |
|
| |
|
| |
|
| |
| 534
| |
|
| |
|
| |
| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
| |
|
| |
|
| |
| (c) gives origin to adult notochord of Amphioxus and cyclostomatous fishes
| |
| and to notochordal portions of adult vertebral column of gnathostomous
| |
| fishes and water-living amphibia, and
| |
|
| |
| (d) also, comprises the remains of the notochord in land vertebrates, such
| |
| as “nucleus pulposus†in man.
| |
|
| |
|
| 5. Mesodermal Areas | | 5. Mesodermal Areas |
|
| |
| These areas give origin to:
| |
|
| |
| (a) Epimeric, mesomeric, and hypomeric areas of primitive mesodermal
| |
| tube,
| |
|
| |
| (b) epimeric portion also aids in induction of central nerve tube,
| |
|
| |
| (c) muscle tissue, involuntary and voluntary,
| |
|
| |
| (d) mesenchyme, connective tissues, including bone, cartilage,
| |
|
| |
| (e) blood and lymphoid tissue,
| |
|
| |
| (f) gonads with exception of germ cells, genital ducts, and glandular tissues of male and female reproductive ducts, and
| |
|
| |
| (g) kidney, ureter, musculature and connective tissues of the bladder,
| |
| uterus, vagina, and urethra.
| |
|
| |
|
| 6. Germ-cell Area | | 6. Germ-cell Area |
|
| |
| This area gives origin to:
| |
|
| |
| (a) Primordial germ cells and probably to definitive germ cells of all vertebrates below mammals and
| |
|
| |
| (b) primordial germ cells of mammals and possibly to definitive germ cells.
| |
|
| |
|
| F. Metamerism | | F. Metamerism |
|
| |
| 1. Fundamental Metameric Character of the Trunk and
| |
| Tail Regions of the Vertebrate Body
| |
|
| |
| Many animals, invertebrate as well as vertebrate, are characterized by the
| |
| fact that their bodies are constructed of a longitudinal series of similar parts
| |
| or metameres. As each metamere arises during development in a similar
| |
| manner and from similar rudiments along the longitudinal or antero-posterior
| |
| axis of the embryo, each metamere is homologous with each of the other
| |
| metameres. This type of homology in which the homologous parts are arranged serially is known as serial homology. Metamerism is a characteristic
| |
| feature of the primitive and later bodies of arthropods, annelids, cephalochordates, and vertebrates.
| |
|
| |
| In the vertebrate group, the mesoderm of the trunk and tail exhibits a type
| |
| of segmentation, particularly in the epimeric or somitic area. Each pair of
| |
| somites, for example, denotes a primitive body segment. The nervous system
| |
|
| |
|
| |
|
| |
|
| |
|
| |
| ^OPTIC VESICLE
| |
| LENS PLACODE .
| |
|
| |
| ^ nasal placode
| |
| — —maxillary
| |
| process
| |
|
| |
| mandibular arch
| |
|
| |
|
| |
| branchial arch
| |
|
| |
|
| |
|
|
| |
|
| |
|
| |
| nasal placode
| |
|
| |
|
| |
| ORAL OPENING
| |
|
| |
| Laxillary PROCESstl^
| |
|
| |
| .
| |
|
| |
|
| |
|
| |
| mandibular ARCH
| |
|
| |
|
| |
| \ ^nasolateral
| |
| PROCESS
| |
|
| |
|
| |
| ^ NaSOMEDIAL -*
| |
|
| |
| process I
| |
| ''naso-optic furrow
| |
| 'maxillary process
| |
| "mandibular arch
| |
|
| |
|
| |
| hyomandibular cleft
| |
|
| |
|
| |
| NASOMEDIAL
| |
|
| |
| process
| |
|
| |
|
| |
| NASOLATERAL
| |
|
| |
| process
| |
|
| |
|
| |
| naso-optic
| |
|
| |
| furrow
| |
|
| |
|
| |
| 'hyomandibular
| |
|
| |
| CLEFT
| |
|
| |
|
| |
|
| |
|
| |
|
| |
| tubercles around
| |
| ^ hyomandibular CLEFT
| |
| § fusing to form
| |
|
| |
| f external EAR'
| |
|
| |
|
| |
|
| |
| ...»
| |
|
| |
|
| |
| NASOLATERAL PROCESS^
| |
|
| |
| NASOMEDIAL PROCESSES
| |
|
| |
| fusing to form PHILTRUM-.
| |
| OF LIP
| |
|
| |
| EXTERNAL EAR
| |
|
| |
| ear tubercles around
| |
| hyomandibular cleft
| |
|
| |
| -hyoid bone REGlONr
| |
|
| |
|
| |
| ih j 1
| |
|
| |
|
| |
|
| |
| F.O. 256. Developmental features of the human fac. Modified slightly from models by
| |
| B. Ziegler, Freiburg, after Karl Peter.
| |
|
| |
|
| |
| 535
| |
|
| |
|
| |
|
| |
| 536
| |
|
| |
|
| |
| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
| |
|
| |
|
| |
| also manifests various degrees of segmentation (Chap. 19), although the
| |
| origin and arrangement of the peripheral nerves in the form of pairs, each
| |
| pair innervating a pair of myotomic derivatives of the somites, is the most
| |
| constant feature.
| |
|
| |
| In the cephalochordate, Amphioxus, the segmentation of the early mesoderm is more pronounced than that of the vertebrate group. As observed in
| |
| Chapter 10, each pair of somites is distinct and entirely separate from other
| |
| somitic pairs, and each pair represents all the mesoderm in the segment or
| |
| metamere. That is, all the mesoderm is segmented in Amphioxus. However,
| |
| in the vertebrate group, only the more dorsally situated mesoderm undergoes
| |
| segmentation, the hypomeric portion remaining unsegmented.
| |
|
| |
| 2. Metamerism and the Basic Morphology of the
| |
| Vertebrate Head
| |
|
| |
| While the primitive, metameric (segmental) nature of the vertebrate trunk
| |
| and tail areas cannot be gainsaid, the fundamental metamerism of the vertebrate head has been questioned. Probably the oldest theory supporting a
| |
| concept of cephalic segmentation was the vertebral theory of the skull, propounded by Goethe, Oken, and Owen. This theory maintained that the basic
| |
| structure of the skull demonstrated that it was composed of a number of
| |
| modified vertebrae, the occipital area denoting one vertebra, the basisphenoidtemporo-parietal area signifying another, the presphenoid-orbitosphenoidfrontal area denoting a third vertebra, and the nasal region representing a
| |
| fourth cranial vertebra. (Consult Owen, 1848.) This theory, as a serious
| |
| consideration of vertebrate head morphology was demolished by the classic
| |
| Croonian lecture given in 1858 by Huxley (1858) before the Royal Society
| |
| of London. His most pointed argument against the theory rested upon the
| |
| fact that embryological development failed to support the hypothesis that the
| |
| bones of the cranium were formed from vertebral elements.
| |
|
| |
| A factor which aroused a renewal of interest in a segmental interpretation
| |
| of the vertebrate head was the observation by Balfour (1878) that the head
| |
| of the elasmobranch fish, Scy Ilium, contained several pairs of pre-otic (prootic) somites (that is, somites in front of the otic or ear region). Since Balfour’s
| |
| publication, a large number of studies and dissertations have appeared in an
| |
| endeavor to substantiate the theory of head segmentation. The anterior portion of the central nervous system, cranial nerves, somites, branchial (visceral)
| |
| arches and pouches, have all served either singly or in combination as proffered
| |
| evidence in favor of an interpretation of the primitive segmental nature of
| |
| the head region. However, it is upon the head somites that evidence for a
| |
| cephalic segmentation mainly depends.
| |
|
| |
| A second factor which stimulated discussion relative to head segmentation
| |
| was the work of Locy (1895) who emphasized the importance of so-called
| |
| neural segments or neuromcres (Chap. 19) as a means of determining the
| |
|
| |
|
| |
|
| |
|
| |
|
| |
| ARROWS SHOW water CURRENTS
| |
|
| |
|
| |
| Fig. 257. Drawings of early frog tadpoles showing development of early systems.
| |
| (A) Frog tadpole (R. pipiens) of about 6 7 mm. It is difficult to determine the exact
| |
| number of vitelline arteries at this stage of development and the number given in the
| |
| figure is a diagrammatic representation. {A') Shows right and left ventral aortal divisions of bulbus cordis. (B) Anatomy of frog tadpole of about 10-18 mm. See also
| |
| figures 280 and 335.
| |
|
| |
|
| |
| 537
| |
|
| |
|
| |
|
| |
|
| |
|
| |
|
| |
| 540
| |
|
| |
|
| |
| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
| |
|
| |
|
| |
| primitive segmental structure of the vertebrate brain. It is to be observed that
| |
| the more conservative figure 253, taken from Goodrich, does not emphasize
| |
| neuromeres, for, as observed by Kingsbury (’26, p. 85), the evidence is overwhelmingly against such an interpretation. The association of the cranial nerves
| |
| with the gill (branchial) region and the head somites, shown in figure 253,
| |
| will be discussed further in Chapter 19.
| |
|
| |
| A third factor which awakened curiosity, concerning the segmental theory
| |
| of head development, is branchiomerism. The latter term is applied to the
| |
| development of a series of homologous structures, segmentally arranged, in
| |
| the branchial region; these structures are the visceral arches and branchial
| |
| pouches referred to above. As mentioned there, the branchial pouches or outpocketings of the entoderm interrupt a non-segmented mass of lateral plate
| |
| (hypomeric) mesoderm, and this mesoderm secondarily becomes segmented
| |
| and located within the visceral arches. These arches when formed, other than
| |
| possibly the mandibular and the hyoid arches (fig. 253), do not correspond
| |
| with the dorsal somitic series. Consequently, “branchiomerism does not, therefore, coincide with somitic metamerism.†(See Kingsbury, ’26, p. 106.)
| |
|
| |
| Undoubtedly, much so-called “evidence†has been accumulated to support
| |
| a theory of head segmentation. A considerable portion of this evidence apparently is concerned more with segmentation as an end in itself than with a
| |
| frank appraisal of actual developmental conditions present in the head (Kingsbury and Adelmann, ’24 and Kingsbury, ’26). However, the evidence which
| |
| does resist critical scrutiny is the presence of the head somites which includes
| |
| the pre-otic somites and the first three or four post-otic somites. While the
| |
| pre-otic somites are somewhat blurred and slurred over in their development
| |
| in many higher vertebrates, the fact of their presence in elasmobranch fishes
| |
| is indisputable and consistent with a conception of primitive head segmentation.
| |
|
| |
| Furthermore, aside from a possible relationship with head-segmentation
| |
| phenomena, the appearance of the pre-otic and post-otic head somites coincides with basic developmental tendencies. As observed above, for example,
| |
| there is a tendency for nature to use generalized developmental procedures in
| |
| the early development of large groups of animals (see von Baer’s laws, p. 522,
| |
| and also discussion relative to Haeckel’s biogenetic law in Chap. 7). Nature,
| |
| in other words, is utilitarian, and one can be quite certain that if general
| |
| developmental procedures are used, they will prove most efficient when all
| |
| factors are considered. At the same time, while generalized procedures may
| |
| be used, nature does not hesitate to mar or elide parts of procedures when
| |
| needed to serve a particular end. The obliteration of developmental steps
| |
| during development is shown in the early development of the mesoderm in
| |
| the vertebrate group compared to that which occurs in Amphioxus. In the
| |
| vertebrate embryo, as observed previously, the hypomeric mesoderm is unsegmented except in a secondary way and in a restricted area as occurs in
| |
| branchiomerism. However, in Amphioxus, early segmentation of the meso
| |
|
| |
|
| |
| METAMERISM
| |
|
| |
|
| |
| 541
| |
|
| |
|
| |
| derm is complete dorso-ventrally, including the hypomeric region of the
| |
| mesoderm. It becomes evident, therefore, that the suppression of segmentation
| |
| in the hypomeric area in the vertebrate embryo achieves a precocious result
| |
| which the embryo of Amphioxus reaches only at a later period of development. Presumably in the vertebrate embryo, segmentation of the epimeric
| |
| mesoderm is retained because it serves a definite end, whereas segmentation
| |
| of the hypomeric mesoderm is deleted because it also leads to a necessary end
| |
| result in a direct manner.
| |
|
| |
| When applied to the developing head region, this procedure principle means
| |
| this: A primitive type of segmentation does tend to appear in the pre-otic
| |
| area as well as in the post-otic portion of the head, as indicated by the pre-otic
| |
| and post-otic somites, and secondarily there is developed a branchial metam
| |
|
| |
| GASSERIAN GANGLION I
| |
| ME TENCEPHAUON
| |
|
| |
| geniculate GANGLION OF NERVE Stt
| |
| ACOUSTIC GANGLION OF NERVE :
| |
|
| |
| MYf lencephalon
| |
| OTIC VESICLE
| |
|
| |
| SUPERIOR GANGLION OF NERVE H
| |
| JUGULAR GANGLION OF NERVE X
| |
| PETROSAL GANGLION OF NERVE IX ^
| |
|
| |
| NERVE :
| |
|
| |
| NOOOSE ganglion OF nerve::
| |
|
| |
| NERVE
| |
|
| |
| SPINAL CORO-^
| |
|
| |
| pharyngeal pouch in-<:
| |
| pharyngeal POUCHBC;
| |
| thyroid BODY
| |
| BUL0US COROIS
| |
|
| |
|
| |
| MESENCEPHALON
| |
|
| |
|
| |
| â– NERVE 33
| |
|
| |
| NERVE m
| |
|
| |
| infundibulum
| |
| E’S POCKET
| |
| SEESSEL'S POCKET
| |
| CHOROID FISSURE
| |
| OlENCEPHALON
| |
|
| |
|
| |
|
| |
| DORSAL aorta
| |
| NOTOCHORD
| |
| stomach
| |
| LIVER
| |
|
| |
|
| |
| ventral pancreasdorsal pancreas
| |
| gall blaode
| |
| MESONEPHROS- —
| |
|
| |
|
| |
| MIDGUT
| |
| AN DUCT
| |
|
| |
| glomeruli
| |
|
| |
| COLLECTING DUCT
| |
| HINDGUT
| |
|
| |
| SPINAL GANGLION
| |
|
| |
|
| |
| Fig. 259. Chick embryo reconstruction of about 100 hrs. of incubation with special
| |
| reference to the nervous and urinary systems. See also fig. 336D.
| |
|
| |
|
| |
|
| |
|
| |
| bation. Reference should
| |
| 5
| |
|
| |
|
| |
| BASIC HOMOLOGY OF ORGAN SYSTEMS
| |
|
| |
|
| |
| 545
| |
|
| |
|
| |
| erism (branchiomerism) . However, all these segmental structures serve a
| |
| definite end. In other areas, head development proceeds in a manner which
| |
| obscures segmentation, for the probable reason that segmentation does not fit
| |
| into the developmental pattern which must proceed directly and precociously
| |
| to gain a specific end dictated by problems peculiar to head development.
| |
|
| |
| {Note: For a critical analysis of the supposed facts in favor of segmentation,
| |
| together with a marshaling of evidence against such an interpretation, consult
| |
| Kingsbury and Adelmann (’24) and for a favorable interpretation of the segmental nature of the head region, see Goodrich (’18) and Delsman (’22).
| |
| Figure 253 is taken from Goodrich (’18), and the various structures which
| |
| favor a segmental interpretation of the head region are shown.)
| |
|
| |
|
| G. Basic Homology of the Vertebrate Organ Systems | | G. Basic Homology of the Vertebrate Organ Systems |
Line 1,737: |
Line 642: |
| 1. Definition | | 1. Definition |
|
| |
|
| Homology is the relationship of agreement between the structural parts of
| | 2. Basic Homology of Vertebrate Blastulae, Gastrulae, and Tubulated Embryos |
| one organism and the structural parts of another organism. An agreeable
| |
| relationship between two structures is established if:
| |
| | |
| ( 1 ) the two parts occupy the same relative position in the body,
| |
| | |
| (2) they arise in the same way embryonically and from the same rudiments, and
| |
| | |
| (3) they have the same basic potencies.
| |
| | |
| By basic potency is meant the potency which governs the initial and fundamental development of the part; it should not be construed to mean the
| |
| ability to produce the entire structure. To the basic potency, other less basic
| |
| potencies and modifying factors may be added to produce the adult form of
| |
| the structure.
| |
| | |
| 2. Basic Homology of Vertebrate Blastulae, Gastrulae, and | |
| Tubulated Embryos | |
| | |
| In Chapters 6 and 7, the basic conditions of the vertebrate blastula were
| |
| surveyed, and it was observed that the formative portion of all vertebrate
| |
| blastulae presents a basic pattern, composed of major presumptive organforming areas oriented around the notochordal area and a blastocoelic space.
| |
| During gastrulation (Chap. 9), these areas are reoriented to form the basic
| |
| pattern of the gastrula, and although round and flattened gastrulae exist, these
| |
| form one, generalized, basic pattern, composed of three germ layers arranged
| |
| around the central axis or primitive notochordal rod. Similarly, in Chapter
| |
| 10, the major organ-forming areas are tubulated to form an elongated embryo,
| |
| composed of head, pharyngeal, trunk, and tail regions. As tubulation is effected in much the same manner throughout the vertebrate series and as the
| |
| pre-chordal plate mesoderm, foregut entoderm, notochord, and somitic meso
| |
| | |
| | |
| 546
| |
| | |
| | |
| BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
| |
| | |
| | |
| geniculate ganglion of seventh nerve
| |
| | |
| ACOUSTIC GANGLION OF EIGHTH NERVE
| |
| AUDITORY VESICLE
| |
| | |
| | |
| JU^dLAR GANGLION
| |
| | |
| SUPERIOR GANGLION
| |
| NINTH NERV
| |
| ACCESSORY ganglion
| |
| BASILAR ARTERY
| |
| DORSAL ROOT
| |
| GANGLION OP FIRST
| |
| | |
| cervical nerve
| |
| aortal arch I
| |
| | |
| AORTAL ARCH II
| |
| AORTAL ARCH III
| |
| AORTAL ARCH IV
| |
| AORTAL ARCH VT
| |
| PULMONARY ARTERY
| |
| TRACHEA
| |
| NOTOCHORD
| |
| RIGHT ATRIUM
| |
| LUNG
| |
| | |
| | |
| | |
| SMALL
| |
| | |
| INTESTINE
| |
| | |
| hepatic
| |
| | |
| PORTAL VEIN
| |
| DORSAL AORTA
| |
| | |
| | |
| OMPHALOMESENTERIC
| |
| ARTERY
| |
| | |
| (FUTURE SUPERIOR
| |
| MESENTERIC ARTERY)
| |
| | |
| | |
| GLOMERULUS
| |
| MESONEPHRIC TUBULE
| |
| | |
| | |
| DORSAL AORTA
| |
| MESONEPHRIC DUCT
| |
| | |
| | |
| Fig. 261. Drawings of pig embryos of about 9.5 to 12 mm. (A) Reconstruction of about
| |
| 9.5 to 10 mm. pig embryo with special emphasis on the arterial system.
| |
| | |
| derm appear to be the main organizing influence throughout the series (Chap.
| |
| 10), the conclusion is inescapable that the tubulated embryos of all vertebrates
| |
| are homologous basically, having the same relative parts, arising in the same
| |
| manner, and possessing the same basic potencies within the parts. To this
| |
| conclusion must be added a caution, namely, that, although the main segments
| |
| or specific organ regions along each body tube of one species are homologous
| |
| with similar segments along corresponding tubes of other species, variations
| |
| may exist and non-homologous areas may be insinuated or homologous areas
| |
| | |
| | |
| | |
| BASIC HOMOLOGY OF ORGAN SYSTEMS
| |
| | |
| | |
| 547
| |
| | |
| | |
| may be deleted along the respective tubes. Regardless of this possibility, a
| |
| basic homology, however, appears to exist.
| |
| | |
| During later development through larval and definitive body-form stages,
| |
| a considerable amount of molding or plasis by environmental and intrinsic
| |
| factors may occur. An example of plasis is given in the development of the
| |
| forelimb rudiment of the fish, frog, bird, and pig. In the definitive form, these
| |
| structures assume different appearances and are adapted for different func
| |
| | |
| METENCEPHALON
| |
| | |
| | |
| BASILAR ARTERY
| |
| NOTOCHORD
| |
| | |
| ROOT OF TONGUE
| |
| THYROID GLAND
| |
| developing epiglottis
| |
| AORTIC ARCH III
| |
| L ARYN X
| |
| | |
| | |
| ESOPH AGU S
| |
| VALVES OF
| |
| | |
| SINUS 'VENOSUS/
| |
| LUNG bud'
| |
| | |
| SPINAL CORD
| |
| SINUS VENOSU;
| |
| | |
| | |
| GALL BLADDER
| |
| | |
| NOTOCHORDOORSAL AORTA
| |
| | |
| DEVELOPING
| |
| VERTEBRAE
| |
| | |
| MESONEPHRIC
| |
| KIDNE
| |
| | |
| | |
| | |
| MESENCEPHALON
| |
| | |
| | |
| TUBERCULUM
| |
| | |
| ju / POSTERIUS
| |
| ~ — ^INFUNDIBULUM
| |
| OIEUCEPHALON
| |
| | |
| rathke's pocket
| |
| | |
| SEESSEL'S POCKET
| |
| — -OPTIC CHIASMA
| |
| | |
| -RECESSUS OPTICUS
| |
| TELENCEPHALON
| |
| AMINA TERMINALIS
| |
| TONGUE
| |
| BULBUS CORDIS
| |
| | |
| | |
| EXTRA-EMBRYONIC COELOM
| |
| UMBILICAL CORD
| |
| | |
| | |
| ALLANTOIC DIVERTICULUM
| |
| GENITAL EMINENCE
| |
| PROCTODAEUM
| |
| | |
| CLOACA
| |
| | |
| | |
| ALLANTOIC STALK
| |
| | |
| | |
| B.
| |
| | |
| | |
| metanephrogenous
| |
| | |
| TISSUE SPINAL GANGLION
| |
| | |
| | |
| Fig. 261 — (Continued} (B) Median sagittal section of 10 mm. embryo.
| |
| | |
| | |
| | |
| VEIN OF maxillary REGION
| |
| (BRANCH OF INTERNAL JUGULAR)
| |
| | |
| | |
| OTIC VESICLE
| |
| | |
| | |
| VEIN OF
| |
| | |
| MANDIBULAR REGION
| |
| BRANCH OF EXTERNAL
| |
| JUGULAR)
| |
| | |
| INTERNAL JUGULAR
| |
| VEIN
| |
| DORSAL
| |
| | |
| jSEGMENTAL VEINS
| |
| EXTERNAL
| |
| JUGULAR VEIN
| |
| | |
| | |
| | |
| LEFT DUCT OF CUVIER
| |
| RIGHT VITELLINE VEIN
| |
| LIVER
| |
| DUCTUS VENOSUS
| |
| HEPATIC VEINS
| |
| PORTAL VEIN
| |
| | |
| | |
| UMBILICAL
| |
| | |
| ARTERY
| |
| | |
| | |
| 'TRANSVERSE ANASTOMOSIS
| |
| OF SITBCAROINALS
| |
| | |
| | |
| POSTERIOR CARDINAL
| |
| VEIN
| |
| | |
| | |
| PIG EMBRYO SHOWING RIGHT HALF
| |
| OF VENOUS SYSTEM
| |
| | |
| | |
| Fig. 261 — (Continued) (C) Lateral view of 12 mm. embryo showing venous system.
| |
| (C is redrawn and modified from Minot; 1903, A Laboratory Text-book of Embryology,
| |
| Blakiston, Philadelphia.)
| |
| | |
| | |
| 548
| |
| | |
| | |
| | |
| | |
| Fig. 262. Sections and stereograms of 10 mm. pig embryo.
| |
| | |
| | |
| MCSCNCHYME^
| |
| | |
| | |
| | |
| Ibl— (Continued) Sections and stereograms of 10 mm. pig embryo
| |
| | |
| | |
| | |
| | |
| BIBLIOGRAPHY
| |
| | |
| | |
| 551
| |
| | |
| | |
| tional purposes. Basically, however, these structures are homologous, although
| |
| plasis produces adult forms which appear to be different.
| |
| | |
| A further statement should be added, concerning that type of molding or
| |
| plasis of a developing structure which produces similar structures from conditions which have had a different genetic history. For example, the bat’s fore
| |
| limb rudiment is molded to produce a structure resembling superficially that
| |
| of the bird, although modern bats and birds have arisen through different lines
| |
| of descent. Similarly, the teeth of certain teleost fishes superficially resemble
| |
| the teeth of certain mammals, an effect produced from widely diverging lines
| |
| of genetic descent. These molding effects or homoplasy, which produce superficially similar structures as a result of adaptations to certain environmental
| |
| conditions, are called convergence, parallelism, and analogy. An example of
| |
| experimental homoplasy is the induction of eye lenses in the embryo by the
| |
| transplantation of optic-cup material to a place in the epidermis which normally does not produce a lens.
| |
| | |
| {Note: For a discussion of homology, homogeny, plasis, convergence, etc.,
| |
| see Tait, ’28.)
| |
| | |
| | |
| Bibliography
| |
| | |
| | |
| Adelmann, H. B. 1925. The development
| |
| of the neural folds and cranial ganglia
| |
| of the rat. J. Comp. Neurol. 39:19.
| |
| | |
| . 1927. The development of the eye
| |
| | |
| muscles of the chick. J. Morphol. 44:29.
| |
| | |
| . 1932. The development of the
| |
| | |
| prechordal plate and mesoderm of Ambly stoma piinctatum. J. Morphol. 54:1.
| |
| | |
| Baer, K. E. von. 1828-1837. liber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. Erster Theil,
| |
| 1828; Zweiter Theil, 1837. Konigsberg,
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Part III The Development of Primitive Embryonic Form
Part III - The Development of Primitive Embryonic Form: 6. Cleavage (Segmentation) and Blastulation | 7. The Chordate Blastula and Its Significance | 8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning | 9. Gastrulation | 10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form | 11. Basic Features of Vertebrate Morphogenesis
The general procedures leading to the development of primitive embryonic body form
in the chordate group of animals are:
( 1 ) Cleavage. Cleavage is the division of the egg into progressively smaller cellular
units, the blaslomeres (Chap. 6).
(2) Blastulation. Blastulalion results in the formation of the blastula. The blastula
is composed of a cellular blastoderm in relation to a fluid-filled cavity, the blastocoel.
The blastoderm of the late blastula is composed of neural, epidermal, notochordal,
mesodermal, and entodermal major presumptive organ-forming areas. In the phylum
Chordata, the notochordal area is the central region around which the other areas are
oriented (Chap. 7). The major presumptive organ-forming areas of the late blastula
exist in various degrees of differentiation (Chap. 8).
(3) Gastrulation. This is the process which effects a reorientation of the presumptive
organ-forming areas and brings about their axiation antero-posteriorly in relation to the
notochordal axis and the future embryonic body (Chap. 9). During gastrulation the
major organ-forming areas are subdivided into minor areas or fields, each field being
restricted to the development of a particular organ or part. (Pp. 378, 446, 447.
(4) Following gastrulation, the next step in the development of embryonic body form
is tubulation and extension of the major organ-forming areas (Chap. 10).
(5) As tubulation and extension of the organ-forming areas is effected, the basic or
fundamental conditions of the future organ systems are established, resulting in the
development of primitive body form. As the development of various vertebrate embryos
is strikingly similar up to this point, the primitive embryonic body forms of all vertebrates
resemble each other (Chap. II).
In the drawings presented in Part III, the following scheme for designating the major
organ-forming areas existing within the three germ layers is adhered to:
Cleavage (Segmentation) and Blastulation
6. Cleavage (Segmentation) and Blastulation
A. General considerations
1. Definitions
2. Early history of the cleavage (cell-division) concept
3. Importance of the cleavage-blastular period of development
a. Morphological relationships of the blastula
b. Physiological relationships of the blastula
1 ) Hybrid crosses
2) Artificial parthenogenesis
3) Oxygen-block studies
4. Geometrical relations of early cleavage
a. Meridional plane
b. Vertical plane
c. Equatorial plane
d. Latitudinal plane
5. Some fundamental factors involved in the early cleavage of the egg
a. Mechanisms associated with mitosis or cell division
b. Influence of cytoplasmic substance and egg organization upon cleavage
1) Yolk
2) Organization of the egg
c. Influence of first cleavage amphiaster on polyspermy
d. Viscosity changes during cleavage
e. Cleavage laws
1 ) Sach’s rules
2) Hertwig’s laws
6. Relation of early cleavage planes to the antero-posterior axis of the embryo
B. Types of cleavage in the phylum Chordata
1. Typical holoblastic cleavage
a. Amphioxus
b. Frog (Rana pipiens and R. sylvatica)
c. Cyclostomata
2. Atypical types of holoblastic cleavage
a. Holoblastic cleavage in the egg of the metatherian and eutherian mammals
1 ) General considerations
2) Early development of the rabbit egg
a) Two-cell stage
b) Four-cell stage
c) Eight-cell stage
d) Sixteen-cell stage
e) Morula stage
f) Early blastocyst
3) Types of mammalian blastocysts (blastulae)
b. Holoblastic cleavage of the transitional or intermediate type
1) Amhystoma maculatum (punctatum)
2) Lepidosiren paradoxa
3) Necturus maculosus
4) Acipenser sturio
5) Amia calva
6) Lepisosteus (Lepidosteus) osseus
7) Gymnophionan amphibia
3. Meroblastic cleavage
a. Egg of the common fowl
1 ) Early cleavages
2) Formation of the periblast tissue
3) Morphological characteristics of the primary blastula
4) Polyspermy and fate of the accessory sperm nuclei
b. Elasmobranch fishes
1 ) Cleavage and formation of the early blastula
2) Problem of the periblast tissue in elasmobranch fishes
c. Teleost fishes
1) Cleavage and early blastula formation
2) Origin of the periblast tissue in teleost fishes
d. Prototherian Mammalia
e. Cleavage in the California hagfish, Polistotrema (Bdellostorna) stouti
C. What is the force which causes the blastomeres to adhere together during early
cleavage?
D. Progressive cytoplasmic inequality and nuclear equality of the cleavage blastomeres
1. Cytoplasmic inequality of the early blastomeres
2. Nuclear equality of the early blastomeres
E. Quantitative and qualitative cleavages and their influence upon later development
The Chordate Blastula and Its Significance
7. The Chordate Blastula and Its Significance
A. Introduction
1. Blastulae without auxiliary tissue
2. Blastulae with auxiliary or trophoblast tissue
3. Comparison of the two main blastular types
B. History of the concept of specific, organ-forming areas
C. Theory of epigenesis and the germ-layer concept of development
D. Introduction of the words ectoderm, mesoderm, endoderm
E. Importance of the blastular stage in Haeckel's theory of The Biogenetic Law of Embryonic Recapitulation
F. Importance of the blastular stage in embryonic development
G. Description of the various types of chordate blastulae with an outline of their organforming areas
1. Protochordate blastula
2. Amphibian blastula
3. Mature blastula in birds
4. Primary and secondary reptilian blastulae
5. Formation of the late mammalian blastocyst (blastula)
a. Prototherian mammal, Echidna
b. Metatherian mammal, Didelphys
c. Eutherian mammals
6. Blastulae of teleost and elasmobranch fishes
7. Blastulae of gymnophionan amphibia
Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning
8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning
A. Introduction
B. Problem of differentiation
1. Definition of differentiation; kinds of differentiation
2. Self-differentiation and dependent differentiation
C. Concept of potency in relation to differentiation
1. Definition of potency
2. Some terms used to describe different states of potency
a. Totipotency and harmonious totipotency
b. Determination and potency limitation
c. Prospective potency and prospective fate
d. Autonomous potency
c. Competence
D. The blastula in relation to twinning
1. Some definitions
a. Dizygotic or fraternal twins
b. Monozygotic or identical twins
c. Polyembryony •
2. Basis of true or identical twinning
3. Some experimentally produced, twinning conditions
E. Importance of the organization center of the late blastula
Gastrulation
| 9. Gastrulation
A. Some definitions and concepts
1. Gastrulation
2. Primitive vertebrate body plan in relation to the process of gastrulation
a. Fundamental body plan of the vertebrate animal
b. The gastrula in relation to the primitive body plan
c. Chart of blastula, gastrula, and primitive, body-form relationships (fig. 188)
B. General processes involved in gastrulation
C. Morphogenetic movement of cells
1. Importance of cell movements during development and in gastrulation
2. Types of cell movement during gastrulation
a. Epiboly
b. Emboly
3. Description of the processes concerned with epiboly
4. Description of the processes involved in emboly
a. Involution and convergence
b. Invagination
c. Concrescence
d. Cell proliferation
e. Polyinvagination
f. Ingression
g. Delamination
h. Divergence
i. Extension
D. The organization center and its relation to the gastrulative process
1. The organization center and the primary organizer
2. Divisions of the primary organizer
E. Chemodifferentiation and the gastrulative process
F. Gastrulation in various Chordata
1. Amphioxus
a. Orientation
b. Gastrulative movements
1 ) Emboly
2) Epiboly
3) Antero-posterior extension of the gastrula and dorsal convergence of the
mesodermal cells
4) Closure of the blastopore
c. Resume of cell movements and processes involved in gastrulation of Amphioxus
1 ) Emboly
2) Epiboly
2. Gastrulation in Amphibia with particular reference to the frog
a. Introduction
1) Orientation
2) Physiological changes which occur in the presumptive, organ-forming areas
of the late blastula and early gastrula as gastrulation progresses
b. Gastrulation
1) Emboly
2) Epiboly
3) Embryo produced by the gastrulative processes
4) Position occupied by the pre -chordal plate material
c. Closure of the blastopore and formation of the neurenteric canal
d. Summary of morphogenetic movements of cells during gastrulation in the frog
and other Amphibia
1) Emboly
2) Epiboly
3. Gastrulation in reptiles
a. Orientation
b. Gastrulation
4. Gastrulation in the chick
a. Orientation
b. Gastrulative changes
1) Development of primitive streak as viewed from the surface of stained
blastoderms
2) Cell movements in the epiblast involved in primitive-streak formation as
indicated by carbon-particle marking and vital-staining experiments
3) Cell movements in the hypoblast and the importance of these movements
in primitive-streak formation
4) Primitive pit notochordal canal
5) Resume of morphogenetic movements of cells during gastrulation in the
chick
5. Gastrulation in mammals
a. Orientation
b. Gastrulation in the pig embryo
c. Gastrulation in other mammals
6. Gastrulation in teleost and elasmobranch fishes
a. Orientation
b. Gastrulation in teleost fishes
1) Emboly
2) Epiboly
3) Summary of the gastrulative processes in teleost fishes
a) Emboly
b) Epiboly
4) Developmental potencies of the germ ring of teleost fishes
c. Gastrulation in elasmobranch fishes
7. Intermediate types of gastrulative behavior
G. The late gastrula as a mosaic of specific, organ-forming territories
H. Autonomous theory of gastrulative movements
I. Exogastrulation
J. Pre-chordal plate and cephalic projection in various chordates
K. Blastoporal and primitive-streak comparisons
Development of Primitive Body Form
10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form
A. Introduction
1. Some of the developmental problems faced by the embryo after gastrulation
a. Tabulation
b. Increase in size and antero-posteri(*)r extension of the tubulated, major organforming areas
c. Regional modifications of the tubulated areas
2. Common, vertebrate, embryonic body form
3. Starting point for tabulation
4. Developmental processes which accomplish tabulation
a. Immediate processes
b. Auxiliary processes
5. Blastocoelic space and body-form development
6. Primitive circulatory tubes or blood vessels
7. Extra-embryonic membranes
B. Tabulation of the neural, epidermal, entodermal, and mesodermal, organ-forming
areas in the vertebrate group
1. Neuralization or the tabulation of the neural plate area
a. Definition
b. Neuralizative processes in the Vertebrata
1) Thickened keel method
2) Neural fold method
c. Closure of the blastopore in rounded gastrulae, such as that of the frog
d. Anterior and posterior neuropores; neurenteric canal
2. Epidermal tabulation
a. Development of the epidermal tube in Amphibia
b. Tabulation of the epidermal area in flat blastoderms
3. Formation of the primitive gut tube (enteric tabulation)
a. Regions of primitive gut tube or early metenteron
b. Formation of the primitive metenteron in the frog
c. Formation of the tubular metenteron in flat blastoderms
4. Tabulation (coelom formation) and other features involved in the early differentiation of the mesodermal areas
a. Early changes in the mesodermal areas
1) Epimere; formation of the somites
2) Mesomere
3) Hypomere
b. Tabulation of the mesodermal areas
C. Notochordal area
D. Lateral constrictive movements
E. Tubulation of the neural, epidermal, entodermal, and mesodermal, organ-forming
areas in Amphioxus
1. Comparison of the problems of tubulation in the embryo of Amphioxus with that
of the embryos in the subphylum Vertebrata
a. End-bud growth
b. Position occupied by the notochord and mesoderm at the end of gastrulation
2. Neuralization and the closure of the blastopore
3. Epidermal tubulation
4. Tubulation of the entodermal area
a. Segregation of the entoderm from the chordamesoderm and the formation of
the primitive metenteric tube
b. Formation of the mouth, anus, and other specialized structures of the metenteron
5. Tubulation of the mesoderm
6. Later differentiation of the myotomic (dorsal) area of the somite
7. Notochord
F. Early development of the rudiments of vertebrate paired appendages
G. The limb bud as an illustration of the field concept of development in relation to the
gastrula and the tubulated embryo
H. Cephalic flexion and general body bending and rotation in vertebrate embryos
I. Influences which play a part in tubulation and organization of body form
J. Basic similarity of body-form development in the vertebrate group of chordate animals
Basic Features of Vertebrate Morphogenesis
11. Basic Features of Vertebrate Morphogenesis
A. Introduction
1. Purpose of This Chapter
2. Definitions
a. Morphogenesis and Related Terms
b. Primitive, Larval, and Definitive Body Forms (see fig. 255)
1) Primitive Body Form.
2) Larval Body Form.
3) Definitive Body Form.
3. Basic or Fundamental Tissues
B. Transformation of the Primitive Body Tubes into the Fundamental
or Basic Condition of the Various Organ Systems
Present in the Primitive Embryonic Body
1. Processes Involved in Basic System Formation
(a) extension and growth of the body tubes,
(b) saccular outgrowths (evaginations) and ingrowths (invaginations)
from restricted areas of the tubes,
(c) cellular migrations away from the primitive tubes fo other tubes and
to the spaces between the tubes, and
(d) unequal growth of different areas along the tubes.
2. Fundamental Similarity of Early Organ Systems
C. Laws of von Baer
D. Contributions of the Mesoderm to Primitive Body Formation and
Later Development
1. Types of Mesodermal Cells
2. Origin of the Mesoderm of the Head Region
a. Head Mesoderm Derived from the Anterior Region of the Trunk
b. Head Mesoderm Derived from the Pre-chordal Plate
c. Head Mesoderm Contributed by Neural Crest Material
d. Head Mesoderm Originating from Post-otic Somites
3. Origin of the Mesoderm of the Tail
4. Contributions of the Trunk Mesoderm to the Developing Body
a. Early Differentiation of the Somites or Epimere
b. Early Differentiation of the Mesomere (Nephrotome)
c. Early Differentiation and Derivatives of the Hypomere
5. Embryonic Mesenchyme and Its Derivatives
1. Neural Plate Area (Ectoderm)
2. Epidermal Area (Ectoderm)
3. Entodermal Area
4. Notochordal Area
5. Mesodermal Areas
6. Germ-cell Area
F. Metamerism
G. Basic Homology of the Vertebrate Organ Systems
1. Definition
2. Basic Homology of Vertebrate Blastulae, Gastrulae, and Tubulated Embryos
Bibliography