Book - Comparative Embryology of the Vertebrates 3: Difference between revisions

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1. Purpose of This Chapter  
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  
2. Definitions  


a. Morphogenesis and Related Terms  
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
518
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
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)  
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
1) Primitive Body Form.  
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
2) Larval Body Form.
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.  
 
INTRODUCTION
 
 
519
 
 
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  
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 de
520
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
veloping 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  
B. Transformation of the Primitive Body Tubes into the Fundamental  
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1. Processes Involved in Basic System Formation  
1. Processes Involved in Basic System Formation  
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,  
(a) extension and growth of the body tubes,  
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(d) unequal growth of different areas along the tubes.  
(d) unequal growth of different areas along the tubes.  


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  
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
LAWS OF VON BAER
521
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  
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
522
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
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  
D. Contributions of the Mesoderm to Primitive Body Formation and  
Later Development  
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  
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  
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.
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
523


a. Head Mesoderm Derived from the Anterior Region of the Trunk  
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  
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.
NEURAL ECTODERM
.RANCHIAL POUCHBRANCHIAL GROOVE
OR gill- SL 1 T AREA
HYPOMERICl
MESOOERMAI^
CONTRIBUTION ^
TO LATERAL
BODY WALL
MESENCHYMAL
CONTRIBUTION
FROM SPLANCHNIC
LAYER OF HYPOMERE
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.
524
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
525
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.


c. Head Mesoderm Contributed by Neural Crest Material  
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  
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  
3. Origin of the Mesoderm of the Tail  


In the Amphibia, the tail mesoderm has been traced by means of the Vogt
4. Contributions of the Trunk Mesoderm to the Developing Body  
staining method to tail mesoderm in the late blastular and early gastrular
 
 
 
526
 
 
BASIC FEATURES OF VERTEBRATE MORPHOGENESIS
 
 
stages. At the time of tail-rudiment formation, this mesoderm forms two bilateral masses of cells located within the “tail bud” or “end bud.” These cellular
masses proliferate extensively as the tail bud grows caudally and give origin
to the mesoderm of the tail. Similarly, in other vertebrates, the mesoderm of
the future tail is present as mesenchyme in the terminal portion of the tail
bud. These mesenchymal cells proliferate, as the tail grows caudalward, and
leave behind the mesoderm, which gradually condenses into the epithelial
masses or segments (myotomes) along either side of the notochord and
neural tube.
 
4. Contributions of the Trunk Mesoderm to the  
Developing Body  
 
The mesoderm of the trunk area contributes greatly to the development of
the many body organs and systems in the trunk region. Details of this contribution will be described in the chapters which follow, but, at this point, it
is well to survey the initial activities of the mesodermal tubes of the trunk
area in producing the vertebrate body.


a. Early Differentiation of the Somites or Epimere  
a. Early Differentiation of the Somites or Epimere  
The somites (figs. 217, 237, 252) contribute much to the developing structure of the vertebrate body. This fact is indicated by their early growth and
differentiation. For example, the ventro-mesial wall of the fully developed
somite gradually separates from the rest of the somite and forms a mass of
mesenchymal cells which migrates mesad around the notochord and also
dorsad around the neural tube (fig. 252A-C). The mesenchyme which thus
arises from the somite is known as the sclerotome. In the somite of the higher
vertebrates just previous to the origin of the sclerotome, a small epithelial
core of cells becomes evident in the myocoel; this core contributes to the
sclerotomic material (fig. 252B). As a result of the segregation of the sclerotomic tissue and its migration mesad to occupy the areas around the notochord
and nerve cord, the latter structures become enmeshed by a primitive skeletogenous mesenchyme. The notochord and sclerotomic mesenchyme are the
foundation for the future axial skeleton of the adult, including the vertebral
elements and the caudal part of the cranium as described in Chapter 15.
After the departure of sclerotomic material, myotomic and dermatomic
portions of the somite soon rearrange themselves into a hollow structure (fig.
252C, D), in which the myotome forms the inner wall and the dermatome
the outer aspect. This composite structure is the dermomyotome, and the
cavity within, the secondary myocoei. In many vertebrates (fishes, amphibia,
reptiles, and birds), the dermatome gives origin to cells which migrate into
the region of the developing dermis (Chap. 12) and contributes to the formation of this layer of the skin.
CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION
527


b. Early Differentiation of the Mesomere (Nephrotome)  
b. Early Differentiation of the Mesomere (Nephrotome)  
The differentiation of the nephrotome or intermediate mesoderm will be
considered later (Chap. 18) in connection with the urogenital system.


c. Early Differentiation and Derivatives of the Hypomere  
c. Early Differentiation and Derivatives of the Hypomere  


The lateral-plate mesoderm (hypomere), figure 252A, performs an extremely important function in embryological development. The cavity of the
hypomere (splanchnocoel) and the cellular offspring from the hypomeric mesoderm, which forms the wall of this cavity, give origin to much of the structural material and arrangement of the adult body.


1) Contributions of the Hypomere (I^ateral Plate Mesoderm) to the Developing Pharyngeal Area of the Gut Tube. The developing foregut (Chap.  
1) Contributions of the Hypomere (I^ateral Plate Mesoderm) to the Developing Pharyngeal Area of the Gut Tube. The developing foregut (Chap.  

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

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Part III The Development of Primitive Embryonic Form

Part III - The Development of Primitive Embryonic Form: 6. Cleavage (Segmentation) and Blastulation | 7. The Chordate Blastula and Its Significance | 8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning | 9. Gastrulation | 10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form | 11. Basic Features of Vertebrate Morphogenesis

The general procedures leading to the development of primitive embryonic body form in the chordate group of animals are:

( 1 ) Cleavage. Cleavage is the division of the egg into progressively smaller cellular units, the blaslomeres (Chap. 6).

(2) Blastulation. Blastulalion results in the formation of the blastula. The blastula is composed of a cellular blastoderm in relation to a fluid-filled cavity, the blastocoel. The blastoderm of the late blastula is composed of neural, epidermal, notochordal, mesodermal, and entodermal major presumptive organ-forming areas. In the phylum Chordata, the notochordal area is the central region around which the other areas are oriented (Chap. 7). The major presumptive organ-forming areas of the late blastula exist in various degrees of differentiation (Chap. 8).

(3) Gastrulation. This is the process which effects a reorientation of the presumptive organ-forming areas and brings about their axiation antero-posteriorly in relation to the notochordal axis and the future embryonic body (Chap. 9). During gastrulation the major organ-forming areas are subdivided into minor areas or fields, each field being restricted to the development of a particular organ or part. (Pp. 378, 446, 447.

(4) Following gastrulation, the next step in the development of embryonic body form is tubulation and extension of the major organ-forming areas (Chap. 10).

(5) As tubulation and extension of the organ-forming areas is effected, the basic or fundamental conditions of the future organ systems are established, resulting in the development of primitive body form. As the development of various vertebrate embryos is strikingly similar up to this point, the primitive embryonic body forms of all vertebrates resemble each other (Chap. II).

In the drawings presented in Part III, the following scheme for designating the major organ-forming areas existing within the three germ layers is adhered to:


Cleavage (Segmentation) and Blastulation

6. Cleavage (Segmentation) and Blastulation

A. General considerations

1. Definitions

2. Early history of the cleavage (cell-division) concept

3. Importance of the cleavage-blastular period of development

a. Morphological relationships of the blastula

b. Physiological relationships of the blastula

1 ) Hybrid crosses

2) Artificial parthenogenesis

3) Oxygen-block studies

4. Geometrical relations of early cleavage

a. Meridional plane

b. Vertical plane

c. Equatorial plane

d. Latitudinal plane

5. Some fundamental factors involved in the early cleavage of the egg

a. Mechanisms associated with mitosis or cell division

b. Influence of cytoplasmic substance and egg organization upon cleavage

1) Yolk

2) Organization of the egg

c. Influence of first cleavage amphiaster on polyspermy

d. Viscosity changes during cleavage

e. Cleavage laws

1 ) Sach’s rules

2) Hertwig’s laws

6. Relation of early cleavage planes to the antero-posterior axis of the embryo

B. Types of cleavage in the phylum Chordata

1. Typical holoblastic cleavage

a. Amphioxus

b. Frog (Rana pipiens and R. sylvatica)

c. Cyclostomata

2. Atypical types of holoblastic cleavage

a. Holoblastic cleavage in the egg of the metatherian and eutherian mammals

1 ) General considerations

2) Early development of the rabbit egg

a) Two-cell stage

b) Four-cell stage

c) Eight-cell stage

d) Sixteen-cell stage

e) Morula stage

f) Early blastocyst

3) Types of mammalian blastocysts (blastulae)

b. Holoblastic cleavage of the transitional or intermediate type

1) Amhystoma maculatum (punctatum)

2) Lepidosiren paradoxa

3) Necturus maculosus

4) Acipenser sturio

5) Amia calva

6) Lepisosteus (Lepidosteus) osseus

7) Gymnophionan amphibia 3. Meroblastic cleavage

a. Egg of the common fowl

1 ) Early cleavages

2) Formation of the periblast tissue

3) Morphological characteristics of the primary blastula

4) Polyspermy and fate of the accessory sperm nuclei

b. Elasmobranch fishes

1 ) Cleavage and formation of the early blastula

2) Problem of the periblast tissue in elasmobranch fishes

c. Teleost fishes

1) Cleavage and early blastula formation

2) Origin of the periblast tissue in teleost fishes

d. Prototherian Mammalia

e. Cleavage in the California hagfish, Polistotrema (Bdellostorna) stouti

C. What is the force which causes the blastomeres to adhere together during early cleavage?

D. Progressive cytoplasmic inequality and nuclear equality of the cleavage blastomeres

1. Cytoplasmic inequality of the early blastomeres

2. Nuclear equality of the early blastomeres

E. Quantitative and qualitative cleavages and their influence upon later development

The Chordate Blastula and Its Significance

7. The Chordate Blastula and Its Significance

A. Introduction

1. Blastulae without auxiliary tissue

2. Blastulae with auxiliary or trophoblast tissue

3. Comparison of the two main blastular types

B. History of the concept of specific, organ-forming areas

C. Theory of epigenesis and the germ-layer concept of development

D. Introduction of the words ectoderm, mesoderm, endoderm

E. Importance of the blastular stage in Haeckel's theory of The Biogenetic Law of Embryonic Recapitulation

F. Importance of the blastular stage in embryonic development

G. Description of the various types of chordate blastulae with an outline of their organforming areas

1. Protochordate blastula

2. Amphibian blastula

3. Mature blastula in birds

4. Primary and secondary reptilian blastulae

5. Formation of the late mammalian blastocyst (blastula)

a. Prototherian mammal, Echidna

b. Metatherian mammal, Didelphys

c. Eutherian mammals

6. Blastulae of teleost and elasmobranch fishes

7. Blastulae of gymnophionan amphibia

Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning

8. The Late Blastula in Relation to Certain Innate Physiological Conditions: Twinning

A. Introduction

B. Problem of differentiation

1. Definition of differentiation; kinds of differentiation

2. Self-differentiation and dependent differentiation

C. Concept of potency in relation to differentiation

1. Definition of potency

2. Some terms used to describe different states of potency

a. Totipotency and harmonious totipotency

b. Determination and potency limitation

c. Prospective potency and prospective fate

d. Autonomous potency c. Competence

D. The blastula in relation to twinning

1. Some definitions

a. Dizygotic or fraternal twins

b. Monozygotic or identical twins

c. Polyembryony •

2. Basis of true or identical twinning

3. Some experimentally produced, twinning conditions

E. Importance of the organization center of the late blastula

Gastrulation

| 9. Gastrulation

A. Some definitions and concepts

1. Gastrulation

2. Primitive vertebrate body plan in relation to the process of gastrulation

a. Fundamental body plan of the vertebrate animal

b. The gastrula in relation to the primitive body plan

c. Chart of blastula, gastrula, and primitive, body-form relationships (fig. 188)

B. General processes involved in gastrulation

C. Morphogenetic movement of cells

1. Importance of cell movements during development and in gastrulation

2. Types of cell movement during gastrulation

a. Epiboly

b. Emboly

3. Description of the processes concerned with epiboly

4. Description of the processes involved in emboly

a. Involution and convergence

b. Invagination

c. Concrescence

d. Cell proliferation

e. Polyinvagination

f. Ingression

g. Delamination

h. Divergence

i. Extension

D. The organization center and its relation to the gastrulative process

1. The organization center and the primary organizer

2. Divisions of the primary organizer

E. Chemodifferentiation and the gastrulative process

F. Gastrulation in various Chordata 1. Amphioxus

a. Orientation

b. Gastrulative movements

1 ) Emboly

2) Epiboly

3) Antero-posterior extension of the gastrula and dorsal convergence of the mesodermal cells

4) Closure of the blastopore

c. Resume of cell movements and processes involved in gastrulation of Amphioxus

1 ) Emboly

2) Epiboly

2. Gastrulation in Amphibia with particular reference to the frog

a. Introduction

1) Orientation

2) Physiological changes which occur in the presumptive, organ-forming areas of the late blastula and early gastrula as gastrulation progresses

b. Gastrulation

1) Emboly

2) Epiboly

3) Embryo produced by the gastrulative processes

4) Position occupied by the pre -chordal plate material

c. Closure of the blastopore and formation of the neurenteric canal

d. Summary of morphogenetic movements of cells during gastrulation in the frog and other Amphibia

1) Emboly

2) Epiboly

3. Gastrulation in reptiles

a. Orientation

b. Gastrulation

4. Gastrulation in the chick

a. Orientation

b. Gastrulative changes

1) Development of primitive streak as viewed from the surface of stained blastoderms

2) Cell movements in the epiblast involved in primitive-streak formation as indicated by carbon-particle marking and vital-staining experiments

3) Cell movements in the hypoblast and the importance of these movements in primitive-streak formation

4) Primitive pit notochordal canal

5) Resume of morphogenetic movements of cells during gastrulation in the chick

5. Gastrulation in mammals

a. Orientation

b. Gastrulation in the pig embryo

c. Gastrulation in other mammals

6. Gastrulation in teleost and elasmobranch fishes

a. Orientation

b. Gastrulation in teleost fishes

1) Emboly

2) Epiboly

3) Summary of the gastrulative processes in teleost fishes

a) Emboly

b) Epiboly

4) Developmental potencies of the germ ring of teleost fishes

c. Gastrulation in elasmobranch fishes

7. Intermediate types of gastrulative behavior

G. The late gastrula as a mosaic of specific, organ-forming territories

H. Autonomous theory of gastrulative movements

I. Exogastrulation

J. Pre-chordal plate and cephalic projection in various chordates

K. Blastoporal and primitive-streak comparisons

Development of Primitive Body Form

10. Tubulation and Extension of the Major Organ-forming Areas: Development of Primitive Body Form

A. Introduction

1. Some of the developmental problems faced by the embryo after gastrulation

a. Tabulation

b. Increase in size and antero-posteri(*)r extension of the tubulated, major organforming areas

c. Regional modifications of the tubulated areas

2. Common, vertebrate, embryonic body form

3. Starting point for tabulation

4. Developmental processes which accomplish tabulation

a. Immediate processes

b. Auxiliary processes

5. Blastocoelic space and body-form development

6. Primitive circulatory tubes or blood vessels

7. Extra-embryonic membranes

B. Tabulation of the neural, epidermal, entodermal, and mesodermal, organ-forming areas in the vertebrate group

1. Neuralization or the tabulation of the neural plate area

a. Definition

b. Neuralizative processes in the Vertebrata

1) Thickened keel method

2) Neural fold method

c. Closure of the blastopore in rounded gastrulae, such as that of the frog

d. Anterior and posterior neuropores; neurenteric canal

2. Epidermal tabulation

a. Development of the epidermal tube in Amphibia

b. Tabulation of the epidermal area in flat blastoderms

3. Formation of the primitive gut tube (enteric tabulation)

a. Regions of primitive gut tube or early metenteron

b. Formation of the primitive metenteron in the frog

c. Formation of the tubular metenteron in flat blastoderms

4. Tabulation (coelom formation) and other features involved in the early differentiation of the mesodermal areas

a. Early changes in the mesodermal areas

1) Epimere; formation of the somites

2) Mesomere

3) Hypomere

b. Tabulation of the mesodermal areas

C. Notochordal area

D. Lateral constrictive movements

E. Tubulation of the neural, epidermal, entodermal, and mesodermal, organ-forming areas in Amphioxus

1. Comparison of the problems of tubulation in the embryo of Amphioxus with that of the embryos in the subphylum Vertebrata

a. End-bud growth

b. Position occupied by the notochord and mesoderm at the end of gastrulation

2. Neuralization and the closure of the blastopore

3. Epidermal tubulation

4. Tubulation of the entodermal area

a. Segregation of the entoderm from the chordamesoderm and the formation of the primitive metenteric tube

b. Formation of the mouth, anus, and other specialized structures of the metenteron

5. Tubulation of the mesoderm

6. Later differentiation of the myotomic (dorsal) area of the somite

7. Notochord

F. Early development of the rudiments of vertebrate paired appendages

G. The limb bud as an illustration of the field concept of development in relation to the gastrula and the tubulated embryo

H. Cephalic flexion and general body bending and rotation in vertebrate embryos

I. Influences which play a part in tubulation and organization of body form

J. Basic similarity of body-form development in the vertebrate group of chordate animals

Basic Features of Vertebrate Morphogenesis

11. Basic Features of Vertebrate Morphogenesis

A. Introduction

1. Purpose of This Chapter

2. Definitions

a. Morphogenesis and Related Terms

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

1) Primitive Body Form.

2) Larval Body Form.

3) Definitive Body Form.


3. Basic or Fundamental Tissues

B. Transformation of the Primitive Body Tubes into the Fundamental or Basic Condition of the Various Organ Systems Present in the Primitive Embryonic Body

1. Processes Involved in Basic System Formation

(a) extension and growth of the body tubes,

(b) saccular outgrowths (evaginations) and ingrowths (invaginations) from restricted areas of the tubes,

(c) cellular migrations away from the primitive tubes fo other tubes and to the spaces between the tubes, and

(d) unequal growth of different areas along the tubes.


2. Fundamental Similarity of Early Organ Systems

C. Laws of von Baer

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

1. Types of Mesodermal Cells

2. Origin of the Mesoderm of the Head Region

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

b. Head Mesoderm Derived from the Pre-chordal Plate

c. Head Mesoderm Contributed by Neural Crest Material

d. Head Mesoderm Originating from Post-otic Somites

3. Origin of the Mesoderm of the Tail

4. Contributions of the Trunk Mesoderm to the Developing Body

a. Early Differentiation of the Somites or Epimere

b. Early Differentiation of the Mesomere (Nephrotome)

c. Early Differentiation and Derivatives of the Hypomere


1) Contributions of the Hypomere (I^ateral Plate Mesoderm) to the Developing Pharyngeal Area of the Gut Tube. The developing foregut (Chap. 13) may be divided into four main areas, namely, (1) head gut, (2) pharyngeal, (3) esophageal, and (4) stomach areas. The head gut is small and represents a pre-oral extension of the gut; the pharyngeal area is large and expansive and forms about half of the forming foregut in the early embryo; the esophageal segment is small and constricted; and the forming stomach region is enlarged. At this point, however, concern is given specifically to the developing foregut in relation to the early development of the pharyngeal region.

In the pharyngeal area the foregut expands laterally. Beginning at its anterior end, it sends outward a series of paired, pouch-like diverticula, known as the branchial (pharyngeal or visceral) pouches. These pouches push outward toward the ectodermal (epidermal) layer. In doing so, they separate the lateral plate mesoderm which synchronously has divided into columnar masses or cells (fig. 252E, F). Normally, about four to six pairs of branchial (pharyngeal) pouches are formed in gnathostomous vertebrates, although in the cyclostomatous fish, Petromyzon, eight pairs appear. In the embryo of the shark, Squalus acanthias, six pairs are formed, while in the amphibia, four to six pairs of pouches may appear (fig. 252F). In the chick, pig, and human, four pairs of pouches normally occur (figs. 259, 261). Also, invaginations or inpushings of the epidermal layer occur, the branchial grooves (visceral furrows); the latter meet the entodermal outpocketings (figs. 252F; 262B).

The end result of all these developmental movements in the branchial area is to produce elongated, dorso-ventral, paired columns of mesodermal cells (figs. 252E; 253), the visceral or branchial arches, which alternate with the branchial-groove-pouch or gill-slit areas (figs. 252F; 253). The most anterior pair of visceral arches forms the mandibular visceral arches; the second pair forms the hyoid visceral arches; and the succeeding pairs form the branchial (gill) arches (figs. 239C, D; 240; 244; 246; 252E; 253). The branchial arches with their mesodermal columns of cells will, together with the contributions from the neural crest cells referred to above, give origin to the connective, muscle, and blood-vessel-forming tissues in this area.


528


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS


2) Contributions of the Hypomere (Lateral Plate Mesoderm) to the Formation of the Gut Tube and Heart Structures. Throughout the length of the forming gut tube, from the oral area to the anal region, the lateral plate mesoderm (mesoblast) contributes much to the forming gut tube. This is occasioned to a great extent posterior to the pharyngeal area by the fact that the inner or mesial walls of the two hypomeres enswathe the forming gut tube as they fuse in the median plane (fig. 241), forming the dorsal and ventral mesenteries of the gut. However, in the heart area, due to the dorsal displacement of the foregut, the dorsal mesentery is vestigial or absent while the ventral mesentery is increased in extent. Each mesial wall of the hypomeric mesoderm, forming the ventral mesentery in the region of the developing heart, becomes cupped around the primitive blood capillaries, coursing anteriad in this area to form the rudiments of the developing heart. The ventral mesentery in the heart area thus gives origin to the dorsal mesocardium, the ventral mesocardium, and the rudimentary, cup-shaped, cpimyocardial structures around the fusing blood capillaries (figs. 236C-D; 254A). The primitive blood capillaries soon unite to form the rudiment of the future endocardium of the heart, while the enveloping epimyocardium establishes the rudiment of the future muscle and connective tissues of the heart (Chap. 17).

On the other hand, in the region of the stomach and continuing posteriorly to the anal area of the gut, the movement mediad of the mesial walls of the two lateral plate (hypomeric) mesodermal areas occurs in such a way as to


Fig. 253. Diagram illustrating the basic plan of the vertebrate head based upon the shark, Scy Ilium canicula. (Modified from Goodrich: 1918, Quart. Jour. Micros. Science, 63.)


CONTRIBUTIONS OF THE MESODERM TO PRIMITIVE BODY FORMATION


529


the hypomeres to the developing heart and gut structures in reptiles, birds, and mammals. Sections are drawn through the following regions: (A) Through primitive tubular heart anterior to sinus venosus. (B) Through caudal end of sinus venosus and lateral meso* cardia. (C) Through liver region. (D) Through region posterior to liver. (E) Through posterior trunk in region of urinary bladder.

envelop or enclose the gut tube. This enclosure readily occurs because in this region of the trunk, the gut tube lies closer to the ventral aspect of the embryo than in the heart area. Consequently, a dorsal mesentery above and a ventral mesentery below the primitive gut tube are formed (fig. 25 4C). The dorsal and ventral mesenteries may not persist everywhere along the gut (fig. 254D). The degree of persistence varies in different vertebrates; these variations will be mentioned later (Chap. 20) when the coelomic cavities are discussed. However, there is a persistence of the ventral mesentery below the stomach and anterior intestinal area of all vertebrates, for here the ventral mesentery (i.e., the two medial walls of the lateral plate mesoderm below the gut) contributes to the development of the liver and the pancreas. These matters are discussed in Chapter 13.

Aside from the formation of the dorsal and ventral mesenteries by the inward movement and fusion of the medial walls of the lateral plate mesoderm above and below the primitive enteron or gut tube, that part of the medial walls of the lateral plate mesoderm which envelops the primitive gut itself is of great importance. This importance arises from the fact that the entoderm of the gut only forms the lining tissue of the future digestive tract and its various glands, such as the liver, pancreas, etc., whereas mesenchymal contributions from the medial wall of the lateral plate mesoderm around the


530


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS


entodermal lining give origin to smooth muscle tissue, connective tissue, etc. (figs. 254C, D; 258; 260; 262; 278C). It is apparent, therefore, that the gut throughout its length is formed from two embryonic contributions, namely, one from the entoderm and the other from the mesenchyme given off by the medial walls of the lateral plate or hypomeric mesoderm.

{Note: The word splanchnic is an adjective and is derived from a Greek word meaning entrails or bowels. That is, it pertains to the soft structures within the body wall. The plural noun viscera (singular, viscus) is derived from the Latin and signifies the same structures, namely, the heart, liver, stomach, intestine, etc., which lie within the cavities of the body. It is fitting, therefore, to apply the adjective splanchnic to the medial portion of the hypomere because it has an intimate relationship with, and is contributory to, the development of the viscera. The somatic mesoderm, on the other hand, is the mesoderm of the lateral or body-wall portion of the hypomere. The word splanchnopleure is a noun and it designates the composite tissue of primitive entoderm and splanchnic mesoderm, while the word somatopleure is applied to the compound tissue formed by the primitive lateral wall of the hypomere (somatic mesoderm) plus the primitive ectoderm overlying it. The coelom proper or spianchnocoel is the space or cavity which lies between the splanchnic and somatic layers of the lateral plate or hypomeric mesoderm. During later development, it is the cavity in which the entrails lie.

3) Contributions of the Hypomere (Lateral Plate Mesoderm) to the External (Ectodermal or Epidermal) Body Tube. The somatopleural mesoderm gives origin to a mass of cellular material which migrates outward to lie along the inner aspect of the epidermal tube in the lateral and ventral portions of the developing body (fig. 252A, D). In the dorsal and dorso-lateral regions of the body, contributions from the sclerotome and dermatome apparently aid in forming this tissue layer. The layer immediately below the epidermis constitutes the embryonic rudiment of the dermis. (See Chap. 12.)

4) Contributions of the Hypomere or Lateral Plate Mesoderm to the Dorsal Body Areas. Many cells are given off both from splanchnic and somatic layers of the hypomeric mesoderm to the dorsal body areas above and along either side of the dorsal aorta (fig. 254), contributing to the mesenchymal “packing tissue” in the area between the notochord and differentiating somite, extending outward to the dermis.

5) Contributions of the Lateral Plate Mesoderm to the Walls of the Coelomic Cavities. The pericardial, pleural, and peritoneal cavities are lined, as stated above, by an epithelial type of tissue called mesothelium (fig. 254A-E). These coelomic spaces (see Chap. 20) are derived from the fusion of the two primitive splanchnocoels or cavities of the two hypomeres. External to the mesothelial lining of the coelomic spaces, there ultimately is developed a fibrous, connective tissue layer. Thus, mesothelium and connective tissue form.



Fig. 255. This figure illustrates different types of body form in various vertebrates during embryonic development. A, D, H, M, and Q show primitive embryonic body form in the developing shark, rock fish, frog, chick, and human. B, larval form of shark; E and F, larval forms of rock fish; I and J, larval forms of frog; N and O, larval forms of chick; R, larval form of human. C, G, K, L, P, and S represent definitive body form in the above species. (Figures on rockfish development (Roccus saxatilis) redrawn from Pearson: 1938, Bull. Bureau of Fisheries, L). S. Dept, of Commerce, vol. 49; figures on chick redrawn from Hamburger and Hamilton: 1951, J. Morphol., vol. 88; figure Q, of developing human embryo, redrawn and modified from model based upon Normentafeln of Keibel and Elze: 1908, vol. 8, G. Fischer, Jena; Dimensions of human embryos in R and S, from Mall: Chap. 8, vol. 1, Human Embryology, by F. Keibel and F. P. Mall, 1910, Lippincott, Philadelphia.)


531


532


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS


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

5. Embryonic Mesenchyme and Its Derivatives

The mesenchymal cells given off from the mesodermal tubes of the trunk area, namely, (1) sclerotomic mesenchyme, (2) dermatomic mesenchyme, (3) mesenchymal contributions from the lateral plate mesoblast (hypomere) to the gut, skin, heart, and (4) the mesenchyme contributed to the general regions of the body lying between the epidermal tube, coelom, notochord, and neural tube, form, together with the head and tail mesoderm, the general packing tissue which lies between and surrounding the internal tubular structures of the embryo (fig. 254). Its cells may at times assume polymorphous or stellate shapes. This loose packing tissue of the embryo constitutes the embryonic mesenchyme. (See Chap. 15.)

This mesenchyme ultimately will contribute to the following structures of the body:

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

(b) endothelium of blood vessels, blood cells (Chap. 17),

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

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

(e) connective tissues of voluntary and involuntary muscles (Chap. 16),

(f) connective tissues of soft organs, exclusive of the nerve system (Chap. 15),

(g) connective tissues in general, including bones and cartilage (Chap. 15),

(h) smooth musculature of the gut tissues and gut derivatives (Chap. 16),

(i) voluntary or striated muscles of the tail from tail-bud mesenchyme (Chap. 16),

(j) striated (voluntary) musculature of face, jaws, and throat, derived from the lateral plate mesoderm in the anterior pharyngeal region (Chap. 16),

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

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

(m) tongue and musculature of bilateral appendages, derived from somitic muscle buds (sharks) or from mesenchyme possibly of somitic origin (higher vertebrates) (Chap. 16), and

(n) chromatophores or pigment cells of the body from neural crest mesenchyme (Chap. 12).


SUMMARY OF DERIVATIVES OF ORGAN-FORMING AREAS


533


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

1. Neural Plate Area (Ectoderm)

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. Entoderm AL Area

From this area the following arise:

(a) Epithelial lining of the primitive gut tube or metenteron, including: (1) epithelium of pharynx; epithelium pharyngeal pouches and their derivatives, such as auditory tube, middle-ear cavity, parathyroids, and thymus; (2) epithelium of thyroid gland; (3) epithelial lining tissue of larynx, trachea, and lungs, and (4) epithelium of gut tube and gut glands, including liver and pancreas,

(b) most of the lining tissue of the urinary bladder, vagina, urethra, and associated glands,

(c) Seessel’s pocket or head gut, and

(d) tail gut.

4. Notochordal Area

This area:

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

(b) aids in induction of central nerve tube.


534


BASIC FEATURES OF VERTEBRATE MORPHOGENESIS


(c) gives origin to adult notochord of Amphioxus and cyclostomatous fishes and to notochordal portions of adult vertebral column of gnathostomous fishes and water-living amphibia, and

(d) also, comprises the remains of the notochord in land vertebrates, such as “nucleus pulposus” in man.

5. Mesodermal Areas

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



^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.


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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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