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==Basic Features of Vertebrate Morphogenesis==
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A. Introduction
{{Nelsen1953 TOC}}
{{Historic Disclaimer}}
=Part III The Development of Primitive Embryonic Form=
[[Book - Comparative Embryology of the Vertebrates 3|'''Part III - The Development of Primitive Embryonic Form''']]: [[Book - Comparative Embryology of the Vertebrates 3-6|6. Cleavage (Segmentation) and Blastulation]] | [[Book - Comparative Embryology of the Vertebrates 3-7|7. The Chordate Blastula and Its Significance]] | [[Book - Comparative Embryology of the Vertebrates 3-8|8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning]] | [[Book - Comparative Embryology of the Vertebrates 3-9|9. Gastrulation]] | [[Book - Comparative Embryology of the Vertebrates 3-10|10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form]] | [[Book - Comparative Embryology of the Vertebrates 3-11|11. Basic Features of Vertebrate Morphogenesis]]


1. Purpose of This Chapter
=Basic Features of Vertebrate Morphogenesis=


In this chapter, the basic morphogenetic features which give origin to the
==A. Introduction==
later organ systems are emphasized. These features arise from the stream of
morphogenetic phenomena which come down from the fertilized egg through
the periods of cleavage, blastulation, gastrulation, and tabulation. This chapter
thus serves to connect the developmental processes, outlined in Chapters 6
to 10, with those which follow in Chapters 12 to 21. As such, it emphasizes
certain definitions and basic structural features involved in the later morphogenetic activities which mold the adult body form.  


2. Definitions
===1. Purpose of This Chapter===


a. Morphogenesis and Related Terms
In this chapter, the basic morphogenetic features which give origin to the later organ systems are emphasized. These features arise from the stream of morphogenetic phenomena which come down from the fertilized egg through the periods of cleavage, blastulation, gastrulation, and tabulation. This chapter thus serves to connect the developmental processes, outlined in Chapters 6 to 10, with those which follow in Chapters 12 to 21. As such, it emphasizes certain definitions and basic structural features involved in the later morphogenetic activities which mold the adult body form.


The word morphogenesis means the development of form or shape. It involves the elaboration of structural relationships. The morphogenesis of a
===2. Definitions===
particular shape and structure of a cell is called cytomorphosis or cytogenesis
and is synonymous with the term cellular differentiation, considered from the
structural aspect. In the Metazoa, the body is composed of groups of cells,
each cellular group possessing cells of similar form and function. That is, each
cell group is similarly differentiated and specialized. A cellular group, composed of cells similar in form (structure) and function, is called a tissue.
Histology is the study of tissues, and the word histogenesis relates to that
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.
For example, the esophagus, stomach, liver, etc., are organs of the body.
During development, each of the major organ-forming areas, delineated in


a. Morphogenesis and Related Terms


The word morphogenesis means the development of form or shape. It involves the elaboration of structural relationships. The morphogenesis of a particular shape and structure of a cell is called cytomorphosis or cytogenesis and is synonymous with the term cellular differentiation, considered from the structural aspect. In the Metazoa, the body is composed of groups of cells, each cellular group possessing cells of similar form and function. That is, each cell group is similarly differentiated and specialized. A cellular group, composed of cells similar in form (structure) and function, is called a tissue. Histology is the study of tissues, and the word histogenesis relates to that 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. For example, the esophagus, stomach, liver, etc., are organs of the body. During development, each of the major organ-forming areas, delineated in Chapters 6, 7, 9 and 10, produce several specific organs. Organogenesis is concerned with the formation of these specific organs. A group of organs which are associated together to execute one general function form an organ system. The digestive system, for example, has for its general function that of obtaining nourishment for the body. It is composed of a series of o'-gans integrated toward this end. The nervous system, similarly, is an assemblage of specific organs devoted to the discharge of nervous functions. So it is with the other systems of the organism. System development is concerned with the genesis of such systems. The association of various systems, integrated together for the maintenance of the body within a particular habitat, constitutes the organism. Finally, the organism acquires a particular body form because of the form, structure, and activities assumed by its organ systems as a result of their adaptation to the functional necessities of the particular habitat in which the organism lives. It should be urged further that this nice relationship between form and structure, on the one hand, and functional requirements, on the other, is a fundamental principle of development from the egg to the adult. It is a principle intimately associated with the morphogenesis of the organ systems described in Chapters 12 to 21.


518
During development from the egg to the adult form, three major types of body form are evolved in the majority of vertebrate species.


b. Primitive, Larval, and Definitive Body Forms (see fig. 255)


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
1) Primitive Body Form. The condition of primitive or generalized, embryonic body form is attained when the embryo reaches a state in which its developing organ systems resemble the respective developing organ systems in other vertebrate embryos at the same general period of development. (See p. 520.) Superficially, therefore, the general structure of the primitive embryonic body of one species resembles that of the primitive embryonic bodies of other vertebrate species. Such comparable conditions of primitive, body-form development are reached in the 10 to 15-mm. embryo of the shark, Squalus acanthias, of the frog embryo at about 5 to 7 mm., the chick at about 55 to 96 hrs. of incubation, the pig at 6 to 10 mm., and the human at 6 to 10 mm.


2) Larval Body Form. Following primitive body form, the embryo gradually transforms into a larval form. The larval form is present in the period between primitive body form and definitive body form. The larval period is that period during which the basic conditions of the various organ systems, present in primitive body form, undergo a metamorphosis in assuming the form and structure of the adult or definitive body form. In other words, during the larval period, the basic or generalized conditions of the various organ systems are changed into the adult form of the systems, and the larval period thus represents a period of transition. Embryos which develop in the water (most fishes, amphibia) tend to accentuate the larval condition, whereas those which develop within the body of the mother (viviparous teleosts, sharks, mammals) or within well-protected egg membranes (turtle, chick) slur over the larval condition.


Chapters 6, 7, 9 and 10, produce several specific organs. Organogenesis is
concerned with the formation of these specific organs. A group of organs
which are associated together to execute one general function form an organ
system. The digestive system, for example, has for its general function that
of obtaining nourishment for the body. It is composed of a series of o’-gans
integrated toward this end. The nervous system, similarly, is an assemblage
of specific organs devoted to the discharge of nervous functions. So it is with
the other systems of the organism. System development is concerned with
the genesis of such systems. The association of various systems, integrated
together for the maintenance of the body within a particular habitat, constitutes the organism. Finally, the organism acquires a particular body form
because of the form, structure, and activities assumed by its organ systems
as a result of their adaptation to the functional necessities of the particular
habitat in which the organism lives. It should be urged further that this nice
relationship between form and structure, on the one hand, and functional
requirements, on the other, is a fundamental principle of development from
the egg to the adult. It is a principle intimately associated with the morphogenesis of the organ systems described in Chapters 12 to 21.


During development from the egg to the adult form, three major types of  
The larval stage in non-viviparous fishes (see Kyle, '26, pp. 74-82) and in the majority of amphibia is a highly differentiated condition in which the organs of the body are adapted to a free-living, watery existence. The tadpole of the frog, Rana pipiens, from the 6-mm. stage to the 11 -mm. stage, presents a period during which the primitive embryonic condition, present at the time of hatching (i.e., about 5 mm.), is transformed into a well-developed larval stage capable of coping with the external environment. From this time on to metamorphosis, the little tadpole possesses free-living larval features. Another example of a well-developed, free-living, larval stage among vertebrates is that of the eel, Anguilla rostrata. Spawning occurs in the ocean depths around the West Indies and Bermuda. Following the early embryonic stage in which primitive body form is attained, the young transforms into a form very unlike the adult. This form is called the Leptocephalus. The Leptocephalus was formerly classified as a distinct species of pelagic fishes. After many months in the larval stage, it transforms into the adult form of the eel. The latter migrates into fresh-water streams, the American eel into streams east of the Rockies and the European eel into the European streams (Kyle, '26, pp. 54-58). The larval stages in most fishes conform more nearly to the adult form of the fish.
body form are evolved in the majority of vertebrate species.  


b. Primitive, Larval, and Definitive Body Forms (see fig. 255)


1) Primitive Body Form. The condition of primitive or generalized, embryonic body form is attained when the embryo reaches a state in which its
The embryo of Squalus acanthias at 20 to 35 mm. in length, the chick embryo at 5 to 8 days of incubation, the pig embryo of 12- to 18-mm. length, and the human embryo of 12 to 20-mm. length may be regarded as being in the stage of larval transition. The young opossum, when it is born, is in a late larval state. It gradually metamorphoses into the adult body form within the marsupium of the mother (Chap. 22).
developing organ systems resemble the respective developing organ systems
in other vertebrate embryos at the same general period of development. (See p.
520.) Superficially, therefore, the general structure of the primitive embryonic
body of one species resembles that of the primitive embryonic bodies of other
vertebrate species. Such comparable conditions of primitive, body-form development are reached in the 10 to 15-mm. embryo of the shark, Squalus  
acanthias, of the frog embryo at about 5 to 7 mm., the chick at about 55 to  
96 hrs. of incubation, the pig at 6 to 10 mm., and the human at 6 to 10 mm.  


2) Larval Body Form. Following primitive body form, the embryo gradually transforms into a larval form. The larval form is present in the period
between primitive body form and definitive body form. The larval period is
that period during which the basic conditions of the various organ systems,
present in primitive body form, undergo a metamorphosis in assuming the
form and structure of the adult or definitive body form. In other words, during
the larval period, the basic or generalized conditions of the various organ
systems are changed into the adult form of the systems, and the larval period
thus represents a period of transition. Embryos which develop in the water
(most fishes, amphibia) tend to accentuate the larval condition, whereas
those which develop within the body of the mother (viviparous teleosts,


3) Definitive Body Form. The general form and appearance of the adult constitute definitive body form. The young embryo of Squalus acanthias, at about 40 mm. in length, assumes the general appearance of the adult shark; the frog young, after metamorphosis, resembles the adult frog (Chap. 21), the chick of 8 to 13 days of incubation begins to simulate the form of the 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, assumes the appearance of a human being. The transformation of the larval form into the body form of the adult is discussed further in Chapter 21 in relation to the endocrine system.


===3. Basic or Fundamental Tissues===


INTRODUCTION
Through the stages of development to the period when the primitive or generalized, embryonic body form is attained, most of the cells which take part in development are closely associated. In the primitive embryonic body, 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 developing head and tail regions, however, mesoderm is present in the form of 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 forms and to have a large amount of intercellular substance between them. The primitive vascular or blood tubes are composed of epithelium in the sense that the cells are closely arranged. However, as these cells are flattened and show specific peculiarities of structure, this tissue is referred to as endothelium. Also, while the cells of the early neural tube show the typical epithelial features, they soon undergo marked changes characteristic of developing neural tissue. The primitive or generalized, embryonic body thus is composed of four fundamental tissues, viz., epithelial, mesenchymal, endothelial, and neural 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==


519
===1. Processes Involved in Basic System Formation===


As the primitive body tubes (epidermal, neural, enteric, and mesodermal) are established, they are modified gradually to form the basis for the various organ systems. While the notochordal axis is not in the form of a tube, it also undergoes changes during this period. The morphological alterations, which transform the primitive body tubes into the basic or fundamental structural conditions of the systems, consist of the following:


sharks, mammals) or within well-protected egg membranes (turtle, chick)  
(a) extension and growth of the body tubes,
slur over the larval condition.


The larval stage in non-viviparous fishes (see Kyle, ’26, pp. 74-82) and in
(b) saccular outgrowths (evaginations) and ingrowths (invaginations) from restricted areas of the tubes,
the majority of amphibia is a highly differentiated condition in which the
organs of the body are adapted to a free-living, watery existence. The tadpole
of the frog, Rana pipiens, from the 6-mm. stage to the 11 -mm. stage, presents
a period during which the primitive embryonic condition, present at the time
of hatching (i.e., about 5 mm.), is transformed into a well-developed larval
stage capable of coping with the external environment. From this time on to
metamorphosis, the little tadpole possesses free-living larval features. Another
example of a well-developed, free-living, larval stage among vertebrates is
that of the eel, Anguilla rostrata. Spawning occurs in the ocean depths around
the West Indies and Bermuda. Following the early embryonic stage in which
primitive body form is attained, the young transforms into a form very unlike
the adult. This form is called the Leptocephalus. The Leptocephalus was formerly classified as a distinct species of pelagic fishes. After many months in
the larval stage, it transforms into the adult form of the eel. The latter migrates
into fresh-water streams, the American eel into streams east of the Rockies
and the European eel into the European streams (Kyle, ’26, pp. 54-58). The
larval stages in most fishes conform more nearly to the adult form of the fish.


The embryo of Squalus acanthias at 20 to 35 mm. in length, the chick
(c) cellular migrations away from the primitive tubes fo other tubes and to the spaces between the tubes, and
embryo at 5 to 8 days of incubation, the pig embryo of 12- to 18-mm. length,  
and the human embryo of 12 to 20-mm. length may be regarded as being
in the stage of larval transition. The young opossum, when it is born, is in a
late larval state. It gradually metamorphoses into the adult body form within
the marsupium of the mother (Chap. 22).


3) Definitive Body Form. The general form and appearance of the adult
(d) unequal growth of different areas along the tubes.
constitute definitive body form. The young embryo of Squalus acanthias, at
about 40 mm. in length, assumes the general appearance of the adult shark;
the frog young, after metamorphosis, resembles the adult frog (Chap. 21),
the chick of 8 to 13 days of incubation begins to simulate the form of the
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,
assumes the appearance of a human being. The transformation of the larval
form into the body form of the adult is discussed further in Chapter 21 in
relation to the endocrine system.  


3. Basic or Fundamental Tissues
As a result of these changes, the primitive neural, epidermal, enteric, and mesodermal tubes, together with the capillaries or blood tubes and the notochord, experience a state of gradual differentiation which is directed toward 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 systems are described in Chapters 12 to 21.


Through the stages of development to the period when the primitive or
===2. Fundamental Similarity of Early Organ Systems===
generalized, embryonic body form is attained, most of the cells which take
part in development are closely associated. In the primitive embryonic body,
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


The general form and structure of each primitive embryonic system, as it begins to develop in one vertebrate species, exhibits a striking resemblance to the same system in other vertebrate species. This statement is particularly 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 if not physiologically. The problem which confronts the embryo of each species, once the basic conditions of the various systems have been established, is to convert the generalized basic condition of each system into an adult 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 the basic or primitive condition of the various systems into the adult form of the systems constitutes the subject matter of Chapters 12 to 21.


520
The basic conditions of the various organ systems are shown in the structure of shark embryos from 10 to 20 mm. in length, frog embryos of 5 to 10 mm., 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. That is to say, the basic or generalized conditions of the organ systems are present when primitive or generalized embryonic body form is developed. It is impossible to segregate any particular length of embryo in the abovementioned series as the ideal or exact condition showing the basic condition of the systems, as certain systems in one species progress faster than those 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).


==C. Laws of von Baer==


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
As indicated above, the species of the vertebrate group as a whole tend to follow strikingly similar (although not identical) plans of development during blastulation, gastrulation, tubulation, the development of the basic plan of the various systems and primitive body form. As observed in the chapters which follow, the fundamental or basic plan of any particular, organ-forming system, in the early embryo of one species, is comparable to the basic plan of that system in other species throughout the vertebrate group. However, after these basic parallelisms in early development are completed, divergences from the basic plan begin to appear during the formation of the various organ systems of a particular species.




veloping head and tail regions, however, mesoderm is present in the form of  
The classical statements or laws of Karl Ernst von Baer (1792-1876) describe a tendency which appears to be inherent in the developmental procedure of any large group of animals. This developmental tendency is for generalized 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 the procedure principle mentioned in Chapter 7, it may be assumed that general, or common, developmental procedures first are utilized, followed by specific developmental procedures which change the generalized conditions into specific conditions.
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
forms and to have a large amount of intercellular substance between them.  
The primitive vascular or blood tubes are composed of epithelium in the sense
that the cells are closely arranged. However, as these cells are flattened and
show specific peculiarities of structure, this tissue is referred to as endothelium.
Also, while the cells of the early neural tube show the typical epithelial features, they soon undergo marked changes characteristic of developing neural
tissue. The primitive or generalized, embryonic body thus is composed of
four fundamental tissues, viz., epithelial, mesenchymal, endothelial, and neural
tissues.  


B. Transformation of the Primitive Body Tubes into the Fundamental
The laws of von Baer ( 1 828-1837, Part I, p. 224) may be stated as follows:
or Basic Condition of the Various Organ Systems
Present in the Primitive Embryonic Body


1. Processes Involved in Basic System Formation
(a) The general features of a large group of animals appear earlier in development than do the special features;


As the primitive body tubes (epidermal, neural, enteric, and mesodermal)  
(b) after the more general structures are established, less general structures arise, and so on until the most special feature appears;
are established, they are modified gradually to form the basis for the various
organ systems. While the notochordal axis is not in the form of a tube, it also
undergoes changes during this period. The morphological alterations, which
transform the primitive body tubes into the basic or fundamental structural
conditions of the systems, consist of the following:


(a) extension and growth of the body tubes,  
(c) each embryo of a given adult form of animal, instead of passing through or resembling the adult forms of lower members of the group, diverges from the adult forms, because


(b) saccular outgrowths (evaginations) and ingrowths (invaginations)
(d) the embryo of a higher animal species resembles only the embryo of the lower animal species, not the adult form of the lower species.
from restricted areas of the tubes,  


(c) cellular migrations away from the primitive tubes fo other tubes and
==D. Contributions of the Mesoderm to Primitive Body Formation and Later Development==
to the spaces between the tubes, and  


(d) unequal growth of different areas along the tubes.  
The mesoderm is most important to the developing architecture of the body. Because the mesoderm enters so extensively into the structure of the many organs of the developing embryo, it is well to point out further the sources of mesoderm and to delineate the structures and parts arising from this tissue.


As a result of these changes, the primitive neural, epidermal, enteric, and
===1. Types of Mesodermal Cells===
mesodermal tubes, together with the capillaries or blood tubes and the notochord, experience a state of gradual differentiation which is directed toward
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
systems are described in Chapters 12 to 21.


2. Fundamental Similarity of Early Organ Systems
Most of the mesoderm of the early embryo exists in the form of epithelium (see p. 519). As development proceeds, much of the mesoderm loses the close arrangement characteristic of epithelium. In doing so, the cells separate and assume a loose connection. They also may change their shapes, appearing stellate, oval, or irregular, and may wander to distant parts of the body. This loosely aggregated condition of mesoderm forms the primitive mesenchyme. Though most of the mesoderm becomes transformed into mesenchyme, the inner layer of cells of the original hypomeric portion of the mesodermal tubes retains a flattened, cohesive pattern, described as mesothelium. Mesothelium comes to line the various body cavities, for these cavities are derived directly from the hypomeric areas of the mesodermal tubes (Chap. 20).


The general form and structure of each primitive embryonic system, as it
===2. Origin of the Mesoderm of the Head Region===
begins to develop in one vertebrate species, exhibits a striking resemblance


The primary cephalic outgrowth (Chap. 10), which later forms the head structures, contains two basic regions, namely, the head proper and the pharyngeal or branchial region. During its early development, the heart lies at the ventro-caudal extremity of the general head region; it recedes gradually backward as the head and branchial structures develop. The exact origin of the mesoderm which comes to occupy the head proper and pharyngeal areas varies in different gnathostomous vertebrates. The general sources of the head mesoderm may be described in the following manner.


a. Head Mesoderm Derived from the Anterior Region of the Trunk


LAWS OF VON BAER
The mesoderm of the branchial area in lower vertebrates, such as the snarks and, to some degree, the amphibia, represents a direct anterior extension of the mesoderm of the trunk (figs. 217D, E; 230D; 252E) . It is divisible into two parts: (1) a ventro-lateral region, the hypomeric or lateral plate mesoderm, and (2) a dorsal or somitic portion. The latter represents a continuation into 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 as part of the mesoderm of the head proper is the mesoderm associated with 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).


In the higher vertebrates (reptiles, birds, and mammals), the mesoderm of the branchial region appears early, not as a continuous epithelium, as in 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 region (figs. 217F; 23 3B; 234B). This mesenchyme assumes branchial region characteristics, for it later condenses to form the mandibular, hyoid, and more posteriorly located, visceral arches. Also, mesenchymal condensations appear which correspond to the pre-otic head somites formed in the early shark embryo. For example, in the chick, there is an abducent condensation, which corresponds to the hyoid somite of the shark embryo, and a superior oblique condensation corresponding probably to the mandibular somite of the shark embryo (cf. fig. 217D, F). (See also Adelmann, '27, p. 42.) Both of these condensations give origin to eye muscles (Chap. 16). Somewhat similar condensations of mesenchyme which form the rudiments of eye muscles occur in other members of the higher vertebrate group.


521
b. Head Mesoderm Derived from the Pre-chordal Plate


The term pre-chordal plate mesoderm signifies that portion of the head 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 with the foregut entoderm and anterior extremity of the notochord in the late blastula and gastrula in the fishes and amphibia. However, in reptiles, birds, and mammals, this association is established secondarily with the foregut entoderm by means of the notochordal canal and primitive-pit invaginations during gastrulation. (See Chap. 9 and also Hill and Tribe, '24.)


to the same system in other vertebrate species. This statement is particularly
(Note: It is advisable to state that Adelmann, '32, relative to the 19-somite embryo of the urodele Ambystoma pimctatum, distinguishes between a prechordal mesoderm, which forms the core of the mandibular visceral arch, and the pre-chordal plate mesoderm, which earlier in development is associated with the dorsal anterior portion of the foregut entoderm. See figure 252E.)
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
if not physiologically. The problem which confronts the embryo of each
species, once the basic conditions of the various systems have been established,  
is to convert the generalized basic condition of each system into an adult
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
the basic or primitive condition of the various systems into the adult form of
the systems constitutes the subject matter of Chapters 12 to 21.  


The basic conditions of the various organ systems are shown in the structure
During the period when the major organ-forming areas are being tubulated the pre-chordal plate mesoderm separates as a mass of mesenchyme from the antero-dorsal aspect of the foregut, anterior to the cephalic terminus of the notochord (fig. 232G, H). It migrates forward as two groups of mesenchyme connected at first by an interconnecting bridge of mesenchyme. Eventually these two mesenchymal masses become separated and each forms a dense aggregation of mesodermal cells over the mandibular visceral arch and just caudal to the eye (fig. 252E). In the shark embryo and in the chick it gives origin to the pre-mandibular somites (condensations) which probably give origin to the eye muscles innervated by the oculomotor or third cranial nerves. 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 mesenchyme ahead of the level of the first (gill) pouch, but its caudal limit cannot be exactly determined.” Thus it appears that a portion of the head mesoderm in the region anterior to the notochordal termination is derived from the pre-chordal plate mesoderm in all vertebrates.
of shark embryos from 10 to 20 mm. in length, frog embryos of 5 to 10 mm.,
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.  
That is to say, the basic or generalized conditions of the organ systems are
present when primitive or generalized embryonic body form is developed.
It is impossible to segregate any particular length of embryo in the abovementioned series as the ideal or exact condition showing the basic condition
of the systems, as certain systems in one species progress faster than those
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).  


C. Laws of von Baer


As indicated above, the species of the vertebrate group as a whole tend to
follow strikingly similar (although not identical) plans of development during
blastulation, gastrulation, tubulation, the development of the basic plan of the
various systems and primitive body form. As observed in the chapters which
follow, the fundamental or basic plan of any particular, organ-forming system,
in the early embryo of one species, is comparable to the basic plan of that
system in other species throughout the vertebrate group. However, after these
basic parallelisms in early development are completed, divergences from the
basic plan begin to appear during the formation of the various organ systems
of a particular species.


The classical statements or laws of Karl Ernst von Baer (1792-1876) describe a tendency which appears to be inherent in the developmental procedure
Fig. 252. Mesodermal contributions to developing body. (A-D) Sections through developing chick of 48-52 hrs. of incubation. (A) Section through somites of caudal trunk area showing primitive area of mesoderm and coelomic spaces. (B) Section through anterior trunk area depicting early differentiation of somite. (C) Section through trunk area posterior to heart revealing later stage of somite differentiation than that shown in B. (D) Section through developing heart area. Observe dermomyotome, sclerotomic mesenchyme, and mesenchymal contributions of hypomere to forming body substance. (E) Mesodermal contributions to anterior end of developing embryo of Ambystoma of about 19 somites. (Redrawn and modified from Adelmann: 1932, J. Morphol. 54.) (F) Frontal section of early post-hatching larva of Rana pipiens show ing mass of mesoderm lying between gut, epidermal and neural tubes, together with the contributions of the mesoderm to the visceral arches.
of any large group of animals. This developmental tendency is for generalized
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  
the procedure principle mentioned in Chapter 7, it may be assumed that
general, or common, developmental procedures first are utilized, followed by






522
c. Head Mesoderm Contributed by Neural Crest Material


A conspicuous phase of the development of the head region in vertebrate 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 from contributing to the nervous system (Chap. 19), a portion of the neural crest material migrates extensively lateroventrally and comes to lie within the forming visceral (branchial) arches, contributing to the mesoderm in these areas (figs. 222C-F; 230D, F). Also, consult Landacre ('21); Stone ('22, '26, and '29); and Raven ('33a and b). On the other hand, Adelmann ('25) in the rat and Newth ('51 ) in the lamprey, Lampetra planeri, were not able to find evidence substantiating this view. However, pigment cells (melanophores) of the skin probably arise from neural crest cells in the head region of all vertebrate groups.


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
d. Head Mesoderm Originating from Post-otic Somites


There is good evidence that the musculature of the tongue takes its origin 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.


specific developmental procedures which change the generalized conditions
===3. Origin of the Mesoderm of the Tail===
into specific conditions.


The laws of von Baer ( 1 828-1837, Part I, p. 224) may be stated as follows:
In the Amphibia, the tail mesoderm has been traced by means of the Vogt staining method to tail mesoderm in the late blastular and early gastrular 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.


(a) The general features of a large group of animals appear earlier in development than do the special features;
===4. Contributions of the Trunk Mesoderm to the Developing Body===


(b) after the more general structures are established, less general structures
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.
arise, and so on until the most special feature appears;


(c) each embryo of a given adult form of animal, instead of passing
a. Early Differentiation of the Somites or Epimere
through or resembling the adult forms of lower members of the group,
diverges from the adult forms, because


(d) the embryo of a higher animal species resembles only the embryo of  
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.
the lower animal species, not the adult form of the lower species.  


D. Contributions of the Mesoderm to Primitive Body Formation and
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.
Later Development


The mesoderm is most important to the developing architecture of the
body. Because the mesoderm enters so extensively into the structure of the
many organs of the developing embryo, it is well to point out further the
sources of mesoderm and to delineate the structures and parts arising from
this tissue.


b. Early Differentiation of the Mesomere (Nephrotome)


1. Types of Mesodermal Cells
The differentiation of the nephrotome or intermediate mesoderm will be considered later (Chap. 18) in connection with the urogenital system.


Most of the mesoderm of the early embryo exists in the form of epithelium
c. Early Differentiation and Derivatives of the Hypomere
(see p. 519). As development proceeds, much of the mesoderm loses the close
arrangement characteristic of epithelium. In doing so, the cells separate and
assume a loose connection. They also may change their shapes, appearing
stellate, oval, or irregular, and may wander to distant parts of the body. This
loosely aggregated condition of mesoderm forms the primitive mesenchyme.
Though most of the mesoderm becomes transformed into mesenchyme, the
inner layer of cells of the original hypomeric portion of the mesodermal tubes
retains a flattened, cohesive pattern, described as mesothelium. Mesothelium
comes to line the various body cavities, for these cavities are derived directly
from the hypomeric areas of the mesodermal tubes (Chap. 20).


2. Origin of the Mesoderm of the Head Region
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.


The primary cephalic outgrowth (Chap. 10), which later forms the head  
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.
structures, contains two basic regions, namely, the head proper and the
pharyngeal or branchial region. During its early development, the heart lies
at the ventro-caudal extremity of the general head region; it recedes gradually
backward as the head and branchial structures develop. The exact origin of  
the mesoderm which comes to occupy the head proper and pharyngeal areas
varies in different gnathostomous vertebrates. The general sources of the head
mesoderm may be described in the following manner.  




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).


CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION


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.


523


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).


a. Head Mesoderm Derived from the Anterior Region of the Trunk
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 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.


The mesoderm of the branchial area in lower vertebrates, such as the snarks
and, to some degree, the amphibia, represents a direct anterior extension of
the mesoderm of the trunk (figs. 217D, E; 230D; 252E) . It is divisible into two
parts: (1) a ventro-lateral region, the hypomeric or lateral plate mesoderm,
and (2) a dorsal or somitic portion. The latter represents a continuation into
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
as part of the mesoderm of the head proper is the mesoderm associated with
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).


In the higher vertebrates (reptiles, birds, and mammals), the mesoderm
of the branchial region appears early, not as a continuous epithelium, as in
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
region (figs. 217F; 23 3B; 234B). This mesenchyme assumes branchial region
characteristics, for it later condenses to form the mandibular, hyoid, and more
posteriorly located, visceral arches. Also, mesenchymal condensations appear
which correspond to the pre-otic head somites formed in the early shark
embryo. For example, in the chick, there is an abducent condensation, which
corresponds to the hyoid somite of the shark embryo, and a superior oblique
condensation corresponding probably to the mandibular somite of the shark
embryo (cf. fig. 217D, F). (See also Adelmann, ’27, p. 42.) Both of these
condensations give origin to eye muscles (Chap. 16). Somewhat similar condensations of mesenchyme which form the rudiments of eye muscles occur in
other members of the higher vertebrate group.


b. Head Mesoderm Derived from the Pre-chordal Plate
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.) 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.


The term pre-chordal plate mesoderm signifies that portion of the head
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
with the foregut entoderm and anterior extremity of the notochord in the late
blastula and gastrula in the fishes and amphibia. However, in reptiles, birds,
and mammals, this association is established secondarily with the foregut entoderm by means of the notochordal canal and primitive-pit invaginations during
gastrulation. (See Chap. 9 and also Hill and Tribe, ’24.)


(Note: It is advisable to state that Adelmann, ’32, relative to the 19-somite
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 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.
embryo of the urodele Ambystoma pimctatum, distinguishes between a prechordal mesoderm, which forms the core of the mandibular visceral arch, and
the pre-chordal plate mesoderm, which earlier in development is associated
with the dorsal anterior portion of the foregut entoderm. See figure 252E.)


During the period when the major organ-forming areas are being tubulated.


{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.




NEURAL ECTODERM
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.


.RANCHIAL POUCHBRANCHIAL GROOVE
OR gill- SL 1 T AREA


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.) in general, the walls of the coelomic spaces. These two tissues arise directly from the hypomeric mesoderm.


HYPOMERICl
===5. Embryonic Mesenchyme and Its Derivatives===
MESOOERMAI^
CONTRIBUTION ^
TO LATERAL
BODY WALL


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.)


MESENCHYMAL
This mesenchyme ultimately will contribute to the following structures of the body:
CONTRIBUTION
FROM SPLANCHNIC
LAYER OF HYPOMERE


(a) Myocardium (cardiac musculature, etc.) and the epicardium or covering coelomic layer of the heart (Chap. 17),


Fig. 252. Mesodermal contributions to developing body. (A-D) Sections through
(b) endothelium of blood vessels, blood cells (Chap. 17),
developing chick of 48-52 hrs. of incubation. (A) Section through somites of caudal
trunk area showing primitive area of mesoderm and coelomic spaces. (B) Section
through anterior trunk area depicting early differentiation of somite. (C) Section
through trunk area posterior to heart revealing later stage of somite differentiation than
that shown in B. (D) Section through developing heart area. Observe dermomyotome,  
sclerotomic mesenchyme, and mesenchymal contributions of hypomere to forming body
substance. (E) Mesodermal contributions to anterior end of developing embryo of
Ambystoma of about 19 somites. (Redrawn and modified from Adelmann: 1932, J.
Morphol. 54.) (F) Frontal section of early post-hatching larva of Rana pipiens show
ing mass of mesoderm lying between gut, epidermal and neural tubes, together with the
contributions of the mesoderm to the visceral arches.


(c) smooth musculature and connective tissues of blood vessels (Chaps. 16 and 17),


524
(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),


CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
(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),


525
(k) striated (voluntary) extrinsic musculature of the eye (Chap. 16),


(l) intrinsic, smooth musculature of the eye (Chap. 16),


the pre-chordal plate mesoderm separates as a mass of mesenchyme from the
(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
antero-dorsal aspect of the foregut, anterior to the cephalic terminus of the
notochord (fig. 232G, H). It migrates forward as two groups of mesenchyme  
connected at first by an interconnecting bridge of mesenchyme. Eventually
these two mesenchymal masses become separated and each forms a dense
aggregation of mesodermal cells over the mandibular visceral arch and just
caudal to the eye (fig. 252E). In the shark embryo and in the chick it gives
origin to the pre-mandibular somites (condensations) which probably give
origin to the eye muscles innervated by the oculomotor or third cranial nerves.
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
mesenchyme ahead of the level of the first (gill) pouch, but its caudal limit
cannot be exactly determined.” Thus it appears that a portion of the head
mesoderm in the region anterior to the notochordal termination is derived
from the pre-chordal plate mesoderm in all vertebrates.


c. Head Mesoderm Contributed by Neural Crest Material
(n) chromatophores or pigment cells of the body from neural crest mesenchyme (Chap. 12).


A conspicuous phase of the development of the head region in vertebrate
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
from contributing to the nervous system (Chap. 19), a portion of the neural
crest material migrates extensively lateroventrally and comes to lie within the
forming visceral (branchial) arches, contributing to the mesoderm in these
areas (figs. 222C-F; 230D, F). Also, consult Landacre (’21); Stone (’22,
’26, and ’29); and Raven (’33a and b). On the other hand, Adelmann (’25)
in the rat and Newth (’51 ) in the lamprey, Lampetra planeri, were not able
to find evidence substantiating this view. However, pigment cells (melanophores) of the skin probably arise from neural crest cells in the head region
of all vertebrate groups.


d. Head Mesoderm Originating from Post-otic Somites
==E. Summary of Later Derivatives of the Major Presumptive Organforming Areas of the Late Blastula and Gastrula==


There is good evidence that the musculature of the tongue takes its origin
===1. Neural Plate Area (Ectoderm)===
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
This area gives origin to the following:


In the Amphibia, the tail mesoderm has been traced by means of the Vogt
(a) Neural tube,
staining method to tail mesoderm in the late blastular and early gastrular


(b) optic nerves and retinae of eyes,


(c) peripheral nerves and ganglia,


526
(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


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
(f) smooth muscles of iris.


===2. Epidermal Area (Ectoderm)===


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
This area gives origin to:
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
(a) Epidermal tube and derived structures, such as scales, hair, nails, feathers, claws, etc.,
Developing Body


The mesoderm of the trunk area contributes greatly to the development of
(b) lens of the eye, inner ear vesicles, olfactory sense area, general, cutaneous, sense organs of the peripheral area of the body,
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
(c) stomodaeum and its derivatives, oral cavity, anterior lobe of pituitary, enamel organs, and oral glands, and


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
(d) proctodaeum from which arises the lining tissue of the anal canal.
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
===3. Entodermal Area===
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.  


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,


CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
(b) most of the lining tissue of the urinary bladder, vagina, urethra, and associated glands,


(c) Seessel's pocket or head gut, and


527
(d) tail gut.


===4. Notochordal Area===


b. Early Differentiation of the Mesomere (Nephrotome)
This area:


The differentiation of the nephrotome or intermediate mesoderm will be
(a) Forms primitive antero-posterior skeletal axis of all chordate forms,
considered later (Chap. 18) in connection with the urogenital system.


c. Early Differentiation and Derivatives of the Hypomere
(b) aids in induction of central nerve tube.


The lateral-plate mesoderm (hypomere), figure 252A, performs an extremely important function in embryological development. The cavity of the
(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
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.
(d) also, comprises the remains of the notochord in land vertebrates, such as "nucleus pulposus” in man.
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
===5. Mesodermal Areas===
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
These areas give origin to:
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.


(a) Epimeric, mesomeric, and hypomeric areas of primitive mesodermal tube,


(b) epimeric portion also aids in induction of central nerve tube,


528
(c) muscle tissue, involuntary and voluntary,


(d) mesenchyme, connective tissues, including bone, cartilage,


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
(e) blood and lymphoid tissue,


(f) gonads with exception of germ cells, genital ducts, and glandular tissues of male and female reproductive ducts, and


2) Contributions of the Hypomere (Lateral Plate Mesoderm) to the Formation of the Gut Tube and Heart Structures. Throughout the length of the
(g) kidney, ureter, musculature and connective tissues of the bladder, uterus, vagina, and urethra.
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
===6. Germ-cell Area===
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


This area gives origin to:


(a) Primordial germ cells and probably to definitive germ cells of all vertebrates below mammals and


Fig. 253. Diagram illustrating the basic plan of the vertebrate head based upon the
(b) primordial germ cells of mammals and possibly to definitive germ cells.
shark, Scy Ilium canicula. (Modified from Goodrich: 1918, Quart. Jour. Micros. Science, 63.)


==F. Metamerism==


===1. Fundamental Metameric Character of the Trunk and Tail Regions of the Vertebrate Body===


CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
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 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.


529






the hypomeres to the developing heart and gut structures in reptiles, birds, and mammals.  
Fig. 256. Developmental features of the human fac. Modified slightly from models by B. Ziegler, Freiburg, after Karl Peter.
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  
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.
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


===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.


530
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 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.


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS




entodermal lining give origin to smooth muscle tissue, connective tissue, etc.  
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.
(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
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.)
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.  




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.




Fig. 255. This figure illustrates different types of body form in various vertebrates
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 mesoderm 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.
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
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 metamerism (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.






532




BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
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




in general, the walls of the coelomic spaces. These two tissues arise directly
from the hypomeric mesoderm.


5. Embryonic Mesenchyme and Its Derivatives
{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.)


The mesenchymal cells given off from the mesodermal tubes of the trunk
==G. Basic Homology of the Vertebrate Organ Systems==
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
===1. Definition===
the body:


(a) Myocardium (cardiac musculature, etc.) and the epicardium or covering coelomic layer of the heart (Chap. 17),
Homology is the relationship of agreement between the structural parts of one organism and the structural parts of another organism. An agreeable relationship between two structures is established if:


(b) endothelium of blood vessels, blood cells (Chap. 17),
# the two parts occupy the same relative position in the body,
# they arise in the same way embryonically and from the same rudiments, and
# they have the same basic potencies.


(c) smooth musculature and connective tissues of blood vessels (Chaps.
16 and 17),


(d) spleen, lymph glands, and lymph vessels (Chap. 17),
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.


(e) connective tissues of voluntary and involuntary muscles (Chap. 16),
===2. Basic Homology of Vertebrate Blastulae, Gastrulae, and Tubulated Embryos===


(f) connective tissues of soft organs, exclusive of the nerve system (Chap.  
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 mesoderm 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 may be deleted along the respective tubes. Regardless of this possibility, a basic homology, however, appears to exist.
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),
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.


(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).


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 functional purposes. Basically, however, these structures are homologous, although plasis produces adult forms which appear to be different.




SUMMARY OF DERIVATIVES OF ORGAN-FORMING AREAS
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.)




533
Fig. 262. Sections and stereograms of 10 mm. pig embryo.




£. Summary of Later Derivatives of the Major Presumptive Organforming Areas of the Late Blastula and Gastrula
Ibl— (Continued) Sections and stereograms of 10 mm. pig embryo


1. Neural Plate Area (Ectoderm)


This area gives origin to the following:


(a) Neural tube,


(b) optic nerves and retinae of eyes,  
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.


(c) peripheral nerves and ganglia,
{Note: For a discussion of homology, homogeny, plasis, convergence, etc., see Tait, '28.)


(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
==Bibliography==


(f) smooth muscles of iris.  
Adelmann, H. B. 1925. The development of the neural folds and cranial ganglia of the rat. J. Comp. Neurol. 39:19.


2. Epidermal Area (Ectoderm)
. 1927. The development of the eye muscles of the chick. J. Morphol. 44:29.


This area gives origin to:  
. 1932. The development of the prechordal plate and mesoderm of Ambly stoma piinctatum. J. Morphol. 54:1.


(a) Epidermal tube and derived structures, such as scales, hair, nails,  
Baer, K. E. von. 1828-1837. liber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. Erster Theil, 1828; Zweiter Theil, 1837. Konigsberg, Borntriiger.
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,  
Balfour, F. M. 1878. Monograph on the development of elasmobranch fishes. Republished in 1885 in The Works of Francis Maitland Balfour, edited by M. Foster and A. Sedgwick, vol. 1. The Macmillan Co., London.


(c) stomodaeum and its derivatives, oral cavity, anterior lobe of pituitary,
Delsman, H. C. 1922. The Ancestry of Vertebrates. Valkoff & Co., Amersfoort, Holland.
enamel organs, and oral glands, and


(d) proctodaeum from which arises the lining tissue of the anal canal.  
Goodrich, E. S. 1918. On the development of the segments of the head of Scy Ilium. Quart. J. Micr. Sc. 63:1.


3. Entoderm AL Area
{{Ref-Hill1924}}


From this area the following arise:  
Huxley, T. H. 1858. The Croonian lecture — on the theory of the vertebrate skull. Proc. Roy. Soc., London, s.B. 9:381.


(a) Epithelial lining of the primitive gut tube or metenteron, including:
{{Ref-Kingsbury1915}}
(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
. 1924. The significance of the so called law of cephalocaudal differential growth. Anat. Rec, 27:305.
associated glands,  


(c) Seessel’s pocket or head gut, and
. 1926. Branchiomerism and the theory of head segmentation. J. Morphol. 42:83.


(d) tail gut.  
and Adelmann, H. B. 1924. The morphological plan of the head. Quart. J. Micr, Sc. 68:239.


4. Notochordal Area
Kyle, H. M. 1926. The Biology of Fishes. Sidgwick and Jackson, Ltd., London.


This area:  
Landacre, F. L. 1921. The fate of the neural crest in the head of urodeles. J. Comp. Neurol. 33:1.


(a) Forms primitive antero-posterior skeletal axis of all chordate forms,
{{Ref-Lewis1910}}


(b) aids in induction of central nerve tube.  
Locy, W. A. 1895. Contribution to the structure and development of the vertebrate head. J. Morphol. 11:497.


Newth, D. R. 1951. Experiments on the neural crest of the lamprey embryo. J. Exper. Biol. 28:17.


Owen, R. 1848. On the archetype and homologies of the vertebrate skeleton. John Van Voorst, London.


534
Raven, C. P. 1933a. Zur Entwicklung der Ganglienleiste. I. Die Kinematik der Ganglienleistenentwicklung bei den Urodelen. Arch. f. Entwlngsmech. d. Organ. 125:210.


. 1933b. Zur Entwicklung der Ganglienleiste. III. Die Induktionsfahigkeit des Kopfganglienleistenmaterials von Rana fusca.


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
Stone, L. S. 1922. Experiments on the development of the cranial ganglia and the lateral line sense organs in Amblystoma pimctatum. J. Exper. Zool. 35:421.


. 1926. Further experiments on the extirpation and transplantation of mesectoderm in Amhlystorna punctatum. J. Exper. Zool. 44:95.


(c) gives origin to adult notochord of Amphioxus and cyclostomatous fishes
. 1929. Experiments showing the role of migrating neural crest (mesectoderm) in the formation of head skeleton and loose connective tissue in Rana paliistris. Arch. f. Entwicklngsmech. d. Organ. 118:40.
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
Tait, J. 1928. Homology, analogy and plasis. Quart. Rev. Biol. Ill: 151.
as “nucleus pulposus” in man.  


5. Mesodermal Areas
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These areas give origin to:
{{Nelsen1953 TOC}}


(a) Epimeric, mesomeric, and hypomeric areas of primitive mesodermal
{{Footer}}
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
 
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
 
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
 
1. Definition
 
Homology is the relationship of agreement between the structural parts of
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,
Borntriiger.
 
Balfour, F. M. 1878. Monograph on the
development of elasmobranch fishes. Republished in 1885 in The Works of
Francis Maitland Balfour, edited by M.
Foster and A. Sedgwick, vol. 1. The
Macmillan Co., London.
 
Delsman, H. C. 1922. The Ancestry of
Vertebrates. Valkoff & Co., Amersfoort,
Holland.
 
Goodrich, E. S. 1918. On the development
of the segments of the head of Scy Ilium.
Quart. J. Micr. Sc. 63:1.
 
Hill, J. P. and Tribe, M. 1924. The early
development of the cat {Felis dornestica).
Quart. J. Micr. Sc. 68:513.
 
 
Huxley, T. H. 1858. The Croonian lecture
— on the theory of the vertebrate skull.
Proc. Roy. Soc., London, s.B. 9:381.
 
Kingsbury, B. F. 1915. The development
of the human pharynx. 1. Pharyngeal
derivatives. Am. J. Anat. 18:329.
 
. 1924. The significance of the so
called law of cephalocaudal differential
growth. Anat. Rec, 27:305.
 
. 1926. Branchiomerism and the
 
theory of head segmentation. J. Morphol.
42:83.
 
and Adelmann, H. B. 1924. The
 
morphological plan of the head. Quart.
J. Micr, Sc. 68:239.
 
Kyle, H. M. 1926. The Biology of Fishes.
Sidgwick and Jackson, Ltd., London.
 
Landacre, F. L. 1921. The fate of the
neural crest in the head of urodeles. J.
Comp. Neurol. 33:1.
 
Lewis, W. H. 1910. Chapter 12. The development of the muscular system in
Manual of Human Embryology, edited
by F. Keibel and F. P. Mall. J. B. Lippincott Co., Philadelphia.
 
Locy, W. A. 1895. Contribution to the
structure and development of the vertebrate head. J. Morphol. 11:497.
 
 
 
552
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
Newth, D. R. 1951. Experiments on the
neural crest of the lamprey embryo. J.
Exper. Biol. 28:17.
 
Owen, R. 1848. On the archetype and
homologies of the vertebrate skeleton.
John Van Voorst, London.
 
Raven, C. P. 1933a. Zur Entwicklung der
Ganglienleiste. I. Die Kinematik der
Ganglienleistenentwicklung bei den Urodelen. Arch. f. Entwlngsmech. d. Organ.
125:210.
 
. 1933b. Zur Entwicklung der Ganglienleiste. III. Die Induktionsfahigkeit
des Kopfganglienleistenmaterials von
Rana fusca.
 
 
Stone, L. S. 1922. Experiments on the development of the cranial ganglia and
the lateral line sense organs in Amblystoma pimctatum. J. Exper. Zool. 35:421.
 
. 1926. Further experiments on the
 
extirpation and transplantation of mesectoderm in Amhlystorna punctatum. J.
Exper. Zool. 44:95.
 
. 1929. Experiments showing the
 
role of migrating neural crest (mesectoderm) in the formation of head skeleton and loose connective tissue in Rana
paliistris. Arch. f. Entwicklngsmech. d.
Organ. 118:40.
 
Tait, J. 1928. Homology, analogy and
plasis. Quart. Rev. Biol. Ill: 151.

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

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

Basic Features of Vertebrate Morphogenesis

A. Introduction

1. Purpose of This Chapter

In this chapter, the basic morphogenetic features which give origin to the later organ systems are emphasized. These features arise from the stream of morphogenetic phenomena which come down from the fertilized egg through the periods of cleavage, blastulation, gastrulation, and tabulation. This chapter thus serves to connect the developmental processes, outlined in Chapters 6 to 10, with those which follow in Chapters 12 to 21. As such, it emphasizes certain definitions and basic structural features involved in the later morphogenetic activities which mold the adult body form.

2. Definitions

a. Morphogenesis and Related Terms

The word morphogenesis means the development of form or shape. It involves the elaboration of structural relationships. The morphogenesis of a particular shape and structure of a cell is called cytomorphosis or cytogenesis and is synonymous with the term cellular differentiation, considered from the structural aspect. In the Metazoa, the body is composed of groups of cells, each cellular group possessing cells of similar form and function. That is, each cell group is similarly differentiated and specialized. A cellular group, composed of cells similar in form (structure) and function, is called a tissue. Histology is the study of tissues, and the word histogenesis relates to that 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. For example, the esophagus, stomach, liver, etc., are organs of the body. During development, each of the major organ-forming areas, delineated in Chapters 6, 7, 9 and 10, produce several specific organs. Organogenesis is concerned with the formation of these specific organs. A group of organs which are associated together to execute one general function form an organ system. The digestive system, for example, has for its general function that of obtaining nourishment for the body. It is composed of a series of o'-gans integrated toward this end. The nervous system, similarly, is an assemblage of specific organs devoted to the discharge of nervous functions. So it is with the other systems of the organism. System development is concerned with the genesis of such systems. The association of various systems, integrated together for the maintenance of the body within a particular habitat, constitutes the organism. Finally, the organism acquires a particular body form because of the form, structure, and activities assumed by its organ systems as a result of their adaptation to the functional necessities of the particular habitat in which the organism lives. It should be urged further that this nice relationship between form and structure, on the one hand, and functional requirements, on the other, is a fundamental principle of development from the egg to the adult. It is a principle intimately associated with the morphogenesis of the organ systems described in Chapters 12 to 21.

During development from the egg to the adult form, three major types of body form are evolved in the majority of vertebrate species.

b. Primitive, Larval, and Definitive Body Forms (see fig. 255)

1) Primitive Body Form. The condition of primitive or generalized, embryonic body form is attained when the embryo reaches a state in which its developing organ systems resemble the respective developing organ systems in other vertebrate embryos at the same general period of development. (See p. 520.) Superficially, therefore, the general structure of the primitive embryonic body of one species resembles that of the primitive embryonic bodies of other vertebrate species. Such comparable conditions of primitive, body-form development are reached in the 10 to 15-mm. embryo of the shark, Squalus acanthias, of the frog embryo at about 5 to 7 mm., the chick at about 55 to 96 hrs. of incubation, the pig at 6 to 10 mm., and the human at 6 to 10 mm.

2) Larval Body Form. Following primitive body form, the embryo gradually transforms into a larval form. The larval form is present in the period between primitive body form and definitive body form. The larval period is that period during which the basic conditions of the various organ systems, present in primitive body form, undergo a metamorphosis in assuming the form and structure of the adult or definitive body form. In other words, during the larval period, the basic or generalized conditions of the various organ systems are changed into the adult form of the systems, and the larval period thus represents a period of transition. Embryos which develop in the water (most fishes, amphibia) tend to accentuate the larval condition, whereas those which develop within the body of the mother (viviparous teleosts, sharks, mammals) or within well-protected egg membranes (turtle, chick) slur over the larval condition.


The larval stage in non-viviparous fishes (see Kyle, '26, pp. 74-82) and in the majority of amphibia is a highly differentiated condition in which the organs of the body are adapted to a free-living, watery existence. The tadpole of the frog, Rana pipiens, from the 6-mm. stage to the 11 -mm. stage, presents a period during which the primitive embryonic condition, present at the time of hatching (i.e., about 5 mm.), is transformed into a well-developed larval stage capable of coping with the external environment. From this time on to metamorphosis, the little tadpole possesses free-living larval features. Another example of a well-developed, free-living, larval stage among vertebrates is that of the eel, Anguilla rostrata. Spawning occurs in the ocean depths around the West Indies and Bermuda. Following the early embryonic stage in which primitive body form is attained, the young transforms into a form very unlike the adult. This form is called the Leptocephalus. The Leptocephalus was formerly classified as a distinct species of pelagic fishes. After many months in the larval stage, it transforms into the adult form of the eel. The latter migrates into fresh-water streams, the American eel into streams east of the Rockies and the European eel into the European streams (Kyle, '26, pp. 54-58). The larval stages in most fishes conform more nearly to the adult form of the fish.


The embryo of Squalus acanthias at 20 to 35 mm. in length, the chick embryo at 5 to 8 days of incubation, the pig embryo of 12- to 18-mm. length, and the human embryo of 12 to 20-mm. length may be regarded as being in the stage of larval transition. The young opossum, when it is born, is in a late larval state. It gradually metamorphoses into the adult body form within the marsupium of the mother (Chap. 22).


3) Definitive Body Form. The general form and appearance of the adult constitute definitive body form. The young embryo of Squalus acanthias, at about 40 mm. in length, assumes the general appearance of the adult shark; the frog young, after metamorphosis, resembles the adult frog (Chap. 21), the chick of 8 to 13 days of incubation begins to simulate the form of the 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, assumes the appearance of a human being. The transformation of the larval form into the body form of the adult is discussed further in Chapter 21 in relation to the endocrine system.

3. Basic or Fundamental Tissues

Through the stages of development to the period when the primitive or generalized, embryonic body form is attained, most of the cells which take part in development are closely associated. In the primitive embryonic body, 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 developing head and tail regions, however, mesoderm is present in the form of 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 forms and to have a large amount of intercellular substance between them. The primitive vascular or blood tubes are composed of epithelium in the sense that the cells are closely arranged. However, as these cells are flattened and show specific peculiarities of structure, this tissue is referred to as endothelium. Also, while the cells of the early neural tube show the typical epithelial features, they soon undergo marked changes characteristic of developing neural tissue. The primitive or generalized, embryonic body thus is composed of four fundamental tissues, viz., epithelial, mesenchymal, endothelial, and neural 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

As the primitive body tubes (epidermal, neural, enteric, and mesodermal) are established, they are modified gradually to form the basis for the various organ systems. While the notochordal axis is not in the form of a tube, it also undergoes changes during this period. The morphological alterations, which transform the primitive body tubes into the basic or fundamental structural conditions of the systems, consist of the following:

(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.

As a result of these changes, the primitive neural, epidermal, enteric, and mesodermal tubes, together with the capillaries or blood tubes and the notochord, experience a state of gradual differentiation which is directed toward 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 systems are described in Chapters 12 to 21.

2. Fundamental Similarity of Early Organ Systems

The general form and structure of each primitive embryonic system, as it begins to develop in one vertebrate species, exhibits a striking resemblance to the same system in other vertebrate species. This statement is particularly 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 if not physiologically. The problem which confronts the embryo of each species, once the basic conditions of the various systems have been established, is to convert the generalized basic condition of each system into an adult 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 the basic or primitive condition of the various systems into the adult form of the systems constitutes the subject matter of Chapters 12 to 21.

The basic conditions of the various organ systems are shown in the structure of shark embryos from 10 to 20 mm. in length, frog embryos of 5 to 10 mm., 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. That is to say, the basic or generalized conditions of the organ systems are present when primitive or generalized embryonic body form is developed. It is impossible to segregate any particular length of embryo in the abovementioned series as the ideal or exact condition showing the basic condition of the systems, as certain systems in one species progress faster than those 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).

C. Laws of von Baer

As indicated above, the species of the vertebrate group as a whole tend to follow strikingly similar (although not identical) plans of development during blastulation, gastrulation, tubulation, the development of the basic plan of the various systems and primitive body form. As observed in the chapters which follow, the fundamental or basic plan of any particular, organ-forming system, in the early embryo of one species, is comparable to the basic plan of that system in other species throughout the vertebrate group. However, after these basic parallelisms in early development are completed, divergences from the basic plan begin to appear during the formation of the various organ systems of a particular species.


The classical statements or laws of Karl Ernst von Baer (1792-1876) describe a tendency which appears to be inherent in the developmental procedure of any large group of animals. This developmental tendency is for generalized 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 the procedure principle mentioned in Chapter 7, it may be assumed that general, or common, developmental procedures first are utilized, followed by specific developmental procedures which change the generalized conditions into specific conditions.

The laws of von Baer ( 1 828-1837, Part I, p. 224) may be stated as follows:

(a) The general features of a large group of animals appear earlier in development than do the special features;

(b) after the more general structures are established, less general structures arise, and so on until the most special feature appears;

(c) each embryo of a given adult form of animal, instead of passing through or resembling the adult forms of lower members of the group, diverges from the adult forms, because

(d) the embryo of a higher animal species resembles only the embryo of the lower animal species, not the adult form of the lower species.

D. Contributions of the Mesoderm to Primitive Body Formation and Later Development

The mesoderm is most important to the developing architecture of the body. Because the mesoderm enters so extensively into the structure of the many organs of the developing embryo, it is well to point out further the sources of mesoderm and to delineate the structures and parts arising from this tissue.

1. Types of Mesodermal Cells

Most of the mesoderm of the early embryo exists in the form of epithelium (see p. 519). As development proceeds, much of the mesoderm loses the close arrangement characteristic of epithelium. In doing so, the cells separate and assume a loose connection. They also may change their shapes, appearing stellate, oval, or irregular, and may wander to distant parts of the body. This loosely aggregated condition of mesoderm forms the primitive mesenchyme. Though most of the mesoderm becomes transformed into mesenchyme, the inner layer of cells of the original hypomeric portion of the mesodermal tubes retains a flattened, cohesive pattern, described as mesothelium. Mesothelium comes to line the various body cavities, for these cavities are derived directly from the hypomeric areas of the mesodermal tubes (Chap. 20).

2. Origin of the Mesoderm of the Head Region

The primary cephalic outgrowth (Chap. 10), which later forms the head structures, contains two basic regions, namely, the head proper and the pharyngeal or branchial region. During its early development, the heart lies at the ventro-caudal extremity of the general head region; it recedes gradually backward as the head and branchial structures develop. The exact origin of the mesoderm which comes to occupy the head proper and pharyngeal areas varies in different gnathostomous vertebrates. The general sources of the head mesoderm may be described in the following manner.

a. Head Mesoderm Derived from the Anterior Region of the Trunk

The mesoderm of the branchial area in lower vertebrates, such as the snarks and, to some degree, the amphibia, represents a direct anterior extension of the mesoderm of the trunk (figs. 217D, E; 230D; 252E) . It is divisible into two parts: (1) a ventro-lateral region, the hypomeric or lateral plate mesoderm, and (2) a dorsal or somitic portion. The latter represents a continuation into 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 as part of the mesoderm of the head proper is the mesoderm associated with 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).

In the higher vertebrates (reptiles, birds, and mammals), the mesoderm of the branchial region appears early, not as a continuous epithelium, as in 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 region (figs. 217F; 23 3B; 234B). This mesenchyme assumes branchial region characteristics, for it later condenses to form the mandibular, hyoid, and more posteriorly located, visceral arches. Also, mesenchymal condensations appear which correspond to the pre-otic head somites formed in the early shark embryo. For example, in the chick, there is an abducent condensation, which corresponds to the hyoid somite of the shark embryo, and a superior oblique condensation corresponding probably to the mandibular somite of the shark embryo (cf. fig. 217D, F). (See also Adelmann, '27, p. 42.) Both of these condensations give origin to eye muscles (Chap. 16). Somewhat similar condensations of mesenchyme which form the rudiments of eye muscles occur in other members of the higher vertebrate group.

b. Head Mesoderm Derived from the Pre-chordal Plate

The term pre-chordal plate mesoderm signifies that portion of the head 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 with the foregut entoderm and anterior extremity of the notochord in the late blastula and gastrula in the fishes and amphibia. However, in reptiles, birds, and mammals, this association is established secondarily with the foregut entoderm by means of the notochordal canal and primitive-pit invaginations during gastrulation. (See Chap. 9 and also Hill and Tribe, '24.)

(Note: It is advisable to state that Adelmann, '32, relative to the 19-somite embryo of the urodele Ambystoma pimctatum, distinguishes between a prechordal mesoderm, which forms the core of the mandibular visceral arch, and the pre-chordal plate mesoderm, which earlier in development is associated with the dorsal anterior portion of the foregut entoderm. See figure 252E.)

During the period when the major organ-forming areas are being tubulated the pre-chordal plate mesoderm separates as a mass of mesenchyme from the antero-dorsal aspect of the foregut, anterior to the cephalic terminus of the notochord (fig. 232G, H). It migrates forward as two groups of mesenchyme connected at first by an interconnecting bridge of mesenchyme. Eventually these two mesenchymal masses become separated and each forms a dense aggregation of mesodermal cells over the mandibular visceral arch and just caudal to the eye (fig. 252E). In the shark embryo and in the chick it gives origin to the pre-mandibular somites (condensations) which probably give origin to the eye muscles innervated by the oculomotor or third cranial nerves. 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 mesenchyme ahead of the level of the first (gill) pouch, but its caudal limit cannot be exactly determined.” Thus it appears that a portion of the head mesoderm in the region anterior to the notochordal termination is derived from the pre-chordal plate mesoderm in all vertebrates.


Fig. 252. Mesodermal contributions to developing body. (A-D) Sections through developing chick of 48-52 hrs. of incubation. (A) Section through somites of caudal trunk area showing primitive area of mesoderm and coelomic spaces. (B) Section through anterior trunk area depicting early differentiation of somite. (C) Section through trunk area posterior to heart revealing later stage of somite differentiation than that shown in B. (D) Section through developing heart area. Observe dermomyotome, sclerotomic mesenchyme, and mesenchymal contributions of hypomere to forming body substance. (E) Mesodermal contributions to anterior end of developing embryo of Ambystoma of about 19 somites. (Redrawn and modified from Adelmann: 1932, J. Morphol. 54.) (F) Frontal section of early post-hatching larva of Rana pipiens show ing mass of mesoderm lying between gut, epidermal and neural tubes, together with the contributions of the mesoderm to the visceral arches.


c. Head Mesoderm Contributed by Neural Crest Material

A conspicuous phase of the development of the head region in vertebrate 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 from contributing to the nervous system (Chap. 19), a portion of the neural crest material migrates extensively lateroventrally and comes to lie within the forming visceral (branchial) arches, contributing to the mesoderm in these areas (figs. 222C-F; 230D, F). Also, consult Landacre ('21); Stone ('22, '26, and '29); and Raven ('33a and b). On the other hand, Adelmann ('25) in the rat and Newth ('51 ) in the lamprey, Lampetra planeri, were not able to find evidence substantiating this view. However, pigment cells (melanophores) of the skin probably arise from neural crest cells in the head region of all vertebrate groups.

d. Head Mesoderm Originating from Post-otic Somites

There is good evidence that the musculature of the tongue takes its origin 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

In the Amphibia, the tail mesoderm has been traced by means of the Vogt staining method to tail mesoderm in the late blastular and early gastrular 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

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.


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

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.


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 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.


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.) 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.


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 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.) in general, the walls of the coelomic spaces. These two tissues arise directly from the hypomeric mesoderm.

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).


E. Summary of Later Derivatives of the Major Presumptive Organforming Areas of the Late Blastula and Gastrula

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)

This area gives origin to:

(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. Entodermal 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

This area:

(a) Forms primitive antero-posterior skeletal axis of all chordate forms,

(b) aids in induction of central nerve tube.

(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

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

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

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 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.



Fig. 256. Developmental features of the human fac. Modified slightly from models by B. Ziegler, Freiburg, after Karl Peter.


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 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.


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.



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 mesoderm 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 metamerism (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.



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


{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

1. Definition

Homology is the relationship of agreement between the structural parts of 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 mesoderm 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 may be deleted along the respective tubes. Regardless of this possibility, a basic homology, however, appears to exist.



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.



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 functional purposes. Basically, however, these structures are homologous, although plasis produces adult forms which appear to be different.


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.)


Fig. 262. Sections and stereograms of 10 mm. pig embryo.


Ibl— (Continued) Sections and stereograms of 10 mm. pig embryo



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, Borntriiger.

Balfour, F. M. 1878. Monograph on the development of elasmobranch fishes. Republished in 1885 in The Works of Francis Maitland Balfour, edited by M. Foster and A. Sedgwick, vol. 1. The Macmillan Co., London.

Delsman, H. C. 1922. The Ancestry of Vertebrates. Valkoff & Co., Amersfoort, Holland.

Goodrich, E. S. 1918. On the development of the segments of the head of Scy Ilium. Quart. J. Micr. Sc. 63:1.

Hill JP. and Tribe M. The early development of the cat (Felis domestica). (1924) Quart. J. Microsc. Sci., 68: 513-602.

Huxley, T. H. 1858. The Croonian lecture — on the theory of the vertebrate skull. Proc. Roy. Soc., London, s.B. 9:381.

Template:Ref-Kingsbury1915

. 1924. The significance of the so called law of cephalocaudal differential growth. Anat. Rec, 27:305.

. 1926. Branchiomerism and the theory of head segmentation. J. Morphol. 42:83.

and Adelmann, H. B. 1924. The morphological plan of the head. Quart. J. Micr, Sc. 68:239.

Kyle, H. M. 1926. The Biology of Fishes. Sidgwick and Jackson, Ltd., London.

Landacre, F. L. 1921. The fate of the neural crest in the head of urodeles. J. Comp. Neurol. 33:1.

Lewis WH. XII. The development of the muscular system pp 454-522 in Keibel F. and Mall FP. Manual of Human Embryology I. (1910) J. B. Lippincott Company, Philadelphia.

Locy, W. A. 1895. Contribution to the structure and development of the vertebrate head. J. Morphol. 11:497.

Newth, D. R. 1951. Experiments on the neural crest of the lamprey embryo. J. Exper. Biol. 28:17.

Owen, R. 1848. On the archetype and homologies of the vertebrate skeleton. John Van Voorst, London.

Raven, C. P. 1933a. Zur Entwicklung der Ganglienleiste. I. Die Kinematik der Ganglienleistenentwicklung bei den Urodelen. Arch. f. Entwlngsmech. d. Organ. 125:210.

. 1933b. Zur Entwicklung der Ganglienleiste. III. Die Induktionsfahigkeit des Kopfganglienleistenmaterials von Rana fusca.

Stone, L. S. 1922. Experiments on the development of the cranial ganglia and the lateral line sense organs in Amblystoma pimctatum. J. Exper. Zool. 35:421.

. 1926. Further experiments on the extirpation and transplantation of mesectoderm in Amhlystorna punctatum. J. Exper. Zool. 44:95.

. 1929. Experiments showing the role of migrating neural crest (mesectoderm) in the formation of head skeleton and loose connective tissue in Rana paliistris. Arch. f. Entwicklngsmech. d. Organ. 118:40.

Tait, J. 1928. Homology, analogy and plasis. Quart. Rev. Biol. Ill: 151.

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